One dimensional nanostructured materials

Progress in Materials Science 52 (2007) 699–913

www.elsevier.com/locate/pmatsci

One dimensional nanostructured materials

Satyanarayana V.N.T. Kuchibhatla a, A.S. Karakoti a,Debasis Bera a,c, S. Seal a,b,*

a Surface Engineering and Nanotechnology Facility, Advanced Materials Processing and Analysis Center,

Mechanical Materials and Aerospace Engineering, University of Central Florida, 4000, Central Fl. Blvd, Eng I,

#381, Orlando, FL 32816, USAb Nanoscience and Technology Center, University of Central Florida, Orlando, FL 32816, USA

c Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32811, USA

Received 1 April 2006; received in revised form 1 June 2006; accepted 1 August 2006

Abstract

The quest for materials with molecular scale properties that can satisfy the demands of the 21stcentury has led to the development of one dimensional nanostructures, ODNS. Nearly, every classof traditional material has an ODNS counterpart. ODNS has a profound impact in nanoelectronics,nanodevices and systems, nanocomposite materials, alternative energy resources and national secu-rity. The interface of nanoscience and technology with biological and therapeutic sciences is expectedto radically improve the ability to provide efficient treatments in otherwise impossible situations. Iron-ically, the huge investment in the past few years across the globe is yet to bring the real benefit of nano-technology in day to day life. While scientists and engineers are working towards this goal, concernsabout the possible harmful effects of the high aspect ratio materials are increasing every day. Follow-ing is an effort to assimilate most of the aforementioned aspects including the entire gamut of ODNS,i.e., elements, ceramics, polymers and composites, with a brief discussion on CNT and toxicology.The focus of this article is mainly on the science behind the synthesis and properties of the ODNSrather than the device fabrication. However, a few challenges in the field of device fabrication arementioned in appropriate contexts. Possible mechanisms of the ODNS evolution from various meth-ods, such as vapor liquid solid (VLS), template based and electrochemically induced growth, havebeen discussed in detail. Electron microscopy analysis has received special focus in determining the

0079-6425/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.pmatsci.2006.08.001

* Corresponding author. Address: Surface Engineering and Nanotechnology Facility, Advanced MaterialsProcessing and Analysis Center, Mechanical Materials and Aerospace Engineering, University of CentralFlorida, 4000, Central Fl. Blvd, Eng I, #381, Orlando, FL 32816, USA. Tel.: +1 407 882 1119; fax: +1 407 8821462.

E-mail address: [email protected] (S. Seal).

mailto:[email protected]

700 S.V.N.T. Kuchibhatla et al. / Progress in Materials Science 52 (2007) 699–913

unique structural features. The article concludes by discussing current research related to environ-ment and toxicology effects and current challenges in this rapidly evolving field.� 2006 Elsevier Ltd. All rights reserved.

Contents

1.

2.

3.

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 706
1.1. Fundamentals of nanostructured materials . . . . . . . . . . . . . . . . . . . . . . . . . . 706
1.1.1. Significance of nanostructured materials . . . . . . . . . . . . . . . . . . . . . . 7061.1.2. Structural aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7071.1.3. Electronic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7081.1.4. Medicine and biosciences. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7081.1.5. Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 708
1.1.6. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 709
1.2. One dimensional nanostructured materials (ODNS). . . . . . . . . . . . . . . . . . . . 7101.2.1. Synthesis and fabrication. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7111.2.2. Properties and applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7121.2.3. Sensing applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7131.2.4. Photochemical and photophysical properties . . . . . . . . . . . . . . . . . . . 716

1 Elemental ODNS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7172.1. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 717
2.2. Synthesis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 718
2.2.1. Template-based synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7182.2.2. Laser ablation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 723
2.2.3. Solution-based synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 724
2.3. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 727
Oxide ODNS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 730 3.1. Titania . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 730
3.1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7303.1.2. Synthesis methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7313.1.3. Structure, morphology, mass transport and phase transformation in

titania nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 748
3.2. Zinc oxide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 752
3.2.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7523.2.2. Structural features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 753
3.2.3. Synthesis processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 753
3.3. Silica . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7593.3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 759
3.3.2. Synthesis process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 760
3.4. Tin oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7623.4.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7623.4.2. Structural features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 763
3.4.3. Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 764
3.5. Vanadium oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7673.5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7673.5.2. Synthesis process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 768
3.5.3. Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 770

S.V.N.T. Kuchibhatla et al. / Progress in Materials Science 52 (2007) 699–913 701

3.6. Other oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7703.7. Applications of oxide ODNS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 772

3.7.1. Catalysis and sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7723.7.2. Energy related applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7753.7.3. Bio-based applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7773.7.4. Other applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 779

4. Nitrides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 782
4.1. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7824.2. Boron nitride. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 782
4.2.1. Synthesis methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7824.2.2. Properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 785

5. Chalcogens and chalcogenides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 788
5.1. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7885.2. Synthesis processes for ODNS of chalcogens . . . . . . . . . . . . . . . . . . . . . . . . 789
5.2.1. Vapor phase routes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7895.2.2. Solution-based methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7905.2.3. Sonochemical synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7935.2.4. Hydrothermal route . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7945.2.5. Biomolecule-assisted synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7955.2.6. Nanoparticles to ODNS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 795

5.3. Synthesis processes for chalcogenides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 797
5.3.1. Template-directed synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7985.3.2. Vapor phase reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7985.3.3. Solid phase reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7985.3.4. Solution-based methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 800
5.4. Properties and applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 801
6. Polymeric ODNS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 802
6.1. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8026.2. Synthesis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 802

6.2.1. Electrospinning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8026.2.2. Tubes from fiber templates (TUFT) . . . . . . . . . . . . . . . . . . . . . . . . . 8076.2.3. Membrane/template-based synthesis . . . . . . . . . . . . . . . . . . . . . . . . . 8096.2.4. Template-free synthesis of polymer ODNS . . . . . . . . . . . . . . . . . . . . 811

6.3. Properties of conducting polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 816
6.3.1. Electronic properties and origin of conductivity in conducting polymers 8186.3.2. Doping of polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8196.3.3. Conduction properties from 3D to 1D . . . . . . . . . . . . . . . . . . . . . . . 820
6.4. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 822
6.4.1. Polymeric ODNS in sensor applications . . . . . . . . . . . . . . . . . . . . . . 8226.4.2. Polymeric nanofibers and nanotubes in medicine . . . . . . . . . . . . . . . . 8236.4.3. Electronic properties of conducting polymers . . . . . . . . . . . . . . . . . . 824
7. Carbon nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824
7.1. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8247.2. History of carbon nanotubes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8257.3. Synthesis processes for one dimensional carbon nanostructures. . . . . . . . . . . . 826
7.3.1. Arc-discharge in gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8277.3.2. Arc-discharge in liquid phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8287.3.3. Laser ablation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8307.3.4. Chemical vapor deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 830
7.3.5. Electrochemical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 833


7.4. Structure of CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 834
7.4.1. Single-walled carbon nanotubes (SWCNTs). . . . . . . . . . . . . . . . . . . . 834 7.4.2. Multi-walled carbon nanotubes (MWCNTs) . . . . . . . . . . . . . . . . . . . 838
7.5. Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 842
7.5.1. Chemical properties, biological applications and toxicity . . . . . . . . . . 842 7.5.2. Chemical sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8457.5.3. Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8477.5.4. Hydrogen storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8497.5.5. Thermal properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8507.5.6. Electronic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8517.5.7. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8567.5.8. Optical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8587.5.9. Magnetic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 858
8. Bio-inspired ODNS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8608.1. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 860
8.2. ODNS sensors in biology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 860
8.2.1. CNT-based DNA and enzyme sensors . . . . . . . . . . . . . . . . . . . . . . . 860
8.2.2. CNT-based DNA biosensor electrodes . . . . . . . . . . . . . . . . . . . . . . . 8618.2.3. Metals and derivatives based biochemical sensors . . . . . . . . . . . . . . . 870
8.3. Drug delivery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 872
8.3.1. Carbon nanotubes in drug delivery. . . . . . . . . . . . . . . . . . . . . . . . . . 872 8.3.2. Metals, polymers and derivatives in drug delivery . . . . . . . . . . . . . . . 873
8.4. Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 875
8.4.1. Peptide nanotubes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 875 8.4.2. Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 878
9. ODNS nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8809.1. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 880
9.2. CNT-reinforced composite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 881
9.2.1. Nonaligned CNTs in composites . . . . . . . . . . . . . . . . . . . . . . . . . . . 881
9.2.2. Aligned CNTs in composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 882
10. Toxicity of nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88410.1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 884
10.2. Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88410.3. Routes of entering human cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 885
10.3.1. Respiratory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 885
10.3.2. Intestinal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88710.3.3. Dermal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 887
10.4. Factors controlling the toxicity of nanomaterials . . . . . . . . . . . . . . . . . . . . . 888
10.4.1. Surface area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 888 10.4.2. Surface chemistry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88810.4.3. Oxidative stress and reactive oxygen species . . . . . . . . . . . . . . . . . . 889
10.5. Future research and trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 891
Summary, challenges and future scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 892 11.Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 893 Appendix A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 894 A.1. Global funding status in nanotechnology. . . . . . . . . . . . . . . . . . . . . . . . . . . 894 A.2. Funding strategy in USA towards nanotechnology from various organiza-
tions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 895A.3. List of organizations and useful websites related to nanotoxicity . . . . . . . . . . 895References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 896

Acronyms

AAO Anodic aluminum oxideAC Alternating currentADS Arc-discharge in solutionAFM Atomic force microscopyAFP Azobenzene functionalized polymerAOT Dioctylsulfosuccinate sodium saltAPS Ammonium peroxydisulfateAPS Ammonium persulphateBCNT Boron neutron capture therapyBET Brunauer–Emmett–TellerBN Boron nitrideCA Contact angleCMC Critical micelle concentrationCNH Carbon nanohornsCNT Carbon nanotubeCNTPE Carbon nanotube paste electrodesCPE Carbon paste electrodesCTAB Cetyltrimethylammonium bromideCV Cyclic voltammetryCVD Chemical vapor depositionDHLA Dihydrolipoic acidDHP Dihexadecyl hydrogen phosphateDNA Deoxy-ribonucleic acidDPV Differential pulse voltametryDS Dodecyl sodium sulphateDSC Differential scanning calorimetryDTAB Dodecyltrimethylammonium bromideEDX/EDS Energy dispersive X-ray spectroscopyEELS Electron energy loss spectroscopyEG Ethylene glycolEIS Electrochemical impedance spectroscopyEL ElectroluminescenceEMI Electromagnetic interferenceEPTC Evaporation physical transport condensationESI Electrostatic interferenceESR Electron spin resonanceFBR Fluidized bed reactorFE-SEM Field emission scanning electron microscopyFET Field effect transistorsFIT Fluctuation induced tunnelingfs Femto secondFTIR Fourier transform infrared spectroscopyGFC Graphitic carbon fiber


HCSA d,l 10-Camphorsulfonic acidHDA HexadecylamineHF Hydrofluoric acidHOPG Highly oriented pyrolytic graphiteHRTEM High resolution transmission electron microscopyHSA Human serum albuminICP Inductively coupled plasmaIF Inorganic fullerenesIPCE Incident photon–photon conversion efficiencyLB Langmuir–BlodgettLED Light emitting diodeLPD Liquid phase depositionMEMS Micro-electro-mechanical devicesMIC Minimum inhibitory concentrationMIT Metal–insulator transitionMR Magnetic resonanceMTP Multiply twinned particlesMWCNT Multi-walled carbon nanotubeNAD Nicotinamide adenine dinucleotideNCA Nanochannel aluminaNEE Nanoelectrode ensembleNEMS Nano-electro-mechanical devicesNF NanofibersNIR Near infraredNP NanoparticleNR Nanorodns NanosecondNSA b-Naphthalene sulfonic acidNT NanotubeNW NanowireOA OleylyamineODNS One dimensional nanostructuresOFET Organic field effect transistorsOLA Oleic acidOTS OctadecyltrichlorosilanePA PolyanilinePAA Porous anodized aluminaPAO Porous aluminum oxidePDLC Polymer dispersed liquid crystalPDT Poly(dodecylthiophene)PECVD Plasma enhanced chemical vapor depositionPEDOT Poly(dioxythiophene)PEI Poly(ethyleneimine)PEO Polyethylene oxidePL Photoluminescence


PLA Poly(lactic acid)PLD Pulse laser depositedPMMA Poly(methylmethacrylate)PPP Poly(p-phenylene)PPV PolyphenylnevinlylenePPX Poly(p-xylene)PPy PolypyrrolePREP Proposed plasma rotating electrode processPSA Potentiometric stripping analysisPSS Poly(sodiumstyrenesulfonate)PV PhotovoltaicPVA Poly(vinyl alcohol)PVD Physical vapor depositionRBM Radial breathing modeRBS Rutherford back scatteringRMSG Reverse micelle mediated sol–gelRu(bpy)3 Runthenium bipyridineSAED Selected area electron diffractionSCCM Standard cubicSCE Standard calomel electrodeSDS Sodium dodecyl sulfateSETOV Single electron trapped oxygen vacanciesSLS Solid–liquid–solidSNF Surface engineering and nanotechnology facilitySPR Surface plasmon resonanceSRG Surface relief graftingssDNA Single stranded DNASSS Solid solution solidSTM Scanning tunneling microscopySvO2 Venous oxygen saturationSWCNT Single wall carbon nanotubeSWV Square wave voltammetryTDPA Tetradecylphosphonic acidTEOS TetraethylorthosilicateTGA Thermogravimetric analysisTHF TetrahydrofuranTIP Titanium isopropoxideTNR Titania nanorodsTON Tungsten oxide nanowireTONF Tungsten oxide nanowire fiberTPR Temperature programmed reductionTS p-Toulenesulfonic acidUV Vis Ultraviolet–visible spectroscopyVGCF Vapor-grown carbon fiberVLS Vapor–liquid–solid


VRH Variable range hoppingVS Vapor solidWGS Water gas shiftXPS X-ray photoelectron spectroscopyXRD X-ray diffraction


1. Introduction

‘‘There’s plenty of room at the bottom’’, the 1959 dream statement [1] of the legendaryRichard Feynman has been realized in less than half a century by consistent efforts andsignificant contributions from the scientific community across the globe. Progress madein past few decades has proven the abysmal nature of matter as a whole and the abilityto achieve exciting technological advancement for the benefit of mankind. From the inven-tion of carbon fullerene structures, carbon nanotubes by Ijima and the equally importantdiscovery of inorganic fullerene structures by Tenne; there have been numerous reportsthat discussed the fundamental and technological importance of novel nanostructuredmaterials. Although the focus of the present article is on the one dimensional nanostruc-tures, to be dealt as ODNS, the significance of nanostructured materials has been brieflydiscussed in the first few pages for completeness. It should be realized that each section inthis article can be written as an extensive review. There are a number of review articlesquoted in the following section that dealt with the ODNS. However, most of them confineto a particular class of material or specific properties but not comprehensively deal withthe entire gamut of the ODNS. The profound importance of synthesis conditions on theproperties has enough appreciation in the materials community. Hence, this article is aneffort to include a brief overview and scientific perspective on the synthesis of every pos-sible ODNS i.e. elements, oxides, nitrides, polymers, chalcogen/chalcogenides, CNTs andcomposites, etc. The readers are highly encouraged to take the lead from this article andbroaden their scope by following the references there in. Important properties and appli-cations of ODNS are also included in the appropriate context to create an appreciation inthe reader about the features that stimulated immense research efforts for ODNS synthe-sis. Authors tried to include as many as possible details in the following pages, however, itshould be understood that an exhaustive description of all the intricacies is beyond thescope of any review article. Hence, an effort has been made to discuss the systems ofour specialization to the maximum extent possible, while briefly outlining the major syn-thetic routes, properties and applications for the other systems.

1.1. Fundamentals of nanostructured materials

1.1.1. Significance of nanostructured materials

The word nano was just a fraction that indicates one billionth of a unit quantity untilrecently; however, the same is redefining the understanding of matter at an extraordinarypace every day. Noble prize winning inventions of bucky balls and carbon fullerene struc-tures, first electron microscope image of the carbon microtubules, later called as carbonnanotubes (CNTs), followed by the invention of inorganic fullerenes and anisotropic


nanostructures can be termed as major breakthroughs in the field of nanoscience and tech-nology. Synthesis of size and shape controlled nanostructures (triangles, cubes, tubes,wires, rods, fibers, etc.), their self-assembly, properties and possible applications are underrigorous research. Realizing the importance of nanotechnology, state of the art technologycenters with excellent processing, characterization and device fabrication facilities arebeing developed.

A number of reviews and text books have been published on the nanoscience and nano-technology, endorsing the status of the 21st century’s leading science and technology basedon fundamental and applied research during the last few decades [2–18]. The significantlydifferent physical properties of these novel materials have been ascribed to their character-istic structural features in between the isolated atoms and the bulk macroscopic materials[9]. ‘‘Quantum confinement’’, the most popular term in the nano-world, is essentially dueto the changes in the atomic structure as a result of the direct influence of the ultra-smalllength scale on the energy band structure. The exceptional electronic, mechanical, opticaland magnetic properties of the nanoscale materials can all be attributed to the changes inthe total energy and structure of the system. In the free electron model the energies of theelectronic states and the spacing between energy levels, both vary as a function of 1/L2,with L being the dimension in that direction [19]. At nanoscale dimensions the normallycollective electronic properties of the solid become severely distorted and the electronsat this length scale tend to follow the ‘‘particle in a box’’ model, might often require higherorder calculations to account for band structure [19]. The electronic states are more likethose found in the localized molecular bonds than the macroscopic solids. The main impli-cation of such confinement is the change in the system total energy; and hence the overallthermodynamic stability. For example, Cr with a bcc structure was reported to be stable inA15 structure at nanoscale and high purity conditions and in a bcc structure in presence ofoxygen impurity [11]. The chemical reactivity, being a function of the system structure andthe occupation of the outermost energy levels, will be significantly affected at such a lengthscale, causing a corresponding change in the physical properties. Recently, size dependantvariation in the oxidation state and the lattice parameter has been reported by our groupin cerium oxide nanoparticles [20]. Combination of both ‘‘bottom-up’’ and ‘‘top-down’’approaches have been efficiently used in producing the functionally important nanoscalematerials and the devices [21,22].

1.1.2. Structural aspects

When the dimensions of a system are reduced to the nanoscale domain number ofatoms at the surface significantly increase along with the increase in surface area per unitvolume.

Specific Surface Area ¼ 4Q

r2

43

Qr3q¼ 3

qr;

hence when the dimensions decrease from micron level to nanolevel the specific surfacearea increases by 3 orders in magnitude. In such a case, large proportions of the atoms willeither be at or near the grain boundaries [10]. While the increase in surface area and thesurface free energy lead to a reduction in the interatomic distance for metals, the oppositewas reported for the semiconductors and metal oxides. Various morphologies obtained inthe nanostructures can be explained through multiple twinned crystalline particles and the


relative surface energies of various crystallographic planes. The dislocations in nanostruc-tures might not be the thermodynamically stable defects; higher internal compressivestrains and reduced vacancy concentration are the characteristics of the nanostructures.These factors were reported to contribute towards the deviations in the mechanical prop-erties of the nanomaterials and an inverse Hall–Petch relation is under rigorous investiga-tion [23]. Another aspect of controversy is the phase stability in systems like tetragonalzirconia, while a clear understanding exists in case of the bulk system, the same has stim-ulated a noteworthy debate in case of undoped nanocrystalline zirconia [24]. It was con-cluded that among various parameters such as nanosize, strain induced grain-growthconfinement, simultaneous presence of monoclinic phase [25]; the oxygen ion vacanciesstabilize the tetragonal phase at room temperature in nanocrystalline system.

1.1.3. Electronic properties

The characteristic electronic properties of the nanostructures are a result of tunnelingcurrents, purely ballistic transport and the coulomb blockade effects. As the dimensionsof the system become comparable with the de Broglie wavelength of the electrons, energybands may cease to overlap. However, owing to their wavelike nature, electrons can tunnelquantum mechanically between two closely adjacent nanostructures. If a voltage is appliedbetween two nanostructures, which aligns the discrete energy levels, resonant tunnelingoccurs abruptly increasing the tunneling current. When all the scattering centers are elim-inated due to extremely small size of the material and purely specular boundary reflectionsresulting from smooth sample boundaries, the electron transport becomes purely ballistic.This particular behavior can be used in wave guide applications. Conduction in highlyconfined structures is very sensitive to the presence of other charge carriers and theircharge state. These coulomb blockade effects result in conduction processes involving asingle electron. Requirement of a very small amount of energy for such conduction canbe utilized in operating switches, transistors or memory elements.

1.1.4. Medicine and biosciencesSignificant number of nanophase materials and nanoscale systems for biological appli-

cations and the use of biological precursors for the synthesis and property enhancement ofthe nanoscale materials are under extensive investigation. One major example is the bio-mineralization of nanocrystallites in a protein matrix, highly important for the formationof bone and teeth. Chemical storage and transport mechanisms within the organs are alsothe fields of major interest. Biomimicry is the process of using biological systems as a guideto synthesize the nanoscale materials. Use of such systems was found to endorse precise-ness and selectivity to the nanostructures synthesized. In addition to the aforementionedaspects biocompatibility of textured implants, cellular engineering, medical sensors anddrug delivery are among the various important fields of interest in the nano-bio-regime[26]. In most of these applications, it is mandatory to have a stable dispersion of the par-ticles with controlled size and physico-chemical properties [27].

1.1.5. Characterization

From the aforementioned arguments and the experience from the dealing with bulkmaterials, it is evident that an appropriate characterization will play a crucial role in deter-mining various properties. Four broadly approved aspects of characterization are


1. Morphology2. Crystal structure3. Chemistry4. Electronic structure

Various techniques that can be used to identify the above mentioned features and thecorresponding resolutions are illustrated in Figs. 1.1.A, 1.1.B and 1.1.C [28].

1.1.6. Applications

Field emission devices for X-ray instruments, flat panel displays and other vacuumnanoelectronic applications are some examples where nanotechnology has already beenintroduced. Nanotechnology has become indispensable for applications to produce probes

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Fig. 1.1.A. Various techniques used to characterize the morphology of materials and their corresponding depthand lateral resolution [28].

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Fig. 1.1.B. Various techniques used to characterize the chemistry of materials and their corresponding depth andlateral resolution [28].

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Fig. 1.1.C. Various techniques used to characterize the crystal structure of materials and their correspondingdepth and lateral resolution [28].


for biological and chemical separation, sensors and their purification, catalysis, energystorage and composites for structural materials, filling materials and coatings. Duringthe last two decades, tremendous research efforts have been dedicated to understand theproperties of nanomaterials, which are of utmost importance for advanced applications.The properties of nanomaterials can be tailored to numerous engineering applicationsvia atomic-level structural control of materials. However, the real challenge lies in the inte-gration of nanostructured materials into the devices and retaining nanostructures in bulkcomponents.

Among the various applications, our group at UCF proved that the nanoscale oxidematerials have better performance ranging from traditional applications like oxidationresistance coatings [29,30] to the mind probing and most sensitive biological applications.We have various collaborative research projects investigating the effect of nanocrystallineparticles in different applications exploring cell longevity [31], radiation protection [32],neuroprotection in retinal cells [33], free radical scavengers [34], etc.

1.2. One dimensional nanostructured materials (ODNS)

A major feature that discriminates various types of nanostructures is their dimension-ality. The word nano, derived from the Greek word nanos, means dwarf. This particularword has been assigned to indicate the number 10�9, i.e., one billionth of any unit. Widelyaccepted threshold for a material to be termed as nano is <100 nm in atleast one dimen-sion [8,9,11,35], hence is the name one dimensional nanostructures for the systems with thelateral dimension in nanometer scale. Although the field of nanotubes (ODNS) hasattained a significant attention after the pioneering work by Ijima [36], formation of hol-low cage polyhedra and fibers from folded chemical compounds is not new to the chemists[37]. Pauling [38] in his article has discussed the formation of fibrous structures in naturallyoccurring minerals like kaolinite, brucite, etc. due to the inherent asymmetry along thec-axis. Tenne [37], has described the origin of misfit compounds analogous to the naturallyoccurring minerals proposed by Pauling. The scope and the exciting properties of the


carbon fullerene structures and the CNT, etc. have driven the chemists and materials sci-entists in quest of the possibilities in the inorganic compounds. After a significant research,the propensity of nanoparticles of 2D layered dichalcogenides to form closed cagestructures was realized. Tenne is the first person to coin the name inorganic fullerene(IF) structures [37], and is still leading the research of ODNS synthesis in variousinorganic systems.

1.2.1. Synthesis and fabrication

Any performance or property of a material primarily depends on the efficiency and pre-cise nature of the synthesis and the fabrication methods. Researchers have studied numer-ous methods to synthesize the ODNS [6,7,39,40]. Different variations of vapor phasesynthesis are widely used along with electrospinning, solution and surfactant based tech-niques and other processes specific to different systems; as discussed in the following pages.Both single and multicomponent ODNS have been synthesized in various exciting andfundamentally different configurations, Fig. 1.2.1 [41].

Xia, Yang and co-workers, from United States, in their excellent review article [7] clas-sified the synthesis of ODNS into four different categories:

1. Anisotropic growth dictated by the crystallographic structure of a solid material.2. Anisotropic growth controlled and directed by various templates.3. Anisotropic growth kinetically controlled by supersaturation or through the use of an

appropriate capping agent, and4. Miscellaneous methods with potential to yield controlled ODNS.

Albeit, the authors have confined their discussion only to the chemical methods, thisclassification takes care of most possible growth mechanisms. Another exhaustive reviewon inorganic nanowires was published by Rao and co-workers [6], from the other part ofthe world, in 2003. To our knowledge this was the first attempt to review almost every pos-sible material synthesized as ODNS.

Along with the synthesis; the fabrication methods have gained importance because ofthe demanding applications. In order to successfully use the unique properties of theODNS it is desired to fabricate them into intricate designs with uniform geometries andprecisely determined structures. Self-assembled high axial ratio nanostructures (HARNS)like twisted ribbons, belts and helical morphologies up to millimeter size have been real-ized. Separation of 1D and quasi-1D nanostructures have been achieved by magneticmethods in specific systems. In order to achieve the difficult task of utilizing ODNS; sci-entists have started developing bio-inspired and bio-mimetic fabrication routes. Comple-mentary biological connectors like antigens, etc. have been used to assemble nanowiresinto specific assemblies. Other side of the spectrum and the most exciting field of researchis the use of ODNS as bio-inspired materials. ODNS are finding crucial applications likemimicking of ion channels, drug delivery and stem cell research, etc. It was claimedrecently that the use of ODNS in drug delivery may soon allow the studies exploringthe brain and nervous system feasible. Use of the nanotube channels as a media for drugdelivery and the 1D photonic materials for imaging is increasing; newer findings can beexpected through this nano-bio-initiative. Recently, a group of scientists have found thatthe white blood cells incubated in dilute solutions of nanotubes treat the nanotubes as theywould the other extra cellular particles – actively ingesting them and sealing them off inside

Fig. 1.2.1. A schematic summary of the kinds of quasi-one dimensional metal oxide nanostructures alreadyreported. (A) Nanowires and nanorods; (B) core–shell structures with metallic inner core, semiconductor, ormetal-oxide; (C) nanotubules/nanopipes and hollow nanorods; (D) heterostructures; (E) nanobelts/nanoribbons;(F) nanotapes; (G) dendrites; (H) hierarchical nanostructures; (I) nanosphere assembly; (J) nanosprings [41].


chambers known as phagosomes. No adverse effects on the cells were reported because ofthe nanotubes.

1.2.2. Properties and applications

Major advantages with the one dimensional nanowires, tubes and fibers are theirextraordinary lengths, flexibility and structure that can allow them to be physically manip-ulated into various shapes according to the design requirements. Besides, quantum effectsare unique in 1D structures resulting in new electronic properties [42]. Mechanical strengthof the ODNS has not only been endorsed to the size but also to the extremely low dislo-cation density. Recently scientists have produced nanotubes of peptides with a Young’smodulus of 19 GPa. In order to measure the properties of such materials; techniques likeatomic force microscopy (AFM), microdevices using ODNS have been developed. These


high strength nanotubes being bio-compatible have made the interest twofolds. ODNS areclaimed to be vital in realizing the fully integrated nano-optoelectronic circuits. Low tem-perature photoluminescence, resolved band gaps and tunable excitonic peaks are proposedas the features that can make the ODNS as excellent components for optoelectronic prop-erties. Photovoltaic and thermoelectric properties of various semiconductor and metallicnanostructures have also been extensively studied. Core–shell and tubular nanowires; witha possible decrease in the thermal conductivity have been studied for their thermoelectricconversion efficiency. CNTs, because of their unique crystal structures, properties (Table1.1) and least boundary scattering, have found applications for their thermal and physicalproperties.

Catalysis and sensors are the major applications for the metal oxide and semiconductorODNS. The fundamental aspects involved in the sensing and catalysis along with the pho-tochemical and photophysical properties of ODNS are briefly outlined below and theapplications are dealt in the sections to follow while discussing various systems.

1.2.3. Sensing applications

Synthesis of the ODNS of almost every class of material has been realized in the pastone decade. Especially metal oxide ODNS have gained a significant attention from the sci-entific community owing to their ability to function as excellent catalysts, bio- and chem-ical sensors [41]. Sensors that work in aqueous media with the ability to sense biological orchemical species have also been developed by Lieber’s group [44], a pioneer in this field.Considering the use of the sensors in biological, environmental, defense and novel energyapplications, it is imperative to have a fundamental understanding of the working princi-ples of these devices [41]. Surface charge dependent and field dependant variation in theconductance of the nanostructures is the major basis of the functioning of the metal oxidenanowire based device configurations. While most of the ODNS configurations; wires,tubes, rods, fibers and belts; have their own fundamental and technological implications;nanowires are most widely recognized for chemical sensing, catalysis, photochemical andphotophysical applications [6]. Precise device fabrication based on the novel ODNS is

Table 1.1Summary of physical attributes of CNTs [43]

Attributes Comments

CNT: metallic, semiconductor and non-conductor (depending on microstructure)

No other known material has this property

Axial electrical conductivity: 108 X�1 m�1 Comparable to that of copperAxial thermal conductivity: 104 W m�1 K�1 Greater than that of diamondCarrier mobility: 104 cm2 V�1 s�1 Greater than that of GaAsSupports a current density of 109 A cm�2 Because of very weak electromigrationOptical-optoelectronic application with

wavelength: 300–3000 nmDirect band gap and one dimensional band structure

Nano-scale heterojunctions Common defect that can create an on-tube heterojunctionAxial Young’s modulus: 1 TPa Stiffer than any other know materialAxial tensile strength: 150 GPa 600 times the strength/weight ratio of steelCNT quantum electronic and low dimensional

transport phenomenaTrue quantum wire behavior, room-temperature field effecttransistor, room-temperature single-electron transistor,Luttinger-liquid behavior, Aharonov–Bohm effect,Fabry–Perot interface


growing from its infancy and a significant progress towards realizing nano-electro-mechanical devices (NEMS) and micro-electro-mechanical devices (MEMS) is underway.Change in the surface electron density by functionalizing the nanostructures can signifi-cantly improve the sensing domain, i.e., material selectivity and the sensitivity. Fabricationof sensing devices, those replace the existing robust designs and have sensitivity down toone molecule, have the precise capability of pattern recognition irrespective of the ambi-ence, demands an ingenious understanding of the underlying mechanism. The use of metaloxide ODNS for the sensing applications has an inherent advantage that comes from thecomprehensive understanding of their physico-chemical properties and existing knowledgein the sensing behavior of nanoparticles and films. Author’s group has proven results forthe use of oxide nanoparticles and ODNS for room temperature sensing applications. Inaddition to the experimental results, Shukla et al. have developed the constitutive equa-tions [45–54] incorporating various parameters involved in the sensing mechanism.

The reasons behind the overwhelming interest in the use of ODNS are multi-fold withthe first being the large surface-to-volume ratio rendering the ability for more surfaceatoms to participate in the surface reactions. The transpiring surface chemistry dependentsensing property of these anisotropic nanoscale materials results from the Debye length,which is comparable to the nanowire diameter. The increased electron and hole diffusionrate to the surface of the device could facilitate the quick desorption of the analyte mol-ecule from the surface, essentially controls the response and recovery times. Higher cry-stallanity in the semiconducting nanowires can potentially reduce the instabilityassociated with the percolation or hopping conduction in the multigranular oxides. Thesurface processes in the nanomaterials can be controlled electronically by varying the posi-tion of the Fermi level within the band gap of the nanowire configured as a three terminalfield-effect transistor, FET. Increasing quantum confinement effects with the decreasingnanowire dimensions is also expected to have a potential impact on the performance ofthe nanowires in various applications. Fig. 1.2.2 shows a few processes that can occurat the surface [41].

Fig. 1.2.2. A summary of a few of the electronic, chemical, and optical processes occurring on metal oxides thatcan benefit from reduction in size to the nanometer range [41].


Sensitivity down to parts per billion can be achieved for CO, ethanol and NO2 usingnanobelt sensor [55]. The variations in the oxygen desorption and adsorption dynamicsas a function of ambient environment was noted to be a result of variation in surface elec-tron density due to a change in surface oxygen vacancy concentration. While the nano-wires were found to behave as highly doped semiconductors or quasi-metals with highconductance, with a weak dependence on temperature, at high temperatures and inertatmosphere, their activation energy was found to significantly increase with oxygen expo-sure. The common features governing the sensing mechanism are (there might be excep-tions that are not mentioned below)

• Stoichiometric oxides are relatively inert.• Moderate annealing in vacuum, inert or reducing atmosphere or UV exposure has the

capability to desorb oxygen from the surface creating surface oxygen vacancies andresult in better sensing capabilities. This emphasized the subtle interplay between theoxygen surface chemistry and its conductance.

• Each vacancy can result in an intragap filled state, near to the conduction band in sucha way that a large fraction of electrons in the donor state is ionized even at lower tem-perature – essentially converting the material into an n-type semiconductor.

• The factors that determine the electron concentration in the bulk of the material andthe relative concentrations of the ionized surface vacancy states also determine the con-ductance of the nanowire, G = pR2eln/L, at a given temperature.

• Presence of a number of species, especially water and organic contaminants, in a realworld environment leads to a complicated situation as the surface hydroxyl ions andthe hydrocarbons might permanently modify the reaction sites.

• External field or doping of the material might result in controlled molecular adsorption;Fig. 1.2.3 shows the alternating donor and acceptor behavior reported for NH3 adsor-bate as a function of doping level of In2O3 NW [56].

A major feature of scientific interest is the inconsistency in the results reported by var-ious groups for closely related systems [41], which could be ascribed to the differences inadsorbed species and the ambient conditions. Contact effects, adsorbed contaminants andimpurities in the support layer in the proximity of the nanostructures can all lead to

Fig. 1.2.3. Alternating donor (right plot) vs. acceptor (left plot) behavior of NH3 adsorbate as a function of thedoping level of an In2O3 nanowire (reproduced with permission from [56]).


current instability in similar systems. Hence, it is always an untold requirement to havesignificantly similar environments before confirming their use in various situations ofinterest. Following are some typical configurations of ODNS sensors from semiconductormetal oxide, Fig. 1.2.4 [57].

1.2.4. Photochemical and photophysical properties

Realizing the size confinement effects in the ODNS has stimulated the researchers toexplore the photochemical and photophysical properties of various anisotropic nanostruc-tures. Some scientific aspects proposed and the observations made in different materialswith the possible reasons and typical applications are summarized below:

• Sharp and discrete features in the absorption spectra and relatively strong ‘‘band-edge’’photoluminescence (PL) are observed – might be a result of quantum confinementeffects and surface states.

• Crystallographic orientation dependant signatures and excitation energies [58] – mightfind applications in future photonic circuits, optical interconnects and switches, etc.

• Anisotropic PL, highly polarized along the longitudinal direction – this feature of nano-wires was explained based on the large dielectric contrast between the nanowire and thesurrounding environment as opposed to quantum mechanical effects such as mixing ofvalence bands [59].

• Efficient migration of electrons and holes to the surface of the nanostructures allowsthem to participate in chemical reactions before recombining – a vital aspect forenhancing the efficiency of solar cells.

• Two possible mechanisms for the photo-response of ODNS [7,41].– Direct excitation of electron–hole pairs, which produce a photo-current that is influ-

enced by the bias.– Photo-induced desorption of ionosorbed species through a photochemical reaction

of the form h+ + A� ! A�, where A� = O�, O2�, NO2�, etc.

Fig. 1.for the

* Above mentioned effects are exploitable in low temperature sensing applications.

2.4. Schematic diagram describing the different forms of semiconductor oxides investigated in the literaturegas sensing applications [57].


• As the nanorods, with properties that can be tuned by changing the aspect ratio, canprovide a direct path for electrical transport at much lower loading – might have impli-cation in photovoltaic applications.

• The enhanced surface plasmon resonance (SPR) and fluorescence in nanorods of Auand Ag, coupled with their bio-inertness make them potential contestants for in vivooptical imaging applications [60,61].

• Single crystalline and well faceted nanowires can function as effective resonance cavi-ties; concluded after studying the lasing properties in various materials [62–64].

• Nonlinear optical properties (NLO) depend on the crystallographic structure of a med-ium, as well as the polarization scheme [65].

• Oxygen chemisorption plays a profound role in enhancing the photosensitivity of oxideODNS.

• The light induced transition between conducting and insulating properties of nanowirescan be exploited in fabricating ON and OFF switches [66].

Aforementioned summary gives a detailed image of the fundamental understanding ofthe nanoscale materials and their integral part, the ODNS. Albeit the attempts are signif-icant, it still seems to be a long way to have a dream device like an electronic nose, a bodychip or NEMS that can have direct impact on the common man’s life. In order to reachsuch a goal at the earliest, the scientific community across the globe has put forward alltheir efforts, supported by the respective governments (see Appendix A). The importanceof controlling the structural features in realizing the precisely controlled properties isuniquely accepted and the efforts are very clear from the ocean of existing and emergingliterature in the field of ODNS synthesis and properties. Following, is an attempt to puttogether the unique features of this 20th centuries rarest academic beasts, (as termed inthe Berkeley science review [67]) the interdisciplinary nanotechnology. While most ofthe previous reviews confined their discussion to one particular class of material or a groupof synthesis strategies, current article includes almost every possible system from elementsto compounds, CNTs, with a special focus on polymer ODNS and the ODNS composites.Bio-nano, the buzz word of the current time is also discussed in a special section. Owing tothe vast nature of the existing literature and other non-technical restrictions, it is also notpossible to have an exhaustive discussion of every possible system. Hence, after emphasiz-ing systems of wide interest along with some basic properties and applications, details ofother systems are tabulated with corresponding references. Authors highly recommend thereaders to use this article as a navigator to explore the horizons of this abysmal topic byreferring to the dedicated groups in systems of their interest. Various challenges, futurescope and the safety concerns pertaining to the nanostructures, particularly the ODNS,are augmented at the end of the article.

2. 1 Elemental ODNS

2.1. Overview

While the solids with layered structures could be formed into nanotubes by carefullycontrolling the experimental conditions, templating against preformed ODNS like nano-wires, nanorods; porous membranes of alumina and polycarbonate, etc. were exploitedto obtain anisotropic nanostructures of other systems; especially the metals. The noble


metals like Au, Ag, Pt and Pd have proven applications in optics, electronics and catalysis.Especially in nanoscale, the physical and the photophysical properties have stimulatedboth fundamental and technological interest. The growing interest in the ODNS of thenoble metals can be ascribed to their enhanced surface plasmon resonance properties thatare important not only for understanding the fundamental behavior of the nanostructuresbut also for novel electronic and optical applications. Arc discharge [68], UV irradiationphotoreduction [69] and reduction of AgNO3 with a developer in presence of AgBr andpoly(vinyl alcohol), laser ablation [70,71] have all been used for the synthesis of noblemetal anisotropic nanostructures. Nanowires of diameters as low as 0.6 nm have been pro-duced, possibly the smallest to the best of our knowledge [72]. Most of the synthetic meth-ods suffer from the lack of morphology control, polycrystalline structure, low yields andunder-controlled aspect ratios, etc. [73]. However, the wet chemical, soft and hardtemplate synthesis of noble metal ODNS have been utilized to conquer these problems[74–77]. Hence, we have discussed these two methods in more detail and the correspondingproperties and applications are elucidated.

2.2. Synthesis

2.2.1. Template-based synthesis

The unique advantages of precise size and shape control directed by the preformed tem-plate have lead the fabrication of single and multi-walled nanotubes, nanowires and nano-rods of noble metals [78,79]. Chemical, electrochemical and vacuum deposition techniqueshave all been used to deposit the metals in the nanoporous templates [80]. This section isdedicated to the synthesis of noble metal ODNS using hard template methods and the softtemplate methods (micelle based) have been dealt under solution based synthesis tech-niques. Researchers [81,82] have exhaustively studied the fabrication of the anisotropicgold nanostructures using the template based synthesis methods. They have used electro-less plating of Au using polymeric and Anodized Aluminum Oxide (AAO) membranes toprepare the gold ODNS [83], functionalized them [84] and explored the various possibleapplications such as chemical and biosensing [85], etc. Some of the applications are dis-cussed in the later sections. Galvanic replacement reaction between the silver nanowiresand the other metals was exploited to fabricate respective metal nanotubes in large quan-tities [86]. The combination of galvanic replacement reaction with electroless plating of sil-ver could result in the multi-walled nanotubes. The silver nanowires were synthesizedthrough a modified polyol process, explained [73,77,87] in the next section. The character-istic pentagonal cross-section and the flat side surfaces were inherited to the nanotubes,Fig. 2.1. The galvanic replacement reaction between the Ag and the HAuCl4 can beexpressed as following, Eq. (2.1):

3AgðsÞ+ HAuCl4ðaqÞ !AuðsÞ+ 3AgClðaqÞ+ HClðaqÞ ð2:1Þ

By repeating the galvanic replacement and the electroless plating reactions co-axialnanotubes with multiple walls were synthesized, Au/Ag and Ag/Pd, etc. to name a few.Assembly of gold nanoparticles on the CNTs, Fig. 2.2 [88] and the DNA assisted self-assembly of gold nanowires [89] have also been achieved. Chloroformic dispersion of goldnanoparticles mixed with CNT was found to show a reduction in the intensity of the char-acteristic dark red color [88]. The decrease in the intensity was imputed to the binding

Fig. 2.2. TEM images of gold nanocrystals self-assembled on a multi-walled carbon nanotube prior to andfollowing firing in air at 300 �C for the indicated time (reproduced with permission from [88]).

A B C

Fig. 2.1. Schematic illustration of the experimental procedure used generate metal nanotubes that arecharacterized by co-axial, multiple walls; (A,C) template engaged replacement reaction between Ag and HAuCl4and (B) electroless plating of Ag note that the cross-sections and side surfaces of the tubular nanostructure aredetermined by the template [86].


between the gold nanocrystals and the CNT, which was continuously monitored spectro-scopically. Gold NWs of up to 30 lm length were realized using the CNT templated self-assembly of the nanocrystals. Polycrystalline nanowires can be obtained by heating the


nanotubes at 300 �C for �120 s. Melting of the surface atoms of nanocrystals was found tooccur at 30 s, followed by fusion of the nanoparticles (35–60 s) and the formation ofunbroken nanowires with further heating, supported by TEM analysis.

Highly ordered metal nanotube arrays with uniform wall thickness and diameter alongthe entire tube were synthesized using a multistep template replication and electrodepos-ition [90]. Metals such as platinum, gold, bismuth, indium and nickel can be electrodepos-ited into preformed nanochannels to form co-axial sandwich nanostructure consisting of anickel oxide core, metal intermediate layer and an AAO sheath. The nickel oxide and theAAO membrane were subsequently dissolved chemically to expose the produced metalnanotubes. The detailed synthesis is schematically shown (Fig. 2.3) with typical micro-structures of the template and the final nanotube.

Dip-coating of the porous alumina membrane in AgNO3 containing solution followedby drying and thermal decomposition, with subsequent dissolution of the AAO templatewas used to produce the Ag nanotubes [91]. Pt was electrochemically deposited in AAOmembranes to produce uniform nanowires [92]. In a typical electroplating cycle, electro-chemical deposition was achieved through a metallic conductive layer thermally evapo-rated on one side of the AAO membrane, which acts as a cathode, and electroplating isfurther carried out through the AAO nanochannels, being controlled by the applied volt-age and the current. Although, nanowires, thin walled nanotubes and thick walled nano-tubes have all been fabricated, because of their relative mechanical stability nanowireswere found as a major component in the final product. Bao [93] and Brumlik [94] haveseparately synthesized Ni and Au tubular structures by modifying the AAO surface withamino group and organocyanide groups respectively. While the need and the role of sur-face functionalization in order to obtain tubular structures is significantly projected, Yooet al. [92] have also shown that the nanotubes are attainable under variable current den-sities. This feature was attributed to the different growth phenomena namely the bottom-up and the wall surface growth. In the bottom-up growth mechanism, low-current–densitycondition, metal gets slowly electroplated and preferentially deposits on the electrode sur-face producing completely packed nanowires in the channel direction. In a high-current–density condition, wall-surface growth pattern, the high electric field concentrated at theedge tips of the tube shape surface structure, the electroplating will be preferential throughthe surface of the AAO channels. The field dependent growth was exploited to producecore–shell nanotubes of Ni, Co, Ag, etc. The role of functionalization of the templatesfor efficient fabrication of ODNS was critically studied with the help of silica template[95]. Silica rod surfaces were functionalized with silanol, amine or thiol groups, and thesilica–silver core–shell nanorods were produced. The core–shell nanorods, when chemi-cally etched with 10 M HF solution result in silver nanotubular structures. While electro-static attraction between the silanol and the amine functional groups lead to theadsorption of silver on the silica surfaces, a chemical interaction between the silver andthiol group achieved the task in case of thiol functionalization. The interaction strengthsbetween the functional group and the silver nanoparticles was found to increase assilanol < amine < thiol. Hence it was confirmed that the formation of silver nanotubesis highly dependant on the type of functionalization. Some established facts for the afore-mentioned process are [95];

• High metal loading capacity because of the densely immobilized functional group.• High sensitivity of metal nanoparticles by changing the reducing agent.

Fig. 2.3. (1) Flow chart of multi-step template replication process for controlled MNT arrays. (2) SEM images of(A) CNT grown on AAO inset – magnified image (micron bar – 200 nm). (B) The metal NT array after thedissolution of AAO template and the nickel oxide cores. (C) Top view of the MNT array (reproduced withpermission from [90]).


• The ease in controlling the size and the shape of resultant structures by adjusting thedimensions of the functionalized templates.

• Wide range of metals can be used by selectively choosing the functionalized templates.


Typical electron micrographs of the silver nanotubular structures, resulted after etchingthe silver coated thiol-functionalized silica with 10 M HF, are shown in Fig. 2.4.

The crystal overgrowth of gold, silver and palladium on preformed Au nanorod seedswere systematically studied [96]. Preferential reduction of metal ions on various Au nano-rod surfaces, providing the information about controlling the composition, aspect ratio,shape and facet of the resulting nanostructure were critically analyzed. It was reported thatthe end {111} facets are more amenable to over growth than the side {110} facets of theAu seeds. The role of silver ions as ‘‘shape regulating agent’’ was also explored in thisstudy along with the synthesis of Au–Ag; Au–Pd core–shell nanorods. The single crystal-line and polycrystalline nature of resulting nanostructures was attributed to the latticeparameter relation between the metals involved.

Photochemical reduction of tetrachloroaurate in presence of highly constrained ionogeltemplates, of 1-decyl-3methyl-imidazolium chloride in water, was used for studying thesynthesis and the organization of gold nanoparticles [97]. After the synthesis of single

Fig. 2.4. SEM and TEM images of silver nanotubes. (A) Entire SEM image (inset; SEM–EDX analysis data);(B) magnified SEM image of specific sample in (A) (inset: the area marked in (B) by box is shown at highmagnification); (C) magnified TEM image of hollow nanostructure in silver nanotubes; (D) selected area electrondiffraction pattern of silver nanotubes (reproduced with permission from [95]).


crystalline Te nanorods and nanowires in an ionic liquid [98]; under micro-wave irradia-tion, there has been growing interest in utilizing these nanostructured anisotropic solventsfor the synthesis of non-spherical metallic materials [97].

2.2.2. Laser ablationAlthough the exact mechanism is still under open discussion, laser assisted techniques

have been widely used in the fabrication of anisotropic nanostructures. While in somecases, the laser beam was used to evaporate the solid precursor for further processing, oth-ers have used to distort (or melt) the precursor particles to yield non-spherical nanostruc-tures. The fluence, wavelength, and pulse duration were found to be the key in achievingthe desired final product through laser ablation. Heating, formation of excited species,phase transformation, and chemical reactions make the ablation process complex; whichwere also found to affect the thermal and the optical properties of the material under con-sideration. The absence of any free (transparent) plasma and negligible heat diffusion intothe material has made the short pulse lasers more attractive. In the fabrication of Aunanorods from the near spherical gold nanoparticles; El-Sayeed [71] has proven the afore-mentioned fact by critically studying the effect of different synthesis parameters. TEM andoptical absorption spectra were used to monitor the changes in the particle morphology. Itwas observed that the use of 100 fs was significantly effective compared to that of 7 ns withthe same energy, having its origin in the possible localized melting and energeticallyfavored diffusion. Later, a group of scientists from Germany [99], have reported the for-mation of metal particle NWs induced by intense, ultra-short, linearly polarized laserpulses to change the form. Metal particle NWs were synthesized in the form of alignednanoparticle array due to laser polarization [99]. Typical electron microscopy images asa result of laser polarization of a plasma polymer film having metal nanoparticles thatalign into parallel rows of nanowires are shown in Fig. 2.5.

Fig. 2.5. TEM micrographs of a plasma polymer film containing silver nanoparticles before (a) and after (b) laserirradiation. (c) The laser SEM micrograph of the gold nanowire ensemble generated by irradiation of linearlypolarized laser pulses. The laser polarization direction (pol) is shown by the arrow (reproduced with permissionfrom [99]).


2.2.3. Solution-based synthesis

The importance of isotropic solution based methods for the synthesis of anisotropicnanocrystals results from a number of unique features. The main factors that determinethe success as summarized by Jana [100] are

1. Control of nucleation and growth, restricting the size to the nanometer regime [101].2. Maintaining a high monomer concentration that induces better stability of anisotropic

nanoparticle embryos [102].3. Use of a suitable surfactant, surfactant mixture, or capping molecules that selectively

adsorb on specific planes of growing particles and induces symmetry-breaking steps[101,103–105].

Scientists have made a significant contribution towards the size controlled synthesis andself-assembly of the anisotropic gold and silver ODNS [74,75,106–108], that can be scaledup to higher yields [100]. Ag seed (4 nm) mediated growth of nanorods and nanowires ofAg was reported in the presence of cetyltrimethylammonium bromide (CTAB) micelles[75]. The reduction of silver salts by ascorbic acid in presence of the rod like micellar solu-tion was found to yield high quantities of ODNS of Ag. Relative amounts of NaOH in thereaction media were found to control the formation of wires and rods. While the nanorodswere found have aspect ratio 2.5–15, 1–4 lm long nanowires were found to have 12–18 nmdiameters. In an almost similar method, cylindrical gold nanorods were produced with�4.6, 13 and 18 aspect ratios [74], Fig. 2.6. Controlled centrifugation was used to separatethe rods and wires from the spherical particles and surfactant.

The synthesis of seed nanocrystals and the growth of nanorods in the micelle templatesis the two step principle behind the seed mediated synthesis of nanorods. Addition of pre-formed seeds is expected to result in an increased reaction rate and the particle size control[106]. Although a clear evidence and mechanism was not presented, presence of Ag+ wasfound to be essential for the aspect ratio control. Authors hypothesized that the Ag+

adsorbs on the surface of Au particle as AgBr and restricts the growth [106].Later it was realized that the role of CTAB is not a ‘‘soft template’’, rather it selectively

adsorbs on the long axis crystal faces of the growing nanorods [109]. It was also reportedthat the addition of heptane could significantly control the aspect ratio of the Au nano-

Fig. 2.6. (a) TEM images of 4.6 aspect ratio gold nanorods, (b) shape separated 13 aspect ratio gold nanorods,and (c) shape-separated 18 aspect ratio gold nanorods. The scale bar (100 nm) applies to all three images(reproduced with permission from [74]).


rods. The bent nanorods obtained in this procedure were proposed to be a result of defectsinduced in the adsorbing CTAB bilayer by the heptane. Slowing the growth steps throughcontrolled variation in the parameters was reported to result in nanowires with the aspectratio �200.

Uniform silver nanowires were produced by reducing AgNO3 with ethylene glycol [87].Presence of poly(vinyl pyrrolidone) and the Pt (or Ag) seed nanocrystals was found to bekey in driving the anisotropic growth of Ag into nanowires with an aspect ratio as a highas �1000. In the proposed polyol process, ethylene glycol plays twin roles as reducingagent and the solvent. Some of the electron microscopy images from the work and differ-ent stages of wire growth are shown in Fig. 2.7.

The crystal structure and the morphology of the gold nanorods have been criticallystudied [110,111]. While the one group has emphasized on the stabilization of the highlyunstable (110) surface by the surfactant molecules, the other have used their electron dif-fraction data to propose the formation of 3D prismatic morphology with ten {111} endfaces and five {100} or {110} side faces or both. The proposed morphology and the cor-responding HRTEM image are shown in Fig. 2.8. It was proposed that the diffusion of Auat the weakly bonded defect sites of multiply twinned particles (MTP) drives the aniso-tropic growth of pentagonal nanorods. It was further claimed, that the data [110] indicated‘‘the symmetry breaking in FCC metallic structures to produce anisotropic nanoparticlesis based on an intrinsic structural mechanism (twinning) that is subsequently modulatedextrinsically during growth in solution by specific adsorption of AuI–surfactant complexeson the side faces/edges of the isometric penta-twinned crystals and is responsible for thepreferential growth along the common [87] axis’’. They also proposed that the couplingof multiple twinning and habit modification is a general mechanism that applies to otherexperimental procedures (electrochemical, inverse micellar media) currently used to pre-pare metallic nanoparticles with a high aspect ratio’’.

Realizing the importance of the field assisted nanorod synthesis, a group of scientistsfrom Australia and Spain [112] have critically studied the process, the controlling

Fig. 2.7. (1) Schematic elucidating the mechanism of nanowire formation [87]; corresponding electronmicroscopy images shown as (2) (A, B) TEM and (C, D) SEM images of four as-synthesized samples of silvernanowires, showing different stages of wire growth (A) 10, (B) 20, (C) 40, and (D) 60 min (reproduced withpermission from [73]).

Fig. 2.8. Model for nanorod growth: (a) MTP. (b) Growth of nanorod along [100] via Au diffusion at twin sites.(c) Projection of the nanorods with (110), (111) faces and [100] direction, some of which are observed in theHRTEM image shown on the right-hand side (reproduced with permission from [111]).


parameters and the kinetics of the nanorod formation. It was proposed that the rate deter-mining step in the nanorod formation is not the electron transfer but the collisionsbetween the micelles. After doing some theoretical analysis, it was proposed that therod formation is a consequence of double layer interaction between the micelles and theparticles. Despite the constant electric potential on the seed, the continuously changingcurvature requires higher electric fields at the points of curvature. The double layer inter-action information was proposed to resolve the effects of salt on the rod formation. Whilelower solubility of the longer chain analogues CTAB hinders their use, the shorter chainanalogues like dodecyltrimethylammonium bromide (DTAB) were found to provide selec-tivity in the gold nanorod formation. In order to keep the essence of the analysis intact, theobservations are reproduced here [112]

1. With colloidally stabilized seeds pre-coated with CTAB, gold nanorods may bereproducibly prepared from the same seed solution for over one month.

2. Decomposition of the reducing agent (NaBH4) in the seed solution reduces second-ary nucleation during rod growth.

3. Higher the stability of the seed, higher the nanorod yield hence, dimers or coales-cence are not precursors to rod formation.

4. Higher yields of nanorods can be achieved with stable seeds and homogeneousgrowth in stirred, convective solution.

5. Despite the inability to yield rods, Br� is a better rod-inducing agent in presence ofCTA+ ions.

6. Use of CTAB and lower ionic strength of the solution lead to better productivity.


7. The aspect ratio is controlled by the seed to HAuCl4 ratio, with the higher surfactantleading to lower aspect ratios.

8. AgNO3 can significantly increase the yield of rods for the CTAB stabilized seeds[113].

9. Both AuCl�4 and AuCl�2 adsorb quantitatively to the CTAB.10. The rate of Au rod formation is orders of magnitude slower than the mass transfer

limit, and significantly, it is also orders of magnitude slower than the rate observedfor the same system when the seeds grow as spheres in the absence of surfactant.CTAB not only directs Au to the particle tips, but also drastically retards the rateof metallic gold formation [114].

11. The higher the curvature of the gold surface, the faster the growth rate of the rods.

So, it is imperative to note the ability and the functioning of surfactant in the synthesisof metallic nanorods, composite nanorods, etc.

2.3. Applications

Shape control of the nanoparticles was expected to result from the dependence of liquidcrystalline ordered structure on the shape anisotropy. Formation of liquid crystallinephases in concentrated solutions of high aspect ratio (13–18) gold nanorods was alsoobserved using polarizing microscopy, electron microscopy and small angle X-ray scatter-ing [108]. The combination of liquid crystalline ordering with electric field induced switch-ing is expected to be a potential contestant in the future optoelectronic applications. Thenanorods were found to show a difference in their optical absorption as a function of theirshape. The morphology dependant UV–Vis absorption spectra are shown in Fig. 2.9.

Controlled synthesis of nanorods and nanowires with variable aspect ratios using aseeded growth mechanism [104] was also achieved. Excitingly different colors of the solu-tions containing the anisotropic nanostructures of silver are shown, with the corresponding

Fig. 2.9. UV–Vis absorption spectra of nanorod solutions before and after shape separation. Concentrationswere arbitrary and solutions were diluted for spectral measurements. D2O was used as the dispersing medium dueto the high absorbance of water at NIR region (reproduced with permission from [105]).


optical spectra, Fig. 2.10. While the left most bottle contains the nanocrystalline seed par-ticles, the right most bottle contains the nanoparticles with highest aspect ratio.

A significant feature of the nanotubes that was explored at very limited scale is the var-iation in the extinction spectra as a function of number of walls [86]. It was shown that themajor surface plasmon resonance (SPR) peak at 710 nm for the single walled nanotubeswas red shifted to 815 and 952 nm for two and three walled nanotubes, Fig. 2.11. It

Fig. 2.10. Aqueous solutions of silver nanoparticles show a beautiful variation in visible color depending on theaspect ratio of the suspended nanoparticles: far left in the photograph, silver nanospheres 4 nm in diameter thatare used as seeds in subsequent reactions; (a–f) silver nanorods of aspect ratio 1–10. The corresponding visibleabsorption spectra for (a)–(f) are also shown (reproduced with permission from [104]).

Fig. 2.11. Normalized peak intensities of extinction spectra of aqueous dispersions containing single-, double-,and triple-walled nanotubes made of Au/Ag alloy (reproduced with permission from [86]).


was speculated that the presence of dielectric liquid in between the walls of dispersed NTseliminates any possible effects of plasmonic coupling between different walls, and hence theextinction peak corresponding to transverse SPR mode must be a characteristic of the out-ermost wall of the nanotubes. These novel composite multilayered nanotubes might serveas substrate for the surface enhanced Raman-spectroscopic detection of molecules.

While the noble metals are seen as the most important materials to study the molecularscale phenomena, use of the ODNS of these materials in a plethora of applications is also

Table 2.1Synthesis methods of various elemental semiconductor ODNS

Material ODNS Synthesis method References

Si NW Thermal evaporation [121–128]V–L–S method [129,58,130–132]Laser ablation [126,133,134,132,135,136]Solution phase [137]

[58,138]

Ge NW V–L–S method [138–142]Solution phase [143–145]Thermal evaporation [146]Laser ablation [147]

B NW Sputtering [148–151]Laser ablation [152,153]Vapor transport, VLS [154]

Table 2.2Synthesis methods of transition metal ODNS


Fe NW Pyrolysis [155]UV irradiation [156]

NR Aqueous chemical growth [157]

Co NW Template based [158–160]

Ni NW Template based [160–165]NT Solution based [93,166]

Template based [93]

Cu NW Template based [167,168]Acqueous reduction [169]

NT Template based [170]

Zn NB Carbothermal reduction [171]NW Template based [172]

Evaporation–condensation–deposition [173]NT Thermal evaporation (catalyst and template-free) [174]

W NW Pyrolysis–carbothermal reduction [175]Hydrothermal treatment [176]

Mo NW Two step deposition and oxide reduction [177]

Pd NW Template assisted deposition [178,179]

Table 2.3Synthesis methods of metallic ODNS (miscellaneous)


In NW Template based [180]Pb NW Template based [181]Sb NW Template based [182]Bi NW Template based [183]

Room temperature extrusion [184]Rh NW Template based [185]


equally stimulating the researchers. As mentioned earlier, a spectrum of applications foranisotropic gold nanostructures have been reported. The ion transport properties of thenanotube ensembles in polymer membranes can be controlled by either chemical function-alization or by potentiostatically charging the membrane in the electrolyte solution[115,116]. A comparative study of the size-based [117], charge-based, chemical- andelectrochemical interaction [118,119] based transport selectivity of the novel gold–polymercomposite membranes [120] are all significant in various applications.

Tables 2.1–2.3 gives the details of the synthesis of ODNS of semiconductors, transitionmetals and some of the other elements (nonexhaustive).

3. Oxide ODNS

Oxide based materials find widespread applications in personal safety, public security,medical diagnosis, detection of environmental toxins, semiconductor processing, agricul-ture, energy related, automotive and aerospace industries, etc. The major goal of oxideODNS is the design of nano-sensors and catalysts that could be easily integrated withmodern electronic fabrication techniques. Most of the applications are highly sensitiveto changes in the surface chemistry of the materials, to their chemical environment andmost sensors are based on appropriately structured doped oxides. Hence, the synthesisof the oxide ODNS plays an important role. Novel synthetic strategies are being exploredto attain this job with precision. Oxides being the field of our major interest, maximumfocus is devoted to this particular section.

3.1. Titania

3.1.1. Overview

Titanium dioxide (titania, TiO2) is one of the most important transition metal oxides,with potential applications in UV-ray shielding, paint, cloth, paper industries, anti-foggingand self-cleaning devices, electrochromic devices and solar cells, etc. Their ability todecompose carbonic acid gases and to protect the environment has increased the interestin novel photocatalytic applications [186]. The crystal phase dimensions and shape controlthe performance in various applications. Higher surface area being the most desired fea-ture, nanoscale anisotropic TiO2 synthesis has received significant attention. Titania nano-tubes [187], nanorods [188], nanowires [189–191] and nanobelts have all been synthesizedalong with various titanates [192] and nanotube composites [193]. Template based, hydro-thermal, sol–gel and anodic oxidation were used for synthesizing the nanotubular struc-


tures of titania and various conditions for tailoring structural, morphological featureswere established. The TiO2 ODNS have proven applications in catalysis, hydrogen sen-sors, tissue growth and oxygen generation, solar cells, rechargeable batteries, etc. In spiteof the research by a number of groups across the globe, the mechanism of nanotubegrowth and the reasons for enhanced performance are still a point of debate. Proposingan early ingenious model might lead to many more characteristic applications in nearfuture.

3.1.2. Synthesis methods

3.1.2.1. Template based synthesis. Hoyer first reported the electrochemical template basedsynthesis of titanium oxide nanostructure [194]. The electrochemical basis for the NT for-mation can be represented as shown below, Eq. (3.1):

nTiOH2þ+ mH2O! [TiIVOx(OH)42x]n + 3nHþ+ ne ð3:1Þ

The schematic shown here and the SEM image indicate the replication process and thecross-section of the titania nanotubes, Fig. 3.1.

A three step NT formation mechanism was originally proposed [194]:

• Partly soluble titania is formed at the electrode surface and is deposited on the polym-ethylmethacrylate (PMMA) rods and on the electrode.

• When the titania deposit on the PMMA rods is thick enough to ensure good conduc-tivity, TiOH2+ is oxidized preferentially at the more elevated tubular parts of thePMMA rods.

• As some of the oxidized species are soluble, titania is deposited on the wall of thePMMA rods. Any Ti(IV) trying to enter the free space between the tubes will be depos-ited at the tube opening; i.e., only the tube grows.

Fig. 3.1. (a) Schematic of the replication process, (b) SEM cross-section image of as prepared titania NT filmafter 20 min of deposition (reproduced with permission from [194]).


Titania nanowires were synthesized using a surface relief grafting (SRG) patterned azo-benzene functionalized polymer (AFP) as template and the sol–gel combination [195].Nanowires were reported to grow in the patterned groves, when the concentration waslow, where as the wires were found to form in the cracks of a thick film developed by vis-cous, and high concentration precursor on a flat AFP substrate. The special feature of theprocess is the combination of sol–gel synthesis with spin on coating. Calcination wasfound to remove the residual precursors while exposing more wires. Crack opening andwire thickness were studied using AFM and conductivity measurements were carried byfour probe method to study the isolation of wires. Electrochemically prepared aluminamembranes from aluminum metal can be used for the synthesis of TiO2 nanotubes andfibrils from the sol–gel precursor [196]. Pore diameters from 10 to 200 nm can be usedto prepare various nanotubes. Time of immersion and temperature of the sol were foundto control the morphology and the diameter of the nanostructures. While 5 s and 25 simmersions have produced thin and thick walled tubules, 60 s immersion has lead to theformation of fibrils (SEM images shown in Fig. 3.2). The length (50 lm) and the diameter(200 nm) were found consistent in all the three cases. While immersion of the membranefor 1 min in the sol at 5 �C lead to the formation of thin walled tubes, a 5 s immersion inthe sol at 20 �C yielded solid fibrils.

TiO2 nanowires can be formed in the AAO templates by a cathodically induced sol–gelmethod [197]. Ability to control the diameter of the wires due to efficient filling of ultra-fine pores in the membrane, control of lengths as a function of time of deposition and ahigh packing density resulting from higher pH gradients from the bottom of the poresare some of the characteristic features. Schematic diagram showing the principle andthe mechanism of deposition is depicted in Fig. 3.3.

Porous nanochannel alumina (NCA) membranes were also used to synthesize the TiO2

nanowire arrays [198]. Anodic oxidative hydrolysis of 0.25 M TiOCl3 solution at 25 �C,with a pH 2.5 and an anodic voltage of 0.1 V vs. a Standard Calomel Electrode (SCE),in argon atmosphere yielded titania NWs. Single crystal anatase NWs of 15 nm diameterand 6 lm length were formed after an annealing treatment at 500 �C. A one step faciletemplate based synthesis of titania NTs and TiO2 NW arrays on a silicon substrate usinga room temperature liquid phase deposition technique was reported [199]. Selective etch-ing of ZnO template and a simultaneous deposition of the TiO2 were found to be the keyfactors that control the formation of nanotubes. The formation mechanism is shown sche-matically in Fig. 3.4.

Fig. 3.2. SEM images showing the nanostructures formed at (A) 5 s, (B) 25 s and (C) 60 s (reproduced withpermission from [352]).

Fig. 3.3. (1) Schematic of the electrochemically induced sol–gel process [197]. (2) (A) SEM image of thenanowires grown in membrane with 50 nm diameter and (B) Raman spectra corresponding to untreated(noncrystalline) and annealed nanowires (crystalline anatase) samples (reproduced with permission from [197]).

TiO2/ZnO Core-sheath

Structure

ZnOnanorod

TiO2nanotube

TiO2nanorod

ZnOdissolution

TiO2deposition

TiO2/ZnO Core-sheath

Structure

ZnOnanorod

TiO2nanotube

TiO2nanorod

ZnOdissolution

TiO2deposition

Fig. 3.4. Step wise description of closed end TiO2 NT and NR [199].


3.1.2.2. Solution based chemical routes. Inability to control the tube dimensions and theinterlayer thickness in the templating method has driven the researchers to use more con-trolled synthesis methods. The first chemical route synthesis of TiO2 NTs used the TiO2–SiO2 nanoparticles produced by sol–gel technique and hydrothermally treated them in


NaOH [200] of different concentrations (2.5–20 M) at 20, 60 or 110 �C for 20 h. The SiO2

doping in the TiO2 particles can increase the photocatalytic activity. Authors have provedthere is no importance for SiO2 in the NT formation by producing NT from the commer-cial TiO2 powders. TEM images of the needle like nanoparticles are shown in Fig. 3.5. Itwas found that the TiO2 (anatase) NTs were consistently �8 nm in diameter and �100 nmin length with 10 M NaOH treatment at 110 �C for 20 h. The detailed flow chart of the NTsynthesis with a starting rutile phase (Fig. 3.6) and the mechanism of the tube formationwere also proposed in a later report [187]. It was observed that the conversion of the Ti–O–Na bonds to Ti–O–H bonds and their subsequent dehydration while treating with aqueousHCl lead to the formation of Ti–O–Ti/Ti–O� � �H–O–Ti bonds. This leads to a decrementin the Ti–Ti bond lengths and subsequent formation of sheet like structure. The tube likemorphology comes from the connection between the ends of the sheets driven by a residualelectrostatic repulsion due to the Ti–O–Na bonds.

However, the mechanism, seamless nanotube formation during washing, proposed byKasuga et al. [187], was contradicted by various researchers with experimental evidenceand questioned the role of washing, chemistry and the purity of the nanotubes[201,202]. TiO2 nanocrystals synthesized using tetrabutyl titanate (C16H36O4Ti) werehydrothermally treated under alkaline conditions to produce titania nanotubes [202].Nanotubular structures were imaged without any washing treatment, whose SAED pat-tern could not be matched to the TiO2 structure. Although the EDX spectra of the wellcrystalline nanotubes have revealed only Ti and O peaks, the O/Ti ratio was not identicalin various tubes. TEM analysis of the product from the hydrothermal treatment, beforewashing, has shown well crystalline nanotubes ruling out the proposed mechanism ofnanotube formation during washing with aqueous HCl. In a later study, Yao et al.[201] have done a detailed TEM analysis to precisely determine the nanotube structureand the formation mechanism. By studying various images and SAED pattern, it was con-firmed that the titania NTs are not seamless. It was proposed that the nanotubes result

Fig. 3.5. TEM photographs of needle-shaped products on the sample holder tilted at angles of �20�, 0�, and+20� (reproduced with permission from [200]).

Rutile

Treatment with 10M-NaOHat 110 oC

Treatment with DW

Measurement of electrical Conductivity( ) of the

Treatment with 0.1 MHCl

TiO2

Treatment with DW

Measurement of electrical Conductivity(σ) of the

When was shown to be 800 µS/cm, ~1g of sample was corrected

When was shown to be10 µS/cm and 70 µS/cm ~1gof sample was corrected

σ

σ

σ

Fig. 3.6. Preparation process of TiO2 nanotubes [187].


from rolling up of delaminated single layer sheets formed from the 3D nanocrystals duringalkali treatment. The higher temperatures were found to provide driving force for thenanotube formation. After comparing the SAED pattern and inter-planar spacing, fromHRTEM images, with the anatase structure; [001], [010] were proposed to be the rollingup vector and tube axis respectively, Fig. 3.7.

Titanate nanotubes were synthesized using the hydrothermal hydrolysis of TiO2 nano-crystals in NaOH solution for studying the ion exchange capability, optical and magneticproperties, etc. [203]. Optimal hydrothermal conditions were found to be 100–180 �C, for48 h and the thermal stability was established. Results obtained by washing the product

Fig. 3.7. Schematic drawing of anatase single layer and the tube wall structure formed by single layer sheets.Shadowed area indicates the unit cell of the anatase phase (reproduced with permission from [201]).


from hydrothermal treatment in acetone and alcohol have supported the nanotube forma-tion during washing. In order to establish the mechanism of formation, pH of the solutionwas slowly altered by adding dilute acid to the suspension of products from the hydrother-mal treatment. While vesicles and agglomerates were found after dispersing the productfrom hydrothermal treatment in alcohol, dispersion in acetone was found to yield crystal-line titanate nanotubes, Fig. 3.8. The ion exchange ability of the nanotubes was establishedby hydrothermally substituting Co2+, Ni2+, Cu2+, Zn2+, Cd2+ and Ag+. The BET analysishas revealed increased surface area (99 m2/g for pure NT compared to the co-substitutedNT 243 m2/g). The XRD analysis has revealed a mixture of Na2Ti6O13 and anatase TiO2

NTs in the resulting products. Different characterizations like TGA, DSC, Raman spec-troscopy and FTIR have been done to precisely determine the nature of the nanotubes.Magnetic susceptibility and the optical properties of pure and transition metal ion substi-tuted nanotubes were also determined.

A most recent study reported [204] the effect of pH on the chemistry of hydrothermallysynthesized titania nanotubes. While a pH of 5.5 lead to �5.5% of Na in the nanotubes,pH of 0.5 has produced nanotubes almost free of Na+. This controlled Na+ was expectedto be useful in potential applications when selectively replaced with various other ions. Thehydrothermal treatment of the TiO2 crystals in alkaline environments was later modifiedand a simplified alkaline treatment was used for nanotube synthesis [205]. After criticallyanalyzing the product obtained, it was confirmed that the structure of the nanotubes issimilar to that of the precursor nanocrystals, i.e., anatase TiO2. It was proposed thatthe NaOH treatment disturbs the crystal structure of anatase TiO2. The Ti–O–Ti bondsbetween the basic octahedral building blocks in the titania structures will re-assembleby forming hydroxyl bridges between Ti ions, resulting in a zigzag structure. While thismechanism leads to the growth of anatase phase in [001] direction, lateral growth wasfound to take place under the influence of oxy bridges between titanium ions. To saturatethe dangling bonds and to lower the surface energy the sheet like structures roll-up to formnanotubes. Formation of the nanotubes is pictorially depicted in Fig. 3.9.

Surface modified titanate sodium nanotubes (Na2Ti2O4(OH)2) were found to assembleleading to the formation of nanorods [206]; when n-octadecyltrichlorosilane (OTS) wasadded to as synthesized nanotubes dispersed in toluene. The nanorod formation can beattributed to a special packing of the surface modified nanotubes, through a hydrophobicinteraction between long fatty chains. Presence of Ti–O, Ti–OH bonds and adsorbed waterthat hydrolyze in toluene due to the addition of OTS lead to the formation of hydrophobicnanotube surfaces. It was revealed by TEM and AFM analysis of the OTS containing dis-persion that the nanorods formed are indeed from the titanate nanotubes, but not just thesurfactant. The self-assembled structures were found to be 1 lm long nanorods that are50–400 nm wide, which could also be realized by Langmuir–Blodgett (LB) assembly pro-cess. Fig. 3.10 shows the mechanism of assembly and a representative AFM image ofnanorods by LB technique.

Titania nanorod films were deposited on glass slides using a low temperature hydro-thermal route, that have a switchable superhydrophobic behavior [207]. It was found thatthe nanorods assembled on the glass slides have morphology similar to papillae, Fig. 3.11.The papillae with 2–6 lm diameter were found to consist of nanorods with 30–60 nmdiameters. These micro- and nano-hierarchical architectures were found to significantlyvary the contact angle (CA) of water droplet on the film as a function of UV exposure.The CA of �154� was found before exposure leading to a superhydrophobicity while

Fig. 3.8. TEM images of precipitate from 10 M NaOH solution dispersed in (a) alcohol and (b) acetone –monodisperse nanotubes (reproduced with permission from [203]).


the same was found to be zero after exposure. This behavior was attributed to the hierar-chical surface microstructures, the orientation of crystal planes and the surface photosen-sitivity of the nanorods.

TiO2 NTs and whiskers can also be synthesized by sonochemical route [208]. Althoughthe basic process that the nanoparticles are treated with the NaOH solution is the same,

Fig. 3.9. Schematic illustrating (a) initial crystal of anatase TiO2, (b) hydroxy bridge formation and thereassembly, (c) growth along [100] direction, (d) lateral growth forming 2D crystalline sheets and (e) anatase TiO2

nanotubes (reproduced with permission from [205]).


sonication could enhance the kinetics and assist the formation of TiO2 whiskers. Themechanism involves breaking of Ti–O–Ti bonds while reacting with NaOH solution andthe formation of titanate layered lattices consisting of titanium–oxygen octahedra havingalkali ions in the interlayer space. The XRD and IR analysis have shown that the whiskershave a composition of H2Ti3O7 Æ 0.5H2O. The stretching at 3400 cm�1 (O–H) and a bend-ing at 1630 cm�1(H–O–H), imply the presence of water. While template based synthesisproduce thick walls (40–50 nm) and large diameters (150–600 nm), and hydrothermaltreatment takes longer times, sonochemical synthesis can achieve the production of precisesize control; 1.5 nm wall thickness and 5 nm diameter, with lower processing times.

Nanotube titanic acid is gaining interest owing to its photocatalytic properties and vis-ible photoluminescence properties [209]. A simple chemical route was adopted to synthe-size the titanic acid nanotubes from the TiO2 nanoparticles under a basic environment.The formation of the Na2Ti2O4(OH)2 and the mechanism of breaking of H2Ti2O4(OH)2

nanotube from vacuum dehydration are shown schematically in Fig. 3.12. Electron spinresonance (ESR) signals were found to be characteristic of the single electron trapped oxy-gen vacancies (SETOV), originating from the vacuum treatment at 100 �C. The formationmechanism of SETOV, XRD, ESR and UV–Vis diffuse reflectance spectra were criticallyanalyzed.

Inherent disadvantages, such as very fast hydrolysis and the formation of amorphous,polydisperse products in the conventional hydrolysis and condensation technique and therelease of environmentally unfriendly byproducts in the alternative non-hydrolytic tech-niques have lead the research towards alternative routes. Any alternate route shouldnot only yield highly crystalline anatase titania nanotubes but also possess the ability tocontrol the size, structure and the morphology. Aminolysis has been reported to have suchunique features [210]. Anatase titania nanocrystals were produced using a high tempera-ture aminolysis of titanium alkoxide. The principle used was the ester aminolysis reaction,which involves the nucleophilic attack of an amine group on the carbonyl carbon atom of

Fig. 3.10. (Top) AFM image of nanorods on Mica using LB technique. (Bottom) mechanism of nanotubeassembly is shown below the image (reproduced with permission from [206]).


titanium carboxylate derivatives and the chemical modification of reactive molecular pre-cursors with oleylamine (OA) as the chelating ligand. Enhanced size control and the aniso-tropic growth are attributed to the controlled aminolysis of the titanium oleate complexesby long-chain OA. A mixture of 1 mM titanium isopropoxide (TIP) and 5 mM of oleicacid (OLA) in 6 ml of 1 octadecene at 80 �C was used to produce the OLA complexesof titanium. 1, 2, and 3 mM OA were injected into the mixture containing titaniumOLA complexes resulting in 12, 30 and 16 nm long nanotubes, with 2 nm diameter, respec-tively; 4 mM OA was reported to produce 2.3 nm diameter nanoparticles. Occasionally,the titania nanorods were found to assemble over small ranges. While studying the mech-anism of nanorod formation, it can be realized that the nanorods result from a synergisticeffect of both OLA and OA. Tight binding of OLA to the nanorod surface was found torestrict their diameters; hence a weaker capping agent cetyltrimethylammonium bromide(CTAB) was used to synthesize tunable nanorods of titania [210]. Proposed aminolysisroute does not yield any environment unfriendly volatile alkyl chlorides or hydrogen chlo-ride make the process more greener (benign). Controlled hydrothermal treatment of var-ious precursors was reported to yield nanotubes, nanofibers, nanoribbons and nanowiresof titania (Table 3.1).

Fig. 3.11. (a) Low magnification FE-SEM image of a TiO2 nanorod film deposited on a glass wafer and(b) morphology of a single papilla at high magnification (reproduced with permission from [207]).


Zn ion surface doped TiO2 nanotubes were synthesized using organic precursors, fol-lowed by a calcination treatment at 500 �C [211].

Zn(acac)2 + Ti–OH!Zn(acac)(–O–Ti) + H(acac) ð3:2Þ

Hydrothermally synthesized TiO2 nanotubes [187] were sonicated in 1 M HCl for 3 h toremove residual Na+ contamination followed by washing with distilled water. Mixing ofthe washed nanotubes with acetonitrile solution containing Zn acetyleacetonate,Zn(acac)2, drives a ligand exchange between the ligands of Zn(acac)2 with the hydroxide

Fig. 3.12. (a) Formation process of nanotubes Na2Ti2O4(OH)2 and (b) mechanism of breaking of the H2Ti2O4

nanotubes (reproduced with permission from [209]).

Table 3.1Details of the precursors, treatment conditions and corresponding TiO2 nanostructures [210]

Precursor Treatment condition Morphology Diameter

Rutile or anatase nanocrystals NaOH solution, 100–160 �C Nanotubes 10 nmAmorphous TiO2 NaOH solution, 100–160 �C Nanofibers-interlinked 5–30 nmTiOSO4 NaOH solution, 100–160 �C Nanofibers-interlinked 5–30 nmAmorphous or crystalline TiO2 NaOH solution 180 �C Pentatitanate nanoribbons 30–500 nm (width)TiO2 particles KOH solution Octatitanate NWs 5–10 nm diameter



radicals on the TiO2 NTs. Zinc ion surface doped nanotubes result after washing and cal-cining the Zn(acac)2 assembled TiO2 NT. Wei et al. have synthesized few hundred nano-meter long and 30–40 nm diameter titania nanotubes hydrothermally, just by using aprecursor Na2Ti3O7 and water [212]. Na2Ti3O7 was synthesized by repeated grindingand mixing of Na2CO3 and anatase TiO2 (1:3), followed by a calcination treatment at1000 �C. The Na2Ti3O7 precursor was then hydrothermally treated in DI water for 5–18days. The filtered, washed and dried product was characterized using SEM, TEM,XRD, BET and UV–Vis spectroscopy. After a critical analysis a model for the formationof the nanotubes by rolling up of the sheet like precursor structures of Na2Ti3O7 have beendeduced. Intercalation of larger size H2O in the neighboring octahedral [TiO6], sheets, inturn leads to the exfoliation of the sheets. Lack of the inversion symmetry and the presenceof intrinsic tension gradually tend to curl up from the edges, while relieving the strain, fol-lowed by nanotube formation. The schematic of the nanotube formation is shown inFig. 3.13.

Effect of temperature on the hydrothermal synthesis of titania NT, has revealed the for-mation of fibrillar bundles and nanobelts [213]. Various morphologies, of anatase phase,as a function of temperature of the hydrothermal treatment and 1 h calcination at 500 �Care shown in Fig. 3.14.

A strong driving force at elevated temperature was expected to be one of the reasons forthe bundle like self-assembly of these nanotubes; however, lack of significant evidence andthe proper mechanism demand further research. Although the actual mechanism is notclear authors have observed titania nanobelts with 30–40 nm thickness, a few hundrednanometer wide and a few micrometers long (Fig. 3.14(D)). When the nanotubes formedthrough hydrothermal treatment and further washing were calcined at 800 �C, researchersfound the disruption of the nanotubes and agglomerated anatase particles [214]. The roleof pH of the washing solution after the hydrothermal treatment can affect the chemistry ofthe nanotubes. Sodium and hydrogen titanate nanotubes along with the titania nanotubeswere produced just by varying the pH of the washing solution and the calcinationtemperature.

Fig. 3.13. An exfoliating-rolling model for the nanotubes formation from layered Na2Ti3O7 particles by softchemistry route (reproduced with permission from [212]).

Fig. 3.14. SEM images of titania nanostructures with different treatment temperatures and then calcined at500 �C for 1 h: (A) 210 �C, (B) 190 �C, (C, D) 170 �C (reproduced with permission from [213]).


3.1.2.3. Electrochemical synthesis. Anodic oxidation of titanium in various electrolytes[215–219], has received a significant focus. The effect of synthesis parameters such as cur-rent density, electrolyte concentration, applied voltage and the time of anodic oxidationhas been critically studied. Among the various groups working on the anodic oxidationprocess for producing titania NTs, Grimes’ group [215], and their co-workers haveobserved the formation of array of titania nanotubes on a thin titanium foil after an anod-ization treatment in HF containing aqueous solutions of different concentrations. Con-stant length arrays of nanotubes with variable diameter (25–65 nm) can be producedunder variable anodizing voltages. As the voltage is increased; particulate or nodularstructure; discrete, hollow cylindrical tube like morphology and finally a sponge like ran-domly porous structure at 40 V were observed. Although a linear scaling was not observedbetween the reduced voltage and the higher concentration, both 0.5 wt% and 1.5% HFhave resulted in similar morphological evolution. However, the addition of chromium tri-oxide was not found to influence the formation of the titania nanotubes. The structuralevolution at different voltages and the cross-sectional view of the nanostructures in0.5 wt% HF, using FE-SEM are shown in Fig. 3.15. An initial oxide film was found to dis-solve with time and lead to nanotube formation. The electric field resulting from high

Fig. 3.15. FE-SEM top-view images of porous titanium oxide films anodized in 0.5 wt% HF solution for 20 minunder different voltages: (a) 3 V, (b) 5 V, (c) 10 V, and (d) 20 V; (e, f) cross-sectional images of titanium oxidenanotubes from the sample was anodized in 0.5 wt% HF solution at 20 V for 20 min (reproduced with permissionfrom [215]).


anodizing voltages is strong enough to drive the titanium ions from the interpore areas tothe oxide/solution interface; this process was proposed as the underlying mechanism toproduce the discrete tube like structures [215].

Thickness of the nanotubes was varied by modifying the pH of the electrolyte from500 nm to almost 4 lm [216], The higher oxide dissolution rate in low pH H3PO4 solutionin contrast to the high pH in NH4H2PO4 was proposed as the reason for the drastic changein length. Authors surmise the high phosphorous content in the nanotubes, found by XPSanalysis, might significantly affect the functional behavior of the nanotubes. The nanotubethickness variation as function of time in (NH4)H2PO4 + 0.5 wt%NH4F electrolyte areshown in the following images, with the XPS graph showing the presence of phosphorous,Fig. 3.16.

It was also reported that the nanotube formation is very sensitive to the sweep rate andthe concentration of the electrolyte [220]. While a low concentration and lower sweep ratesresulted in regular self-organized smooth tubular structures, higher concentration lead torough cross-linked morphologies. It is further [216,220,221] established that the thicknessof the nanotubes is a result of equilibrium between the electrochemical formation of TiO2

at the bottom and the chemical dissolution of TiO2 in fluoride containing solutions. Syn-thesis of highly smooth, bundled nanotubes up to 7 lm long [222] was achieved in presenceof glycerol. A smooth current density profile was found to be the characteristic of the glyc-erol solution, Fig. 3.17, on the contrary to sulfate electrolytes.

Fig. 3.16. (a) Thickness of nanotube layers formed in 1 M (NH4)H2PO4 + 0.5 wt% NH4F for differentanodization times and (b) P 2p XPS spectrum for the samples anodized in 1 M (NH4)H2PO4 + 0.5 wt% NH4F at20 V for 10 h (reproduced with permission from [216]).

Fig. 3.17. (a) Current–time behavior during anodization of Ti samples at 20 V for 13 h. The samples wereanodized in glycerol or in 1 m (NH4)2SO4, in both cases with 0.5 wt% NH4F. The insets show details of thecurrent transients. Current fluctuations can be observed for the anodization in aqueous solution, while they arecompletely suppressed using the glycerol electrolyte. (b) 7 lm long smooth bundled nanotubes, with inset showingthe absence of cross linking (reproduced with permission from [222]).


Furthermore, a detailed study of titania NT array fabrication using anodic oxidation inHF electrolyte is reported [223]. They have systematically studied the formation mecha-nism, morphology and the dimensions of the nanotubes as a function of applied voltage,electrolyte concentration and the oxidation time. The mechanism of nanotube arraygrowth is shown in Fig. 3.18. While 10 · 10 · 0.5 mm3 titanium foils were used as anode,Pt (20 · 20 · 0.1 mm3) foil was used as a cathode with an inter-electrode spacing of 20 mm.Magnetically stirred 0.1–5 wt% HF solution was used as an electrolyte to produce the tita-nia nanotubes at room temperature. It was observed that the smooth and flat (attained bymechanical polishing) foils of titanium will have some non-uniform oxide layer on the sur-face. Selective etching of the oxide by HF was attributed to non-uniform stresses existingin the thin oxide film. The oxide layer color changes during growth swiftly from an initial

Fig. 3.18. Schematic diagram for the formation of titania nanotube arrays (a) oxide layer formation, (b) burst ofoxide by the formation of crystallites (pore formation), (c) growth of the pores due to field assisted dissolution oftitania, (d) immediate repassivation of pore tips, (e) voids formation in the metallic part between the pores, (f)formation of nanotubes of titania, (g) burst of repassivated oxide, (h) formation of new pores inside existing pores(reproduced with permission from [223]).


purple to green via blue and yellow colors in between, reason being the increase in thick-ness of the oxide layer leading to different interference phenomena. The current densitywas found to decrease in the initial stage that again increases and a similar periodic fluc-tuation can be observed throughout the anodization process. This variation in the currentdensity is consistent with the dissolution and growth of oxide layer leading to the nanotubeformation. This can also be ascribed to the competing passivation and depassivation reac-tions. The nanotubes produced [223] were found crystalline from the Raman analysis andthe SAED pattern of the nanotubes, mechanically scraped from the substrate, Fig. 3.19.

Synthesis of ordered titania nanotubular structures by dealloying and subsequent anod-ization of a Ti–8at%Al alloy [224] was also achieved. Al gets selectively dissolved in 1 MNaOH at a critical potential creating a nanoporous structure, which on further anodiza-tion in 1 M H3PO4 + 1–1.5% HF resulted in titania NTs with 40–140 nm diameters andalmost 180 nm length.

It must however be appreciated from the discussion so far that despite extensiveresearch, a common and widely accepted mechanism is far from being acknowledged.Researchers tried to explain it from various mechanistic points of views ranging from sim-ple oxide dissolution and scale formation, pH burst effects and strain relieving to pertur-bation theory. Following the extensive utilization of anodized titania NTs in differentapplications like catalysis and sensors, authors tried to reproduce the results as per theparameters given by various research groups. Serendipitously we have observed an entirely

Fig. 3.19. (A) Raman spectra of (a) anatase, (b) rutile and (c) anodized titanium. (B) SAED of the titania NTs(reproduced with permission from [223]).


new characteristic of anodized titania NT from HF/H3PO4 containing solution [225]. Asillustrated in Fig. 3.20, we have observed these titania NT to be spiral in shape with a welldefined pitch and periodicity. The anisotropic growth could be explained only if thesenanotubes (now nanospirals) follow a specific crystallographic orientation while the newoxide is formed. This could arise from the release in strain energy due the transformation

Fig. 3.20. Nanospirals obtained from anodization of titanium: (a–c) top view of the nanotubes, (d) 30� tiltedview, (e) 20� tilted view and (f) high resolution image illustrating spiral pitch and periodicity (reproduced withpermission from [223]).


of oxide into a partially crystalline oxide during anodization. The partially crystallizedoxide structure could eventually get destroyed as the oxygen diffuses to the bulk fromthe oxide leaving the spirals amorphous as observed under SAED. The amorphous nano-spirals were observed to crystallize under the influence of an electron beam. In our obser-vation we also noted that the current density of around 1.0–1.4 mA/cm2 always resulted inthe formation of nanotubes irrespective of the voltage range [12–20 V]. Schmuki et al.[222] have shown the synthesis of nanotubes from glycerol containing solutions with a verylow current density and current fluctuation. These nanotubes were also very smooth andare in direct contrast to our observation. However, nanospirals of titania observed duringour experiments are under further observation to study the effects of parameters such asultrasonication of the nanotubes in ethanol before imaging under HRTEM. A discreteand widely accepted mechanism is still underway and all the aspects need to be clearlyaddressed for establishing the same.

3.1.3. Structure, morphology, mass transport and phase transformation in titania nanotubes

One of the most interesting aspects in titania is the different phases with different crystalstructure, (anatase, brookite and rutile). It was reported that the gradual increment in thepotential results in a crystalline structure from amorphous. There have been variousreports [217,223,226–230] on the anodization of titania nanotubes, however, the reportsare not always unequivocal. The transformation of phases was imputed to the electricalbreakdown, which further depends on electrochemical parameters such as the electrolyteconcentration and the current density. The discrepancies in the voltage at which the break-down occurs and the crystallization starts must have been a consequence of different sur-face preparation, electrochemical parameters and the temperature of the electrolyte duringdeposition [223].

Detailed study of the crystal structure, high temperature stability and the phase trans-formation of the titania NTs is presented, Fig. 3.21 [228]. The as synthesized nanotubeswere annealed at various temperatures (up to 950 �C) in oxygen, dry and humid argonenvironment. Nanotubes were found to crystallize, to anatase crystallites, around280 �C that completely transform from anatase to rutile phase in the temperature range570–620 �C in humid and dry environments respectively. However, TEM analysis hasrevealed that the transformation of the nanotube substrate interface started as early as470 �C. While the nanotube walls crystallized in case of humid argon environment withoutdestroying the pore structure, the same was not observed in case of oxygen and dry argonenvironments, albeit a significant pore shrinkage. It was proposed, in humid environment,that the adsorbed hydroxyl ions move to the inter-particle neck regions and desorb aswater molecules, creating anionic and cationic vacancies. These vacancies were found toenhance the mass transport kinetics and lead to a quicker transformation in humid envi-ronment. Protrusions on the surface in between the nanotubes were observed to formwhen annealing was done at 580 �C and the tubular structure was found to completely col-lapse when annealed at 880 �C. The crystallite growth studied using XRD was reported asa function of temperature for both the isomorphs, Fig. 3.22. The enthalpy of anatase torutile phase transformation was found to be 8.8 kJ/mol and activation energy to be264 kJ/mol. It was concluded that the nanotubular structure will be stable up to 580 �Cand the phase can be controlled by selective annealing in an appropriate atmosphere.

In a similar work [226], single crystal anatase tubes were reported when the anodizedsurface was annealed in air. Although similar temperature ranges of stability was pro-

Fig. 3.21. GXRD patterns of the nanotube samples annealed at temperatures ranging from 230 to 880 �C in dryoxygen ambient for 3 h. A – anatase; R – rutile; T – titanium (reproduced with permission from [228]).


posed, they have disagreed with the crystallization mechanism proposed by Vargheese,Grimes et al. [228], Fig. 3.23. It was argued that the proposed mechanism cannot explainthe formation of single crystal anatase nanotubes. Solid-state aggregation of small anatasenanocrystals was proposed to follow by an oriented attachment of the aggregates, leadingto the formation of single crystal anatase nanotubes. The TEM images and the SAED pat-terns were used to support the mechanism proposed. Lack of thickness was proposed to bethe reason for the inability to assist the nucleation along the walls of the nanotubes. Thenanocrystal building blocks were expected to drive the formation of single crystal anatasethrough an interfacial nucleation mechanism. Effect of halide ion addition on the anodiza-tion of titanium foils was found to reveal the importance of fluoride ion, as the otherhalides did not result in nanotubular structures [229]. It was also confirmed that onlythe acidified fluoride solution can lead to nanotube formation as the same was notobserved in neutral fluoride solutions. The ordered nanotubular structure formation

Fig. 3.22. Temperature dependence of the anatase and rutile crystallite sizes of samples annealed in oxygen for3 h (reproduced with permission from [228]).

Fig. 3.23. Schematic representation of the crystallization of the titanium oxide nanotube arrays: (a) nucleation ofanatase crystals at temperatures around 280 �C; (b) growth of the anatase crystals at elevated temperatures; (c)nucleation of rutile crystals at around 430 �C; (d) growth of rutile crystals at higher temperatures (the anatasecrystals in the walls in contact with rutile crystals in the interface region are transformed to rutile at temperaturesabove approximately 480 �C); (e) complete transformation of crystallites in the walls to rutile at temperaturesabove approximately 620 �C (reproduced with permission from [228]).


was explained by perturbation theory and the inter-connected nano-porous structure wasimputed to the repulsion in cationic vacancies and various arguments from the pointdefect model were used to support the proposed mechanism. A 5 step nanotubular struc-ture formation was reported [229]:


• Formation of a passive inner barrier-type film during first few seconds of application ofanodization potential.

• Thickening of barrier layer and subsequent micro-fissuring, normally referred as forma-tion of ‘easy paths’.

• Secondary oxide nucleation through these ‘easy paths’ and pore nucleation.• Coverage of the secondary oxide on the entire surface and growth of pores.• Pore separation to form individual, self-ordered nanotubes.

While the stability of the barrier layer was due to surface energy, the instability was pro-posed to be resultant of strain energy from a synergistic effect of electrostriction, electro-static and recrystallization stresses. The total strain energy density can be quantified as:

U s ¼r2

2Yð3:3Þ

where r2 ¼ r2er þ r2

es þ r2vol and r2

er – electrostriction, r2es – electrostatic stress, r2

vol – com-pressive stress due to volume expansion, Y – the Young’s modulus.

When a stable barrier surface with the above mentioned stresses undergoes a perturba-tion with a wavelength k, k 6 8pY c

3r2 ; where c is the surface energy, nanotube formationresults from the mechanism shown schematically, Fig. 3.24.

‘‘Coral reefs’’ structures of p-TiO2 were reported when the anodization was carried in anon-aqueous electrolyte (CH3COOH + NH4F) [218]. The morphology and dimensions ofthe nanotubes synthesized under different conditions in CH3COOH + 0.5 wt% NH4F areclear from Fig. 3.25.

XI

XI

XI

XI

MX

M MO M MO M MO

M MO M MOMO

(1) (2) (3)

(1) (2) (3)M(OH)1-xXx

XI

XI

XI

XI

MX

M MO M MO M MO

M MO M MOMO

(1) (2) (3)

(1) (2) (3)M(OH)1-xXx

a

b

c

Fig. 3.24. Schematic of (a) pit formation (bromide addition) and (b) pore formation (fluoride addition) duringanodization of Ti. The barrier film is intact during porous anodic film formation and substrate metal is notattacked locally; whereas, pitting locally attacks bare metal. Perturbation of the surface shown in (b-2) can lead toadsorption of fluorides at the valleys and develop into nano-tubular structure; (c) is the magnified schematic ofthe perturbed surface. Higher strain energy density at the valleys drives the mass flow to the lower energy crests.Arrows indicate the growth direction of valleys and crests [229].

Fig. 3.25. SEM images of titanium oxide nanotube layers formed at 50 V for 1 h in CH3COOH + 0.5 wt% NH4F:(a) coral reef structures; (b, c) higher magnification of the layers outside and inside the coral reef; (d, e) cross-section image of the layers inside and outside the coral reef; (f) top view image of nanotubes formed at 10 V for5 h (reproduced with permission from [218]).


3.2. Zinc oxide

3.2.1. Overview

Zinc oxide nanostructures have been fabricated as ODNS and proven versatility andcompatibility in numerous applications. ZnO ODNS were synthesized in the form ofnanorods, nanowires, nanotubes, nanobelts, nanocombs, nanosprings, nanorings, nano-


bows and nanopropellers, etc. [231–234]. The wide interest in ZnO has resulted from thefollowing fundamental characteristic features with potential applications in electronic,structural and bio-materials [232]:

• Direct band gap semiconductor (3.37 eV).• Large excitation binding energy (60 meV).• Near UV emission and transparent conductivity.• Piezoelectric property resulting from its non-centrosymmetric structure.• Bio-safe and bio-compatible.

Micro-electro-mechanical devices, sensors, transducers and biomedical applicationswithout coatings are a few among the spectrum of applications for ZnO.

3.2.2. Structural featuresNumber of alternate planes consisting of tetrahedrally coordinated O2� and Zn2+ ions

stacked along the c-axis result in a hexagonal structure of ZnO, with a = 0.3296 andc = 0.52065 nm as the lattice parameters (C6mc space group). The non-centrosymmetricstructure and there by piezoelectric behavior result from the tetrahedral coordination.Spontaneous polarization and diverse surface energies along the �ð0001Þ;�ð01�11Þ havebeen of major interest in establishing the stability and structural evolution in ZnO [232].These polar surfaces always try to arrange the charges in such a way that the electrostaticenergy is always minimized, driving the polar surface dominated nanostructures. The crys-tal structure of ZnO and the projection along ½1�210� direction are shown in Fig. 3.26.

Diverse morphologies achieved in ZnO have been ascribed to its fastest growing direc-tions h0001i; h01�1 0i; h2�1�1 0i along with ±(0001) polar surfaces [232].

3.2.3. Synthesis processes

3.2.3.1. Solution based synthesis. Transition metal doped zinc oxide nanowires were syn-thesized by thermal decomposition of zinc acetate and cobalt(II) acetate in refluxing trioc-tylamine [235]. Uniform transition metal doping and the absence of secondary phases were

Fig. 3.26. (a) Wurtzite structure of ZnO, showing the tetrahedral coordination and (b) the structure model ofZnO projected along ½2�1�10�, with different polar surfaces identified (reproduced with permission from [232]).


confirmed through structural, optical and spectroscopic characterizations. Kahn et al.[236] have done an exhaustive study to establish the factors that control the synthesis ofsize and shape controlled ZnO nanostructures. They have exploited the exothermic reac-tion of the Zn (c-C6H11)2 in moisture and air, assisted by the presence of long-alkyl-chainamines as stabilizing ligands. They have prepared ZnO nanorods, widths from �2–20 nmand up to 200 nm long, using various ligands in the absence of other solvents by varyingthe incubation time. A spectrum of ligands and solvents, at different concentrations, timeand temperature were studied and the conditions for specific morphologies and dimen-sions were established. Well-aligned single-crystalline hexagonal rods of typically 1 lmdiameter were produced on different substrates under a low temperature chemical growth[237]. These highly crystalline rods were grown by aqueous thermal decomposition ofZn2+ amino complex with reagent grade chemicals. Surfactant tetradecylphosphonic acid(TDPA) was used in a non-hydrolytic sol–gel synthesis of ZnO nanorods, Fig. 3.27 [238].Excellent binding ability and the ability to induce anisotropic growth of TDPA wasreported to result in highly crystalline nanorods along h0001i direction.

Homogeneous and dense ZnO nanowire arrays were fabricated on various substrates,silicon wafers and plastic substrates, etc. to name [239]. These largest surface area, ori-ented nanowire films synthesized by two step procedure have ease to commercial scaleup. These nanowire arrays were also found stable when annealed in various environmentsup to 800 �C. Pulsed laser deposited ZnO layer on Si was reported to yield aligned nano-tube arrays from aqueous Zn nitrate, under hydrothermal conditions [240].

ZnðOHÞ2�4 precursor solution, CTAB, co-surfactant n-hexanol, and solvent n-heptanewere used in various molar ratios to synthesize ZnO ODNS by microemulsion method[241]. A directed aggregation process mediated by microemulsion droplets was proposedas one of the mechanisms driving the anisotropic nanostructure formation. Given enoughtime, the nanoclusters can grow anisotropically leading to ZnO one dimensional nano-structure. The surfactant controls the nucleation, size and morphology of the products.Synthesizing the nanostructures in controlled dimensions can be achieved by tailoringthe treatment time, choice of surfactant and the co-surfactant. A systematic SEM analysisrevealing different stages in the growth of ODNS from initial nuclei is depicted inFig. 3.28.

– Zn – O

H – Ö

Ö

OH

OOH

O

+– Zn – OH

– Zn – OH + HO – Zn –H2O – Zn – O – Zn –

– Zn – O

H – Ö

Ö

OHH – Ö

Ö

H – Ö

Ö

OH

OOH

O

+– Zn – OH OOH

O

+– Zn – OH OOH

O

+– Zn – OH

– Zn – OH + HO – Zn –H2O – Zn – O – Zn –– Zn – OH + HO – Zn –H2O – Zn – O – Zn –

Fig. 3.27. A typical ester elimination reaction involving zinc acetate and 1, 12 dodecane diol, used in non-hydrolytic sol–gel synthesis of ZnO nanostructures [238].

Fig. 3.28. Series of SEM images of shape evolution of 1D ZnO with stepwise prolonging of the reaction time(a) 10 min, (b) 30 min, (c) 1 h, (d) 2 h, (e) 4 h, and (f) 8 h, exhibiting the evolution of ZnO nanoparticles to 1Dnanorods (reproduced with permission from [241]).


3.2.3.2. Vapor phase growth. In one of the early reports on the synthesis of ZnO nanowires[242,243], Huang et al. have used a thermal evaporation method. It was believed that theNWs grow by VLS mechanism. In catalysts assisted VLS mechanism, catalyst nanoparti-cle serves as a preferential site for the absorption and dissolution of reactants from thevapor phase, controls the NW’s growth direction, and defines the diameter of the nano-wire. A common morphological feature of VLS grown NWs is that each nanowire is


terminated at one end by a catalyst nanoparticle with a diameter similar to that of thenanowire.

ZnO NWs were produced in presence of Au [242], Ge [244], etc. as catalysts. Althoughthe ZnO has a higher melting point, its carbothermal reduction in presence of graphitepowder was expected to assist the formation of Zn–Au alloy. It was also surmised thatthe Au–Zn phase diagram is essentially the same even in the presence of CO/H2O. Inabil-ity to produce NWs in absence of Au catalyst was an important aspect supporting theaforementioned arguments. It was also reported that the NWs could be grown on specificpatterns, with and without the Cu grid masks on the sapphire substrates, and controlledfashions by controlling the catalyst morphology. Direct hydrogen reduction and use of Auclusters as catalyst can also produce thinner nanowires (�15 nm). Controlled nanorods of�1 lm in length with hexagonal cross-section were fabricated on a substrate with pat-terned Ge spots as a catalyst [244], Fig. 3.29.

Abundant hollow ZnO NTs were produced by thermal evaporation of Zn/ZnO pow-ders in �1:3 ratio under an argon stream at 1300 �C [245]. Maintaining a wet oxidationenvironment was reported to be useful in generation of ZnO nanotubes. Formation ofNTs with polyhedral morphology was related to the hexagonal crystal structure of ZnOand peculiar growth conditions. The TEM analysis has also revealed that the nanotubesare often irregular in shape with both poly- and single-crystalline nature with wall thick-ness approximately 8 nm. TEM analysis of bulk white product formed after a two stagethermal evaporation of ZnO powder in presence of graphite (for 1150 �C for �3 h) hasrevealed the formation of nanoplate–nanorod junctions that can be potential membersfor electronic devices [246]. In a typical junction, while one nanorod has grown epitaxiallyon the corner of (0001) plane, the other nanorod has twinned relationship with respect tothe plate surface. Based on the electron microscopy and XRD analysis an interfaceassisted nucleation mechanism was proposed for the aligned growth of ZnO nanorods

Fig. 3.29. SEM images of a ZnO nanorod pattern grown on a Ge-dot pattern. The top-right inset shows theGe-dot pattern. The bottom-left insets show two typical morphologies of the nanorods. The bottom right inset isthe top view of a hexagonal nanorod, showing a hemispherical hollow at the upper end of the nanorod(reproduced with permission from [244]).


with a buffer layer. Epitaxially grown Au/Zn alloy droplets (nuclei) on the buffer layer(ZnO) surface drive the VS growth of the nanorods. Easy nucleation and higher densityof the nanorod array were attributed to the buffer layer. Low cost and extreme controlon the process parameters are the major advantages of the process. Authors expectedthe presence of buffer layer provides aligned nanorods on the substrate with minimum lat-tice mismatch that could be potential members for quantum well and superlattice devices[247]. Lin et al. [248], have reported the synthesis of arrayed ZnO NWs using a buffer layermade of TiN. Need for the deposition of the nanorods and the driving force for using theTiN, stems from its applicability as an electrode material. A self-catalytic growth of ZnOnanowires using a simple thermal evaporation process resulting in urchin like assembly ofthe nanowires was proposed [249]. Vaporization of Zn powder, solidification of liquiddroplets, surface oxidation, sublimation, and self-catalytic growth of one dimensionalnanowires are the major steps in the process of ZnO nanorod assemblies, Fig. 3.30.

While a significant number of articles have focused on the fabrication of ZnO ODNS,Ye et al. have studied the role of various parameters that govern the morphology [250]through Evaporation Physical Transport Condensation (EPTC). The competition of thecapture of impinged molecules by different surface planes under certain supersaturationdetermines the final morphology of the nanostructures i.e., nanowires, nanorods, nano-belts, or nanoplatelets. To elucidate this aspect, supersaturation of the ZnO vapors as afunction of distance from the source at different carrier gas pressures was studied. Differ-ent regimes of supersaturation and corresponding morphologies are depicted in Fig. 3.31.

Various experimental parameters, such as the temperature of the source material andthe substrate, the temperature gradient in the tube furnace, the distance from the sourcematerial and the substrate, the gas flow rate, the inner diameter of the ceramic tube,and the starting material were all found to play a role in influencing the final morphologyof the structure. Various morphologies produced are shown in Fig. 3.32 [250]. Bae et al.[251], have used thermal CVD method to grow ZnO nanorods (80–150 nm diameterand 3 lm long) on prefabricated ODNS such as CNT, GaP, GaN, SiC nanowires.

Fig. 3.30. Schematic illustration of the formation of hollow ZnO urchins (reproduced with permission from[249]).

Fig. 3.31. Qualitative supersaturation profile of ZnO vapor in the reactor under a different flow rate of carrier gasalong the distance downstream from the source material. Regions I, II, III, and IV correspond to microrods,nanoplatelets, nanobelts, and nanowires, respectively, with a low flow rate of carrier gas (reproduced withpermission from [250]).

Fig. 3.32. Typical SEM images showing morphologies of ZnO structures: dense filmlike rods (a), dense filmlikenanoplatelets (c), flowerlike nanoplatelets (e), nanobelts (g), and nanowires (i). The corresponding highmagnification images are displayed in (b), (d), (f), (h), and (j), respectively (reproduced with permission from[250]).


3.2.3.3. Template-assisted synthesis. Template assisted synthesis was coupled with vaporphase growth and the wet chemical routes for obtaining ZnO ODNS, Fig. 3.33. Nanosized

Fig. 3.33. Schematic illustration of the growth process of the aligned ZnO nanorods on PAO template. (A)Empty PAO template at the beginning of reaction. (B) Nanosized Zn liquid droplets precipitated from thesupersaturated Zn vapor and deposited onto the surface of PAO template. (C) While air was introduced into thetube furnace, Zn liquid droplets were oxidized and formed ZnO nuclei. (D) With the reaction going on, ZnOnanorods would grow from the ZnO nuclei because of the intense c-axis orientation growth [252]. (2) Schematicillustration of the growth of nanotube and nanowire structures by a template assisted sol–gel technique [253].


Zn liquid droplets from the supersaturated Zn vapor were deposited onto the surface ofempty PAO template, which further form ZnO nuclei through airflow assisted oxidation.ZnO nanorods were formed from the ZnO nuclei resulting from an intense c-axis orienta-tion growth [252]. Polycrystalline ZnO nanowires and nanotubes with wurtzite structurewere synthesized using a template assisted sol–gel technique [253]. While Zn nitrate precur-sor acts as a source for Zn ions, urea was used for achieving basic environment through itshydrolysis above 60 �C. Controlling the time is a key factor in achieving the morphologicalcontrol. The porous structure of alumina with positive charged walls attracts the ions hav-ing negative charge from the solution leading to a preferential deposition at the walls thatfurther grow towards the core. The schematic of the mechanism for nanotube and nano-wire generation is shown in Fig. 3.33.

3.3. Silica

3.3.1. Overview

The most abundant oxides in the earths crust, silica and its related oxides, are alsoknown for their outstanding structural features. The layered and fibrous structures ofmaterials like asbestos, mica, etc. are known much earlier than the first CNT [36,38]. Whilethe tube walls consisting of curved crystalline layers occur in the fibrous asbestos mineralsserpentine and chrysotile, artificial silica nanotube walls were found to be amorphous[254]. The origin and structural feature of various silicate based minerals was summarizedpreviously [254]. The structural anisotropy that causes the bending of the layers is due tothe presence of octahedra MO6 (M = Mg, Al) on one side and tetrahedra SiO4 on theother side of the layers. Co-existence of a variety of structural variations, including spiralsand concentric layers was highlighted

• Excellent PL properties.• Nano interconnection integrated optical devices.• Host material in bio-analysis, bio-catalysis and bio-separation [255,256].


3.3.2. Synthesis process

3.3.2.1. Solution and template-based synthesis. Researchers have studied the structural fea-tures of the silica nanotubes synthesized through template-assisted route [254,257,258].They have used nanoscale anisotropic structures of V3O7 Æ H2O and [Pt(NH3)4](HCO3)2

for the synthesis of ODNS of silica. Several mm long nanotubes with 50–150 nm diameterwere reported. While hydrogen peroxide was used to remove the vanadium oxide core, thePt salt was thermally evaporated. The thermal evaporation of the template lead to theauto-reduction of metallic Pt and was found embedded in the nanotubes of silica,Fig. 3.34. Thick tubes (100–500 nm) and thin tubes (50 nm) with almost 0.5–3 lm lengthwere found from the SEM and TEM analysis. Some features of the nanotubes with Ptbased template with reference to Fig. 3.34(1):

• Overview image. Most NTs are filled with Pt particles, which can be recognized by theirblack contrast. Besides that, a thick NT in the centre appears empty. (a)

• Pure SiO2-NTs, i.e., without any filling. (b)• SiO2-NTs filled with Pt particles. The SiO2-NTs on the lower left side contains a Pt

nanowire that is about 0.5 lm long and has a diameter of about 10 nm. On the top,two NTs are grown together, forming a unique arrangement. (c)

Fig. 3.34. (1) Transmission electron microscopy (TEM) images of the synthesis product with V3O7 Æ H2O: (a, b)SEM images of the nanotubes and (c) TEM image showing hollow nanotubes. (2) [Pt(NH3)4](HCO3)2 templates(reproduced with permission from [254,257]).


The nanotubes synthesized with the Pt mineral template have revealed some structuralvariants with respect to the dimensions and the Pt particle filling. While most of the largertubes with open ends have no particles inside, the smaller nanotubes with closed ends werefilled with Pt nanoparticles, Fig. 3.34(2) [254]. Sol–gel electrophoretic deposition was usedfor the synthesis of silica nanorods [259]. The sol–gel silica particles will be drawn towardsthe anode upon the application of appropriate electric potential, which will eventually fillthe pores in the membrane, which upon a subsequent calcination treatment lead to densenanorods. Reverse micelle mediated sol–gel (RMSG) synthesis of silica nanotube synthesiswas reported with size modification [260]. The hydrolysis of tetraethyl orthosilicate(TEOS) was carried in AOT soft-template (cylindrical micelles) containing solution (con-taining apolar solvents like hexane, heptane or isooctane). The tunability of the micellesize by varying the composition and the dynamics of the micelle solution might proveeffective in size tuning of the fabricated nanotubes.

Bright blue light emitting single-crystalline SiOx (x = 1.68) nanowires were synthesizedusing a template-free, low temperature Fe-assisted vapor phase hydrothermal technique[261]. The SiOx nanowires were found to have a orthorhombic crystal structure belongingto P21212 space group with the lattice parameters a = 9.685, b = 14.870, and c = 16.898 A.Proposed synthesis from Fe-pre-embedded bimodal silica in ethylenediamine at 200 �Clead to the formation of silica nanowires with diameters less than 150 nm and more than4 lm length. A novel facile sol–gel method using inorganic CaCO3 needles as a templatewas used to produce porous hollow silica nanostructures [262,263]. BET surface area of upto �974 m2/g, with specific synthesis parameters was reported. The amount of surfactantC16TMABr, was found to play an important role in determining the properties. Typicalgel composition used in the synthesis was 1SiO2:1.4CaCO3:xC16TMABr:11NH3H2O:58EtOH:144H2O, where x = 0 for ratio D, 0.139 for ratio C, 0.185 for ratio B, and0.278 for ratio A. In addition to the surfactant molar ratio, acid etching treatment was alsofound to affect the features of the final product. The pore distribution and BET surfacearea obtained by using composition B with different methods are tabulated, Table 3.2.Further silver nanoparticles were immobilized on the hollow silica nanotubes usingAgNO3 and [Ag(NH3)2]NO3; with higher Ag loading in the later case.

3.3.2.2. Vapor phase synthesis. As in many other systems, VLS growth of silica nanotubesassisted with In was reported [264] with 30–120 nm diameters and almost �1 mm lengthand occasional nanobelts on the top surface. The contrast difference along the length ofthe nanotube was due to filled and unfilled portions of these anisotropically grown struc-tures. Si, In2O3 powder mixture in the ratio 1:1 was thermally treated in vacuum inductionfurnace at �1400 �C. The vapors of Sn–In–O condense on the substrate as liquid dropletsand the SiOx precipitates from the mixture resulting from the higher bond energy of Si–O

Table 3.2The pore structural analysis of samples prepared at ratio B with Methods I–III [263]

Feature Method I Method II Method III

BET surface area (m2/g) 774.9 974.3 895.6Pore volume (cm3/g) 0.696 0.916 0.742Average pore size (nm) 3.52 3.05 2.47


(773 kJ/mol) compared to that of In–O (105 kJ/mol). TEM images reveal the nanotubeswith included indium metal in their core, Fig. 3.35.

It is also possible [265], to synthesize a spectrum of morphologies like nanowires, filledand hollow nanotubes of silicon and germanium oxides, that have self-assembled to formnumber of morphologies like feather, maize, brush, bamboo, etc., Fig. 3.36. Laser ablationof germanium in combination with thermal evaporation of SiOx was used for this purpose.Ge catalyzed VLS or Ge nanowire templated VS growth are the mechanisms driving theseexciting self-assembled structures. The variation in the morphology and the self-assemblyis a function of the temperature i.e., specific regions in the chamber can yield some char-acteristic morphology. Schematic of the growth mechanism and a typical furnace set-upare shown in Fig. 3.36(2,3).

3.4. Tin oxide

3.4.1. Overview

SnO2, an important large band gap n-type semiconductor, is widely known for its use insensors, transparent conducting electrodes for organic light emitting devices and solar cells.

Fig. 3.35. (a–e) TEM images of as prepared nanotubes (reproduced with permission from [264]).

Fig. 3.36. (1) Various morphologies as a function of temperature, (2) schematic of the growth mechanism and (3)typical experimental set-up (reproduced with permission from [265]).


Some features of scientific and technological importance:

• Large band gap n-type semiconductor (Eg = 3.6 eV at 300 K).• Relatively higher theoretical capacities for SnO2 anodes (�790 mA h/g).• Widely used as a transparent conducting oxide.• Potential applications: catalysis, sensors, electronic devices, Li ion batteries.

3.4.2. Structural features

The thermodynamically stable crystal structure of SnO2 is rutile (tetragonal crystal sys-tem), with lattice parameters a = 4.737 A, and C = 3.186 A having an axial ratio of1:0.672. The crystal structure of SnO2 belongs to the point group symmetry 4/mmm andspace group P42/mnm; with tin and oxygen atoms in 2a and 4f positions, respectively.With a unit cell consisting of two tin atoms and four oxygen atoms, with metal and oxygenatoms having an octahedral coordination. Oxygen atoms are surrounded by three tinatoms, which approximate the corners of an equilateral triangle. It was observed thatthe SnO2(11 0) surface has no net dipole moment making it non-polar. The low axial ratiosof centrosymmetric structure was proposed as the reason for reduced ease to form aniso-tropic nanostructures along [001] direction. The ODNS of SnO2 grow along the [101],


[301], [200], or [112] directions [266]. Considering the crystal-symmetry and the surface-energy of different planes; a typical crystal habit should be acicular and tubular with asquare cross-section.

3.4.3. Synthesis

3.4.3.1. Vapor phase methods. Nd:YAG laser was used to ablate a Sn target in order tosupply Sn vapor that is further transported down stream using an Ar/O2 mixture downstream [267]. The Sn vapors diffuse in to catalyst Au particles on the Si–SiO2 substrateforming a Sn–Au alloy. SnO2 nanowires grow from the supersaturated Sn–Au alloy, withcatalyst particles on the tip, controlling the dimensions of the nanowires SnO2. A typicalVLS growth mechanism was reported. Size tunable box beams of rutile–SnO2 werereported by Liu and co-workers [268,269]. They have produced nanotubes with squareor rectangular cross-section using a combustion chemical vapor deposition technique.These box type nanotubes were grown on quartz substrate via vapor–solid mechanism.The 50 nm-sub-micrometer diameter nanostructures were found to grow along h00 1idirection with {110} peripheral surfaces. The nanoboxes grow from the bottom througha self-catalyzed surface diffusion mechanism. In addition to the aligned square and boxtyped structures, co-axial and secondary tubes perpendicular to the periphery of primarynanotubes were also reported. The time, temperature, supersaturation, availability ofnucleation sites, crystallographic orientation have all been discussed [269], Fig. 3.37.

Nanowires of Ru doped SnO2 were reported through high temperature evaporation ofprecursor oxide powders [270]. It was expected that the Ru acts as a nucleating agent and

Fig. 3.37. Proposed steps in the growth mechanism for SnO2 nanoboxes: (a) accumulation of polycrystallineSnO2 layer, (b) nucleation of end caps on the surface of large grains by Ostwald ripening and (c, d) growth ofSnO2 tube arrays by lifting tubes up from bottom (reproduced with permission from [269]).


enhances the anisotropic growth. 72% and 65% nanotube yield was obtained for 20% and10 wt% Ru respectively. Dai et al. [190] have reported ODNS of SnO2 with orthorhombicstructure at a pressure in excess of 150 kbar. The orthorhombic structure can form in a thinnanowire, coexist with the normal rutile structured SnO2 in a sandwiched nanoribbon, oroccur in the form of nanotubes, Fig. 3.38. It was observed that the use of interlayeredSn/SnO mixtures have a higher ease to form nanowires compared to the pure SnO precur-sor. The microdiffraction patterns of the nanobelts have revealed strong superlattice ortho-rhombic structure at the sides of the sandwiched nanobelts and strong rutile structure withweak superlattice reflections at the center of the core layer, Fig. 3.38. Another importantfeature of interest in this study is the dimensional similarity in orthorhombic and rutilestructures, which might assist an easy transformation between orthorhombic and rutilestructures. (a = 0.4714 nm, b = 0.5727 nm, c = 0.5214 nm, for orthorhombic structure).

SnO powder and a mixture of basic ZnCO3 and graphite powders was thermally evap-orated to produce uniformly distributed ZnO nanocrystals with wurtzite structure on thesurfaces of the tetragonal SnO2 nanowires [271]. A 20:1 ratio of Sn and Zn precursors wereused and the products were collected on a stainless steel boat. The wires were expected to

Fig. 3.38. (a) TEM image of an individual sandwiched nanoribbon. (b) Select area electron diffraction patternfrom the ribbon. (c, d) Microdiffraction patterns taken from the center and edge of the nanoribbon, respectively.The sample was made using a layered Sn foil/SnO reactant mixture. The furnace temperature was 1050 �C as thechamber ranged from 250 to 700 Torr over the course of the run. The sample was collected from the cold plate(reproduced with permission from [190]).


follow a VS growth mechanism and replacement reaction leading to the surface depositionof ZnO nanocrystals is:

ZnðgÞ þ SnO2ðsÞ ! ZnOðsÞ þ SnOðgÞ ðDG ¼ 51:8 kJ=molð900 �CÞÞ ð3:4Þ

The HRTEM image and a micro-SAED pattern of the ZnO coated SnO2 nanowires areshown in Fig. 3.39.

SnO2 nanowires and nanobelts synthesized by thermal evaporation were coated with Pdthrough vapor deposition [272]. Following images show the amount of Pd deposited onthe surface of SnO2 NWs and NBs, Fig. 3.40.

3.4.3.2. Solution and template-based synthesis. Molten salt based synthesis of SnO2 nano-rods (15 nm diameter) was reported at moderate temperatures (320–700 �C) [273]. Thisnon-hydrothermal route yielded nanorods with aspect ratios of �50–100. Ordered arraysof stannic oxide nanotubes with the diameters matching the pore diameters of the PAAtemplate (�20 nm) were obtained through sol–gel/template based synthesis [274]. The syn-thesis of arrays of SnO2 nanorods is feasible [266]. A one-step, aqueous, low-temperaturegrowth process for the inexpensive fabrication of large (several tens of cm2) 3D arrays ofhighly ordered and crystalline SnO2 nanorods is practiced (50 nm in width and 500 nm inlength). Significance of this work is the uniquely designed architecture without the need oftemplate, surfactant, applied field, or undercoating on various substrates. Controlled orga-

Fig. 3.39. High-resolution TEM image of the SnO2 nanowire and the formed ZnO nanocrystals. Inset a micro-SAED image of a ZnO nanocrystal on the surface of a SnO2 nanowire. The black parallelogram and whiterectangle denoted the pattern of rutile SnO2 and wurtzite ZnO, respectively (reproduced with permission from[271]).

Fig. 3.40. HAADF-STEM (high-angle annular dark field) images of a nanowires (top row; a, c, e) and ananobelts (bottom row; b, d, f) recorded after different palladium deposition times, as indicated: 210 s (a, b),1000 s (c, d) and 5000 s (e, f) (reproduced with permission from [272]).


nization and preferential crystallographic orientation of Fe2O3/SnO2 composite nanorodswas also studied [275] and hetero-structures with sixfold symmetry through a solutionbased synthesis. Fig. 3.41 depicts the interfacial mismatch (c) and the correspondingTEM images.

Electrospun hydroxylpropyl cellulose (HPC) fibers were used as a template to synthe-size nanoscale SnO fibers by Seal’s group. In this work HPC and SnCl2 precursors weremixed in ethanol and the electrospinning was carried at predetermined voltages. The com-posite fibers were calcined to burn off the polymer and the resultant SnO2 fibers were ana-lyzed, work is under progress to enhance the sensing properties of SnO2 in the fiber formby integrating them on MEMS devices [276]. Fluorinated tin oxide fractals were synthe-sized using a simple anodization technique. Bera et al. proposed that the fluorinated tinoxide fractals grown with �25% fluorine follow diffusion limited aggregation (DLA)model [277].

3.5. Vanadium oxide

3.5.1. Overview

Vanadium oxide, a well known functional ceramic, has significant applications in thefields of catalysis, electrochemistry, and potential possibilities in the rechargeable ion bat-teries and electronics.

Fig. 3.41. (a) A closer TEM observation from the interfacial region of an individual hierarchical structure.(b) HRTEM image recorded from the white frame in (a). (c, d) Schematics of the interfacial lattice mismatchbetween (110) group of planes of R-Fe2O3 and [101], [001] growth of SnO2, respectively (reproduced withpermission from [275]).


3.5.2. Synthesis process3.5.2.1. Solution and template-based synthesis. CNTs were used as templates for the synthe-sis of V2O5 NT in the first attempt [278]. Subsequently, the solution based synthesis routeshave taken the lead and various possibilities were realized [279]. The credit for the synthe-sis of first redox active transition metal oxide goes to the scientists from ETH Zurich. Var-ious amines and the alkoxides of vanadium were used to synthesize the tubular VOx

structures [280]. It was realized that the (CH2) chain length in the primary or diamine tem-plate will control the inter layer spacing in the tubular structures. Morphology of the tubeswas varied by the choice of diamine or monoamine. Diameters of up to 150 nm andlengths of a few hundred nanometers could be produced. Later, the same group has pro-duced the VOx NT using non-alkoxide precursor [281]. Hydrolysis and hydrothermaltreatment of V2O5 or VOCl3, under a ligand exchange templating mechanism was usedto produce variable length nanotubes. High crystallanity and the amine intercalation


are the characteristic features of these ‘‘anisotropic scroll’’ like structures. Aging of thehydrolyzed sol, duration of the hydrothermal treatment were found to control the vana-dium oxide/surfactant composite and their subsequent scrolling up to form nanotubularstructures. The intermediate lamellar composite of V2O5/surfactant from VOCl3/dodecyl-amine is shown in Fig. 3.42.

The role of amines in controlling the dimensions and morphologies of the VOx NT canbe found in Table 3.3.

Irrespective of the mechanism of formation of the NT, the morphology and structuralfeatures were found to be the same. Initial vanadium to template ratio and the similarintermediate lamellar structure lead to uniform features in all the methods. The NTs wereshown to be scrolled sheets with different layers, rather than the concentric circles. Inanother work, VOx NTs were synthesized using hexadecylamine (HDA) as a templatefrom the V2O5 sol [282]. Co-condensation of the HDA with the water molecules in theinorganic cluster was found to form the lamellar structures that rollup to form the nano-tubes. A detailed characterization (FTIR, TGA and XPS) have strengthened the argumentof variable oxidation states in the VOx NT and ascertained the presence of predominant4+ oxidation state in the NTs. Template removal was made difficult by structural

Fig. 3.42. TEM image of the product obtained from VOCl3 dodecyllamine, without washing, the gel washydrothermally treated at 180 �C for 3 days. The lamellar arrangement of vanadium oxide layers started to bendbut then the tube formation stopped (reproduced with permission from [281]).

Table 3.3Morphological characteristics of the vanadium oxide nanotube depending on the precursor and template [281]

Precursor TemplateCnH2n+1NH2n

Outer diameter(nm)

Inner diameter(nm)

Number oflayers

Length(lm)

VOðO=PRÞ233 4 6 n 6 22 15–150 5–50 2–30 1–15

VOCl3 11 50–100 15–35 10–20 1.5–5VOCl3 16 80–100 20–35 11–14 0.8–1.5V2O5 11 70–40 20–35 9–13 2–12V2O5 12 50–90 15–35 7–13 1–3V2O5 16 70–100 20–45 6–11 1–2V2O5 20 60–90 20–45 6–10 1–3


instability above 250 �C and the collapse of the NTs. Use of proton exchange with neutralalkyldiamines and the cation exchange with transition metals are the possible alternatives.A significant feature of the VOx NT is the ease for functionalization resulting from its pref-erence to participate in the exchange reaction with mono-, di- or trivalent cations.Although, drastic reduction in the interlayer structures (almost 50%) were observed themorphology was quite stable.

Di-vanadium pentoxide nanorods were synthesized in a microemulsion based method[283] using vanadium alkoxide precursors. A detailed XPS and XRD analysis concludedthat the predominant oxidation state for vanadium in these structures is 5+. Authors, aftera detailed analysis, take credit for the synthesis of c-V2O5 for the first time. Scientists alsoclaim that the interatomic spacing in the nanorods is completely different from that of thebulk structure, a key for understanding the catalytic behavior of these anisotropic nano-structures. The size of the nanorods can be altered by varying the ageing time in the micel-lar solution.

3.5.3. Modeling

The electronic properties of pure and Mo doped vanadium pentoxide nanotubes wereanalyzed through theoretical studies [284,285]. Tight binding approximation was used tocalculate the binding energy of the scroll like nanotubes and attributed higher stabilitycompared to the cylindrical nanotubes. Studies were carried out considering pure V2O5

nanostructures. However, most of the vanadium oxide nanotubes are non-stoichiometricand are composite structures with intercalated amines, etc. [284]. Hence, the future studiesshould address the non-stoichiometry and model the VOx type of nanotubes.

3.6. Other oxides

Tungsten oxide is known for its high structural flexibility, switchable optical propertiesand catalytic behavior. Tungsten oxide nanotube bundles were synthesized at room tem-perature using a solution phase approach by a group of scientists from Germany [286].They have used tungsten isopropoxide in benzyl alcohol as a medium for one pot synthesisof anisotropic nanostructures. Their high surface-to-volume ratio combined with the highpurity of the material was expected to enhance the sensing capability of the nanowire bun-dles. Elemental tungsten was thermally evaporated to yield nanowire networks [287]. 3Dnetwork of the nanostructures was reported to form with a preferred growth in h100idirection. Nanowires of this transition metal oxides along with nanowires doped withpotassium were grown on a W plate with out any catalyst or template [288]. The nanowireswere expected to follow a vapor–solid growth mechanism. It was soon realized that thepresence of potassium halide was necessary to generate nanowires [289]. The interactionbetween salt, oxygen and tungsten were discussed and the temperature was reported tobe below the melting temperature of the salt. These nanostructures grown on the tips ofW were expected to be potential contestants for high resolution STM and nanosensorapplications. Native oxides on CVD tungsten films when exposed to methane/hydrogenwere found to result in the formation of crystalline WO3 nanowires, through a whiskergrowth mechanism [290]. The nanowire growth in the process was driven by the interfacialstrains and stimulated by the tungsten carbide.

The synthesis details of various other oxide systems are given in Table 3.4 and generalgas sensing results and configurations in Table 3.5.

Table 3.4Synthesis of oxide ODNS


MgO NW Vapor–solid process [291,292]NT Thermal evaporation of elemental Mg [291]NB Evaporation of elemental Mg powders [293,294]NR Mg(OH)2 Evaporation of Mg, VS method [295]

Al2O3 NF VLS [296]NT Hydrothermal, solvothermal [297,298]

Wet etching if ZnO/Al2O3 NFs [299]Oxidation of Al4O4C NWs [300]

NW Etching of porous alumina membranes [301,302]Carbothermal process [303]VLS growth [304]

Ga2O3 NW Thermal evaporation of oxide, metal [305–311]Carbothermal reduction [311]Thermal annealing [312]Microwave plasma based synthesis [313]Arc discharge [314,315]

NT, NB Carbothermal reduction [316]

In2O3 NW Thermal evaporation of oxide [306–310]Laser ablation [317,318]Template based [319]

NF Thermal evaporation–oxidation [320]

MnO2/Mn3O4 NR Hydrothermal [321–323]NW Solid state reaction (ostwald ripening) [324]

Electrochemical synthesis [325]Hydrothermal treatment, with Na+ asstructure directing agent

[326]

CuxO NW Thermal oxidation (VS) [327]Hydrothermal conditions [328]Solid state reduction with hydrazine [329]Electrodeposition from surfactant liquidcrystalline phase

[330]

NT Template MOCVD [331]NR Wet chemical synthesis at 100 �C in basic

environment[332]

Sb2O3, Sb2O5 NR Microemulsion in presence of AOT [333]NF Heating of Sb2S3 nanopowders in Ar

atmosphere[334]

Fe2O3 (Hematite) NT Hydrothermal synthesis [335]Template based [336]Wet etching in (NH4)2SO4 (magnetite) [337]

NW Oxidation of pure iron [338]NR/NB Low-temperature iron water reaction

(350–450 �C)[339]

ZrO2/YSZ NT CNT template based [340]Electrochemical anodization [217,219]

HfO2 NT Electrochemical anodization [341](continued on next page)


Table 3.4 (continued)


Y2O3 NR Hydrothermal synthesis [342]

CeO2 NR Microemulsion – AOT [343]Hydrothermal [344]

NT Controlled annealing of Ce(OH)3 fromhydrothermal synthesis

[345]

Sonochemical [346]

Table 3.5Typical gas sensing results reported recently for the various forms of Oxide ODNS

Sensormaterial

Form Synthesismethod

Operatingtemperature(�C)

Gas(amount)

Sensitivity(Rair/Rgas)

Responsetime (s)

References

SnO2 NB Vapor phaseevaporation

200 C2H5OH(250 ppm),NO2

(0.5 ppm)

2.0, 30 Fewseconds

[347]

SnO2/Pd NW Thermalevaporation

200 H2 (?) 2.5 2.5 [272]

Pd NW Electrochemicaldeposition

25 H2 (5%) 3.5 75.0 (ms) [348]

In2O3 Randomnetwork ofNW

Thermalevaporation

370 C2H5OH(1000 ppm)

30.0 10.0 [349]

ZnO Randomnetwork ofNW

Carbothermalreduction

370 C2H5OH(200 ppm)

50.0 15.0 [350]

TiO2 NT arrays Anodization 290 H2

(1000 ppm)10,000.0 200.0 [351]


3.7. Applications of oxide ODNS

3.7.1. Catalysis and sensorsThe key to the success of any photocatalytic activity is the realization of the importance

of immobilization of high surface area nanostructures. The photocatalytic properties andthe enzyme immobilization by stannous bridges inside of the TiO2 (anatase) nanostruc-tures are reported [196]. The rate of decomposition of salicylic acid in presence of sunlightwith the TiO2 fibers (rate constant 0.03 min�1) as catalyst was shown to be an order ofmagnitude higher than the sol–gel synthesized thin film of TiO2 (rate constant0.003 min�1) [352]. The reason was found to be the most obvious increase in the surfacearea up to �315 cm2. The rate of decomposition was found follow pseudo-first-orderkinetics. In addition to the photocatalysis, these anisotropic template synthesized nano-tubes were used to immobilize alcohol dehydrogenase, with the help of Sn2+ as linkingagent. This might find applications in the bioreactors. The open ends of the tubes wereproposed to assist the easy flow of the substrate solution, while the effluents will be prod-


uct after a bioconversion. Annealing treatment was found have a role in determining themorphology, structure and photocatalytic behavior of nanotubes H2Ti2O4(OH)2 are alsoreported [353].

Rechargeable lithium ion batteries are another field, where the TiO2 ODNS might havepotential utilization. Further, the use of TiO2 in other electrochemical devices also resultsfrom its active lithium intercalation ability [354,355]. It was found that the lithium inser-tion coefficient ‘‘x’’ (LixTiO2) depends on the microstructure and the crystal nature. Usu-ally, the best performance can be found with an anatase crystal structure resulting in aninsertion coefficient of 0.5 [356]. Large lithium intercalation capacity, high discharge/charge rate capabilities and excellent cycling stabilities were reported for the electrodemade of H-titanate nanotubes. Well-dispersed nanostructured electrodes were preparedby in situ ultrasonic dispersion in N-methyl pyrrolidone. An initial discharge capacityof 282.2 mA h/g at a current density of 0.24 A/g, and a stable cycling discharge capacitiesof 210, 185.7 and 165.9 mA h/g at current density of 0.24, 1.0 and 2.0 A/g, respectively,were reported. Extraordinary cycling efficiencies, excellent capacity and the electrode sta-bility were attributed to the layered, open ended nature of the nanotubes. Few hundrednanometers length of these novel nanotubes provides significantly smaller solid state dif-fusion distances in addition to much larger intercalation lengths resulting from 0.78 nminterlayer spacing in the nanotubes (compared to the commercial LiCoO2 NT). The micro-structure and the morphology of the electrode lamina were found stable even after 50charge/discharge cycles [355]. Charge/discharge capacity, cycling capacity and the effi-ciency are shown in the following, Fig. 3.43.

Zn ion surface doped TiO2 NT synthesized by Xu et al., have shown a better catalyticactivity compared to the pure TiO2 nanoparticles, Zn ion surface doped nanoparticles, andthe pure TiO2 NTs [211]. The degradation of methyl orange in water was used to charac-terize the photocatalytic ability of various titania based structures. Various catalysts forwater gas shift (WGS) reaction are under rigorous investigation, for their use in H2 pro-duction and fuel processing. Extremely high activity and the structural stability are thestringent requirements [357]. Titania nanotubes synthesized from the particle precursor

Fig. 3.43. (A) Charge and discharge capacity cycling performance of H-titanate electrode at various currentdensities and the charge/discharge efficiency [354]. (B) Residual methyl orange at different irradiation time for (a)pure TiO2 nanoparticles, (b) Zn ions surface-doped TiO2, nanoparticles, (c) pure TiO2 NTs, and (d) Zn ionssurface-doped TiO2 NTs (reproduced with permission from [211]).


through hydrothermal synthesis were used to support the Au particles for the WGS reac-tion [358]. Observed improvement in the catalytic activity of titania NTs supported goldnanoparticles was attributed to the peculiar structure, and the nature of the nanotubes.The gold nanoparticles were identified to fit in the nanotubes, hence an increase in thediameter of the nanotubes was expected to further enhance the catalytic activity of theAu/titania NT. Nitrogen absorption isotherms and the temperature programmed reduc-tion (TPR) were used to characterize the catalytic activity, while XRD, TEM were usedto carry the structural and morphological analysis. It was reported that the titania NTsupported Au nanoparticles have shown improved WGS catalytic activity compared tothe traditional Au/Al2O3 or Au/TiO2 high surface area and mesoporosity. However, theauthors believe that attaining anatase structure in nanotubes can further improve the cat-alytic activity [358].

A low temperature, functionalization of anodized titania nanotubes with Ba and Srproducing barium strontium titanate (BST) thin film pattern in the from of nanotubesis also reported [359]. The anodized titania nanotubes were subjected to hydrothermaltreatment in aqueous Ba(OH)2 + Sr(OH)2 at 200 degrees for transformation to crystallineBST nanotubes. Because of the piezoelectric properties of BST these BST nanotubes canfind useful applications in electronic, optoelectronic and sensor devices.

Electro- and bio-electro-catalysis and bio-sensors are expected to be the potential appli-cations for immobilized titania NTs [360]. Ready and reversible adsorption of three redoxsystems, namely, Meldola’s blue, Ni2+, and cytochrome c, was reported. Adsorption dataare shown in Table 3.6.

Intriguing electrochromism can be seen for transparent titanate nanotube film whencompared to the anatase nanocrystals [361], resulting from its layered structure. Signifi-cant color change observed after cathodic polarization was attributed to reversible valencychange of Ti(IV).

Alternatively, tunable gas sensors and catalysis applications of SnO2 nanowires werealso explored [362]. Individual tin oxide nanowires were fabricated into a FET and theelectron transport properties were studied at different temperatures and gaseous environ-ments. It was proposed that the bulk electronic properties of the nanowires were entirelydependent on the surface chemical reaction taking place that could be modified by gatepotential. An enhancement in the gas sensing by SnO2 NW/NBs when functionalized withPd catalyst on the surface has a significant impact on the gas sensor technology [272]. Theenhanced sensitivity of these metal coated nanostructures is exclusively from Pd nanopar-ticles on the nanowire, as they create Schottky barrier-type junctions resulting in the for-mation of electron depletion regions within the nanowire, constricting the effective

Table 3.6Data for adsorption of aqueous cationic redox systems (5 min adsorption time) onto TiO2 nanotubes [360]

Redox system Binding constanta

(mol�1 dm3)Binding sitesb

(mol g�1)Mass ofabsorbance (lg)

Meldola’s blue 2500 ± 700 7 ± 2 · 10�5 1.5Ni2+ 250 ± 100 4 ± 2 · 10�5 15Cytochrome c 2 ± 1c 0.97 ± 0.2 · 10�5 30

a Approximate, assuming Langmuir adsorption (errors estimated).b Number of +1 binding sites determined voltammetrically (errors estimated).c This apparent binding constant does not correspond to a true equilibrium value.


conduction channel and reducing the conductance. The dramatic improvement in the sen-sitivity from Pd-functionalization was due to the enhanced catalytic dissociation of themolecular adsorbate on the Pd nanoparticle surfaces and the subsequent diffusion of theresultant atomic species to the oxide surface. However, the sensing is inherent to the nano-belts rather than a result of phenomena occurring at the Schottky barriers [363]. NO2

detection limits of 3 ppm and sensitivity of the order of a few seconds are achieved. Excel-lent sensitivity toward CO, ethanol, and NO2, was reported for SnO2 nanobelts betweenplatinum interdigitated electrodes; at 300–400 �C under a constant flux of synthetic air[55]. A significant difference in the LPG and NO2 sensing was observed for SnO2 NWswith the amount of Ru doping [270], Fig. 3.44. Active sites present on the surface ofthe SnO2 nanowires were expected to work as humidity sensor by dissociating water.

Alcohol sensors have a major demand in the biomedical, chemical, food industriesalong with the wine quality and breath analysis. Most existing sensors being sensitive tovarious other trace elements, anisotropic V2O5 structures like fibers, ribbons or belts havebeen explored as possible alternatives. V2O5 nanobelts can be used as highly stable ethanolsensors with typical response and recovery times in the range of 30–50 s and sensitivitybelow 10 ppm [364]. Fig. 3.45 is a schematic showing the image of the sensor and the sens-ing mechanism.

3.7.2. Energy related applicationsThe proton exchange capability of the titania nanotubes were examined after realizing

the presence of OH groups on the surface of nanotubes that can be easily dissociated torelease protons [204]. Titania NT with adsorbed oxyacid molecules were realized whenthe solution pH, containing hydrothermally treated nanotubes, was changed to 1.5 bythe addition of sulfuric acid, phosphoric acid and perchloric acid. When the hydrother-mally treated TiO2 nanoparticles were washed using oxoacids (phosphoric acid, sulfuricacid and oxalic acid), the titania NT were found to be useful as solid electrolytes in protonexchange fuel cell. Pellets made of powders containing nanotubes were used to measure theconductivity using AC impedance method. Higher conductivities in the range of 100 �C orhigher were reported for the oxoacid treated nanotubes. The presence of adsorbed oxoac-ids was expected to increase the proton concentration that further lead to an increase in

Fig. 3.44. Temperature dependent sensor response of SnO2 with different Ru contents: (a) NO2 and (b) LPG(reproduced with permission from [270]).

Fig. 3.45. (A) Photograph and SEM image of gas sensor. (B) Schematic diagram showing the structure of atypical gas sensor based on the nanobelts and a longitudinal cross-section of nanobelts. Current passes throughthe belts by surface contact and crosses the belts bulk along the axis. (C) Schematic representation of themechanism of reaction of vanadium oxide sensors to ethanol. In air, negatively charged oxygen adsorbates(O�, O2�) cover the surface of the belt and result in an electron depleted surface layer due to electron transferfrom the belt surface to the adsorbates. In ethanol gas, oxygen adsorbates react with the adsorbed ethanolmolecules attached by hydrogen bonds, release the trapped electrons and cause the current to increase(reproduced with permission from [364]).


the electric conductivities of the nanotubes. Direct methanol oxidation fuel cells are gain-ing interest in the vague of increasing demand for the alternate fuels. The excellent cata-lytic nature of the titania NT in combination with palladium particles was explored inenhancing the performance of methanol oxidation [365]. Electro-oxidation of methanolin sulfuric acid solution was studied using cyclic voltametry. It was reported that a com-posite structure consisting of 3 wt% Pd in titania nanotubes has the best activity in meth-anol oxidation.

Incident photon–photon conversion efficiencies (IPCE) of 3.3% and 1.6% were reportedrespectively for long and short titania NTs at 540 nm and 530 nm wavelength [366]. Both2.5 lm and 500 nm tubes of 100 nm diameter and 15 nm wall thickness were sensitized byRu–dy (N3) in different concentrations. High degree of control over device implementa-tion and higher light absorption lengths are considered as beneficial effects for significantimprovements in IPCE. It was also established that higher packing density of the dye inlonger tubes is the key in tuning the performance. Addition of 10% of titania nanorodsto the template grown titania NTs could enhance the solar cell efficiency by 40%, as aresult of short circuit photo-current [188].

Unusually high specific capacities (1100 mA h/g) and excellent cyclic stability werereported for SnO2 nanorod anodes in the rechargeable Li-ion batteries [273], in the voltagerange of 5 mV–2 V. This excellent capacity and cycling performance were attributed to thehigher crystallanity combined with very low dimensionality. The reversible insertion of Li+

ion was expected to follow the sequence reduction of SnO2, alloying and dealloying withLi. Another significant feature observed by the researchers was the distortion of the nano-rods leading to the formation of shortened nanorods, cavities, bulged edges and the nano-particles, Fig. 3.46.

Their intercalation ability has driven the scientists to explore the electrochemical prop-erties of these vanadium oxide nanotubes [367,368]. The ability to insert/disinsert Mg2+

into its tubular structure was exploited for using the Mg2+ intercalated V2O5 NT as analternative to Li ion batteries. The cyclic voltametry (CV) and electrochemical impedance

Fig. 3.46. Illustrations showing (a) the formation of shortened nanorods, isolated nanoparticles, and cavities onthe nanorod surface and (b) the expansion of diameters near the ends of the nanorods relative to the midsections(reproduced with permission from [273]).


spectroscopy (EIS) data has shown faster diffusion rates in the NT than the polycrystallinebulk material. Mg2+ intercalation was supported by the XRD, XPS data along with thecyclic voltammograms. The electrode reaction for Mgj0.25 M Mg (AlBu2Cl2)2/THFjVOx

NT secondary battery can be written as

xMg2þ+ 2xe + VO2:37 (TEMP)0:26 () MgxVO2:37(TEMP)0:26 ð3:5Þ

In the following cyclic voltammogram (Fig. 3.47) the reduction peak at 0.9 V indicatesthe electrochemical insertion of Mg2+ in the vanadium oxide nanostructures.

3.7.3. Bio-based applications

Use of silica in biological applications results from its characteristic ability towards sur-face functionalization, based on simple silane chemistry [256]. Based on this concept,nanotubes were prepared with the green fluorescent silane N-(triethoxysilylpropyl)dan-sylamide attached to their inner surfaces (A), and the hydrophobic octadecyl silane(C18) to their outer surfaces (B). Their clear separation, when dispersed in an immisciblemixture of water and cyclohexane, can be observed from Fig. 3.48. The silica nanotubewere also used for the enantiomeric drug separation [255].

Titania nanotubes synthesized with controlled Na+ content were treated with calciumacetate during acid treatment resulting in the formation of Ca2+ ion doped titania NTs.ICP analysis has revealed 8 wt% of Ca2+ and 0.2 wt% of Na+ in the nanotubes. Thesenanotubes when immersed, as a pellet of 15 mm diameter, in simulated body fluid for1 min, petal like apatite layers were found to cover the entire surface of the nanotubes sub-merging the needle like structure. However, the same was not observed when commerciallyavailable apatite, b-TCP and the titania nanocrystals. Hence it was proposed that the Ca2+

doping can significantly enhance the apatite growth and proved that Ca-titania NT caninduce excellent bone tissue regeneration in the initial stages of implantation in a rat[204]. Titania nanotubes treated with NaOH were also shown to significantly improve

Fig. 3.47. Cyclic voltammogram of vanadium oxide nanotubes at 0.1 mV s�1 in 0.25 M Mg(AlBu2Cl2)2/THF(reproduced with permission from [367]).

Fig. 3.48. Ten milligrams of both A and B nanotubes (200-nm diameter) nanotubes dispersed in cyclohexane–water mixture (reproduced with permission from [256]).


the kinetics of in vitro hydroxyapatite formation and osteoblast cell growth [369,370].Growth of sodium titanate nanofibers on titania NTs was expected to enhance the bioac-tivity of the nanotubes. Following graphs, Fig. 3.49, show the number of cells adhered perunit area on titanium and titania nanotubes.

A group of scientists from Japan [371] have established the bio-compatibility and oxy-gen generation of titania nanotubes in vivo. Venous oxygen saturation (SvO2) measured 4weeks after the implantation of the nanotube samples implanted under the inguinal skin ofthe nude mouse, was found to be 30–40% more than the control region. Absence of deathor destruction of cells also established the bio-compatibility of the nanotubes.

Jeng and co-workers from Taiwan have demonstrated the feasibility of performingprotein analysis with ultra low sample volume by combining a tungsten oxide nanowirefiber (TONF) with a miniaturized electrospray ionization interface [372]. The abilityto pick up 50 nL of methanol solution was expected to outperform the existing dispos-

Fig. 3.49. Number of adhered cells per unit area on titanium and titania nanotubes respectively (reproduced withpermission from [369]).


able ESI emitters. The TONFs, 30–50 nm in size were uniformly grown on the tungstenfibers.

3.7.4. Other applications

Excellent n-type semiconduction was reported for field effect transistors (FETs) madeout of SnO2 nanowires with threshold voltage ��50 V and on/off ratios of 103 at roomtemperature [210]. UV illuminated modulation in conduction was expected to be usefulas polarized UV detectors.

Tuning of the field emission properties of WO2.9 nanorods was reported by Liu et al.[373]. They have shown a low turn on field (�1.2 V lm�1) which can be controlled byvarying the amount of WO2 content in the nanorod, with possible implications in fieldof cold-field electron emitters.

Unidimensional nanostructures of silica are expected to be potential members for bio-logical and/or chemical functionalization and as templates for further synthesis of ODNS[257]. The photoluminescence property of the silica nanotubes was reported to vary withthe synthetic method and the post treatment, Fig. 3.50 [374,257]. While Zygmunt reportedemission wavelengths of 410, 435, 460 nm in heptane based RMSG method, Zhang et al.have reported the PL peaks at 486 and 539 nm for as grown and annealed nanotubes. Thehigher intensity in case of annealed nanotubes was attributed to the removal of physi-sorbed H2O from the inner surface of the nanotubes. The discrepancy in the emissionwavelengths can be imputed to the solvent chemistry, pH, water content and the other pos-sible experimental parameters. The shoulder peaks at 410 (violet) and 460 nm (blue)observed in case RMSG silica NT were ascribed to intrinsic damaging defect centersand the neutral oxygen vacancy respectively [257].

Actuation strains of up to 0.21% and a force-generation capability up to 5.9 MPa werereported for the V2O5 nanofiber sheets [375]. These excellent characteristics might makeV2O5 and other mixed valence metal oxide nanowires as actuators, with much lowerapplied voltages than commercial ferroelectric and electrostrictive materials.

Photoluminescence (PL) of ZnO ODNS has received a maximum attention of theresearchers owing to its potential use in new generation optoelectronic and advanced

Fig. 3.50. PL spectra for silica nanotubes synthesized using (a) RMSG [257], (b) SGTM (sol–gel templatemethod) (reproduced with permission from [374]).


electronic devices [249]. Increase in the intensity of the green emission as a function of syn-thesis time relates to the evolution of ZnO nanorods, wires in the urchin like structureswith increased crystallanity [249]. UV emission at �380 nm and the green emission at�520 nm, Fig. 3.51(a), were correlated with the near band-edge emission and the deepor trap-state emission respectively [242]. The enhanced deep level emission as a function

Fig. 3.51. (a) Photoluminescence spectra of ZnO nanowires of different diameters recorded at room temperature.Spectra a, b, and c correspond to nanowires with average diameters of 100, 50, and 25 nm, respectively [242].(b) Photoluminescence spectra of four arrays grown over 1.5 h after annealing treatments (reproduced withpermission from [239]).


of decreasing thickness of these nanowires was expected to be the consequence ofincreased surface area and the higher oxygen vacancy concentration. Yang’s group[242,243] has done PL measurements and demonstrated excitonic lasing action in ZnOnanowires with a threshold of 40 kW/cm2 under optical excitation. Excitonic stimulatedemission, due to giant oscillator strength effect, was expected in these well crystalline nano-wires with diameters greater than the Bohr exciton radius and smaller than the opticalwavelength. Inherent ability to serve as laser cavity mirrors was attributed to the refractiveindices of epitaxial ZnO/sapphire interface and the sharp (0001) plane of ZnO. Relativelylonger lifetimes (350 s) compared to the thin film counterparts and the room temperaturelasing without any fabricated mirrors, resulting from a well crystalline nature, can beexpected to find applications as natural resonance cavities, etc. The temperature depen-dence and the effect of annealing in various environments at different temperatures ofemission (400 �C in 10%H2/90% Ar for 15 min, 500 �C in 10% H2/90% Ar for 15 min,or 800 �C at 5 · 10�6 Torr for 2 h) was reported [239], Fig. 3.51(b). By analyzing the lasingdata, activation energy for the non-radiative mechanisms that quench the orange lumines-cence was found to be 71 meV.

Same group has also reported a complex magnetic behavior for the transition metaldoped ZnO ODNS, that could not be modeled as either ferromagnetic or paramagnetic[235]. Effect of annealing treatment on the PL emission of the ZnO nanotube arraysand the nanowires was reported to show significant differences, Fig. 3.52 [240]. The ratioof peak intensities (IUV:IGY) of as grown nanorods (0.32) and nanotubes (3.4) was used toascertain the higher crystallanity of the nanotubes, a possible consequence of the synthesisconditions. However, the same were increased to 1.4 and 20.5 for nanorods and nanotubesrespectively while no change was observed in the PL spectra of PLD ZnO film. This par-ticular feature of the aligned nanotubes might find applications in new generation gas sen-sors and field emission devices. UV illumination and dark storage controlled reversiblesuper-hydrophobicity and super-hydrophlicity transition was reported for the ZnO nano-
rod films formed on glass wafers [376].
Fig. 3.52. Room-temperature PL spectra from a thin ZnO template film, and from nanorod and nanotube arraysgrown on such thin ZnO films: (a) as grown and (b) annealed under vacuum for 1 h at 300 �C. All spectra, afterscaling as indicated, are plotted on a common vertical scale but have been offset vertically, for clarity (reproducedwith permission from [240]).


4. Nitrides

4.1. Overview

Nitride ODNS of B, Al, Si, Ga, etc. were all synthesized and their important technolog-ical and fundamental contributions were studied by the researchers. Among all these, theunique features of boron nitride (BN) have received a significant attention from the scien-tific community [377–387].

4.2. Boron nitride

While the high temperature oxidation resistance of the BNNTs is an inheritance fromits bulk counter part, their other exceptional electronic, thermal and mechanical featureshave been predicted to have their origin in the nanoscale. Some specific features reportedfor the BNNTs are listed below and a comparative history of BNNTs with CNTs are givenin Table 4.1.

• Behaves as an insulator (band gap �5.5 eV) irrespective of the chirality andmorphology.

• Better thermal and chemical stability relative to their C counter parts.• Young’s modulus exceeding �1 TPa.

4.2.1. Synthesis methods

4.2.1.1. Solid-state synthesis. Chen and co-workers [406,407] from Australia have synthe-sized BNNT using ball milling technique. Elemental boron powders were ball milled in anammonia gas for 150 h at room temperature, which on a subsequent annealing treatmentat >1000 �C were found to result in the formation BNNTs [407]. Thermal annealing ofmetastable products from ball milling was proposed to be the key in this solid-state syn-thesis method. Nitrogen atmosphere maintained during the annealing treatment convertsresidual boron, from the ball milling, to well crystalline hexagonal BN. The 20–150 nmdiameter nanotubes resulting from the low temperature mechanochemistry were under-stood to follow a completely different mechanism compared to laser ablation and arc-discharge techniques [388].

Table 4.1Comparative history of CNT and BNNTs

Parameter/property CNT BNNT

Multi-wall tube 1991, Iijima [36] 1995, Chopra et al. [388]Single-wall tube 1993, Iijima and Ichihashi [389] 1996, Loiseau et al. [390]High yield 1992, Ebbesen and Ajayan [391] 2000, Cumings and Zettl [392]Filling 1994, Tsnag et al. [393] 2001, Bando et al. [394]Cap morphology 1992, Iijima et al. [395] 1996, Loiseau et al. [390]Helicity 1993, Zhang et al. [396] 1999, Golberg et al. [397]Thermal stability 1993, Ajayan et al. [398] 2001, Goberg et al. [399]Young modulus 1996, Treacy et al. [400] 1998, Chopra, Zettl [401]Electrical conductivity 1996, Ebbessen et al. [402] 2001, Cumings and Zettl [403]Thermal conductivity 1999, Hone et al. [404] 2005, Chang and Zettl [405]


Further, BNNTs are produced by ball milling BN powder in a nitrogen environment.The as-produced powders were found to be highly disordered or amorphous [406] whichon further annealing in a nitrogen ambience were found to form nanotubular structures.The need for milling and annealing steps was found essential as neither alone could resultthe nanotubes. There is an emphasis on catalytic effects of transition metal particles likeFe, Cr, Ni on the nanotube formation. It was also found that the bamboo like nanostruc-tures were often capped with the metal nanoparticles, where as the same was not foundwith the cylindrical nanotubes (Fig. 4.1).

4.2.1.2. Vapor phase/high temperature synthesis. The synthesis [388] of the BNNTs usingarc-discharge technique is also possible. Multiwalled nanotubes with �1–3 nm inner diam-eters and 200 nm length, using a carbon-free arc-discharge between BN packed tungstennanorod and a cooled copper electrode, are produced. The sp2 bonded hexagonal BNnanotubes with excellent semiconducting behavior were expected to contribute towardsnovel device applications. Several researchers have extensively studied the synthesis ofBNNT using high temperature vapor phase techniques [399,408–411]. The basic steps inthe transition metal oxide assisted synthesis of BNNT are:

1. Chemical reaction between C and metal (Me) oxide vapors

MeO + C!Me + CO ð4:1Þ

2. Nitridation reaction between C, boron oxide vapor and nitrogen

B2O3 + C + N2!BN + CO ð4:2Þ

It was believed that the reaction (2) is enhanced due to the open ends of the C tube viathe openings generated during reaction (1). The openings were expected to allow easyvapor flow along external and internal C tube surfaces. In addition to this, numerous
defects in the CVD nanotube templates were also expected to assist the gas flow across.
Fig. 4.1. TEM images of ball milled products after annealing in N2 atmosphere: (a) at 1200 �C for 10 h, SAEDpattern from the two filament type structure in the image and (b) 1300 �C for 10 h, SAED pattern from thelongest nanotube (reproduced with permission from [406]).


Presence of predominant tubes with even number of layers (�61%) compared to the oddnumber of layers (39%) having open ends have drawn the attention of researchers [408].The even and odd number of tubular layers as a function of the promoter is reported inTable 4.2.

Ropes of BN through nanotube self-assembly [399,410] assisted by PbO, MoO3 andV2O5 in a flowing N2 atmosphere was reported. The nanorope structures were expectedto show a significant improvement in the mechanical properties. The average diameterof the ropes was found to extend from �30 nm to few microns. A typical rope image witha possible assembly is shown in Fig. 4.2.

The approximate B to N ratio was found to be 1 from the EELS analysis. The tubesconstituting the rope structures were found to have zigzag chirality (almost 80% of theyield), with the [10�10] graphitic sheet orientation parallel to the tube/rope axis. Nano-beam diffraction patterns (probe diameter 1.6 nm) and the corresponding structural fea-tures are shown in Fig. 4.3.

The partially open ended CNTs after conversion to BNNT were found to have com-pletely open ended structures. The open cap structures were imputed to the efficientannealing effects through metal particles; the continuous oxidation of caps and efficient

Table 4.2Synthesis of even vs. odd number of layers in 600BNNT using various promoters [408]

Promoter Even Odd

MoO3 63 ± 7% 37 ± 7%PbO 56 ± 7% 44 ± 7%CuO 65 ± 7% 35 ± 7%

Fig. 4.2. HRTEM image of a representative rope made of BN multiwalled NTs synthesized using a PbOpromoter. The rope displays the uniform zigzag NT helicity (the [1010] direction of the graphitic shells is parallelto the tube/rope axis) in various areas across its cross-section, as confirmed by the nano-diffraction patternsshown in the insets. The proposed assembly of the individual NTs within the rope is shown on the left of themicrograph (reproduced with permission from [399]).

Fig. 4.3. Characteristic nanobeam diffraction patterns (NBDs), taken with an electron probe diameter 1.6 nm, ofthe individual BN nanotubes in the ropes: (a) armchair; (b) zigzag; and (c) helical atomic arrangements. Thecorresponding 3D structural models of the armchair {10,10}, zigzag {20,0} and helical {14,5} nanotubes aredisplayed for clarity below the experimental NBDs (reproduced with permission from [410]).


removal of caps by oxide vapors. Seed atomic ring formation of BN tubular shell and itscrystallization with the progress in the oxidation process were elucidated with schematics,Fig. 4.4.

In order to produce high volumes of the BNNTs, researchers have thermally (1100–1700 �C) evaporated a mixture of FeO, MgO and elemental boron powders in a BN cru-cible [412]. The Mg, Fe and B2O2 vapors when reacted with incoming ammonia vaporsform pure nanotubes of �50 nm. Long ropes of BN were also realized through a contin-uous CO2 laser ablation treatment of hot pressed, out gazed and thermally shocked h-BN[413]. In addition to the nanotubes, faceted boron nitride onion rings were also found atspecific regions in the furnace. From the HRTEM analysis it was observed that the BNpolyhedra are covered by B crystal, which undergoes nitridation on further treatment.Nitrogen plasma treatment of FeB nanoparticles at the eutectic temperature was alsoreported to yield BN nanotubes and BNNT with Fe nanowire core [414]. Kim et al.[415] have synthesized B–C–N nanotubes and studied the effect of rotating anode onthe nucleation and growth of anisotropic structures. Proposed Plasma Rotating ElectrodeProcess (PREP) can lead to improvement in discharge characteristics and/or the increasedcatalytic effect of metal species (cobalt in this case). Golberg’s group has also reported thesynthesis of AlN–BN composite nanotubes using a two step process [416].

4.2.2. Properties

4.2.2.1. Electronic properties. In addition to their synthesis and characterization, variousscientists are actively involved in exploring possible features of the BNNTs i.e., structure,

Fig. 4.4. Schematics showing the gas species flow within an opened tube and the formation of seed atomic ringsfor BN tubular shell crystallization due to gas attacks from the sides of a tube and from the edge of a tube(reproduced with permission from [410]).


electronic, and thermomechanical properties, etc. [417–420]. The prime importance ofBNNT comes from its stable band gap (5.5 eV, bulk band gap �5.8 eV) that is indepen-dent of the diameter, chirality and the number of layers of the nanotubes [421], a featurein contrast to that of CNT. Constant energy SW and MW NTs of BN were predicted toshow excellent performance under n-type doping [421]. Possible differences in the elec-tronic behavior of the BNNT with similar CNT can be attributed to the higher ionicityof the former [422]. Owing to its excellent dielectric behavior the possibility of usingBN as an insulating sheathing for conducting CNT and their combined thermal stabilitywas studied using a density functional tight binding method [419,423,424]. An excellentmechanical and thermal stability of the electronic properties, for the hetero-structuredC/BN nanostructures, was observed up to �3500–3700 �C [419,424]. However, the bandgap width between the highest occupied N 2p and lowest unoccupied B 2p states decreasesfrom 4.0 to 1.2 eV, and was expected to be a consequence of band-structure modificationupon the thermal deformation of the atomic structure of the walls [419]. The threefoldsymmetry observed in the BN, prohibiting any ground state polarization, was found tobe broken when these graphine like sheets wrap up to form BNNT ensuing in a polariza-


tion along the tube axis [418]. The polarization is controlled by quantum mechanicalboundary conditions on its electronic states around the tube circumference. Effect ofF-doping on the electronic properties of the BNNT [381] was studied through the energeticaspects involved in substitution or adsorption of F on B, N sites. It was proved that the Fatoms prefer to substitute N-atoms resulting in substantial changes in the structural fea-tures without affecting the conductivity. However, the p-type conductivity of F-dopedBNNTs was found to be a consequence of preferred adsorption of F on B sites.

4.2.2.2. Mechanical properties. While the electronic properties of the BNNTs are alluring,their main application is to use them as sheathing to the metallic nanowires as an insula-tion [399]. In addition to well known oxidation resistance of bulk BN, the BN nanotubeshave been predicted to exhibit significantly higher strengths [401,425], withstanding higherstrains compared to CNTs. This unique feature was reported by observing the thermalvibration amplitude of a cantilevered multi-wall BNNT. An axial Young’s modulus of1.22 ± 0.24 TPa is consistent with theoretical predictions and the highest ever reportedfor any insulating fiber. The agglomeration behavior of the nanotubes is a major hin-drance in identifying the properties of the individual nanotubes. Often the properties

Table 4.3Synthesis of nitride ODNS


GaN NW Thermal evaporation [427]Sublimation, VLS method [428–433]Hot filament CVD (VLS) [434–437]Catalysts assisted strategies [138,438–440,432,441–443,443–445]Template based (CNT) [446]MOCVD [447]

NR Direct vapor phase reaction [448]Molecular beam epitaxy [449,450]Pyrolysis [451]

NT Interface chemical reaction [452]

InN NW Single precursor decomposition [453,454]Vapor–solid method [455]Thermal evaporation [456]

NR Two step growth on GaN NW substrates (VLS) [457]NF Solution–liquid–solution [453]

AlN NW Two step sublimation process [458]CNT templating [459]Carbothermal reduction [460]Alumina, silica assisted catalytic growth [445,461]

Si3N4 Whiskers Carbothermal reduction [462]Thermal reduction of Si/SiO2 in NH3 [463]CNT based/assisted/templated [438,464]

NR CNT templated [465]NT Thermal heating CVD (VLS) [466]

Ga2O3 assisted hot-wall CVD [467]Liquid metal assisted route [468]

Ge3N4 NW Thermal reduction of Ge/SiO2 in NH3 [469]

Table 4.4Synthesis of carbide ODNS


SiC NW Carbothermal reduction [464,470–472]CNT confined reaction [473]Laser ablation (VLS) [474]Thermal evaporation in presence of Fe catalyst [475]

NR Floating catalyst method [476]Carbothermal reduction of silica xerogels [470]

BxC NW Fe-catalyzed growth [477]

TiC NW Fe-catalyzed reduction of TiO with CH4 [478]


measured for a random collection will not be representative of individual nanotubes. Tak-ing this into account, researchers [401] have explored several methods for measuring theelastic properties of individual nanotubes using in situ high resolution transmission elec-tron microscopy (HRTEM). Measuring the deflection due to external forces applied tonanotubes (deflections due to gravitational and electron wind forces were neglected),determining the resonant frequency of a particular tube by driving it with a forced oscil-lator, measurement of the thermal vibration amplitude of a cantilever nanotube are thedifferent methods proposed. It was noted that the measurement of thermal vibration ismost straight forward and easy to implement. The significant improvement, 14 timesgreater, in the in-plane Young’s modulus of BNNT relative to bulk hexagonal BN wasattributed to their defect-free single crystalline structure. Later Golberg et al., have pre-dicted further increment in the Young’s modulus for the BNNT when formed as ropes[399]. However, in a theoretical prediction it was reported that the BNNT might sufferfrom instability and much smaller elastic limits against bond rotations resulting from afast generation of defects [426].

Synthesis of various other nitride and carbide ODNS is summarized in Tables 4.3 and4.4 respectively.

5. Chalcogens and chalcogenides

5.1. Overview

The invention of IF structures by Tenne, as mentioned in Section 1, has significantlywidened the scope of the fullerenes, nanotubes and the nanostructures with negative cur-vature. The quest for inorganic nanotubes and other ODNS was mainly focused on thematerials with layered structures. Fig. 5.1 indicates the obvious similarities between aninorganic layered structure and the sp2 graphite.

Owing to the fact that the origin of CNT is from the fullerene type structures, under-standing the formation of IF structures and their synthesis is a key in understandingthe formation of nanotubes, whiskers and other one dimensional nanostructures. Chalco-gens and chalcogenides are not the field of our group’s expertise; hence we have onlydiscussed the properties and applications very briefly. Articles by Xia, Yang, Tenne andco-workers discuss the properties and applications of chalcogens/chalcogenides critically.

Fig. 5.1. Schematic drawings of (a) WS2 and (b) graphite nanoclusters showing the layered structures(reproduced with permission from [37]).


5.2. Synthesis processes for ODNS of chalcogens

Due to their excellent ability to control the chemistry, scalability and cost effectiveness;solution phase synthesis routes have always been a first choice. There is no requirement forusing physical templates and high quantities can be obtained as the seeds are not confinedto a 2D plane, [479]. Se, Te both have a tendency to grow anisotropically, hence ideal forODNS. Hexagonal Se (commonly known as trigonal Se, [t-Se]) has extended spiral chainsof covalently bound Se atoms parallel to c-axis [480]. Similarly, an inherent chiralityresults from the highly anisotropic helical chain of covalently bound atoms, which arein turn bound together by van der Waals interactions into a hexagonal lattice in t-Te.

5.2.1. Vapor phase routes

The importance of vapor phase routes and the parameters to obtain controlled mor-phologies were highlighted [481]. A medium supersaturation with an optimum carriergas flow was reported to grow whiskers. Researchers have reported the formation of net-works of Y type junctions of the NWs. The mechanism was categorized under VLS growthand TEM analysis at intermediate levels has confirmed the chain like structure from solid-ified liquid droplets. Si powder was used as a catalyst to reduce the Se vapor pressure andthereby assist the formation of Se nanowires from Se powders [482]. A simple carbother-mal chemical vapor deposition (CTCVD) was reported recently for the synthesis of t-Senanowires and the proposed mechanism is shown in Fig. 5.2 [480].

Various morphologies of the Tellurium whiskers were extensively studied in 1970s. Thewhiskers were grown [483] by sublimation of metallic tellurium on the surface of platinumsubstrate at various temperatures and studied by electron microscopy. Spine-(s), filament-(f) and needle-(n) like whiskers were observed in substrate temperature range from 90 to110 �C, 100 to 140 �C and 130 to 200 �C, respectively. Growth parallel to [001] direction

Fig. 5.2. Schematic illustration of a plausible growth mechanism for 1D Se nanostructures through the CTCVDroute. (A) Active carbon and excess selenium are reacted to generate CSe2 or other non-stoichiometriccompounds followed by evaporation. (B) The experimental conditions were suitable for the produced compoundsto decompose into elemental selenium instead of solidification into compounds; may be the selenium was liquidbecause of its low melting point. (C) With the decrease in temperature, nucleation and solidification began, andthen (D) the preferential growth occurred because of the inherent extended helical chains in the t-Se structure. (E,F) The growth rate along the [001] direction should be faster than that along the [100] and [110] directions. Aftera period of time, 1D Se nanostructures are formed (reproduced with permission from [480]).


was reported with various dislocation characteristics. Fig. 5.3, shows the anisotropic struc-ture of t-Te and different morphologies under the favorable growth conditions.

A group of scientists from Korea [484] for the first time have reported the synthesis oftrigonal and hexagonal nanotubes of tellurium by depositing metallic Te vapors onSi(100) substrate at 150–250 �C. A down stream of Ar was used to carry the vapors tothe substrate. Ar flow rate, temperature of the substrate and the appropriate substrateswere found to be dictating the morphology of the tubes. For example, when Ar flow rateincreased to 500SCCM or when the Si(1 00) was replaced with Si(1 11) or sapphire (0001),nanowires and nanorods were produced. Fig. 5.4 shows the nanotubes and rods at differ-ent temperatures.

5.2.2. Solution-based methods

t-Te ODNS were successfully produced in three different solvents, (water, EG andwater–EG mixture) [479]. Hydrazine was used to reduce the orthotelluric acid and reflux-ing was done between 20 and 200 �C. The reaction mixtures were stirred and heated in first10 min to complete the reduction and then the product was aged in dark at room temper-ature. The morphology and the length of the nanowires were manipulated by varying thecomposition of solvent, refluxing temperature, and the ageing time (Fig. 5.5). Ostwald rip-ening was proposed as a mechanism by which initial amorphous (a)-Te colloid particlesdissolve and crystallize to trigonal (t)-Te nanowires to attain lower free energy. A morpho-logical variation was reported with synthesis using different solvents and temperatures.Table 5.1 compares the conditions of various morphologies in vapor phase and solutionphase synthesis.

Fig. 5.3. (A) The crystal structure of t-Te is highly anisotropic, with the basic unit being helical chains ofcovalently bound tellurium atoms. These chains are held together to form a hexagonal lattice via weaker van derWaals forces. Such an inherently anisotropic structure tends to grow along the [001] direction even in an isotropicmedium. Depending on the substrate temperature (Ts), t-Te crystals grown from the vapor phase were shown toexhibit a number of different morphologies: (B) triangular, spine-shaped whiskers at Ts � 90–100 �C; (C)filamentary, blade-shaped whiskers when Ts was raised to 100–140 �C; and (D) needle-like whiskers when Ts wascontrolled in the range of 130–200 �C. The inset of each plate indicates the cross-section of that particular type ofwhisker (reproduced with permission from [479]).


A similar chemistry has been predicted for the synthesis of both t-Te and t-Se.

2TeðOHÞ6 þ 3N2H4 ! 2Teð#Þ þ 3N2ð"Þ þ 12H2O ð5:1ÞH2SeO3aq þN2H4aq ! Seð#Þ þN2ð"Þ þ 3H2O ð5:2Þ

Synthesis of Se nanowires of wide range of diameters, 10–800 nm, and few hundredmicron length was achieved [485,486]. Formation of initial amorphous a-Se, its subsequentdissolution and deposition onto crystalline seeds has been proposed as the predominantmechanism driven by the preferred anisotropic growth in along the c-axis. This underlyingmechanism in both Se and Te nanowires is pictorially depicted in Fig. 5.6.

As the amorphous solid seed dissolves and grows as crystalline solid, the process iscoined with a name solid–solution–solid process (SSS), in contrast to the vapor–liquid–solid growth (VLS). A similar mechanism was predicted to yield t-Te nanorods in presenceof a surfactant [487], that can assemble into smectic-like arrays over a few hundred

Fig. 5.4. FESEM image of (a) tellurium nanotubes synthesized on a Si(100) substrate, (b) nanotube withtriangular and (c) hexagonal cross-section, (d) Te nanorods with triangular and hexagonal cross-sectionsdeposited on a Si(100) substrate, (e) Te nanowires deposited on a Si(111) substrate (reproduced with permissionfrom [484]).

Fig. 5.5. The boiling points of ethylene glycol and water mixtures having different ratios. The generalmorphologies of Te nanostructures observed at different solvent compositions (and thus refluxing temperatures)were also indicated in this drawing. Needle-shaped nanowires were only formed in a very narrow range oftemperature (174–182 �C). Below this temperature range, filamentary nanowires were generated. Above thistemperature range, tubular nanostructures with tri-tipped whiskers were observed (reproduced with permissionfrom [479]).


Table 5.1Various morphologies as a function of temperature and solvent used

Morphology Futura et al. [483] Mayers et al. [479]

Spine like 90–110 �C 20–100 �C (Water)Filamentary 100–140 �C 100–196� (Ethylene glycol)Needle like 130–200 �C 178 �C (Water/ethylene glycol)

Fig. 5.6. Schematic illustration of a general mechanism for the formation of t-Se nanowires via a solid-solution-solid pathway: (A) reduction of aqueous selenious acid with excess hydrazine yields a dispersion of amorphouscolloids that are 0.1.2 lm in diameter; (B) rapid cooling of the colloid mixture leads to the nucleation of trigonalseed crystals, and the colloids become irregular due to Rayleigh instability; (C) material from the amorphouscolloids dissolves and grows onto the seeds; (D) growth continues until all of the amorphous material is consumedand only nanowires of trigonal Se remain (reproduced with permission from [39]).


nanometers. The synthesis of super long nanoribbons of t-Te in tetraethylene pentaminesolution [488] and their stability in ethanol and water as a function of time were estab-lished. Taking advantage of their similar structural and chemical features scientists havealso produced the Se/Te alloy nanotubes with controlled morphology, composition andlateral dimensions [489].

5.2.3. Sonochemical synthesis

t-Se nanowires and nanotubes were synthesized at room temperature using sonication[490,491]. Use of room temperature is reported to hinder the nucleation of crystallineseeds. The precipitation and redispersion of the a-Se of a few microns size in ethyl alcohol,followed by mechanical agitation and a short sonic pulse were found to initialize the crys-tallization event on the surface of a-Se colloids and lead to nanowire growth. Theenhanced kinetics of growth can be attributed to the higher solubility and mobility ofSe atoms in alcohol. The mechanism of sonochemical synthesis and the correspondingSEM images are shown in Fig. 5.7.

Interwoven 2D networks of these nanowires contribute towards the self-guided growthof interconnects between nanoelectronic devices. This can be achieved by depositing thesonicated a-Se on a solid substrate and subsequently submerging the dried substrates inalcohol. Ability to produce large (order of grams) quantities with this process and the var-iation in the thickness of the nanowire as a function of Se concentration are specific attri-butes of this process. For example the diameter was increased from �40 nm to 100 nm by

Fig. 5.7. (1) A schematic of the illustration of three major steps involved in the sonochemical approach (2) andthe SEM images that confirm different proposed stages (reproduced with permission from [490]).


a threefold increase in Se concentration. Xia’s group [492] have reported an exhaustivestudy of sonochemical method for a large scale production of Se NWs. Results from theirwork using various solvents are tabulated in Table 5.2.

5.2.4. Hydrothermal route

Thin single crystal Nano Belt (NB)s (quasi-ODNS) and NTs of t-Te were synthesizedthrough hydrothermal reduction of sodium tellurate (Na2TeO4 Æ 2H2O) with aqueousammonia solution at �180 �C for 36 h in an autoclave [493]. Two different mechanismsfor the formation of nanobelts and nanotubes, namely, template-roll-growth and tem-plate-twist-join-growth, respectively, are shown in Fig. 5.8.

Hydrothermal method was used to synthesize the t-Te nanotubes, that are groove like,by reducing sodium tellurate with formamide (HCONH2) in sodium hydroxide solution[494]. The hydrothermal treatment was carried at 160 �C for 20 h and the products wereanalyzed at various time intervals. The mechanism of formation was reported to be dif-ferent from that proposed by previous researchers [479,493], nucleation–dissolution–recrystallization mechanism in this case. The initial nuclei were reported to be sphericalnanoparticles which further dissolve into the solution as atoms and recrystallize intoODNS due to the preferred anisotropic growth along c-axis. SEM images taken at dif-ferent time intervals (Fig. 5.9) have revealed the ODNS to be groove like nanotubes.non-ionic surfactant polyoxyethylene sorbitan monolaurate (Tween-20), was used inanother facile hydrothermal synthesis method to obtain ultra-long submicometer SeNTs [495].

Both hydrothermal and sonochemical techniques were used in sequence for producingchalcogen ODNS [496]. Depending on the product size from the hydrothermal treatmentboth tubes and wires of t-Se were obtained. Larger particles were found to break downearly during sonication and assemble edge-wise leading to nanotubes, whereas, the smallerparticles directly assemble together to form nanowires.

Table 5.2Comparison of t-Se grown in various solvents [492]

Solvent Viscosity Lowenergyabs, nm

Highenergyabs, nm

Structure Morphology

Ether 0.224 650 450 Crystalline NanowiresMethanol 0.54 630 450 Crystalline NanowiresEthanol 1.07 600 450 Crystalline NanowiresPropanol 1.95 600 450 Crystalline NanowiresButanol 2.54 610 450 Crystalline Slightly tapered

nanowiresHexanol 4.58 630 B Crystalline Slightly tapered

nanowires2-Propanol 2.04 645 415 Crystalline Tapered nanowiresIsobutanol 4.31 640 >415 Crystalline Tapered nanowires2-Butanol 3.1 665 >415 Crystalline Tapered nanowiresCyclohexanol 57.5 630 404 Crystalline Tapered nanowiresEthylene glycol 16.1 >690 B Mixed

amorph/crystallineTapered nanowires

Diethylene glycol 30.2 >690 B Mixedamorph/crystalline

Tapered nanowires

Triethylene glycol 50 >690 B Mixedamorph/crystalline

Tapered nanowires

Tetraethylene glycol >60 >690 B Mixedamorph/crystalline

Tapered nanowires

PEO(MW-150) >80 >690 B Crystalline Tapered rods3-(Dimethylamio)-propanol >10 A A Crystalline Non-uniform

microcrystals2-Methylamino-ethanol >10 A A Crystalline Nonunifrom

microcrystals18 MX Water 0.89 >690 B Amorphous No reactionUnsubstituted alkanes >690 B Amorphous No reactionAcetone 0.31 A A Crystalline Polydispersed rodsChloroform 0.54 A A Crystalline Polydispersed rodsAniline 2.03 A A Crystalline Tapered rodsAcetonitrile 0.37 A A Crystalline Tapered rodsEthylenediamine Soluble

A – Unable to disperse and attain adsorption spectra, B – no peak observed.


5.2.5. Biomolecule-assisted synthesis

Biomolecules are used for the synthesis of nanowires under hydrothermal conditions[497,498]. The strong morphology directing nature of various biomolecules such as algenicacid, cellulose, mild biopolyhydric chemicals like sorbitol and polygalacturonic acid, etc.was exploited to produce these ODNS. Various biomolecules used and the resultantODNS are tabulated, Table 5.3.

5.2.6. Nanoparticles to ODNSSe and Te NWs can also be produced by the spontaneous transformation of their respec-

tive chalcogenides, namely, CdSe and CdTe respectively [499]. In this study, the stabilizersused while synthesizing the CdTe and CdSe NPs were depleted by using different alcoholsand the particles were, separately, redispersed in potassium ethylenediaminetetraacetate

Fig. 5.8. Schematic illustration of (a) template-roll-growth mechanism, (b) template-twist-join-growth mecha-nism (reproduced with permission from [493]).

Fig. 5.9. SEM images of five samples collected after hydrothermal treating for (a) 5 h, (b) 8 h, (c, d) 12 h, (e) 16 h,and (f) 20 h (reproduced with permission from [494]).


(EDTA) solutions at pH 9. The stabilizer depletion was expected to result in low concen-tration chalcogenide molecular species, which subsequently deposit on to the nanoparticles.

Table 5.3Various biomolecules used and the resultant ODNS

Biomolecule Morphology/dimensions

Algenic acid Te NWs of 80 nm diameter and few microns lengthCellulose Se NBs of 200–1500 nm width, tens of nanometer thickness and few

micrometers lengthsSorbitol Se NWs of 60 nm diameter and few microns lengthPolygalacturonic acid Wire like architectures of Se, few hundred nm long and centepede shapes with

dozens of aligned nanowiresFructose, lactose, algenic acid

and starchNonuniform, thick and rodlike shapes of Se


This process was expected to reduce the energy for recrystallization due to higher mobility.TEM images at different times confirm the evolution of the t-Se NWs from the assembly ofNPs (Fig. 5.10). Simultaneous change in crystal packing and the chemical reaction of pre-cursor are unique combination in this NP! NW transformation. Presence of oxygen wasthe key to the mechanism leading to the formation of CdE complexes and atomic chalco-gens in solution. As the dimensions of the product are time and temperature dependant,uniform dimensions and 100% conversion efficiencies can be achieved. Protein CytochromeC3 was used as a catalyst to produce Se nanowires from selenate ðSeO2�

4 Þ, which are actuallythe assembly of Se nanoparticles into long chains [500].

5.3. Synthesis processes for chalcogenides

Realization of IF structures has accelerated the research towards the synthesis of var-ious ODNS of chalcogenides, a technologically important group of materials [501,502].The IF-MX2 (IF dichalcogenides, where M = Mo, W, Bi, etc. and X = S, Se, Te) and their

Fig. 5.10. Evolution of Se-NWs from stabilizer depleted CdSe NPs redispersed in EDTA solution a 25 �C after(A) 10 min, (B) 1 h and (C) 24 h (reproduced with permission from [499]).


one dimensional nanostructures have been synthesized by various methods [6,39,503]. Thebasic chemistry is almost similar in most of these processes [37].

5.3.1. Template-directed synthesis

The chalcogen ODNS [39], especially Se and Te nanowires were successfully used forthe synthesis of chalcogenides ODNS. Chemical transformation of Se nanowires into sil-ver selenide (Ag2Se) and mercury selenide chloride were reported (Hg3Se2Cl2). The differ-ences in the mechanisms were attributed to the similar lattice spacing (to that of Se), fasterdiffusion of Ag species in the prior case and the lower mobility, dissimilar lattice distancesin mercury in the later case. Porous alumina membrane was used to synthesize single crys-tal CdS nanowires array [504]. Preferred attachment of S2� ion to the Al3+ walls (Lewisacid in nature) with the Cd2+ ions was proposed as a preferred mechanism for the forma-tion of the nanowires. Gavrilov et al. [505], have sulfidized the porous anodized alumina(PAA) membranes filled with metal precursors to obtain the CdS and CuxS nanowiresarrays.

5.3.2. Vapor phase reactionThe solid precursor (MoO3) of nanoparticles are evaporated and collected on a quartz

surface while passing through a stream of H2S and a forming gas (5%H2/95%N2). Theoxide clusters in gas phase react with these gasses and form an oxide core covered witha single MoS2 shell. These clusters on the quartz tube continue to react slowly and formhollow sulfide IF structures. A non-uniform oxide to sulfide conversion rate may be sig-nificant as the sublimation of the precursor is not instantaneous. This leads to the varia-tion in the number of MoS2 layers at different times. Also, the size of the IF particledepends on the incipient oxide nanoparticles that sublimate from the precursor, whichin turn will be a function of temperature.

As2S3 NWs were synthesized through an evaporation and condensation of the corre-sponding glass and the elemental powder mixtures [503]. The actual mechanism of growthwas completely different from the established mechanisms like VLS, SLS and VS. Unstablethin film growth resulting from variations factors were proposed to drive the anisotropicNW growth. Bi2S3 NTs were also produced by evaporation of the precursor sulfide pow-ders [506].

5.3.3. Solid phase reactionThe solid precursor (WO3 nanoparticles) remains intact and the H2S gas reacts with the

nanoparticles uniformly. Hence there will be no size distribution or non-uniformity in thesulfide layers at intermediate stages. The solid phase reaction can be achieved in a Fluid-ized Bed Reactor (FBR). The possible growth mechanism is given in Fig. 5.11.

Modifying the parameters in the solid phase synthesis like precursor feed rate, gas flowrate, etc.; both on higher side to those used for fullerene like particles; lead to the forma-tion of WS2 nanotubes. This process is capable of producing many grams of multiwallWS2 nanotubes [507] and their bundles in a single step. The FBR for this process was con-structed using quartz glass. The typical parameters are listed in Table 5.4.

The forming gas mixture was supplied from the bottom of the 60 cm long reactor, at atemperature of 800–840 �C, to keep the particles in suspension. A ceramic filter at the bot-tom of the tube was used to collect the powder and analyzed after cooling. A significantand repeatable yield of nanotubes along with few millimeter thick and �1 cm long wires is

Sublimation of MoO3 With 5μm

particle size

MoO2

MoS2

MoO3-x<100nm

MoS2

secondstens of seconds

tens of minutes

MoS2

NestedFullerene – like


tens of minutesWO3

< 200nm WO3-x W18O49

WS2WS2 WS2


(B) Solid-phase reaction

(A) Gas-phase reaction


particle size

MoO2

MoS2

MoO3-x<100nm

MoS2


tens of minutes

MoS2



tens of minutesWO3

< 200nm WO3-x W18O49

WS2WS2 WS2



particle size


particle size

MoO2

MoS2

MoO3-x<100nm

MoS2

secondsMoO3-x<100nm

MoS2

secondssecondstens of secondstens of seconds

tens of minutestens of minutes

MoS2



tens of minutesWO3

< 200nm WO3-x W18O49

WS2WS2 WS2



tens of minutesWO3

< 200nm WO3-x W18O49


tens of minutesWO3

< 200nm WO3-x W18O49

WS2WS2 WS2


(B) Solid-phase reaction

(A) Gas-phase reaction

Fig. 5.11. Schematic representation for growth model of the inorganic fullerene-like nested polyhedra of(A) MoS, and (B) WS, from oxide nanoparticles [37].

Table 5.4Typical parameters and their quantities in a solid phase growth process for synthesizing chalcogenide (WS2)nanostructures, [507]

Parameter Quantity

Precursor (WO3 nanoparticles) 60–70 mg/minFeed gas (nitrogen) 60–70 mL/min

Forming gas mixture

• H2S 5–12 mL/min• H2(5%) N2(95%) 60–120 mL/min


possible using this method. The characteristic parameters like helicity and the number ofshells are features of interest. A typical nanotube and a bundle of tubes are shown inFig. 5.12.

Remskar and his co-workers from Switzerland, have done an extensive work on syn-thesis and structural stabilization of chalcogenides micro and nanotubes [502,508–510].They have synthesized MoS2 microtubes [509] and SWNT bundles [502] in large quanti-ties. The SWNTs were prepared using a C60 catalyzed growth realizing 15% of the startingmaterial as nanotubes. This process took 22 days at 1010 K in an evacuated silicaampoule at a pressure of 10�3 Pa. They have reported bundles of nanotubes growing per-pendicular to the substrate surface that could be separated to individual nanotubes by dis-solving in ethanol. An extensive characterization has not revealed the presence of any C60

trace indicating its role as a catalyst. Syntactic coalescence of WS2 nanotubes and theirself-assembly to form WS2 nanoropes was observed [508]. The origin of nanoropes wasexplained by the effect of long range attraction forces and the charge modulations ofnanotubes in the growing process. They have synthesized gold and silver alloyed ODNSof MoS2 and WS2.

Fig. 5.12. (A) TEM images of a bundle of three very long WS2 nanotubes; (B) high-resolution image of a WS2

nanotube and its electron diffraction (inset) showing the helicity of the nanotube. (C) Fissured nanobelts(reproduced with permission from [507]).


The synthesis of Ag and Au alloyed nanotubes is reported by the same group [510]. Thechirality and the variation in the growth of the nanotube as a function of the alloying ele-ment have been critically analyzed with the help of electron diffraction techniques. Ballmilling and sintering route was used to synthesize nanotubes and nanowires of TiSe2 fromelemental powders [511]. Rolling up of the layered sheets to form the tubules was con-firmed by TEM analysis. Hydrogen reduction of MS3 type chalcogenides under controlledconditions has resulted in the NbS2 and TaS2 nanotubes [512].

5.3.4. Solution-based methods

Micelle-template-assisted method was used to synthesize the Bi2S3 nanotubes of �8 lmlong and �120 nm diameter [513]. Laser irradiation of the GaS, GaSe powders dispersedin di-tert-butyldisulfide was reported to yield nanotube like structures [514]. The roll up ofexfoliated sheets was proposed as a possible mechanism. Use of toxic precursors like H2Sand noxious surfactants like TOPO/TOP has driven the researchers towards more benignmethods. Cadmium sulfide and selenide nanotubes and nanowires were synthesized byc-irradiation. The 60Co c-ray source was used for irradiation [515].

5.3.4.1. Solvothermal/hydrothermal synthesis. Rod-shaped MnSe2 and a-MnSe were syn-thesized from elemental selenium and manganese acetate under hydrothermal conditionsin presence of NaOH [516]. ODNS of NiE2(E = Se, Te) [517]; PbSe [518]; CdS [519] weresynthesized using the ethylenediamine. Double N-chelation and the special structure ofethylenediamine were proposed to drive the ODNS formation. MSe (M = Pb, Zn, Cd)nanorods were synthesized by using the monodentate ligand n-butylamine as shape con-troller [520]. The interaction of surface metal ion with only one anchor atom in the ligandis sufficient for the formation of nanorods. The close interaction between the anchoratoms in ligands and the metal ions on the surface (Lewis base and Lewis acid respec-tively) stimulate the nanorod growth and suppress the nucleation and growth of nanopar-ticles. Schematic in Fig. 5.13 shows the possible interaction between the metal ion and theligand.

NH2 NH2

NH2 NH2

NH2NH2

Cd2+ Cd2+ Cd2+ Cd2+

M2+ M2+

NH2 NH2

NH2NH2

NH2 NH2

NH2 NH2

NH2NH2

Cd2+ Cd2+ Cd2+ Cd2+

M2+ M2+

NH2 NH2

NH2NH2

NH2 NH2

NH2NH2

NH2 NH2

NH2NH2

Cd2+ Cd2+Cd2+ Cd2+ Cd2+ Cd2+Cd2+ Cd2+

M2+ M2+M2+ M2+

NH2 NH2

NH2NH2

NH2 NH2

NH2NH2

Fig. 5.13. Possible surface coordination modes: (a) monodentate mode of en molecules; (b) polydentate mode ofen molecules; (c) monodentate mode of n-butylamine [520].


5.4. Properties and applications

Some common features and potential applications of the chalcogenide ODNS:

1. While most of them show a semiconducting behavior irrespective of their structure,with a band gap that diminishes with decreasing diameter of the nanotube, NbS2

NTs were found to be metallic.2. CdSe nanorods with polythiophenes were reported to show power conversion efficien-

cies as high as 1.7%, when used in solar cells [521].3. Arrays of Bi2Te3 and BiSb wires were expected to show significant improvements in

thermo-electric properties [522,523].4. Coulomb blockade effects in InP nanowires at 0.35 K drive their use in future quantum

electronic devices like resonant tunneling diodes and single electron transistors [524–526].

It is quite evident from the discussion in all the previous pages that the synthesis ofnanoscopic one dimensional objects has fascinated the scientific community for almosttwo decades now. ODNS have found application in almost all the fields of science andengineering from semiconductors to drug delivery, from sensors to catalysis and fromoptics to photonics. The gamut of their properties (chemical, mechanical, electrical orthermal) is extending its arms into more complex and more evolved form of science thatis becoming unpredictable but yet so reliable that it can shape the future. There are nano-tubes beyond carbon which are present in this article. The potential of ODNS which con-tain other materials as building blocks is unlimited. However, there is another class of onedimensional materials that have the same ‘‘carbon’’ as the backbone but still not a carbonnanotube. These materials are classified as polymeric or organic ODNS and have evolvedas a separate science parallel to the growth of carbon nanotubes.


6. Polymeric ODNS

6.1. Overview

The first application of polymeric nanofibres was demonstrated in a pioneering work byMartin et al. [527] where they reported extraordinary increase in the electronic conductiv-ity of polymeric fibers with the confinement of dimension and size in mesoscopic range.Conducting polymers (doped or undoped) [528] science has progressed in leaps andbounds to a stage where scientists are now looking them as a possible candidate to oustthe silicon, which has reached the limits of miniaturization, from semiconductor industry.Polymeric nanotubes, nanowires and nanorods have since then flooded the scene ofresearch with unprecedented growth. This rapid growth rate has been fueled by syntheti-cally novel approaches and the importance to various cross disciplines. Number of scien-tific groups across the globe have been working successfully in developing novel routes forsynthesis of OD Polymers such as electrospinning [529], co-electrospinning [530], mem-brane based [78], tubes by fiber templates (TUFT) [531], electrochemical polymerization[532] within templates and chemical and self-assembly routes [533,534], etc. to quote a few.

All the processes described above are able to yield polymeric ODNS with desired prop-erties and precise aspect ratios. In the present section, general synthetic strategies and theadvancement made in each field will be discussed briefly. In addition, application of suchnanofibres and nanotubes in medicine, catalysis, electronics and sensors shall be discussedwith special reference to the electronic properties of conducting polymers.

6.2. Synthesis

6.2.1. Electrospinning

Electrospinning is one of the most widely used techniques for synthesis of polymericfibers. This technique is a novel yet simple and capable of assembling fibrous polymersto diameters from micron level down to less than 100 nm. It is a high voltage process toform ultra-fine solid fibers from a stream of polymeric solution or melt onto a target cath-ode through a nozzle. As the process is based on electric field, it can be controlled orguided to yield several geometric three dimensional shapes. However, our description willbe restricted to the synthesis of nanofibres which are one dimensional ultra-fine solid fibershaving large surface to volume/mass ratio (approximately 100 m2/g for a fiber with adiameter of 100 nm [535]). MacDiarmid et al. were able to electrospin polyaniline-basednanofiber with diameter below 30 nm [536]. The elegance of the electrospinning processlies in its ability to form a wide range of polymeric fibers [537] depending upon few phys-ical and chemical parameters. However, precise control of these parameters is required forobtaining specificity in the product shape, size or properties. A variety of particles can beadded to the polymeric blend, solution or melt to reinforce the fiber matrix or encapsulatespecial materials in the fibers. One of the advantages of electrospinning is that water can beused as a solvent and hence water soluble polymers such as poly-ethyl oxide (PEO), poly-vinyl alcohol (PVA), poly-lactic acid (PLA), etc. can also be electrospun. Thus the entiregamut of polar and non-polar organic polymers can be spun using this technique [538].

The first patent for electrospinning process was awarded to Formhals (Patent number –1-975-504) in 1934 and since then the process has been revived and reviewed continuouslyby various workers across the globe [528,529,535,537,539–548]. The process has been mod-


ified or used in its prima facie form to yield a variety of polymers which in turn are usedfor multifunctional applications; such as doped polymers [528] in nanoelectronics. Theelectrospinning technique is a high voltage electrostatic method in which a polymer solu-tion (or melt) maintained at a high positive potential, in a variety of different polar/non-polar solvents is placed in a hypodermic syringe at a fixed distance from a metal cathode[549,550]. The resulting fibers are collected on a grounded plate as shown in the schematicof the process in Fig. 6.1 [548]. There are various arguments for positioning the capillaryor needle at different inclinations [531,543,549]. The polymeric solution or melt coming outof the syringe or needle forms a droplet owing to its surface tension, which gets charged tovarious degrees depending on applied voltage and the concerned polymer. The mutualcharge repulsion causes a force directly opposite to the surface tension [551]. As the inten-sity of applied field is increased the droplet changes its shape from hemisphere to an elon-gated cone like structure popularly known as Taylor Cone [552]. As the applied voltagecrosses a critical value of potential, the surface charge overcomes the surface tension ofthe droplet on the tip and a jet of fluid is ejected from the tip of Taylor cone. This dis-charged polymer undergoes a whipping process (or solidification in case of melt) in whichthe solvent evaporates within the flight distance. The charged polymer left behind is highlystretched as it lands on the cathode. Dry fibers accumulate on the surface of the cathoderesulting in a non-woven mesh of nano- to micron-diameter fibers depending upon exper-imental parameters. Though the process is very simple in operation but is very complex inactual mechanism of spinning of fibers and is controlled by various types of instabilitiessuch as the Rayleigh instability, an axisymmetric instability and the whipping or bendinginstability [541,553–555]. The whipping action is attributed as the major factor governingthe elongation and thinning of the electrospun fibers. The other instabilities are responsi-ble for fluctuations of the radius of the jet and may result in a droplet formation.

Electrospinning process parameters can be divided into (a) Electromechanical: appliedpotential, flow rate of polymer, distance between the cathode and the anode and motion ofthe screen, (b) Chemical: molecular weight of the polymer, molecular weight distribution,architecture (branched or chain polymer), co- or mono-polymer, conductivity and

V

Collector Plate

Flowing semi-liquid jet

Taylor cone

Polymer or metal solution

Syringe

High Voltage

Deposited polymer

V

Collector Plate

Flowing semi-liquid jet

Taylor cone

Polymer or metal solution

Syringe

High Voltage

Deposited polymer

Fig. 6.1. A schematic diagram of electrospinning process.


dielectric constant of the polymer solution (depending on the functional groups), (c)Rheological: viscosity, concentration of solution, surface tension, etc. and (d) External:

temperature, humidity, air velocity/vacuum of the chamber.Based on the experimental studies it has been found that these parameters must have an

equilibrium value as they may act contrary to each other; for instance, the polymer solu-tion must have a high concentration to cause entanglement but a very high viscosity pre-vents polymer motion by electric field. Similarly a solution with low surface tension andhigh charge density is preferred but at the same time viscosity should be high enough toprevent the jet from collapsing into droplets. There are several systematic studies for find-ing the correlation between various parameters. One of the studies on the effects of spin-ning voltage and solution concentration on the morphology of the fibers reported [540]that the bead defects (Fig. 6.2) are strongly related to spinning voltage. The fiber size isrelated to solution concentration through a power law relationship with fiber diameterincreasing with increase in concentration. Applied electric field can produce several differ-ent cross-sectional morphologies, such as nanoribbons (Fig. 6.3 [535]).

Fig. 6.2. Example of bead formation during electrospinning of 13 wt% PET in 1:1 dichloromethane andtrifluoroacetic acid (reproduced with permission from [540]).

Fig. 6.3. Nanoribbons of polyaniline fibers synthesized using electrospinning (reproduced with permission from[535]).


As stated earlier the electrospinning process is characterized by rapid evaporation of thesolvent or solidification of the melt. The nanostructure formation occurs on a millisecondtime scale and hence one would expect that the nucleation and growth of the crystalswould be different from their bulk or macro counterparts. However this may not be thecase and the mechanism for nucleation has found to be the same as that observed forthicker fibers. The nanofeature is attributed to the whipping mode that induces bendingand has been studied mathematically including rheological complexity of the polymersolution allowing consideration of viscoelastic jets [550]. Three dimensional paths of con-tinuous jets in the straight region and in the region of instability were calculated. Thismodel provides a reasonable representation of the experimental data and jet paths. Inanother similar study [556,541,546,555] the stability characteristics of jet were studied asa function of realistic and measurable values of the fluid properties. The most importantcharacteristics of the fluid were found to be viscosity and conductivity. It was proposedthat the mechanism of electrospinning is governed by a rapidly whipping jet and thatthe classical Rayleigh instability is axisymmetric and is suppressed when the applied elec-tric potential and surface charge density exceed a threshold value. The air drag force perunit jet length which tends to compress the jet along its axis is given by the equation

fa ¼ Paqat20:65

2ta

ta

� ��0:81

ð6:1Þ

where qa and ma are the air density and kinematic viscosity, respectively.The growth of the small bending perturbation that is characterized by d is governed in

the linear approximation by the equation

md2ddt2¼ 2e2

‘31

d ð6:2Þ

where e is the charge and ‘ is length of ideal rectilinear jet and m is mass.The growing solution of this equation is given by

d ¼ d0 exp½ð2e2=‘31Þ

1=2t� ð6:3Þ

At high field a second axisymmetric and a third non-axisymmetric instability dominate.Which instability dominates depends upon the surface charge density and the radius ofthe jet. A quantitative model was developed subsequently for calculating the shape andcharge density of a steady jet. The static and dynamic properties of amorphous polyeth-ylene fibers have been studied using Monte Carlo simulations [557,558]. It described theeffect of confinement and curvature of the surface of nanofibres on the chain propertiesof polymer. It was found that anisotropy in the orientation of bonds and chains at the sur-face and mobility of the chain in nanofibers increases as the diameter decreases.

Several modifications have been proposed to the setup of conventional electrospinningprocess. Reneker et al. have introduced rotating drum technique to collect electrospunfibers as uniform mats [559]. A process of using multiple jets (US patent Number6713011) to increase the efficiency of electrospinning process is patented. Craigheadet al. demonstrated the use of pyramidal silicon tip used as a scanning tip electrospinningsource for deposition of oriented fibers [560]. In another unique attempt [542] single poly-mer fibers on the surface of a microchip without using high voltage and syringe pump orneedle though the fibers were produced at microscale and not nanoscale. But this also


promises the combination of electrospinning technique with the stereolithographictechniques.

Recently a new version of electrospinning termed as co-electrospinning has been devel-oped wherein the researchers have produced PEO–PEO type and PEO–PDT type core–shell nanofibers [530]. The experimental set-up is as shown in Fig. 6.4 and is characterizedby two air inlets. This process is capable of producing core–shell nano/meso fibers. Bothliquids flowing from the core and the surrounding concentric annular nozzles form a com-pound droplet which undergoes a transformation into a compound Taylor cone with acompound jet co-electrospun from its tip. This jet when pulled by electric field undergoesthe similar instability effects, as in the traditional process. As the solvent evaporates thecompound jet solidifies resulting in compound core–shell nanofibers as shown inFig. 6.5. The high speed of electrospinning process prevents the mixing of core and shellpolymers. Co-electrospinning can also be applied to polymer–metal salt systems as shownby synthesis of PLA–Pd(OAc)2 system [530]. On annealing the coaxial fibers for 2 h at170 �C metallization of Pd(OAc)2 into Pd was observed. Co-electrospinning can also beused to form the polymer nanotubes and hence can be of extensive use in a variety of sys-tems and applications. There are similar modifications of the electrospinning process thatare of importance and the readers are encouraged to refer the literature [561–563].

The process of electrospinning cannot be summarized without mentioning the discoveryand application of electrically conductive polymers. Though conducting polymers shall bediscussed vide infra, it would be worth mentioning their efforts in this discussion using theelectrospinning process. MacDiarmid et al. [528] coined the term intrinsically conductingpolymer (ICP) or synthetic metal for an organic polymer that possesses electrical, elec-

OuterChamber outlet

Inner chamberoutlet

Inner and outerpolymer solution

Air pressureinlet of theouter chamber

Rubber band

Air pressureinlet of theinner chamber

Electrode

OuterChamber outlet

Inner chamberoutlet



Rubber band


Electrode

OuterChamber outlet

Inner chamberoutlet



Rubber band


Electrode

OuterChamber outlet

Inner chamberoutlet



Rubber band


Electrode

Fig. 6.4. Experimental set-up for co-electrospinning of compound core–shell nanofibers [530].

Fig. 6.5. TEM of unstrained samples of co-electrospun PEO (shell) and PDT (core) (reproduced with permissionfrom [530]).


tronic, magnetic and optical properties of a metal while retaining the mechanical proper-ties and processibility of a polymer. Its properties are intrinsic to a doped form of polymerand are completely different from the class of polymers called as conducting polymers. Thelatter are merely a physical mixture of a non-conducting polymer with a conducting mate-rial such as carbon powder distributed throughout the material. The scientists were suc-cessful in increasing the conductivity of the electronic polymers by many orders ofmagnitude as illustrated in Fig. 6.6. The concept of doping is central and unifying themewhich distinguishes conducting polymers from all other types of polymers. The doping ofpolymeric fibers was obtained using an already existing process of producing highly con-ducting sulphuric acid doped polyaniline nanofibers [529]. Polyaniline doped with 10-cam-phorsulphonic acid (HCSA) has been synthesized ranging from 0.5 to 2 wt% and blendedwith 2–4 wt% PEO as shown in Fig. 6.7 [543]. These polymer blend solutions were pre-pared by first dissolving the exact amount of HCSA required to fully dope the emeraldinebase in chloroform. By this process separate individual nanofibers can be collected andexamined. These fibers are found to be sufficiently conducting and could be imaged underan SEM without the necessity of applying a gold coating.

6.2.2. Tubes from fiber templates (TUFT)

Synthesis of nanotubes by using fibers as template is a multiple step and another highlyversatile technique for producing polymer, metal and hybrid nano- and meso-size tubes[531]. It utilizes degradable fibers produced by electrospinning as template. These degrad-able polymeric fibers can then be coated with the desired wall materials using variousdeposition techniques as shown in Fig. 6.8. This type of morphology gives a core–shelltype fiber as already discussed in co-electrospinning. Selective removal of the core material

Fig. 6.6. Conductivity of electronic polymers (reproduced with permission from [528]).

Fig. 6.7. Nanofiber blend (50 wt%) of PAn Æ HCSA fabricated from 2 wt% PAn Æ HCSA and 2 wt% PEO fromchloroform solution at 25,000 V (anode/cathode separation, 25 cm). Scale bar: 100,000 nm (reproduced withpermission from [543]).


then produces the nanotubes structure. The only limitation of the process at the time ofinception was the mesoscopic diameters rather than nanoscopic. Since the external shellis coated on the fibers, the inner diameter of the tubes is strictly limited to the outer diam-eter of the nanofibers. A distribution of the tube diameters is given in Fig. 6.9. In general,low melting polymers are used as the core which in the initial study was chosen as PLA.The coating of the fiber template was achieved by three different processes viz. CVD, dipcoating, PVD, etc. The degradable polymer was then decomposed thermally to obtain ametal, polymer or core–shell tube like structure. The wall thickness was in the range of0.1–1 lm. Though these tubes cannot really be called as nanotubes but they offered a

Template fiber

Polymer tube

Metal tube

A B C D

–

+

Template fiber

Polymer tube

Metal tube

A B C D

–

+

Fig. 6.8. (A) Schematic diagram of the apparatus used for electrospinning. It consists of a syringe with a metalcapillary (diameter 0.3 mm) and a pressure supply on the piston of the syringe. A high voltage field, typically inthe range of 4 kV/cm, with the anode on metal capillary was applied to the polymer solution has been formed atthe tip of the capillary. These fibers are deposited on a glass substrate. (B) Concept for the preparation of polymertube. (C) Concept for the preparation of polymer/metal hybrid tubes. (D) Concept for the preparation of metaltubes [531].

Fig. 6.9. Distribution of tube diameters (in lm) of PPX tubes obtained by coating electrospun PLA fibers andsubsequent degradation of PLA template fibers [531].


potential for synthesizing various metallic, polymeric and core–shell nano-tubes by aunique multiple step process which can be modified to obtain nanotubular structures.

6.2.3. Membrane/template-based synthesis

The significance of the template-based method has been emphasized in almost everyinorganic system. The same was also found to have a prominence in case of the polymerODNS, as a result of its reliability and preciseness in the product. However, polymericsolutions or melts have an advantage over metallic or metal oxides structures synthesizedusing this method because of their inherently high wettability. Thus formation of poly-meric ODNS using membrane based or template based synthesis is relatively easy andhence popular. Martin et al. have pioneered the use of this technique to form ordered


meso–nano-structured polymers and their use thereof in the synthesis of conductive poly-mers [564] or in bioseparations or biocatalysis [256]. Various synthetic strategies havealready been discussed in this article, a few more significant details about the membraneor template based synthesis are presented below.

Most of the polymeric ODNS work in template based synthesis has been limited to twotypes of membranes, the ‘‘track-etch’’ polymeric membranes and the porous alumina/silicamembranes (the latter is less popular in polymeric systems). Track etch membranes infertheir name from their synthesis via the track-etch [565] method. These membranes containcylindrical pores of uniform diameters and the most common material used to preparethese membranes is polycarbonate. Pore diameters down to 10 nm with a pore densityof 109 pores/cm2 are commercially available. On the other end are the porous aluminaor silica membranes. These types of membranes are prepared by electrochemical etching(or anodization) of the base metals aluminum or silicon to produce ordered nanoporosityon the surface of these materials. The processes involved are discussed at length in litera-ture elsewhere [566–569]. Historically, the conductive polymers were synthesized using oxi-dative polymerization of polymers either electrochemically or using chemical oxidizingagent within the pores of these templates [570]. For electrochemical template synthesis;a metal film is coated onto one side of the membrane and then this film can be used toelectrochemically synthesize the desired polymer within the pores of the membrane. Chem-ical template synthesis can be accomplished by immersing the membrane into a solution ofdesired monomer and its oxidizing agent. It was also observed that the polymer generallynucleates and grows on the pore walls. As a result polymeric tubules sizes are obtained atshort polymerization times. The preferential nucleation of polymer on the pore walls wasattributed to the solubility difference of these polymers in their monomer and polymericforms. While the monomers are soluble, the polycationic forms of the monomers are insol-uble and hence there are solvophobic and electrostatic components of interaction betweenthe polymer and the pore walls. Essentially there can be other synthetic strategies also thatuse the template-based synthesis. CVD has also been one of the major contributors forproducing polymeric ODNS from templates. Process is simple and involves placing ofthe membrane in a high temperature furnace while reactive gases are passed throughthe furnace. Thermal decomposition of gas occurs in the pores resulting in the decompo-sition/deposition of polymeric films along the walls of the pores. Synthesis of PPV nano-tubes and nanorods using CVD polymerization methods in the pores of alumina andpolycarbonate filters having pore diameters of 10–200 nm [571] have been reported. In asimilar effort polypyyrole was synthesized using track etched membranes and their furtherelectrochemical polymerization [572]. Different electrolytes were used to see the effect ofelectrolyte on the morphology and properties of polypyrrole nanotubes. Authors alsomade a morphological time dependant study of growth of polypyrrole tubes. A systematicRaman study found the results to be in agreement with the increase of conductivity forsmall diameter tubules as shown in Table 6.1.

The template-based process is further analyzed [538,573]. A phenomenon of precursorfilm of submicron-size thickness is exploited in this method. Synthesis by template wettingtakes place, if liquid droplets spread on flat substrates which happen even in the case ofviscous liquids. A precursor film of thickness less than 100 nm stems from the macroscopicdroplets of the wetting liquid and eventually covers the substrate. It is possible to preparepolymer nanotubes by wetting of an array of cylindrical nanoporous membranes by poly-mers in the molten state or viscous solution form. It is expected that a complete wetting of

Table 6.1Raman bands intensity ratio in terms of PPy tubule diameter for different electrolytes [572]

PPy tubule diameter I1595/1500

PPy=ClO�4 PPy/DS PPy/TS PPy/PSS

110 1.4 1.3 1.4 1.265 1.9 1.4 1.5 1.335 2.4 1.5 1.7 1.4

DS – Dodecyl sodium sulfate, TS – p-toulenesulfonic acid sodium salt, PSS – poly(sodium styrenesulfonate).


the pore walls by polymers and the thickness of the polymer wetting layer is controlled bythe polymer–wall material interaction. The driving forces for complete filling are muchweaker than for the coating of the pore walls by a thin wetting film. Wall wetting and fill-ing thus take place on different time scales.

Polymer nanotubes can be realized if the flow of the polymer is quenched after wall iswetted prior to the complete filling. The diameter of the nanotubes, the distribution of thediameter and the homogeneity along the tube walls as well as their length are all controlledby the template matrix. Polymer nanotubes can be prepared by placing the polymer solu-tion as a powder or pellet on the top of a pore array within a thermostat and heating it to atemperature above the glass transition temperature. The walls of the pores are wetted bythe polymer through this process until the entire surface is covered. The shape of the poresis thus reproduced resulting in the nanotubes with a wall thickness of 10 nm and diametersranging from less than 100 nm up to a few microns depending on the template. The lengthof the nanotubes is controlled by the depth of the porous template. On successful removalof the template, arrays of free standing polymer nanotubes can be obtained.

6.2.4. Template-free synthesis of polymer ODNSThough template-based synthesis and electrospinning techniques are very popular

because of the ease of operation, they suffer from a serious drawback of integration ontoa device for application. Recent progress in the field of nanolithography and othernanopatterning applications has outperformed the limitations of these techniques. Theelectrospinning process suffers from outsized apparatus and inefficient control on thedirectionality of the spinning fibers in terms of a specific mode of coating on the miniatur-ized device is difficult. On the other hand the template-based methods are limited to thesize and shape of the template and moreover several precautionary measures are to betaken to preserve the nanostructure from destroying while separating them from template.Outer and/or inner diameters that can be achieved from these templates lack completetransformation below 30 nm scale. Researchers across the globe opened various frontsof research out of their own scientific interests or to overcome the above mentioned draw-backs in the techniques available for synthesis of polymeric ODNS. These processes, com-monly known as ‘‘template-free synthesis’’ or ‘‘self-assembly’’ have successfully been ableto produce polymeric ODNS. Their major advantage is the reduction in diameters up to orbelow 50 nm; however, the distribution of diameters of the nanowires, nanotubes or nano-rods is not uniform.

Major contribution to this field was made by a group of scientists from China throughan in situ doping and polymerization method in the presence of selective acids [534]. Poly-pyrrole microtubules were synthesized by this method using b-naphthalene sulfonic acid


(NSA) and ammonium persulphate (APS) as oxidant. The internal diameter of tubes wasfound to be 30–100 nm. ‘‘In situ doping polymerization’’ technique substitutes the use oftemplating and detemplating steps with a single step process through simultaneous dopingand nanostructure formation. Comparing with the traditional processes it can be realizedthat the nanotubular structures through this process can be achieved while pyrrole poly-merizes in presence of NSA as the dopant, whereas, the same is not true in the absence ofNSA. This confirms that NSA can act as a template because of its plate like shape andlarge molecular size. The internal diameter of the microtubules could be controlled byvarying the reaction temperature, while the microtubules formed during in situ polymer-ization at 0 �C have a diameter of 100 nm, the same synthesized at 25 �C have an internaldiameter of 400 nm. However, the internal diameter for a given temperature was found toincrease with increasing NSA concentration. It must be noted that room temperature con-ductivity of so formed microtubules was found to be slightly less than granular polypyr-role at room temperature and the major charge carriers in NSA doped polypyrrole wereidentified as both polarons and bipolarons. The temperature dependency of the polypyr-role was in agreement with the 3D variable range hopping (VRH) model proposed byMott [574]. It was reported that at low temperature, the first hopping barrier T0 of micro-tubules is higher than that of granular polypyrrole and they obey VRH model at suffi-ciently high temperatures only (equation provided in Section 6.3.2). The effects ofchanging various parameters on the morphology and control of different conducting poly-mers have been discussed in a series of publications that followed. Synthesis of NSA/poly-aniline (PA) tubes with diameters ranging from 650 nm to 76 nm [575] have been reported.Analysis of the influence of NSA/polyaniline ratio shows that diameter of tubes decreaseswith decrease in NSA/PA ration, Table 6.2.

But the conductivity also decreases due to reduced doping levels. It has been theorizedthat the formation mechanism of NSA/PA tubules was different in high and low ratio ofdoping resulting in different diameter of the tubes. Another set of experiments reported theeffect of polymerization time and rate as well as solution’s ionic strength on the morphol-ogy, conductivity and molecular structure of NSA/polypyrrole tubes [576]. It wasobserved that the formation of NSA/polypyrrole microtubules via a template-free methodwas a slow and self-assembled process. Typical SEM images as a function of polymeriza-tion time are shown in Fig. 6.10. When the rate of reaction was increased, the morphologychanged from tube to grains. Such observations were ascribed to the anionic surfactantbehavior of b-NSA which tends to form a micelle in high concentration. This micelle actsas template in formation of NSA/polypyrrole tubule. Effect of salt concentration on themorphology showed that adding inorganic salt changes the b-NSA micelles to ‘‘templatelike’’. This results in morphology changes and did not have any effect on the polymerbackbone and crystallanity of the resulting NSA/polypyrrole. It must be noted that thetubules synthesized using the above template-free methods are in the nano-micro range

Table 6.2Effect of molar ratio on the diameter of PA tubules [575]

NSA/PA ratio Diameter

2 650 nm1/2 or 1/4 <100 nm

Fig. 6.10. SEM images of PPy-NSA synthesized at different polymerization times: (a) 0.5 h, (b) 1.5 h, (c) 3.5 h,and (d) 6.5 h (other reaction conditions: pyrrole 0.73 mol/L b-NSA 1.0 mol/L, and APS 0.52 mol/L) (reproducedwith permission from [576]).


as the outer diameters may not be within the boundary of present definition of nanoma-terials but the inner diameters fluctuate between the nano-micro regimes.

The formation of micro-nano-tubules of polyaniline and polypyrrole in different acidssuch ad D-10-camphorsulfonic acid [577], azobenzene sulfonic acid [578] and p-toluenesulf-onate acid have been studied [579]. SA/PA–TiO2 and NSA/PA–Fe3O4 nanotubes can berealized through the same process in the presence of their corresponding nanoparticles[579]. Junction dendrites of polypyrrole and polyaniline have been synthesized by stirringduring the synthesis of nanotubules Fig. 6.11 [580]. Recently chiral polyaniline nanotubeswere also synthesized via self-assembly process by using D- and L-10-camphorsulfonic acid[581]. Optical activity of chiral nanotubules arises from electrostatic binding of the chiralCSA-1 sulfonated ions to –NH –+Æ centers and the hydrogen bonding of the chiral CSA-carbonyl groups to –NH– sites along the polymer backbone.

Jang’s group pioneered the synthesis of polymeric nanotubes using reverse microemul-sion polymerization [582]. The polypyrrole nanotubes were synthesized using reversemicroemulsion in an apolar solvent and AOT as surfactant. FeCl3 added during the syn-thesis can aid in decreasing the critical micelle concentration (CMC) value and increasingthe solvent’s ionic strength. The synthetic procedure follows the scheme as depicted in

Fig. 6.11. Self-assembled sub-micrometer sized tube junctions and their aggregated dendrites (SEM image ofPANi-(D-CSA) (reproduced with permission from [580]).


Fig. 6.12. The diameter of the nanotubes was 95 nm and the length was more than 5 lmwhich was significantly higher aspect ratio than the in situ doping self-assembly method.The effect of molar ratio on the diameter of nanotubes observed was correlated with theconductivity which was reported to be higher than the granular nanotubes in this case(Table 6.3).

The electrical conductivity in this case was measured using a four probe method. Theauthors [583] also reported the synthesis of poly(3,4-ethylene-dioxythiophene) (PEDOT)using reverse cylindrical micelles for their use in chemical sensors. Diameters of the nano-rods were in range of 20–60 nm while the lengths were varying between 180 and 230 nm.

Fig. 6.12. Schematic diagram of PPy nanotube fabrication using reverse microemulsion polymerization(reproduced with permission from [582]).

Table 6.3Diameter and conductivity of polypyrrole nanotubes with respect to the AOT/FeCl3 ratio [582]

AOT (g) FeCl3 solution (ml) Diameter (nm) Conductivity (S cm�1)

9 1 95 30.49 0.5 90 26.57 1 135 17.8


The conductivity was reported as 72 S cm�1. Nanorods sensors thus synthesized also gavemeasurable response to NH3 and HCl vapor.

Surfactants have also been used to produce polyaniline and polypyrrole ODNS on flatsurfaces [584]. Engineering ordered structures on flat surfaces finds widespread applicationin electronics, photonics and microelectronics industry which as discussed earlier was oneof the driving forces for looking synthetic options beyond template-based synthesis. Anillustration of the process is shown in Fig. 6.13 which shows a three step approach to formordered films from nanorods or nanowires on flat surfaces. It has been observed that incase of polymeric nanowires the fabrication of aligned arrays over flat surfaces and overa large area was quick and the alignment is parallel to the surface. The film morphologycould be controlled by the addition of co-adsorbing molecules which induce the phasetransitions from spherical to cylindrical to planar. The film morphology was also shownto be sensitive to the length of the surfactant hydrophobe. In addition, the interactionbetween the surface and the surfactant was also identified as a parameter to control themorphology.

A host of other synthetic methods which are either a modification or completely newprocesses were explored. For example, synthesis of dendritic polyaniline in a surfactantgel and [585] synthesis of large array of nanowires by a three step electrochemical deposi-tion process [586] where a large current density was used to create nucleation sites on thesubstrate. Bulk synthesis of polypyrrole nanofibers was also reported [587] by a seedingapproach in which only a catalytic amount of seed template was required. Zhu et al.[588] described the synthesis of polydiacetylene nanowires by a self-polymerization and

Fig. 6.13. Illustration of the process to fabricate morphologically controlled nanostructures of electricallyconducting polymers on surfaces using surfactant templates. This particular schematic represents the proposedschematic of wire formation on (A) chemically treated HOPG and (B) HOPG (reproduced with permission from[584]).


self-assembly process. Self-polymerization of monomer by UV light and self-assembly bynon-covalent forces such as hydrogen bonds is the desired condition. The diameters of theobtained nanowires varied from 110 nm to 150 nm. A liquid crystal templating methodhas also been used for the synthesis of nanowires by a well known hexagonal (H1) lyotro-pic liquid crystal (LC). This crystal consists of cylindrical hydrophobic cores parallel toone another and separated by hydrophilic continuum. This synthesis was unique in thesense that the polymerization occurs in the hydrophobic region and that the use of lesspolar solvent could potentially lead to more useful application in electronics. A schematicdiagram depicting directed aggregation of oriented conducting polymer chains is shown inFig. 6.14 [589]. Purely chemical synthetic methods have also been reported for synthesis ofpolyaniline nanofibers. Synthesis involved oxidative polymerization of polyaniline instrongly acidic environment such as ammonium persoxydisulfate (APS) [590] using animmiscible organic/aqueous biphasic system. The fibers obtained were 80–100 nm in diam-eter and about 0.5 lm long. Major advantages of this process were (a) yield of productnear 90%, (b) easy and scalable synthesis approach, and (c) small and almost uniformdiameter considering this is a template-free approach. Another interesting research inthe synthesis of polymeric ODNS is based on self-organization of hydrogen bonds leadingto formation of nanoscale conducting cylinders [591]. The comb shape properties ofcopolymers was utilized in designing a general framework for formation of complex supra-molecules starting from the nanoregimes, Fig. 6.15 [592].

Though the template-free methods offer another versatile and applicable approach tosynthesis of polymeric ODNS, they are limited by the fact that they are applicable tofew selective chemicals and polymeric systems. The field is fast evolving and the entiregamut may soon be realized.

6.3. Properties of conducting polymers

Out of several applications of one dimensional polymeric nanostructures the researchon conducting polymers has been phenomenal. In the present section, we will discuss some

Fig. 6.14. Schematic diagram depicting directed aggregation of oriented conducting polymer chains [589].

Components Supramoleculedue to recognition

Self-organization(hierarchy)

AmphiphilesCleaved

(Optional)

Functionalizablenanoporous materials

Individualnano-objects



AmphiphilesCleaved

(Optional)




AmphiphilesCleaved

(Optional)



G

A

B

C

E F

H

I

D



AmphiphilesCleaved

(Optional)





AmphiphilesCleaved

(Optional)




AmphiphilesCleaved

(Optional)



G

A

B

C

E F

H

I

D

Fig. 6.15. Comb-shaped supramolecules and their hierarchical self-organization, showing primary and secondarystructures. Similar schemes can, in principle, be used both for flexible and rod like polymers. In the first case,simple hydrogen bonds can be sufficient, but in the latter case a synergistic combination of bondings(recognition) is generally required to oppose macrophase separation tendency. In (A through C), the self-organized structures allow enhanced processibility due to plastization, and solid films can be obtained after theside chains are cleaved (D). Self-organization of supramolecules obtained by connecting amphiphiles to one ofthe blocks of a diblock copolymer (E) results in hierarchically structured materials. Functionalizable nanoporousmaterials (G) are obtained by cleaving the side chains from a lamellae-within-cylinders structure (F). Disk-likeobjects (H) may be prepared from the same structure by crosslinking slices within the cylinders, whereasnanorods (I) result from cleaving the side chains from a cylinder-within-lamellae structure. Without loss ofgenerality, (A) is shown as a flexible polymer, whereas (B) and (C) are shown as rod like chains (reproduced withpermission from [592]).



of the properties of conducting polymers by comparing them with their coarse counter-parts. Major emphasis has been given to the understanding of the basic conduction mech-anism of the conducting polymers and their applicability to the one dimensionalnanostructures.

As discussed previously, there is a rapid advancement in the different synthesis tech-niques for the development of polymeric ODNS and most of them have been acceleratedby the fast growing needs of the semiconductor/electronic industry which has almostreached its limit with the present conventional semiconductors. The limitation has stimu-lated the researchers to look beyond the metal oxide/metal semiconductors whose proper-ties can be controlled at the scale of the miniaturization required in the electronic devicesi.e., the nanolevel. One such alternative could be provided by the conducting polymerswhich have been used in the electronic industry in applications like electromagnetic inter-ference (EMI) shielding [593], organic field effect transistors (OFET) [594], etc. though notextensively. A common feature of these conducting polymers is that their conductivity canbe increased manifold upon appropriate doping. Polymeric nanowires and nanotubes, etc.have recently flooded the research arena of conducting polymers. The transport propertiesof these low dimensional systems could be of great interest owing to their potential infuture nanoelectronics circuitry. Out of several conducting polymers available, polyacety-lene (PA), polyaniline (PANi), polydioxythiophene (PEDOT), polypyrrole (PPy) andpolyphenylnevinlylene (PPV) in their doped or undoped form have attracted most ofthe attention. The synthetic methods for obtaining these polymers have already beendiscussed.

6.3.1. Electronic properties and origin of conductivity in conducting polymers

The discovery of an inorganic polymer polysulfur nitride (SN)x in 1973 lead the scien-tific community to a new world of conducting materials. The room temperature conduc-tivity of (SN)x is of the order of 103 (X cm)�1 which is reasonably comparable with that ofthe copper; approximately 6 · 105 (X cm)�1 [595]. Below a temperature limit of 0.3 K itbehaves as a superconductor [596]. These findings stimulated the enormous amount ofwork that has been put into the field of conducting polymers. Most of the polymers inas synthesized state are either insulators or semiconductors. Table 6.4 lists the band gapsfor some of these polymers.

It is evident from the band gap energy that these polymeric materials have very lowconductivity and fall in the insulator/semiconductor range. In order to make them appli-cable as conducting materials, their basic poor intrinsic conductivity must be increased.This can be achieved by doping these polymeric materials with various oxidizing or reduc-ing agents.

Table 6.4Band gap of various conducting polymers

Polymer Band gap (eV)

Polyacetylene 1.5–1.7 [539]Polypyrrole 3.2 [597]Polythiophene 2.0 [597]Polyaniline 3.6 [598]


6.3.2. Doping of polymers

Though the term doping is used synonymously with the semiconductor counterparts,there is a difference in the charge conducting mechanism upon doping. Hence the termdoping can often be misleading in the case of organic polymers. Doping in polymers isa redox process involving charge transfer with subsequent creation of charged species[599]. The insulating polymer is converted into a charged ionic complex consisting of apolymeric cation/anion and its counterpart. As compared to the conduction mechanismin the inorganic semiconductors (where an electron can be removed from the valence bandand placed in the conduction band with the whole structure still remaining intact) conduc-tion in polymers involves electronic excitations that are accompanied by a disorder orrelaxation of the polymeric lattice resulting in defect states (polaron states or solitons)along the polymer chain. The conduction in polymer is believed to operate through thesedefect states which are in between the valence and the conduction band, and originate withthe equilibrium geometry in the ionized state. This equilibrium geometry in the ionizedstate is different from that in the ground or unionized state (Fig. 6.16) [600]. Thus a pbonded polymer becomes an obvious choice for the conducting polymers because of therelative ease with which the electron cloud can be delocalized to obtain resonance states.These p conjugated molecules have small ionization potential and thus an electron can beeasily added or removed from the polymeric ion without much disruption of the r bondswhich form the polymer backbone. For a review of the electronic properties of individualpolymers readers are advised to refer the listed articles and the references therein[601,539,597,600]. Though this is the general concept of conduction in 3D conductingpolymers and has been studied thoroughly for past several decades, it still remains a topicof significant controversies because of the strong influence of disorder on the transportproperties. Depending upon the degree of disorder, one can observe three different trans-port methods viz. metallic, critical and insulating. These regimes are identified by the equa-tions governing their behavior. Variable range hopping (VRH) equation [574] for theinsulating regime, Eq. (6.4),

r / exp½�ðT 0=T Þy � ð6:4Þ

Fig. 6.16. PPP and evolution of energy levels with p-doping (top) and n-doping (bottom) (reproduced withpermission from [600]).


where T0 is the VRH parameter, r0 is a prefactor with an algebraic temperature depen-dence, and y = 1/(d + 1), and d is the dimensionality of the system (one, two or threedimensional).

A power law equation for the critical regime is given in Eq. (6.5)

qðT Þ ¼ ðe2pF=�h2ÞðkbT =EFÞ�1=g ¼ T�c ð6:5Þwhere e is the electron charge, �h is the Planck’s constant, EF is the Fermi energy, c is powerexponent, g = 1/c, pF is the Fermi momentum, KbT is the Boltzmann temperature, and1 < g < 3 [602].

In the heterogeneous systems, if the metallic islands are large, the conductivity isdescribed by a fluctuation induced tunneling (FIT)

rðT Þ ¼ r1 exp½�T t=ðT þ T sÞ� ð6:6Þwhere Tt represents the temperature at which the thermal fluctuations become large tocross the tunneling junction by raising the energy of the electronic states, and Ts is the tem-perature over which the thermally activated condition begins to occur [603]. In the absenceof fluctuations the ratio Tt/Ts determines the tunneling.

6.3.3. Conduction properties from 3D to 1D

The charge transport in 3D polymeric films have been studied extensively, however,similar magnitude of work has not been done for polymeric ODNS and much need tobe explored. Experimentally verified data is available on the electronic properties of afew polymers; however, a general trend that can lead to a dominant mechanism is stillawaited.

Electronic and spectrochemical properties of LiClO4 doped PPy nanotubes have beenstudied using ultraviolet–visible spectroscopy (UV–Vis), Raman spectroscopy and X-rayphotoelectron spectroscopy (XPS) [604]. The concentration of polarons was found toincrease as the oxidation potential was increased. At higher potentials, however, the pola-rons concentration decreases and the bipolarons concentration increases indicating therecombination of polarons into bipolarons. This trend intuitively proposes the bipolaronsas the conducting medium at high oxidation potentials. This was further reinforced by thereduction experiments which showed that the concentration of bipolarons progressivelydecreased and that of polarons increased to a maximum as the reduction proceeded. Itwas observed that as compared to films, with zero bipolarons concentration, there wasa significant bipolaron contribution for the nanotubes. The p–p* transition occurs at a sig-nificantly lower energy for PPy nanotubes than for PPy films. It has also been found thatthe nanotubes of PPy can also be switched from insulating to conducting state like the PPyfilms. These results indicated that the high conductivity of polypyrrole nanotubes could beattributed to the higher bipolaron content, higher relative conjugation length and lowerenergy of p–p* transition. The current voltage characteristics of PPy nanotubes depositedon Au were studied by Park et al. using an AFM tip [605,606]. Results indicated the roomtemperature conductivity of PPy nanotubes as 1 S cm�1 at 300 K which is consistent withthe values obtained for their coarse film counterparts.

The temperature dependence on conductance for PEDOT thin films and nanofibers wasalso reported [607]. It was reported from the temperature dependence that PEDOT wasnot in metallic state. Relatively large room temperature conductivity for the film(37 S cm�1) was observed. PEDOT was reported to be in insulating state in the metal–


insulator transition (MIT). The temperature dependence of the fibers is larger than forfilms and this variation increases with decrease in diameter of the tubes.

Conductivity of a single polyaniline doped with camphorsulfonic acid has also beenreported [608]. The conductivity of the single nanotubes was found to be 31.4 S cm�1. Thisvalue of PANi synthesized by self-assembly method (175 nm outer diameter) is rather highas compared to the nanotubes obtained by template methods. It was found that the tem-perature dependence of the PANi follows the VRH model (Eq. (6.4)). In a similar exper-iment with measurement of polyaniline/polyethylene oxide spun nanofibers doped withCSA were studied [536]. However in this case the single fiber I–V measurement showedthe conductivity of fibers of 20 nm and 70 nm diameter as 10�3 and 10�2 S cm�1 respec-tively, which was sharply less than the bulk material (1 S cm�1). The reduction in conduc-tivity with diameter was ascribed to the formation of opaque Schottky barriers at thenanofiber electrode contact which are similar to those present in the SWNTs. Scanningconductance measurements in this case indicated the crossover from conducting to insu-lating fibers as the diameter is reduced below 15 nm.

The gating effect in the I–V characteristics of iodine doped PA nanofibres shows thatthe room temperature resistivity for a 20 nm diameter fiber is 100 X cm which is higherthan that of a PA fiber network [609–611]. Gate voltage dependence showed that the cur-rent is enhanced at negative gate voltage and suppressed at positive gate voltage. This canbe taken as a direct proof of charge carrier being the hole in the iodine doped PA nanof-ibers. A nonlinear trend in the I–V curve as shown in Fig. 6.17 was obtained for PA whichcould be originated from the soliton tunneling conduction in the PA nanofiber. Akagiet al. [612] treated the nanofibers by stretching and reported a relatively high conductivity(1 S cm�1) in the helical PA nanofibers.

The geometry, conductance and conductivity was summarized by Aleshin et al. [539] fora group of nanofiber/nanotubes and is shown in Table 6.5.

As can be inferred from the above discussion that there is considerable amount ofresearch data available for polymeric ODNS but no general or specific trend has evolved.

Fig. 6.17. I–V characteristics of iodine doped PA nanofiber. A gap structure of approximately 0.7 eV is observed(reproduced with permission from [607]).

Table 6.5Parameters for polymer fibers and tubes

S. no. Sample Diameter orcross-section [nm]

G300 K [S] r300 K

[S cm�1]A B [a] References

1 R hel PA fiber 65 · 290 8.4 · 10�7 1.13 2.2 – [613,614]2 R hel PA fiber 60 · 134 1.1 · 10�7 0.85 5.5 4.8 (50 K) [613,614]3 R hel PA fiber 47 · 312 2.1 · 10�9 0.0036 7.2 5.7 (95 K) [613,614]4 Single PA fiber 20 7.3 · 10�9 0.01 5.6 2.0 (95 K) [609,611]5 PPy tube 15 1.7 · 10�8 0.83 5.0 2.1 (56 K) [615]6 PPy tube 50 2.8 · 10�8 0.83 4.1 2.8 (50 K) [615]7 R hel PA 4

fibers– 2 · 10�8 3.7 2.3 (90 K) [613,614]

8 PANi tube 180 (140) 31.4 [608]9 PANi/PEO

fiber20 0.001 [536]


Most of the data has trend comparable to that of the polymeric films but their temperaturedependence seems to be different. The nature and the dependence of conductivity areshown to follow 3D models such as VRH and FIT. Further investigations of individualpolymer fibers is needed to identify the complete intrinsic properties of organic conductingpolymers and these nanoscale measurements could then uncover the novel behavior ofpolymeric ODNS compared to the one observed in individual CNTs.

6.4. Applications

The present day research, as indicated in the discussion about conductive polymers, isdriven by applications. Either it is the drive to replace the existing materials by new andbetter property materials or it can be inspired by the limitation of a multi-step systemto be made simpler to one single step. Integration of the materials directly on the deviceor synthesis of the material onto the device surface is one of the major goals of the presentday electronics and photonics industry. Stringent safety considerations in the industry andhas fueled the research in sensor industry. Similar to these are several research frontsworking parallel to each other to either find new and better property materials or to pushthe existing materials to their limits when we know that even space is not the limit.

6.4.1. Polymeric ODNS in sensor applications

Chemically synthesized polyanilinile nanofibers were shown to incite resistance changeswith respect to doping and dedoping in HCl [590]. The diameters of the nanofibers were inthe range of 30–50 nm and the length was reported to be 500 nm. Fig. 6.18(A) shows theresistance changes of an emeraldine nanofiber based thin film and the conventional filmupon exposure to 100 ppm HCl vapor in nitrogen and Fig. 6.18(B) corresponds to the fullydoped films exposed to 100 ppm of NH3 vapor in nitrogen. The nanofiber thin filmresponds quickly than the conventional film to both the acid doping and dedoping eventhough the nanofiber film was made twice as thick to conventional film.

In a similar, effort PEDOT nanorods synthesized using a reverse microemulsion tech-nique were used as a sensing material to HCl and ammonia [583]. The conductivity ofthe as prepared PEDOT nanorods with diameters in the range of 20–60 nm and lengths

Fig. 6.18. Resistance changes of a nanofiber emeraldine base thin film (solid line) and conventional (dotted line)film upon exposure to 100 ppm HCl vapor in nitrogen (A) and the same fully HCl doped films exposed to100 ppm NH3 vapor in nitrogen (B). R/R0 is the resistance R normalized to the initial resistance (R0) prior to gasexposure (reproduced with permission from [590]).


varying from 170 to 230 nm was found to be 72 S cm�1. Fig. 6.19 shows the response toNH3 and HCl vapor with continuous on and off cycles. Compared to the conventionalPEDOT films, which saturate at concentration above 20 ppm of ammonia, the PEDOTnanorods were able to detect and sense ammonia for a range of concentrations from 20to 100 ppm. Similar approaches can be used in the challenging and animated fabricationof sensors from polymeric nanotubes.

6.4.2. Polymeric nanofibers and nanotubes in medicineThere is a considerable commercial market for biotechnical and biomedical applications

of nanofibers. Nanofibers, nanofiber matrix composites and mats have been researchedextensively for their use in drug delivery and tissue engineering applications. Most of thesenanofibers are synthesized by electrospinning of polymers from their solutions.

Zong and co-workers [547] have fabricated bioabsorbable amorphous poly(D, L lacticacid) nanofiber non-woven membranes. Electrospun fibers have been demonstrated asa model for drug delivery [616]. The sustained release of proteins from electrospun

Fig. 6.19. (a) Reversible and reproducible response to periodic exposure of ammonia PEDOT nanofibers basedsensor and (b) response to on and off cycle of HCl (reproduced with permission from [583]).


biodegradable fibers is possible [617]. Also, poly(vinyl alcohol) nanofibers as a proteindelivery system is made feasible [618]. The electrospinning of collagen nanofibers for tissueengineering scaffold applications are found to be attractive [544]. The structural propertiesof electrospun collagen vary with the tissue of origin, the isotope and the concentration ofcollagen solution. Scientists [619–632] have reported extensive work on the preparation ofscaffold for tissue engineering using nanofibers and nanofiber composites. Peptide nanof-ibers have also been investigated for their potential use as magnetic resonance (MR) con-trast agents [633]. The discussion above suggests the enormous use of nanofibers in thefield of biomedical engineering and biotechnology. The field being too vast to be coveredin a very short discussion, readers are directed to above mentioned references for adetailed structural, mechanical, morphological and biological characterization of polymerODNS in bio-applications.

6.4.3. Electronic properties of conducting polymers

Integration of conducting polymers in electronic devices is a challenging task. Line pat-terning techniques have been used as a tool for development of low cost polymeric elec-tronic devices [528]. In line patterning the substrate and the insulating lines printed by aconventional printing press are exposed to a fluid (or vapor). Due to different chemicalreactivity of the substrate and the insulating line they react differently and result in anon-uniform deposition on the substrate as opposed to the insulating lines. Different mate-rials can be used to deposit on the substrate to obtain a pattern that can then be reprinted.Upon removal of the printed lines a clean pattern of the deposited material can beobtained. The use of PEDOT in developing a free standing polymer dispersed liquid crys-tal (PDLC) display device has been exploited. The response of PEDOT when exposed to apositive gate potential in a field effect transistor (FET) configuration was also reported[528]. PEDOT nanowires of approximately 200 nm have been used in designing nanotipsfor application in field emission displays [634]. Similar observations have also beenreported for transport in doped PPy [605,606] and PANi/PEO [536,635], PANi-CSA/PEO nanofibers [635]. It can thus be said that the properties of conducting polymerscan be combined with the present state of the art techniques for development of futurenano-electronic devices with enhanced properties.

7. Carbon nanotubes

7.1. Overview

Although, the properties and applications of a spectrum of materials are discussed inthe previous pages; carbon nanostructures are perhaps the most important discovery inthe 21st century. They have already spun off the augmentation of the nanotechnologyresearch in a new dimension. A brief discussion on the structure and synthesis processof the one dimensional carbon nanostructures is given in this section followed by its sev-eral important properties and various technological applications.

Carbon, (atomic weight = 12.0) is one of the most abundant elements in the universe. Ithas four valence electrons. Carbon can form various structures with entirely differentproperties using these valence electrons. The versatility of carbon stems from its abilityof rehybridization among sp, sp2, and sp3. It is important to note that existence of severalintermediate degrees of hybridization between sp2 and sp3 is possible as out-of-plane flex-


ibility is present in the graphene sheet [636]. Until the last decade, two bulk solid phases ofcarbon were known: graphite and diamond. Graphite, a soft material, is based on sp2

hybridization of carbon electrons, whereas carbon in isotropically strong diamond hassp3 hybridization [637]. The structure of fullerenes and related derivatives was discoveredin late 1990s. These structures are recognized as a different phase from the graphite, eventhough such structures maintain the architecture of sp2. Fig. 7.1 represents the architectureof three different bulk solid phases of carbon [638]. After discovery of fullerene, scientistsfound a number of different carbon nanostructures with similar architecture. Carbononions, carbon nanohorns (CNHs), carbon nanorods and CNTs are few of them. Carbonnanotubes (CNTs) are seamless cylinders derived from a honey comb lattice representing asingle atomic layer or multiple atomic layers of crystalline graphite, called graphene sheets.The nanometer-scale size and hollow cylindrical shape of carbon nanotubes suggests thatthey may have many potential applications as molecular sieves, nano-test-tubes, andhydraulic actuators. The bonding is also sp2, although it is mixed with an extent of sp3

character because of its high curvature. This cap-end structure is formed with the combi-nations of 12 pentagonal (five-) and 20 hexagonal (six membered) carbon rings.

In comparison with graphite, the CNTs show significantly different properties. It hasgreater interplanar distance, smaller work function, steeper Fermi edge, negative core-levelshift and stronger plasma excitation. Their valence band is basically same as that of graph-ite with lower intensity in the binding energy region of 2–7 eV. The CNTs exhibit a strongoptical limiting effect, superior to both carbon black and fullerene [639].

7.2. History of carbon nanotubes

In 1953, Davis et al. [640] found a deposition of an unusual form of carbon from carbonmonoxide at an optimized temperature of 450 �C on an iron oxide substrate. However,their transmission electron microscopic work could not reveal the architectures of suchan unusual form of carbon. In 1960, Bacon [641] found the growth of graphite whiskersin direct current (DC) arc under a pressure of 92 atm of argon at 3900 K. Formation ofthese graphite whiskers was believed to be through a scroll mechanism. After few years, car-bon filaments with hollow tubes of diameter were reported to be in the range of 2–50 nm.Tibbetts [642] reported tubular carbon filaments or carbon whiskers which were formedfrom the decomposition of hydrocarbon at 900 �C with submicron catalytic particles. In1985, Kroto et al. [643] discovered the fullerenes. In general, the structure of the fullerene(C60) is similar to a common soccer ball. In the mean time, Speck et al. [644] tried to cor-relate the growth mechanism of carbon whisker with experimental parameters, but nodetailed systematic studies of such nano-sized materials were carried till date. Direct stim-ulus to study such filaments in nanometer dimension came more systematically from thediscovery of fullerenes [645]. In 1991, a breakthrough in the research on one dimensionalcarbon nanostructures came when Iijima [36] reported the arc-discharge synthesis andhigh-resolution electron microscopic characterization of such ‘helical microtubules’. Thesemicrotubules, later known as carbon nanotubes (CNTs), are molecular-scale fibers withstructures related to fullerenes. First HRTEM images of CNTs [36], are shown in Fig. 7.2.

The first application of CNTs can be attributed to the Massachusetts-based companyHyperion Catalysis International Inc., which owns the first nanotube patent. The inven-tion was credited to Tennent, who discovered a way to produced CNTs using catalyticmethod [646].

Fig. 7.1. The architecture of three different forms of carbon: (a) graphite, (b) diamond and (c) carbon nanotubes(figures drawn using Material Studio� software) [638].


7.3. Synthesis processes for one dimensional carbon nanostructures

Carbon nanostructures can be synthesized by several methods such as: arc-discharge invacuum, arc-discharge in solution (ADS), laser ablation, chemical vapor deposition(CVD) and electrochemical deposition. The manipulation of the process variables, such

Fig. 7.2. Iijima’s first TEM images of MWCNTs: (a) diameter 6.7 nm, (b) diamter 5.5 nm and (c) diameter 6.5 nm(reproduced with permission from [36]).


as electrode materials, catalyst and other experimental parameters including pressure andtemperature, leads to a particular shape and the size of the carbon nanostructure.Although CNTs can be synthesized using a variety of techniques, following three methodsare used for commercial and bulk production of carbon nanotubes: arc-discharge, chem-ical vapor deposition (CVD) and laser ablation. There are several reviews on differentmethods of CNT synthesis reported in the literature [647–649]. Bulk-synthesis processesalong with other methods are discussed briefly in the following section.

7.3.1. Arc-discharge in gases

The arc-discharge technique for synthesizing MWCNTs appears to be a simple and tra-ditional tool, but obtaining high yield is difficult and requires careful control of experimen-tal conditions [36,391,650,651]. During discovery of CNTs [36], an argon-filled vessel of100 Torr was used to synthesize CNTs with diameter in the range of 4–30 nm and lengthof �1 lm. In the most common laboratory scale production scheme, the DC arc operatesin a 1–4 mm wide gap between two graphite electrodes of diameter 6–12 mm that are


vertically or horizontally installed in a water cooled chamber filled with helium gas at sub-atmospheric pressure. Helium gas and DC current are more important to maximize theyield. Typical conditions for operating a carbon arc for the synthesis of CNTs includethe use of graphite rod with a voltage 20–25 V across the electrodes and a DC electric cur-rent of 50–120 A flowing between the electrodes. The arc is typically operated in�500 Torr helium with a flow rate 5–15 ml s�1 for cooling purposes. As the CNTs form,the length of the positive electrode (anode) decreases. A schematic diagram of a arc-dis-charge apparatus is presented in Fig. 7.3, used to synthesize MWCNTs and SWCNTsin nitrogen atmosphere [652].

SWCNTs are produced in the arc-discharge process utilizing covaporization of graphiteand metal in a composite anode (positive electrode), commonly made by drilling an axialhole in the graphite rod and densely packing it with a mixture of metal and graphite pow-der. Although, almost all transition metal particles, some rare-earth metals, alkaline earthmetal and some p-block metal particles and also their mixture can be used for synthesizingSWCNTs, presently only Ni/Y and Co/Ni catalysts are commonly used for SWCNT pro-duction [653].

7.3.2. Arc-discharge in liquid phase

Although different methods have been employed for the synthesis of multi-walled car-bon nanotubes (MWCNTs), arc-discharge in water and in solution (ADS) methods haveproven to be highly efficient. During the last couple of years, arc-discharge in the liquidphase has attracted considerable attention due to its simplicity [654–660]. In several pub-lications [661–665], Bera et al. discussed the arc-discharge in solution (ADS) procedureelaborately. We successfully demonstrated that carbon nanostructures can be synthesized

Fig. 7.3. Schematic diagram of the arc-discharge apparatus: (A) small snowflakes-like soot obtained around themetal rod behind the cathode, (B) puffy powders obtained on the inner wall of the large copper pipe, (C) rod-shaped deposit formed on the surface of the cathode, (D) puffy powders obtained on the inner wall of the steelcontainer and (E) small snowflakes-like soot obtained on the anode [652].


with a simple technique avoiding costly and time consuming equipment and without main-taining a vacuum. The method is not only efficient in production of clean CNTs, but alsosuccessful in the synthesis of in situ decorated CNTs with metallic and ceramic nanopar-ticles [666]. A schematic diagram of such a nanotube production setup located at the Sur-face Engineering and Nanotechnology Facility (SNF), University of Central Florida(UCF) is illustrated in Fig. 7.4 [663].

Continuous synthesis of MWCNTs in liquid nitrogen using a dc power supply of20–25 V operating at 60 A was first reported in the year 2000 [654]. Afterwards, severalattempts were taken including the synthesis of metal-filled CNTs using arc-discharge ina cobalt sulfate solution, which resulted in the formation of CNTs-filled with not onlymetallic cobalt but also cobalt sulfide particles [667]. The encapsulated particles weremostly rod-shaped. Moreover, the mechanism of the encapsulation of such particles wasnot clear from the aforementioned attempts. During the synthesis of carbon-onions inwater, Sano and coworkers found rod-shaped carbon nanostructures, carbon nanotubesand carbon onions. The diameter of CNTs also depends on the hydrostatic pressure dur-ing arc-discharge in water [658]. In another attempt, SWCNTs along with carbon nano-horns were found in liquid nitrogen with Ni as a catalyst [668]. Such a process confirmsthe need for a relatively inert environment to synthesize SWCNTs and carbon nanohornsselectively. Production of well-crystallized nano-onions, nanotubes, fully and partiallyfilled with crystalline metallic particles could be achieved using DC as well as AC[660,657]. In addition, lower current values can contribute towards higher fraction ofCNTs in the product. Antisari et al. [659] analyzed the MWCNTs synthesized by arc-dis-charge in liquid media, liquid nitrogen and water. Among different media, water providesa suitable environment to synthesize high-quality CNTs. In a recent work, by Montoro

Fig. 7.4. A schematic diagram of the whole set-up for synthesis of carbon nanotubes (CNTs) and nanoparticlesdecorated CNTs using arc-discharge in solution (ADS) method (reproduced with permission from [663]).


et al. [669], describing the synthesis of SWCNTs and MWCNTs by the arc-discharge inwater, reports a large quantity of graphite particles with estimated diameters ranging from10 nm to 600 nm in the sample. Moreover, it was found that the concentration of elon-gated straight structures was low. Recently, we have reported a parametric study consid-ering four different current values. The rates of yield and their corresponding currentvalues are tabulated (Table 7.1) and the size distribution measured by light scatteringmethod is presented in Fig. 7.5 [664].

7.3.3. Laser ablation

During fullerene production experiments using a laser vaporization apparatus with anablated graphite sample positioned in an oven, it was found that closed-ended MWCNTswere produced in the gas phase through homogeneous carbon-vapor condensation in a hotargon atmosphere. The laser-produced MWCNTs are relatively short, approximately300 nm, although the number of layers, ranging between 4 and 24, and the inner diameter,varying 1.5–3.5 nm, are similar to those of arc-produced MWCNTs. A prevalence ofMWCNTs with an even number of layers was observed, but no SWCNTs were detected.Evaporation of a carbon-metal composite target with laser light pulses, either separate orfrequently repeated, and with continuous illumination by laser and solar light, can bringabout SWCNT formation. A 70–90% conversion of graphite to SWCNTs was reported inthe condensing vapor of the heated flow-tube operating [671] at 1200 �C. A Co–Ni/graph-ite composite laser vaporization target was used, consisting of �1 at% Co–Ni alloy (equalpercent) added to graphite. Two sequenced laser pulses were used to evaporate a targetcontaining carbon mixed with a small amount of transition metals from the target. Flow-ing of argon gas sweeps the entrained nanotubes from the high-temperature zone to thewater cooled Cu-collector downstream, just outside the furnace [672]. A schematic illustra-tion of the laser ablation deposition setup is presented in Fig. 7.6.

7.3.4. Chemical vapor deposition

Chemical vapor deposition (CVD) is a heterogeneous reaction process to synthesizeCNTs from volatile precursors. Compared to arc-discharge and laser ablation methods,the main advantages of this process are [648]:

(1) Scaling up of production to industrial level (approximately a pound per day scalesynthesized by Carbon Nanotechnology Inc., Houston, Texas).

(2) Appreciable control over growth of desired (diameter, length and position) CNTswhich is more important for electronic applications.

Chemical vapor deposition (CVD) can be categorized depending on the energy sources:plasma-enhanced CVD (PECVD), thermal CVD, etc.

Table 7.1The rate of production of carbon nanostructures with corresponding currents

Current (A)

25 35 50 75

Yield rate (mg min�1) Value 0.96 2.03 4.54 5.89Standard deviation 0.06 0.26 0.65 0.28

Fig. 7.5. Dynamic light scattering analysis of suspended carbon nanostructures in water synthesized at variousvalues of current: (a) 75 A, (b) 50 A, (c) 35 A and (d) 25 A. Sizes of nanostructure increase with decrease incurrent values (reproduced with permission from [664]).


7.3.4.1. PECVD. The plasma-enhanced CVD (PECVD) first emerged in microelectronicsto avoid the elevated temperatures of the thermal CVD, which might be detrimentalfor the devices. In CNT growth, precursor dissociation in the gas phase is not necessary;

Fig. 7.6. Schematic illustration of CNT growth setup using laser ablation method [672].


however, dissociation at the catalytic particle surface appears to be the key for nanotubegrowth. The growth temperature must be maintained below the pyrolysis temperature ofthe particular hydrocarbon to prevent excessive production of amorphous carbon.

It has emerged as a key growth technique to produce vertically aligned CNTs [666,673].The ability to grow CNTs with a high degree of uniformity is necessary to various tech-nologically important applications. Such structural uniformity of CNTs, including specificarrays and single-standing nanotubes can be achieved through this technique [674].Fig. 7.7 shows such a highly aligned CNTs deposited by PECVD method using Ni-cata-lyst. The diameter (standard deviation 4.1) and height (6.3%) of CNTs were found to bevery precise in the case of single isolated growth.

Tubular graphitic cones are a specific morphology of CNTs. These one dimensionalcarbon nanostructures were grown [675] using a microwave PECVD system with nitrogenand methane as reaction gas at a ratio of 200:3 and iron needles as substrates.

Despite the hundreds of scientific reports on the synthesis and properties of CNTs, pre-cise control on selective growth of metallic or semiconducting SWCNTs has not yet beenachieved [648]. Preferential growth of semiconducting SWCNTs (>85%) observed in aPECVD process at 600 �C, reported by Li et al. is one of very few works in this direction[676].

7.3.4.2. Thermal CVD. When a conventional heat source, such as a resistive or inductiveheater, furnace or infra-red (IR) lamp, is used, the technique is called thermal CVD orsimply CVD. In 1986, the growth mechanism of carbon whiskers from mainly organicprecursor was proposed [642]. The thermal CVD apparatus for CNT growth is very sim-ple. It consists of a quartz tube of 1–2 diameter, inserted into a tubular furnace capableof maintaining a constant temperature over a 25 cm long zone. In thermal CVD, hydro-carbons or CO are used as precursor. A typical growth run would involve first purgingthe reactor with some inert gas. Then the gas flow is switched to the feedstock for thespecified growth period. At the end, the gas flow is switched back to the inert gas whilethe reactor cools down. For growth on the substrates, the catalyst mixtures need to beapplied to the substrate before loading it inside the reactor. Typical temperatures forcatalytic chemical vapor deposition in CNT growth are in the range of 800–1500 K.A schematic illustration of the thermal CVD deposition setup is presented in Fig. 7.8[677].

The CNT could be synthesized precisely as high purity (>95%) tubes in desired sizeranges from 7 to 35 nm under controlled conditions through catalytic CO-disproportion-ation [678].

Fig. 7.7. (a) Highly aligned CNTs were grown from a 100 nm wide, 7 nm thick Ni catalyst line. (b) CNTs grownfrom the dots of Ni catalyst with 7 nm thickness (reproduced with permission from [674]).

Fig. 7.8. Schematic illustration of the CVD process [677].


7.3.5. Electrochemical methods

An electrochemical method for the synthesis of MWCNTs has been described in the lit-erature [679]. This process involves the electrolysis of molten lithium chloride using a


graphite cell in which the anode is a graphite crucible and a graphite rod immersed in themelt is cathode. A current of 30 A is generally used through the cell for 1 min, after whichthe electrolyte is allowed to cool down and then added to water to dissolve the lithiumchloride and react with lithium metal. The solution is treated with toluene to form CNTs.

Taking another innovative approach, Pal et al. [680] attempted to grow CNTs on a sil-icon (001) substrate by an electrodeposition technique using acetonitrile (1% v/v) andwater as electrolyte at an applied potential of 20 V. The electrolysis of acetonitrile wascarried out at atmospheric pressure and the bath temperature of �300 K. CNTs weredeposited onto a Si wafer (resistivity � 15 kV) of dimension 10 mm · 8 mm · 0.3 mmattached to a copper cathode. The inter-electrode distance was �8 mm and the depositiontime was 4–6 h. The typical thickness of the CNT film was �300 nm, measured byinterferometry.

It is very clear from the aforementioned discussion that there are numerous groupsworking on a wide variety of synthesis methods to realize efficient ways that can yield pre-cise structural, morphological and chemically controlled carbon nanostructures. Now it isimperative to discuss the unique structural features in carbon nanostructures and the cor-responding properties and applications. As mentioned previously, every topic below canbe written as a separate review article. However, the following is an effort to summarizeunique properties and applications of CNTs.

7.4. Structure of CNTs

CNTs are the hollow one dimensional carbon nanostructures. These hollow, cylindricaltubes of graphitic carbon are characterized by a single tube wall or a large amount ofordered tube walls. Their length can vary from a few hundred nanometers to several hun-dred microns. The diameter can be varied from 0.37 nm to 100 nm. Bonding in the nano-tubes is essentially through sp2 hybridization. The sp2 hybrid orbital allows carbon atomsto form pentagon and hexagon units by in-plane r-bonding and out-of-plane p-bonding.However, the circular curvature will cause quantum confinement and r–p rehybridization,in which three r bonds are slightly out of plane [681]. Consequently, the p orbital is moredelocalized outside the tube, that is, the p-character in the hybridization is increased tosome extent than that of the graphite. This makes nanotubes mechanically stronger, elec-trically and thermally more conductive and chemically and biologically more active thangraphite.

CNTs can exist as single tubes (called single-walled nanotubes, SWCNT) or in the formof concentric tubes (termed multi-walled nanotubes, MWCNT). A SWCNT is a hollowcylinder of graphite sheet whereas a MWCNT is a group of coaxial SWCNTs. SWCNTswere discovered [650] in 1993, two years after the discovery of MWCNTs. Brief descrip-tions of these two are presented in the following subsections.

7.4.1. Single-walled carbon nanotubes (SWCNTs)

The discovery of SWCNTs was first reported by Iijima and Ichihashi [650] in 1993.Such ‘single-shell’ CNTs can be visualized as a hollow cylinder, formed by rolling overa graphite sheet. The structure is one dimensional with axial symmetry exhibiting a spiralconformation called chirality [671]. The body of the tubular shell is mainly made of a hex-agonal ring of carbon atoms, whereas the ends are capped by dome-shaped half-fullerenemolecules composed of hexagonal and pentagonal rings of carbon atoms. The role of the


pentagonal ring is to give positive curvature to the surface which helps in closing of thetube at the two ends and also makes the end caps chemically more reactive comparedto cylindrical walls of CNTs [682].

7.4.1.1. Nomenclature. A nomenclature (n,m) used to identify each SWCNT, in the liter-ature, refers to the integer indices of two graphene unit lattice vectors corresponding tothe chiral vector of the nanotubes [40]. The chirality, a single vector called the chiral vector(shown in Fig. 7.9), can be expressed by Eq. (7.1)

Ch ¼ na1 þ ma2 � ðn;mÞ ð7:1Þwhere Ch is called chiral vector that connects crystallographically equivalent sites on a twodimensional sheet, where a1 and a2 are graphene lattice vector and n and m are integers.The chiral vector determines the direction along which the graphene sheets are rolledup to form tubular shell structures perpendicular to the axis vectors [653]. The primarysymmetry classification of a SWCNT is either achiral (symmorphic) or chiral (asymmor-phic). Achiral CNTs, whose mirror image can be superimposed to the original one, are twotypes: armchair and zigzag nanotubes. Chiral nanotubes exhibit a spiral symmetry whosemirror image cannot be superposed on to the original one. Thus, depending on chirality, avariety of geometries in SWCNTs can be obtained. A classification of SWCNT is given inTable 7.2.

Zigzag (n, 0) SWCNTs exhibit two distinct behaviors as a metal when n/3 is an integer,or as semiconductor otherwise. Similarly, a chiral (n,m) SWCNT can be metallic in naturewhen (2n + m)/3 is integer. As mentioned in Table 7.2, when chiral angle is 30� withrespect to (n, 0), n becomes m. Such a SWCNT, called armchair, is metallic with bandcrossing at k = ±2/3 of the 1D Brilluoin zone.

Fig. 7.9. Schematic of 2D graphene sheet showing the lattice vector a1 and a2 and roll-up vector ch = na1 + ma2.The limiting cases of (n, 0) zigzag and (n,n) armchair tubes are indicated with dashes lines. The nanotubes axis isindicated by the vector T (reproduced with permission from [40]).

Table 7.2Classification of SWCNTs [653,671]

Type Chiral angle (h) Chiral vector/nomenclature Shape of cross-section

Armchair 30� (n,n) cis-typeZigzag 0� (n,0) trans-typeChiral 0� < jhj <30� (n,m) Mixture of cis and trans


In a TEM micrograph of SWCNT, the moire patterns formed by the rolled up graphenelayers are clearly visible between two intense dark lines corresponding to vertical tube wall(Fig. 7.10). Hashimoto et al., recently employed a contrast transfer function to the TEMimages that enabled them to visualize the graphene network of the CNTs. Under this con-dition, each zigzag chain would appear as a dark line in TEM images and a bright spotwould be present at the middle of each hexagonal carbon ring. Two symmetric hexagonson the tube axis were identified using Fourier transformed optical diffraction, as shown inFig. 7.10(b) [683]. Each hexagon in Fourier space corresponds to a single graphene layer(either top layer or bottom layer of a SWCNT) and represents the orientation of the zigzagchain to the tube axis (see Fig. 7.10). From such modification in TEM images, the chiralangle could be determined [683–686]. Recently, a general, systematic and semi-quantitativemethod was proposed for determining the chiral indices of both MWCNTs and SWCNTsusing selective area electron diffraction pattern intensities coupled with extensive compar-ison with kinematic theory [684]. It was claimed that such method could determine the chi-ral angle for both MWCNTs and SWCNTs with radii up to 4 nm.

Junctions among SWCNTs can be produced by introducing pentagon–heptagon pairdefects into the hexagonal network [687] which could change the electronic propertiesof CNTs. Such X, Y and T-like junctions were found during the synthesis of CNTs[680]. A TEM micrograph is illustrated in Fig. 7.11, obtained after the pyrolysis of neck-elocene and thiophene at 1000 �C [688]. Coluci et al. [687] recently proposed a new struc-ture of CNTs called super-carbon nanotubes. These structures were built from SWCNTsconnected Y-like junctions forming a super-sheet that is finally rolled into a seamless cyl-inder. A tight binding total energy calculation and density of states modeling showed thatsuch super-CNTs are stable and predicted to exhibit metallic and semiconducting behavior[687]. Fig. 7.12 presents a schematic atomistic view of a super-CNT. The [6,0] super-CNTwas made by connecting (6,0) SWCNTs.

7.4.1.2. Defects. As mentioned in previous section, a subtle change in structure ofSWCNTs can remarkably change electronic and magnetic behaviors of nanotubes[40,689]. The electronic conduction process in CNTs is unique since in the radial direc-tions, the electrons are confined in the singular plane of the graphene sheet. The conduc-tion occurs in the arm chair tubes through gapless modes as the valence and conductionbands always cross each other at the Fermi energy. In other types of SWCNTs, an openingof the energy gap at the Fermi energy leads to semiconducting properties. As the diameterincreases the band gap tends to zero as it varies inversely with tube diameter. Electrontransport along the tube is ballistic at low temperature. However, introduction of a defectalong the tube would cause scattering of the electron, localization of the electronic wavefunctions [690] and modification of band gap [691]. So, it is important to identify the mostdominant defects in the CNTs before incorporating them in technologically importantdevices and applications. Such defects, which could influence the electrical, mechanical,transport and other properties [692–695] are being studied meticulously for last few years.

Ebbesen and Takada [636] classified such defects into three categories: (1) topologicaldefects, (2) rehybridization defects, (3) incomplete bonding and other defects. Topologicaldefects are observed in CNTs when penta- and/or heptagonal rings are introduced intohexagonal network. Generally nanotube-ends are composed of penta-, hexa- and heptag-onal rings [395]. Euler’s theorem can be used to explain the three dimensional topology inthe approximation that all the individual rings in the sheet are flat. In other words, all the

Fig. 7.10. (a) A typical HRTEM image of a SWNT with enhanced contrast of the zigzag chain (inset). (b) Moirepattern formed by the rolled up graphene layer shows the two intense dark lines correspond to wall of CNT. Insetshows optical diffraction of Moire pattern. (c) A best-fit model of the SWNT with the determined chiral index(13,8). (d) A cross-sectional view of a topological defect at the side of a SWNT. (e) A pentagon–heptagon pair ispresumably responsible for the defect structure and generates a serial junction of two zigzag nanotubes (17,0) and(18,0). (f) A simulated image for the SWNT with the defect rotated by 90�. Scale bar, 2 nm (reproduced withpermission from [683]).


atoms of a given cycle form a plane, although there might be angles between the planesformed in each cycle [636]. The relationship between the number and types of the cycles

Fig. 7.11. TEM micrograph of Y junction MWCNTs obtained by the pyrolysis of neckelocene and thiophene at1000 �C (reproduced with permission from [688]).


(ni) necessary to close a hexagonal graphene network, derived from Euler’s theorem, isgiven in Eq. (7.2).

3n3 þ 2n4 þ n5 � n7 � 2n8 � 3n9 ¼ 12 ð7:2Þ

where subscript (i = 3,4,5,7,8,9) stands for the number of sides to the ring, 12 corre-sponds to the total declination of 4p. Hashimoto et al. [683] recently studied the topolog-ical defects, including vacancies and adatoms, in a graphene sheet. Although it was acommon belief that a CNT does not have many defects along the wall but at the tip, arecent report showed [683,696] that pentagon–heptagon pairs, mono- and multi-vacancies,and adatoms are typical in a CNT wall. Such defects could be identified by scanning tun-neling microscopy (STM) [654,693] and TEM [683].

7.4.2. Multi-walled carbon nanotubes (MWCNTs)7.4.2.1. Structure. A MWCNT is a rolled up stack of at least two graphene sheets into con-centric SWCNTs, with the ends again either capped by half-fullerene or kept open. Thewalls of each layer of the MWCNT, the graphite basal plane, are parallel to the centralaxis (h = 0�). In contrast, a stacked-cone arrangement is also seen where the angle betweenthe graphite basal planes and the tube axis is non-zero. The length of the MWCNTs is in

Fig. 7.12. Atomistic views of a super-carbon nanotubes [6,0] @ (6,0).


the range from a few tens of nanometers to several microns, and the outer diameter is fromas low as 2 nm to more than 100 nm. At high resolution in transmission electron micro-scope (TEM), the individual layers making up the concentric tubes can be imaged directly.It is quite frequently observed that the central cavity of a nanotube is traversed by gra-phitic layers effectively capping one or more of the inner tubes and reducing the total num-ber of layers in the tube. Virtually all of the tubes are closed at both ends with caps whichcontain pentagonal and hexagonal carbon rings. The caps can have a variety of shapesalong with hemispherical shape (see Fig. 7.13) [665].

The distance between the two concentric walls is approximately 0.334 nm. So the suc-cessive tubes should differ in the circumference by (2p · 0.334) nm 2.1 nm. It is assumedthat the walls are not created from scrolling of the graphene sheets. It can readily be seenthat this is not possible for zigzag nanotubes, since 2.1 nm is not a precise multiple of0.246 nm, the width of one hexagon [679]. In the case of armchair tubes, multi-walledstructures can be arranged in such a way that the interlayer distances is 0.34 nm [698]. Thisis because 2.1 nm is close to 5 · 0.426 nm, the length of the repeat unit from which arm-chair tubes are constructed. For chiral nanotubes, the situation is complicated, but it is notpossible to have two tubes with exactly the same chiral angle separated by the interlayerdistance [679] of 0.34 nm. A MWCNT may be achiral as well as chiral and can also exhibitseveral chiral angles. In a MWCNT, the electronic structure of the smallest inner tubes issuperimposed by the outer one. Therefore, the band structure obtained from an individualMWCNT resembles that of graphite [637].

Fig. 7.13. TEM micrograph of (a) a highly defective multiwalled CNT, (b) a multiwalled carbon nanohorn, (c) aCNT with an inner compartment, (d) a CNT with a complex inner-capping, (e) a CNT with kinks and (f) a verysmall CNT; (scale: 5 nm) (reproduced with permission from [665]).


The energetics of MWCNTs have been considered by Charlier and Michenaud [699].They found that the energy gained by adding a new cylindrical layer to a central onewas of the same order as the one in graphite bilayering. The optimum interlayer distancebetween an inner nanotube (5,5) and outer (10, 10) tube was found to be 0.339 nm, differ-


ent from the spacing found for turbostratic graphite [679]. The estimated translational androtational energy barriers for the two coaxial tubes were 0.23 meV per atom and 0.52 meVper atom, respectively. These low values suggest that a significant translational and rota-tional mobility could be present in MWCNTs at room temperature [679].

7.4.2.2. Defects. As mentioned in the previous section, a significant translational and rota-tional mobility could be present in MWCNTs at room temperature. In reality, the pres-ence of caps and defects in the cylindrical structures would limit this mobility [679]. Soit is very important to discuss the defects present in the tubes for any industrial applicationof MWCNTs [700].

A variety of tips of MWCNTs were found in the literature. Most common tip amongmany tube-tip shapes observed in TEM study was hemispherical cap (shown in Fig. 7.13).The extent of defects along the wall of MWCNTs is synthesis dependent. For example,extensive TEM studies of as-synthesized samples through ADS process showed thatalmost 50% of the CNTs are not well-graphitized and with high defect density in theatomic arrangement [665]. One such defective CNT having multiple line-defects alongits whole cross-section is shown in Fig. 7.13(a). These non-organized, defect-enrichedCNTs are always associated with very small bulb-like structures on the outer wall of CNTsas in Fig. 7.13(a).

The second most common tip is the tip of a carbon nanohorn, an ice-cream cone typeasymmetric tip, which is made of pentagonal and heptagonal carbon rings [395]. Carbonnanohorns (CNHs) are made of the same graphitic structure as CNTs. The significantcharacteristic of the CNHs is the formation of an aggregate of approximately 100 nmwhen many of the nanohorns group together. Such well-graphitized carbon species havefree energy of formation so close to that of CNTs that they cannot be used as feedstockin CNT growth. Recently, Sano et al. [701] synthesized single walled CNHs by usingarc-discharge in liquid method. The cone angle of the multi-walled carbon nanohorn ismore than 17� [395]. Unevenly spaced lattice fringes with odd number of fringes on eitherside of the central hollow core could be found in the as-synthesized nanostructures. Thenanohorn with some abruptly terminated inner walls and line defects could help to under-stand the growth mechanism of CNTs during the synthesis.

In several studies, various complicated internal structures were found [395]. Especiallyinteresting are the closed compartments formed inside the carbon nanotubes as shown inFig. 7.13(c). This internally closed compartment is indicated in the rectangular box. A fre-quent observation for nanohorns was capping of an inner wall before the end-capping. ACNT with multiple internal-wall closures before end-capping is presented in Figs. 7.13(c)and 2.6.12(d). These CNT specimens would help to unravel the mechanism of the forma-tion of CNTs during the synthesis process. Recent studies [661,662] revealed that the for-mation of some of the nanotubes remains incomplete and the original graphene sheets arepartially rolled up leaving behind some tub-shaped portions. Fig. 7.13(d) shows aHRTEM micrograph of a CNT with nested cylindrical graphitic layers. A few layersare closed inside the nanotube by caps before final closure of the nanotube. The inner wallsof CNTs in Fig. 7.13(c) and (d) have no access to carbon from outside for capping. It islikely that the inner walls closed before closing of the outer layer. Once a cap is formed onthe tip of an inner cylindrical layer, the layer cannot grow further. Iijima et al. [395]showed such capped CNT which were formed by transformation of the cone to a cylindri-cal structure with the incorporation of a defect, induces a negative curvature at the tip of


the CNT. The formation of a cap on the CNTs could be explained by the deposition ofcarbon atoms followed by their rearrangement. Fig. 7.13(d) shows different number ofwalls on the two sides of a nanotube. Fig. 7.13(b) also reveals the presence of incompleteand bent layers at the inner concentric wall, as marked with an arrow. Such defects couldinitiate an inner cap with time. The formation of a cap on the CNT was possibly from thesystematic deposition of atomic carbon produced during the arc-discharge process [665].Fig. 7.13(e) shows a HRTEM micrograph of a CNT with several inward and outwardkinks. This shape was found to be rare for CNTs from TEM studies [665]. At the junctionof the kinks, the lattice fringes appear to curve smoothly, suggesting the walls are bentwithout any distortion. The outward kinks are believed to result from the presence of asingle pentagon at the tip of the kinks; however, the inward kinks are formed due tothe presence of a heptagon at the tip of the kink. The angles, measured at the point ofthe kinks in the figure, are almost the same at approximately 135�. It is worth mentioningthat the measurement of the exact angle from micrograph will not always be the actualangle at the kink, because it is almost impossible to know if the tube axis was preciselyperpendicular to the electron beam. However, the largest angle at such type of kink wasfound to be 150� in the literature [679]. Helically-coiled CNTs with a diameter of 50 nmwere obtained by using a silicon precursor during ADS [663]. This helical or toroidal struc-ture [702] is well known in carbon fiber growth patterns in which two growth points canoccur at a single nanotube cap. These two growth points provide a mechanism for twistgrowth which characterizes helical carbon nanotubes. Such kind of CNTs were foundwhen grown through catalytic pyrolysis of acetone on a silica substrate [703]. LikeSWCNTs, X and Y junctions were found in the case of MWCNTs produced by ADS.Few junctions are shown in Fig. 7.11 produced through decomposition of metallocene[688].

Our earlier communication [663] reported the presence of carbon nanorods during ADSprocess. A carbon nanorod, scrolled graphene sheet without any inner empty core, israrely found during TEM investigation. Such solid cylindrical structure is regarded as aderivative of CNTs. The terminologies graphitic carbon fiber (GFCs) and vapor-growncarbon fibers (VGCFs) have been used to denote such solid cylinders.

7.5. Properties

7.5.1. Chemical properties, biological applications and toxicity

Cell viability in the presence of CNTs is an important subject of research. The maincauses of toxicity of CNTs in cells are due to the presence of metal catalyst residue andinsolubility of the CNTs [704]. A major drawback in the use of CNTs for biological sys-tems is their complete insolubility in all types of solvents. For solubilization of CNTs,functionalization using relatively large functional groups is required [705–708]. Generally,a variety of oligomeric and polymeric compounds is used in the functionalization of CNTsfor their solubility in common solvents including water. Although such functionalizationopens up a new avenue of medical applications of CNTs [709], questions related to the tox-icity need to be answered before full-fledged incorporation of CNTs into body environ-ment. Recently Ding et al. [710] studied cytotoxic mechanism of MWCNTs on humanskin fibroblast. It was found that the cytotoxic doses induced cell cycle and increasedapoptosis or necrosis. It was also observed that the exposure of MWCNTs to the cellnot only activated genes, which were in cellular transport, metabolism, cell cycle regula-


tion and stress response, but also indicative of a strong immune and inflammatoryresponse within skin fibroblast. In general, functionalized CNTs are highly soluble inaqueous biological media and showed significantly less toxicity. That is why functionalizedSWCNTs and MWCNTs (called f-CNTs) are used in several biological applications, suchas drug delivery [704,711,712], gene delivery [704,713], antigen delivery [704], cancer celldestruction [714], ion channel blocking [715], etc. It is reported [704] that the SWCNTsshowed usually a loading of 0.3–0.5 mmol of functional groups per gram of material,whereas MWCNTs carried 0.5–0.9 mmol g�1. The CNT-lengths and their correspondingcarboxylic-acid loadings, calculated using a quantitative Kaiser test [712], are presentedin Table 7.3. It is worth mentioning that the amount of functional group around the tipsand side walls of the CNTs are different. Recently, the increasing use of functionalized car-bon nanotube-based nanobiotechnology in medical application again warrants the funda-mental understanding of its potential toxic effect. Some common implications of toxicityare referred in the toxicity section of this article and are summarized in a publication byour group [716].

In general, it has been found so far that the functionalized nanotubes had no harmfuleffect on cells [713,714,717,718]. Mattson et al. [718] reported the growth of embryonic rat-brain neurons on functionalized MWCNTs and pristine MWCNTs. Pantarotto [713]showed that the functionalized SWCNTs were able to cross the cell membrane and toaccumulate the cytoplasm or reach the nucleus for cell up to 10 lm. However, the uptakemechanism was not identified.

Functionalization of streptavidin/biotin system to SWCNTs was studied [719] to inves-tigate the adsorption behavior of proteins on the side-wall of SWCNTs. In an independentstudy, the uptake of SWCNTs and SWCNTs-streptavidin conjugates into human promy-elocytic leukemia (HL60) cells and human T-cells (Jurkat) was investigated and found thatthe functionalized SWCNTs could enter the non-adherent as well as adherent cell lines andby themselves were not toxic [717]. Fluoresceinated streptavidin protein conjugated withSWCNTs-biotin transporter could easily enter into cells when fluoresceinated streptavidinprotein itself could not. However, such CNT-complex exhibited a dose-dependent toxicitybecause of the presence of appreciable hydrophobicity along the CNTs. The supramolec-ular complex of functionalized CNTs with plasmid DNA can bind and penetrate within acell through an endosome-independent mechanism [720]. Fig. 7.14 shows interactionsbetween functionalized SWCNTs and plasmid DNA when the solutions were mixed ata charge ratio (±) of 6:1. Functionalized SWCNTs and plasmid DNA were taken in pre-determined amounts, 720 lg mL�1 and 5 lg mL�1, respectively. Kam et al. [714] investi-gated the transportation of oligonucleotides by SWCNTs into living cell using 800 nm

Table 7.3Chemico-physical properties of MWCNTs after acid treatment and carboxylic acid functionalization [712]

Time (h) Length (nm) Loading (mmol g�1)

1 1500–4000 0.063 1000–2000 0.145 200–1000 0.168 180–940 0.2224 160–600 0.2648 140–500 0.34

Fig. 7.14. Transmission electron micrograph of functionalized SWCNTs. (A) Few functionalized SWCNTs bindtogether through the functionalization chains. (B) Globular and supercoiled structures, marked with a blackarrow, were found to be present in different regions of SWCNTs along the surface of SWCNTs. (C) A lowmagnified micrograph showed few sites (with white arrows) where condensation of plasmids onto the SWCNTstook place (reproduced with permission from [864]).


light. They found that the oligos could be translocated into cell nucleus upon endosomalrupture triggered by such near infrared (NIR) laser pulses. However, continuous NIRradiation could cause cell death due to excessive local heating of SWCNTs in vitro. Theyenvisioned that such transporting capabilities of functionalized SWCNTs combined withnanotube’s intrinsic optical properties could replace the conventional classes of materialsfor drug delivery and cancer therapy. Recently, MWCNTs covalently bonded withAmphotericin B (AmB) showed no specific toxic effect in mammalian cell, instead itshowed preservations of its high antifungal activity. Table 7.4 shows the antifungal activ-ities of MWCNTs-AmB conjugates using three species of fungi: Candida parapsilosis

(ATCC 90118), cryptococcus neoformans (ATCC 90112) and candida albicans.Although a significant amount of research effort is focused on the toxicity of CNTs, a

large amount of research needs to be carried out in establishing the toxicity and biocom-patibity of CNTs. The results are often inconclusive and contradictory in many cases [721].The data presented in this review is mainly from initial results and is unrefined. In future, itis expected to have more comprehensive and established results on the biocompatibity ofCNTs.

Table 7.4Antifungal activity of CNTs-AmB conjugates [712]

Materials Minimum inhibitory concentration (MIC)a [lg mL�1]

C. parapsilosis ATCC 90118(collection stains)

C. albicans

(clinical isolate)C. neoformans ATCC 90112(collection stains)

AmB 20 >80 5SWCNT�NHþ3 >80 >80 >80MWCNT-AmBb 18 1.6 6.4 0.8SWCNT-AmBb 19 1.6 13.8 0.8

a The MIC corresponds to the lowest concentration of compounds that inhibited visible growth of theorganism. Results given are mean values of two independent determinations performed in duplicate.

b In this table, the MIC values for MWCNT-AmB and SWCNT-AmB refer to that amount of AmB in theconjugates (approximately one third by weight).


7.5.2. Chemical sensors

Significant interests have been observed to create new types of analytical tools in detec-tion of gas, vapor and bio-molecules using nanostructured materials with the motivationto counter-terrorism and environmental monitoring in recent years. Enhancement of selec-tivity and efficiency of such sensor could be achieved by tailoring the size, structure andshape of the nanomaterial. Numerous studies on sensing properties of these nanoscalematerials have been investigated in the last decade [722–724]. At present, semiconduc-tor-based sensors [44] have been used for the detection of chemical and biological mole-cules. Presently, CNTs-based sensors are implemented for the detection of suchmolecules for better performance, fine accuracy and high precision.

7.5.2.1. Gas. For last few years, it has become evident that the measured electronic prop-erties of CNTs and electrical contacts are profoundly affected by the exposure to humidity,oxygen and other gases. The electronic properties of CNTs are extremely sensitive to thepresence of molecular oxygen due to the formation of the charge transfer complexCþd

p –O�d2 . Nanotubes exhibit ultra-high sensitivity at room temperature to oxygen, nitric

oxide and ammonia [43,725,726]. Most of the CNTs-based sensors reported in the litera-ture are basically the field-effect transistor-based. Such sensors are mostly (�70%) devel-oped using semiconducting CNTs grown using CVD process [727]. It is believed thatthe electrons are being withdrawn when CNT-based FET is exposed to nitric oxide; how-ever, same are being donated to CNTs in the presence of ammonia.

A sensor was developed using SWCNTs and MWCNTs on interdigitated electrode todetect the gases at room temperature. Gases, like ammonia, carbon monoxide, carbondioxide were all detected using this configuration [726–728]. Recently, sensor responseswere found to be linear with the concentration of sub-ppm to hundreds of ppm with adetection limit of 44 ppb nitric oxide gas [655]. The mechanism of the detection of suchgas was attributed to be the direct charge transfer on individual semiconducting SWCNTconductivity with additional hopping effects on inter-tube conductivity through physicallyadsorbed molecules between SWCNTs.

Palladium SWCNTs-based sensor was first [729] introduced in 2001 as part of the questof a high sensitivity, fast response and reversible hydrogen gas sensing system. Sensor, hav-ing a set of five semiconducting SWCNTs decorated with palladium particles of 0.5 nmdiameter was exposed to hydrogen gas at a concentration of 400 ppm. The sensitivities,


defined as the ratio between resistance before and after gas exposure, were found to beapproximately 2. The response and recovery time was found to be approximately 10 sand 400 s respectively. In another study [730], large arrays of CNTs coated with polyethyl-ene imine (PEI) electrodes were used to detect nitric oxide at a concentration less than1 ppb. With such polymer coating, NO2 was detected without interference by NH3, while,with nafion coating, selective sensing of NH3 was observed, as shown in Fig. 7.15.

7.5.2.2. Detection of organic and bio-molecules. Sensors were developed using compositethin-film, like polymethylmethacrylate (PMMA), with MWCNTs and SWCNTs fordetecting organics and vapors, such as, dichloro methane, chloroform, acetone, methanol,ethyl acetate, toluene, and hexane [727,731,732]. Li et al. [655] used SWCNTs depositedinterdigitated electrodes to detect nitrotoluene at room temperature. The detection limit

Fig. 7.15. (a) Optical image showing three devices with polymer coating. (b) Red (top) curve: a device coated withNafion exhibits response to 100 and 500 ppm of NH3 in air, and no response when 1 ppm of NO2 was introducedto the environment. Blue (bottom) curve: a PEI-coated device exhibiting no response to 100 and 500 ppm of NH3

and large conductance decrease to 1 ppm of NO2 (reproduced with permission from [730]). (For interpretation ofthe references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 7.5Values of concentration independent intrinsic response for different organic vapors [736]

Chemical vapor DG/DC

Dinitrotoluene 0.20Dichloropentane 0.10Nitrobenzene 0.08Water 0.045Hexen 0.043Toluene 0.025Benzene 0.0132-Propanol �0.027Acetone �0.03Tetrahydrofuran �0.10DMMP [(CH3O)2P(O)CH3] �0.12


of such organic vapor was 262 ppb with the detection time in the order of seconds. How-ever, the recovery time was in the order of minutes. Through an interesting study, it wasshown that SWCNTs could strongly adsorb the macrocyclic tetraazaannulene complexesfrom ethylene solution because of p–p and hydrophobic interactions [733]. A selective andreal time detection of unlabeled DNA was reported [734] using microfabricated siliconfield-effect sensor. An increasing interest in developing SWCNTs-based optical biosensors[735] stems from their near-IR optical properties.

Using simultaneously measured conductance (DG) and capacitance (DC), a chemicalvapor sensor mechanism was proposed recently [736]. The mechanism behind such sensorwas explained in terms of dielectric effect. It was explained that for most of the vapors,dielectric effect of the molecular adsorbate dominated the capacitance response, whereascharge transfer from the adsorbate controlled the conductance response. The charge trans-fer also perturbed the capacitance via changes in the SWCNTs quantum capacitance. Theratio of the capacitance response (DG) to the conductance response (DC) was a concentra-tion-independent intrinsic molecular property. Such ratios for different chemical vaporsare mentioned in Table 7.5.

In the preceding section, we reviewed a very small portion of CNT-based nanosensorwork from literature to show the versatility and applicability of CNTs in sensors.Although a great deal of research attention needs to be given in several different areasof sensor for high efficiency, and accuracy, it is understood that the CNT is the mostimportant candidate for future nanosensors.

7.5.3. Catalysis

While pristine CNTs have unique properties for numerous applications, CNTs deco-rated with elements and compounds exhibit fascinating and desirable properties to serveas nano-scale chemical reactors or catalyst. Research on decorating CNTs has been oneof the most active fields of nanotubes research. When CNTs were discovered, few attemptswere undertaken to introduce foreign materials into the central cavities. Generally, the fill-ing of CNTs is accomplished using two processes [737–740]. The first involved carrying outarc-discharge in the usual way, but with an anode containing some of the materials to beencapsulated. This technique generally seems to favor the formation of filled particles, andonly applicable to materials which can survive the extreme condition of the electric arc


[679]. The second approach involves several steps after synthesis of CNTs by the usualway. For internal filling of CNTs, at least two steps are required: opening up and fillingthe nanotubes through either capillary action or other chemical means. Such decorationof CNTs with nanoparticles is accomplished as a post-processing step which involvesdeposition of nanoparticles on nanotubes by various means. In the literature, a varietyof decoration techniques for CNTs with nanoparticles was reported through numeroustechniques. Some of the most important procedures are discussed here.

Ang et al. attempted to decorate CNTs using an electroless plating synthesis [741]. Intheir route, the first step was to activate the CNTs with Pd–Sn catalytic nuclei via a sin-gle-step activation approach. Such activated nanotubes were used as precursors forobtaining nickel- and palladium-decorated nanotubes via electroless plating. Activationof the nanotube surfaces promoted specific deposition of the metals on the catalytic tubesurfaces. As a result, CNTs densely coated with metal nanoparticles were obtained withreduced metal deposition in the reaction solution. However, this method needs severalsteps to decorate the CNTs. Ye et al. [742] tried to decorate palladium particles onCVD-grown CNTs by hydrogen reduction of a Pd(II)-b-diketone precursor in super-crit-ical carbon dioxide with precisely controlled temperature and pressure environment. Theirstudy demonstrated the deposition of palladium nanoparticles (5–10 nm) on MWCNTsusing a complicated instrument. The second procedure is not only time consuming but alsoinefficient. Deposition of platinum nanoparticles and nanoclusters on functionalizedCNTs was also attempted [743]. A higher yield of functionalized CNTs was obtained bytreatment of CNTs in HNO3 or H2SO4–K2Cr2O7. The deposition of platinum nanoparti-cles and nanoclusters on these functionalized MWCNTs by electroless plating and two-step sensitization–activation pretreatment. Several complicated steps are required for bulksynthesis using this process. SWCNTs were coated with platinum nanoparticles using athree step process [744]. First step was the purification of SWCNTs using HNO3. Thiswas followed by a weak oxidation to create anchor sites for the platinum. Finally, plati-num was attached to the surface oxidized SWCNTs by reduction of K2PtCl4 in ethyleneglycol. Despite its high efficiency, multiple steps make this process time consuming. Ruthe-nium decorated CNTs were synthesized for heterogeneous catalysis [745] using a ruthe-nium precursor. Ruthenium 2,5-pentanedionate was dissolved in toluene for 72 h incontact with CNTs. The CNTs were separated from toluene by drying toluene. After-wards, CNTs were treated for 3 h under a stream of nitrogen at 523 K and then reducedfor 1 h in a stream of diluted hydrogen. Che et al. [746] used a very complex method todecorate the CNTs. They synthesized ensembles of highly aligned monodisperse CNTswithin the pores of alumina membranes using CVD. The CNT/alumina membranes wereimmersed into a solution of the desired metal ion for 5 h. The membrane was dried in airand the ions were reduced to the corresponding metal or alloy by 3 h exposure to flowinghydrogen at 580 �C. The underlying membrane was dissolved by immersion in 46% HFsolution to obtain the desired free standing CNTs. A series of CNTs decorated with var-ious catalysts were synthesized by a constant pH co-precipitation method [747]. A series ofaqueous solutions of metal precursors was prepared by dissolving appropriate metalnitrate-salts. A 4 N Na2CO3 solution was added drop-wise to the solutions with vigorousstirring at a constant temperature of 353 K. The addition of such solution was adjusted tomaintain a constant pH of 7.0. The precipitation procedure was completed in 1 h.Repeated rinsing of samples was required to remove the sodium ions. In another attempt,Luo et al. [748] synthesized CNTs supporting rhodium for nitric oxide catalysts. First, the

Table 7.6Nanoparticles decorated CNTs and its catalytic activity

Catalyst Reactions Results

Rh-MWCNT No decomposition Higher conversion rate than Rh–Al2O3

Cinnamaldehydehydrogenation

Three times higher rate than Rh-activated carbon

Pt-graphite nano-fiber(GNF)

n-Hexen reaction Higher selectivity than Pt/SiO2

Methanol oxidation Higher rate than Pt-vulcan carbon electrodeCo-MWCNT Cyclohexanol

dehydrogenationHigher activity than cobalt-activated carbon

Rh-GNF Ethylene hydroformylation Activity higher than cobalt-activated carbonNi-GNF Butene hydrogenation Higher conversion rate than Ni–Al2O3 and

Ni-activated carbonMethane decomposition Higher than Ni–Al2O3


CNTs were synthesized by means of CO disproportionation using a hydrogen-reducedNi/La2O3 catalyst at 600 �C for 30 min. The CNT-supported rhodium was prepared bytreating the CNTs with a 5 mM RhCl3 solution. The material was dried at 80 �C andcalcined at 500 �C in air for 2 h. Some results are summarized in Table 7.6.

Hsin and coworkers [667] attempted to produce metal-filled CNTs using arc-dischargein a cobalt sulfate solution which resulted in the formation of CNTs-filled with not onlymetallic cobalt, but also cobalt sulfide particles. The encapsulated particles were mostlyrod-shaped. Moreover, the mechanism of the encapsulation of particles was not clear fromsuch attempts [667]. Preparation of TiC [749] and Cu [750] filled nanotubes using a CVDprocess was attempted, however, the filler material was continuous along the length of thenanotube. Nanowire-filled CNTs posses a surface area less than that of CNTs filled withnanoparticles, hence, the latter is more suitable for gas storage applications.

7.5.4. Hydrogen storage

Hydrogen storage in CNTs has been a subject of intense research since its discovery[751–758]. According to the literature [754], the storage potential of CNTs must exceed8 wt% of hydrogen to power electric vehicles effectively. Recent experimental and model-ing studies [755,756,758–761] confirm that hydrogen uptake in CNTs is up to 20 wt%. It isalso shown that the CNTs combined with selective elements and compounds exhibitenhanced desirable properties to serve as nano-scale chemical reactors for several applica-tions [741,743–747,742]. Lithium-and potassium-doped CNTs have been studied for thedevelopment of materials for hydrogen storage [756] and solid-state batteries [762].Recently, Darkrin et al. [760] reported hydrogen storage values using CNTs at variousthermodynamic conditions, presented in Table 7.7. Such a large variation in experimentalvalues on storage of hydrogen was interpreted [758] in the following ways. (1) CNTs arealways associated with different types of impurities, such as amorphous carbon, water,hydrocarbon, that could influence experimental result. The amount of impurities couldbe different significantly depending on synthesis methods, process parameters, and severalother factors. (2) Distribution in diameter of nanotubes is another major issue, which has asignificant influence on such results.

Mechanism of hydrogen storage was explained through both physisorption and chemi-sorption of hydrogen on CNTs. Theoretical calculation shows that the strength of the

Table 7.7Carbon nanostructures and corresponding amount of hydrogen storage

Materials Hydrogen storage (wt%) Temperature (K) Pressure (MPa) References

SWCNTs 5–10 300 0.04 [751]SWCNTs 8 80 8 [765]SWCNTs 4 300 12 [766]SWCNTs 6.5 300 16 [767]SWCNTs 11 80 10 [768]SWCNTs 6 77 0.1 [769]MWCNTs 0.25 300 0.1 [770]Li-doped-MWCNTs 20 200–400 0.1 [754]K-doped MWCNTs 14 300 0.1 [754]Li-doped MWCNTs 2.5 200–400 0.1 [771]Pd-MWCNTs 1.0 573 0.1 [763]


interaction between physisorption and chemisorption could vary between 0.11 eV perhydrogen molecule and �2.5 eV per hydrogen atom [758]. The physisorption mechanisminvolves the condensation of hydrogen molecules inside or between the CNTs. The chemi-sorption mechanism occurs through dissociation of hydrogen molecules on a catalyst fol-lowed by reaction with unsaturated carbon–carbon bonds to form carbon–hydrogenbonds [763,764]. Recently, a decrease of p* character from unsaturated carbon–carbonbond and an increase in the carbon–hydrogen resonance were shown [758] using X-rayabsorption spectroscopy. It can be concluded from the X-ray photoelectron spectroscopyresults that SWCNT film could store 5.1 ± 1.2 wt% of hydrogen.

7.5.5. Thermal properties

It is very important to understand the thermomechanical properties of devices that aremade of CNTs, since a significant amount of heat generates during device operation andits proper dissipation is of prime importance. Due to the different thermal expansion coef-ficients of CNT and associated materials in a device, a residual stress induces in the CNTwhich may affect the electrical and mechanical properties of the device [772,773]. It hasbeen shown that current saturation is associated with nonlinear transport of metallicSWCNTs. Different experimental results of this were attributed by explaining the onsetof electron back-scattering by high energy optical and zone boundary phonons in high biasregime as well as ballistic transport along the CNTs [773]. Therefore it is probably themost fundamental requirement to understand the thermomechanical properties, especiallyexpansion coefficient, of CNT prior to the device fabrication.

Thermal expansion of MWCNTs with different diameter at high temperature (1373 Kin argon atmosphere) were studied [774]. In general, CNTs are progressively graphitizedwith high temperature [664] and graphitizability of a thin CNT is lower than a thickone because of difference in degree of curvature present in CNTs. Wu et al. reported, asmentioned in Table 7.8, the thermal coefficients became higher and appear to be a singularvalue for MWCNTs with a diameter of 50 nm. In the case of SWCNTs, CNTs could coa-lesce and transform into MWCNTs at high temperature heating (2473 K at vacuum for4 h) [775]. When MWCNTs were heated at such high temperature, the CNTs collapsedinto graphitic nanoribbons (GNR) driven by non-compensated van der Waal forces dueto tube–tube interaction within a bundle. Many reported the irradiation of MWCNTs

Table 7.8The changes in structural parameters and thermal coefficient of MWCNTs during high temperature (1373 K)treatment [774]

Averagediameter (nm)

Before heat treatment After heat treatment

d002a

(A)Thermalcoefficient ac (10�5)

d002

(A)Thermalcoefficient ac (·10�5)

Structuralstrain (ec)

10 3.476 0.73 3.425 1.74 0.009550 3.477 0.99 3.393 1.32 0.01470 3.478 1.27 3.391 1.78 0.016

100 3.480 1.49 3.385 1.74 0.018

a d002: calculated average interlayer spacing.


under electron beam, during TEM analysis, results in the collapse of an isolated CNT(Fig. 7.16). However, such structural transformation is a very anisotropic and a non-equi-librium process [775].

An extensive numerical study on the thermal expansion and vibration characteristics ofmetallic SWCNTs using molecular dynamics simulations [772] was performed. It wasfound that the thermomechanical properties of SWCNTs were exhibited through the com-petition between quasi-static thermal expansion and thermal vibration, although the laterwas more prominent and causes apparent thermal contraction in both radial and axialdirections at below 800 K.

7.5.6. Electronic properties

During the last few years, physicists have been attracted to extraordinary electronicproperties of CNTs. This is because of the hexagonal symmetry of a graphene sheet whichreduces the two dimensional Fermi contours to six points (Fermi Points) [776]. The bandstructure or electronic energy levels, of a graphene sheet includes valence and conductionbands that meet precisely at the Fermi energy at the very edge of the Brillouin zone (themomentum-space unit cell, which corresponds to the periodic unit cell of a real space crys-tal) [777]. Similarly, most ways of rolled graphene sheet produce same type of band struc-ture. Some have moderate band gaps where as the others have almost the same as ingraphene sheets. Semiconducting CNTs could be found as n-doped and p-doped [778].Doping of SWCNTs due to contact with metal electrodes and ambient gases have beenreported [646,778,779]. Doping of CNTs could affect the electron scattering property lead-ing to a change in conductivity.

7.5.6.1. Conduction. When a graphene sheet rolls up into a nanotube, confinement of elec-trons around the circumference in CNTs is observed. The unique electrical properties ofSWCNTs arise from such confinement of the electrons, which allow motion in only twodirections [780] in many micrometer scale at room temperature. When transport is ballis-tic, the terminal conductance (G) of a one dimensional system is given by Landauer’s equa-tion [780] (7.3)

G ¼ 2e2

h

XN

i

T i ð7:3Þ

Fig. 7.16. (a) Schematic representation of chainlike (i) and close-packed (ii) coalescence processes. (b, c)Diagrams of the thermal evolution of SWNT bundles of HiPCO and ARC, respectively. (b) HiPCO SWNTbundles transform into isolated MWNTs. (c) ARC: large SWNT bundles evolve forming MWNT bundles thatcollapse at higher temperatures to form GNR; small SWNT bundles can produce isolated MWNT or diameter-doubled SWNT bundles that eventually also collapse to form GNR (reproduced with permission from [775]).


where 2e2/h is the quantum of conductance and Ti is the transmission of contributing sub-band (conduction channel) produced by the confinement of electrons along the circumfer-ence of the CNT. In the absence of the scattering of electrons, the resistance of metallicSWCNT was calculated to be 6.5 kX using Eq. (7.3). The resistance is due to the quantummechanical coupling of the two conducting subbands in the nanotubes with the leads. This


is unavoidable, and every interconnect made with perfect CNT would have at least similarresistance [646]. An electron microscopic image is presented in Fig. 7.17 showing the pro-cess of measurement of electrical conductivity of a CNT.

It has been shown [690] that the major contribution in conductance of carbon nano-structure comes from the p-electrons and only a very small part from the r-electrons.However, contribution of r and coupling of r–p system were found to be significant inthe case of bent CNTs [781]. The electronic properties of CNTs depend on the detailsof the microstructure of the tube [781], extent of functionalization [782] and doping[783,784]. As mentioned before, a nanotube is metallic at room temperature if (n � m) = 3i

Fig. 7.17. Electron microscopic images of a device with metal electrodes for measuring the electrical properties ofa CNT. The inset shows an atomic resolution scanning tunneling microscopic (STM) image of a CNT(Reproduced with permission from [775]).


is an integer. Otherwise, it is semiconductor with band gap Eg = (0.9/d) eV, where d is thediameter of nanotubes in nm. It is worth mentioning that the band gap of semiconductingCNTs is inversely proportional to the tube diameter. The diameter of a CNT can beexpressed as

p3ac–c (n2 + nm + m2)0.5, where ac–c is the C–C bond length.

The peculiar and remarkable electronic properties of CNTs [785–787] signal the possi-bility of band gap engineering by control of the microstructure. An introduction of smalldefect, like single carbon vacancy, could affect the conductivity [690] and band gap mod-ulation of semiconducting CNTs [691]. Extent of variation in band gap with defects is pre-sented in Table 7.9. It was proposed that the response of the pz-orbital state, contributedmostly by the nearest carbon atoms to the vacancy site, towards the different applied direc-tions of an external field was the main reason for the difference of the band gap variationsin CNTs with vacancy defects [691]. With large field strength, the band gap variation indefective CNTs became similar to the perfect one. Therefore, such flexibility can providea suitable band gap for a specific application as band gap is dependent on the directionand the strength of the transverse electric field.

The electronic current in SWCNTs is found to be saturated at about 20 lA, while cur-rents of up to 1 mA have been reported for low defect arc-produced MWCNTs [646]. Theconductance of MWCNTs rises at a rate of about 0.3G0/V for bias voltages greater than�0.1 V, while the current in arc-produced MWCNTs is found to flow over the outer layeronly. Two reasons are responsible for such property. (1) If the top layer is metallic, statis-tically next layer is semiconducting in two out of three cases, and (2) for perfect planargraphite, transport perpendicular to the planes is strongly suppressed [646]. The resistanceper micron-length was found to be on the order of 1 MX [646]. The important issues andcurrent technologies of high thermal conductivity of materials for electronics applicationshave now been explored. Schelling et al’s review article on this topic should be referred formore details [788].

7.5.6.2. Transistor. The first room temperature operated CNT-based transistor was dem-onstrated in 1998 [789]. A SWCNT-based transistor with gold-electrodes could be gatedby applying a potential to a gate electrode structure below the nanotube [646]. It was alsoreported in literature [789–792], that the SWCNTs could be used as channels of field-effecttransistors (FETs). Such CNTFETs acted as unipolar p-type FETs with on/off currentswitching ratio of �105. During operation, these devices had a high contact resistance(P1 MX) which led to a low transconduction, gm, where gm = dIsd/dVgs [780]. Fig. 7.18shows I–V characteristics of two p-type CNTFETs with metal contacts cobalt and tita-nium carbide. In the titanium carbide-based device, the CNT was passivated by depositinga 10 nm silica-film on top of it, while cobalt-based device was left open to air. The contact

Table 7.9The band gap variation of a zigzag (10,0) SWCNTs under traverse electric field applied in +x-axis direction [691]

Defective atoms inCNTs (10,0)

Applied field strength +x direction (V/A)

0 0.125 0.25 0.375 0.5 0.55 0.6 0.65 0.7 0.75

Band gap (eV)

No defect in 80 atom CNT 0.67 0.67 0.65 0.63 0.56 0.55 0.54 0.41 0.21 0.0079 defective atom 0.19 0.25 0.31 0.37 0.36 0.34 0.31 0.28 0.21 0.00119 defective atom 0.30 0.35 0.40 0.45 0.48 0.49 0.50 0.42 0.38 0.09

Fig. 7.18. Output (top) and transfer (bottom) characteristics of CNTFETs with cobalt and titanium carbidecontacts; Inset: schematic structure of the transistor (reproduced with permission from [780]).


resistance and transconduction for the titanium carbide based device were found to be�25 kX and 3.7 · 10�7 A/V, respectively. According to a recently reported study, a shortchannel length transistor was assembled using bilayers of pentacene onto random arraysof SWCNTs [793]. Such two-orders of magnitude reduction of channel-length enabled theincrease in transconduction without reducing the on/off ratio. Recently state of the artCNT–FET, which behaves as Schottky-barrier modulated transistor, showed very highperformance. Such novel device exhibited n-and p-type unipolar behavior, tunable by elec-trostatic and/or chemical doping with excellent OFF-state performance and a steep sub-threshold swing (S = 63 mV/dec) [792]. There are several reviews on molecularelectronics, reported in literature, that can be used for further understanding [792,794].

7.5.6.3. Capacitor. Significant research efforts are underway for the use of CNT as a capac-itor for microelectronic applications. Recently, fabrication process and electrical charac-teristics of nanocapacitor structure using metal–insulator–CNT–metal layer have beenreported in literature [795] exhibiting a high capacitance value.

7.5.6.4. Field emission. In 1995, field emission property of CNTs was discovered by deHeer et al. [796] by showing that an electric current could be drawn from a surface covered


with CNTs. Films of both MWCNTs and SWCNTs are excellent field emitters althoughthe field emission parameters for nanotube films may vary. According to Fowler–Nordheim equation (Eq. (7.4)), the field emission current (I) of a metal tip is related tolocal field strength (F) at the tip through the following equation:

I ¼ KðF Þ2

/ � exp �B/32

F

� � ð7:4Þ

where B = 6.8 · 109 eV�3/2, / is the work function of CNT, and K is a constant. Thefield emission current density value for CNTs is 1 mA/cm2 with an applied electric fieldof 3 V/lm. According to a recent study, doping in CNTs gives attractive and significantemission properties. A review on emission properties of doped CNT can be studied for fur-ther details [797].

7.5.6.5. Optoelectronic properties. The recent discovery of band gap photoluminescence ofSWCNTs has paved a new way of investigating their unique electronic properties inducedby low dimensionality [798]. Polymer–CNT composite materials have been studied fromthe point of view of applications in electro-optics. Loading layers of organic light emittingdiodes (LEDs) with low concentration of CNTs effectively increased the lifetime of thedevices, by preventing the built-up of local hot-spots through the high thermal conductiv-ity that can be achieved in a nanotube percolation [43].

Photoluminescence experiments were carried out on the SDS wrapped SWCNTs insolution in order to identify the tube characteristics [799]. Recently, enhanced electrolumi-nescence (EL) and photovoltaic (PV) response was found [784] in single-layer LED whenSWCNTs were doped in conjugated polymer, poly[2-methoxy-5-(2 0-ethylhexyloxy)-1,4-phenylenevinylene]. The detailed EL and PV studies indicated that low SWCNTs dopingimproved the bipolar charge injection leading to enhanced reverse and forward EL withreduced threshold voltage. As the concentration of SWCNTs increased, the interfacialexciton dissociation became dominated giving rise to an increased PV response, as shownin Fig. 7.19 [784].

Although, research on photoluminescence of CNTs is significantly increasing, moredetailed exploration is needed in order to commercialize such application. Future directionin this research is expected to be more on the electron–phonon interactions and relatedwork which could be useful for the application of CNTs in photoluminescence.

7.5.7. Mechanical properties

Understanding the mechanical properties of CNTs represents another intellectual chal-lenge that could have significant implications for industrial application [646,40,800]. In1996, CNTs were found to have very interesting mechanical properties, such as high stiff-ness and high axial strength, which were measured using the amplitude of their intrinsicthermal vibration in TEM [400]. The Young’s modulus was found to be in the range ofteraPascal (TPa). The mechanical properties for MWCNTs are very much dependent onthe radius. For example, the Young’s modulus is in the order of 1000 GPa for nanotubeswith diameters of less than 5 nm. The tensile strength of MWCNTs was found to be150 GPa for the diameter of 15 nm [646]. Wong et al. were the first [801] to perform directmeasurement of the stiffness and strength of individual, structurally isolated MWCNTs

Fig. 7.19. EL efficiency and photocurrent are shown as function of the SWCNT concentration in SWCNT-MEHPPV composite LEDs. The external quantum efficiencies (EQE) of EL were measured at the current densityof 10 mV/cm2 while photocurrent was recorded at 0.5 V reverse bias and 0.2 mW/cm2 illumination of 450 nm.The photocurrent density was defined as the difference between currents measured with and withoutphotoillumination (reproduced with permission from [784]).


using atomic force microscope (AFM). The nanotube was pinned at one end to molybde-num disulfide surfaces and load was applied to the CNT by means of AFM tip. The bend-ing force was measured as a function of displacement along unpinned length and a valueof 1.26 TPa was obtained for the elastic modulus. The average bending strength was foundto be 14.2 ± 8 GPa.

Few years ago, TEM was used [802–804] for in situ observation as well as calculation ofmechanical properties. The bending modulus of individual CNTs from aligned arraysgrown by pyrolysis was measured by in situ electromechanical resonance in TEM. Anoscillating voltage with tunable frequency was applied on the nanotubes. Mechanical res-onance could be induced in CNTs if the applied frequency approaches the resonance fre-quency [802]. Therefore, for a tube with uniform structural and mass distribution, therelation between the bending modulus (Eb) and the resonance frequency (mi) could beexpressed through following equation:

mi ¼b2

i

8p� 1

L2

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiD2 þ D2

1

� �Eb

q

sð7:5Þ

where, b for first and second harmonics are 1.875 and 4.694 respectively, D and D1 are theinner and outer diameter respectively. L and q are the length and density of the tube.Using Eq. (7.5), the bending modulus for several different CNTs were calculated and tab-ulated in Table 7.10. Recently, bending modulus of an individual MWCNT,29.6 ± 2.9 GPa, was measured using electric harmonic detection of resonance technique[805]; which is in very good agreement with the result reported in Table 7.10.

The application that exploits the mechanical properties of CNTs, is the use as tips inscanning probe microscopy. The first demonstration of SPM tip using CNTs was the majorcontribution to the field. If the present rate of success is maintained in the field of SPM,CNT-based tips are likely to become the standard for such systems in the long term [807].

Recently, mechanical properties of the aligned MWCNTs films and polymer-reinforcedMWCNTs were reported [808] keeping in the view of application in micro-electromechan-ical system (MEMS) using a commercial thin-film indentation couple with magnetic

Table 7.10Bending modulus for carbon nanotubes with varying sizes and shapes [802,806]

Outer diameter,D (nm) (±1)

Inner diameter,D1 (nm) (±1)

Length (lm)(±0.05)

Frequency(MHz)

Bending modulus(GPa)

33 18.8 5.5 0.658 32 ± 3.639 19.4 5.7 0.644 26.5 ± 3.139 13.8 5 0.791 26.3 ± 3.145.8 16.7 5.3 0.908 31.1 ± 3.550 27.1 4.6 1.420 32.1 ± 3.564 27.8 5.7 0.968 23 ± 2.7


coils as the force induction. Indentation measurements gave a hardness of 0.005 GPafor bare aligned MWCNTs, however, same experiment gave a value of 0.07 GPa forpolymer reinforced MWCNT composite. An elaborate discussion on mechanical proper-ties on CNT-composites is available in the special section (ODNS-nanocomposites) in thisarticle.

7.5.8. Optical properties

Structural characterization, unusual electronic and phonon properties of CNTs havebeen carried extensively using Raman spectroscopy, since its discovery [809,810]. Thecharacterization of radial breathing mode (RBM) at low energy (100–300 cm�1) for theSWCNTs is used to calculate the diameter distribution through following equation(Eq. (7.6)). The relationship between diameter (d, nm) and Raman shift (xRBM, cm�1)has been reported for bundled SWCNTs.

xRBM ¼244

dþ 14 ð7:6Þ

The RBM was not observed for raw MWCNTs, preventing the diameter distribution tobe calculated from Raman spectrum. In contrast, a RBM was found in the innermostdiameters for diameter approximately 1 nm of the MWCNTs. This allows for the diameterdistribution of innermost tubes to be calculated based on an analysis similar to SWCNTs[811].

Optical absorption spectroscopy is another technique for evaluating the diameter distri-bution of SWCNTs since the optical transition are based on the discrete electronic transi-tions associated with Van Hove singularities of semiconducting and metallic tubes [811].

Environment around CNT could affect the structure of the nanotube as recently shownin the literature. A variation in Raman frequency in a SWCNT was found [812] due to itsradial deformation when lying on the substrate. Such a drastic Raman shift could beobserved when the laser spot in Raman spectroscopy moved from middle of a nanotubeto its end as presented in Fig. 7.20.

7.5.9. Magnetic propertiesCNTs are weakly diamagnetic. However, ferromagnetically (Fe, Ni, Co) filled CNTs

usually exhibit a coercivity greater than that of bulk metal. It was envisioned that suchnanostructures could store data at a capacity of 10 Gbit cm�2 [807]. Recently, the exis-tence of a large induced magnetic moment in defected SWCNTs was predicted [689] usingthe Green’s function method. Presence of such magnetic moment is due to quantum inter-

Fig. 7.20. Typical Raman spectra of the SWCNT with laser spot moving along the tube from middle to thetrench of left-side. (a) G-band (G+/G� from 1591.6/1575.7 to 1594.2/1578.3 cm�1); (b) RBM (from 131.4–136.9 cm�1). The left inset gives the legend for the laser spot moving in a backward direction, and the right insetshows the RBM spectra when laser beam moves from the middle of the trench to the right side (reproduced withpermission from [812]).

Fig. 7.21. (A) The geometry of two pair defect orientations for a (5,5) tube: (a) symmetric orientation and (b)asymmetric orientation. (B) The induced magnetic moment in a perfect (5,5) tube is plotted in (a). (b) and (c) areplots for induced magnetic moment due to presence of pair-defect in two orientations, symmetric and asymmetric,respectively (reproduced with permission from [689]).



nal current and influence of pair–defect orientation. The magnetic moment was found tobe increased by several orders of magnitude. Two types of pair–defect orientations fromthe direction of the induced magnetic moment at some specific energy points are presentedin Fig. 7.21.

This review on carbon nanotubes illustrates a few glimpses of advancement in prepara-tion and use of CNTs. The rate of applications of CNT and related technology is farbehind it was anticipated. Several challenges still remain opened. In case of the nanotubegrowth, the real challenge is in obtaining the semiconductor or metallic nanotubes pre-cisely during the synthesis. As far as applications of CNTs are concerned, the CNT-basedtechnology is not foreseen in marketplace in near future as there are several technicalissues need to be solved. For example, although it is well-known that the semiconductingSWCNTs FETs have high mobility and best transconductance, controlling quality of thecontacts to the CNTs is still a significant challenge for the scientific community.

8. Bio-inspired ODNS

8.1. Overview

Nanoscience has extended its horizon into a host of biotechnological and biomedicalapplications. Drug delivery, tissue engineering, scaffold synthesis, biosensors, biosepara-tion, biocatalysis and enzyme immobilization are a few to gain the maximum interestamong one dimensional nanostructure. These bio-inspired ODNS include CNTs and inor-ganic materials on one part and polymers and other organic/biological molecules on theother part. Spherical nanoparticles have typically been a choice prior to the emergenceof one dimensional nanomaterials which are now gaining momentum and have givenimpetus to multitude of bio-applications worldwide. Present discussion will concentrateon major applications of the ODNS in various bio-applications. In general CNT haveattracted researchers more than any other material and are discussed to a greater extentespecially in enzyme/DNA sensors. Though a small discussion has been given in the pre-vious section dealing exclusively with CNT, few more useful aspects have been discussed inthe present section. We have taken extreme care to prevent any overlap of information inboth the sections but due to the multitude of applications of CNT a general overlap isalways inevitable. Tissue engineering and scaffold synthesis have already been discussedin the one dimensional polymeric materials in this article and hence have not been repeatedin the present discussion. Various inorganic/polymeric materials have been introduced anddiscussed appropriately as per the applications.

8.2. ODNS sensors in biology

8.2.1. CNT-based DNA and enzyme sensors

CNT has remarkable electronic properties as described elsewhere in this article. Theseextraordinary properties of CNT make them one of the most promising materials for bio-sensing applications. CNT-based electrochemical transducers offer noteworthy improve-ments in sensing ability of amperometric enzyme electrodes, immunosensors and nucleicacid sensing devices [813]. However, to take the advantages of its unique properties in elec-trochemical sensing applications, CNT needs to be properly functionalized and immobi-lized. It has been demonstrated that CNT can enhance the electrochemical reactivity of


important biomolecules [814,815] by promoting the electron transfer reactions. Function-alization of CNT has paved the way for them to be used in a wide range of electrochemicalbiosensors exhibiting analytical behavior.

8.2.2. CNT-based DNA biosensor electrodes

DNA biosensors are based on nucleic acid recognition and can be used in a host of dif-ferent applications ranging from testing of genetic and infectious disease to food securityand environment protection, etc. [813,816–819]. The main principle behind DNA sensingis the immobilization of single stranded DNA (ssDNA) probe on a transducer surface. Thesingle stranded DNA has the ability to complement itself by recognizing its complemen-tary base sequence during which it generates useful electrical signal [820]. Electrode fabri-cation and hybridization indication techniques are thus crucial in development of highlysensitive and selective DNA electrochemical biosensors. The performance of such devicescan be enhanced by using CNT. These improvements are attributed to enhanced detectionof the target purines or to the product of an enzyme label as well as to the use of CNT ascarrier platforms. The large surface area of CNT can increase the amount of DNAadsorbed on the CNT-based substrate. It can also concentrate the number of enzymesand/or electroactive nanoparticles, at a specific site, to indicate DNA hybridization whichcoupled with the extraordinary electron transfer capabilities of CNT can amplify thehybridization signal. CNT based DNA sensing electrode devices can be categorized into(a) CNT modified electrodes and (b) CNT array electrodes.

8.2.2.1. CNT modified electrodes. The intrinsic electrochemical activity of nucleobasesmainly purines during the hybridization of DNA sequence makes the direct detection ofDNA hybridization possible without any indicators. However, due to high oxidationpotential of purines their peaks are merged into the background noise. These signalscan be enhanced by the use of mercury based electrodes in the presence of copper orRuðbpyÞ2þ3 ions which being mercury based can pose serious threat to the user[821,822]. CNT-based transducers give amplified redox currents of purines with welldefined voltammetric peaks. Immobilization of DNA oligonucleotides onto CNT-basedelectrode is imperative to such selective electrochemical sensing. Immobilization can beachieved by various means;

(a) DNA oligonucleotides can be adsorbed on the exterior surface of CNT by a non-specific adsorption [823].

(b) Covalent linking of oligonucleotides to CNTs through the carboxylic acid functionalgroup. These include oxidizing CNT tips, adsorption of molecules on sidewalls ofCNT, or plasma activation of CNT [824–828].

(c) Coupling of CNT with a conductive polymer in a label-free detection [829,830].

One of the modified electrodes is the carbon nanotube paste electrodes (CNTPE) whichis a modification of the classical carbon paste electrode (CPE). Pedano et al. [823] com-pared the adsorption and electro-oxidation of free purines on classical CPE and CNTPEand suggested that free guanine can be adsorbed on CNTPE while free adenine can beadsorbed on both electrodes. Remarkable improvement in the simultaneous determinationof adenine and guanine from DHP (dihexadecyl hydrogen phosphate) modified glassy car-bon electrode containing MWCNT with COOH group functionalization was also reported


[831]. Fig. 8.1 [831] illustrates the cyclic voltammetric studies of guanine and adenine atthree different electrodes. Ultra-high sensitivity with MWCNT having COOH group func-tionalization of DHP (dihexadecyl hydrogen phosphate) based glassy electrode was attrib-uted to the catalytic effect of MWCNT in purine oxidation and the strong adsorption ofpurine molecules on MWCNT. The amplified guanine signal on the MWCNT modifiedglass electrodes was ascribed to the strong accumulation of the guanine on the CNT sur-face and not because of the accelerated electron transfer [832]. The detection ability ofMWCNT modified glass electrodes was combined with collection and separation abilityof magnetic nanoparticles to fabricate a novel electrochemical DNA biosensor [816]. Aschematic of the process to obtain the sensor is outlaid in Fig. 8.2 [816]. Detection limitof the complementary system from such a sensor was reported to be 2.3 · 10�14 withoutobtaining the hybridization signal for the non-complementary sequence [833]. He et al.[816] summarized various types of CNT based electrodes and sensing techniques, listedin Table 8.1.

8.2.2.2. CNT array electrodes. Electrochemical techniques such as cyclic voltammetry(CV), differential pulse voltammetry (DPV), square wave voltammetry (SWV), electro-chemical impedance spectroscopy (EIS), and potentiometric stripping analysis (PSA) haveshown great potential in effecting the DNA sensing ability of CNT. It is further [837] illus-trated that the exquisite sensitivity and flexibility of electrochemical methods for analyzingDNA. Direct integration of individually addressable microelectrode arrays with electron-ics have been demonstrated for molecular diagnosis. Inspired by the integration of micro-electrode arrays with electronics researchers integrated nanoelectrode arrays into anelectrochemical system for ultra-sensitive chemical and DNA detection [827]. Unlike the

Fig. 8.1. Cyclic voltammograms of 1 · 10�5 mol L�1 guanine (G) and adenine (A) in pH 7.0 phosphate buffer(0.1 mol L�1) at three different electrodes without accumulation: (a) DHP-modified GCE; (b) bare GCE; (d)MWNTs-DHP film-coated GCE. Curve (c) cyclic voltammogram of MWNTs-DHP film-coated GCE in0.1 mol L�1, phosphate solution (pH 7.0) without guanine and adenine. Scan rate: 0.1 V s�1 (reproduced withpermission from [831]).

Fig. 8.2. Schematic representation of electrochemical detection of DNA hybridization based on RSH-coatedmagnetite nanoparticles labeled oligonucleotide probe by MWNTs = PPy CME. Step A: preparation of DNAprobe, Step B: detection of daunomycin connected on DNA probe, Step C: hybridization with target DNA andits detection (reproduced with permission from [816]).


CNT modified electrodes which are unordered, these arrayed carbon nanotubes arealigned vertically on the substrate. These ordered arrays of nanotubes are expected to exhi-bit enhanced electron transfer speed and hence high electrical conductivity. The nanoelec-trode array had been fabricated with a bottom-up approach resulting in accuratelypositioned well aligned MWCNT array embedded in a planarized silica matrix usingCVD [838]. The characteristics of such arrays were studied by combining withRuðbpyÞ2þ3 mediated guanine oxidation. A detection limit of less than few attamoles of oli-gonucleotide targets was reported using this technique. Investigation of two different typesof microelectrodes for studies viz. high density and low density nanotube arrays as shownin Fig. 8.3 [827]. The average tube spacing was 280 nm and the diameter of the MWCNTwas around 100 nm. It was found that the CV curve of high density sample was similar tothat of a macro-electrode due to heavy overlap of the diffusion layer from each CNT elec-trode. However the CV feature changes dramatically to a sigmoidal steady state curve withlow density sample as shown in Fig. 8.4 [827].

Linear scaling of signal with increase in the density can be observed from the CVcurves. It has been reported that the low density nanotube array behaves as independentnanotube electrodes and can be used to immobilize DNA and detect DNA hybridizationwith RuðbpyÞ2þ3 mediated guanine oxidation. It can be reasoned that the higher efficiencyof CNT arrays in sensing DNA hybridization was due to the structural flexibility of the

Table 8.1

Electrochemical DNA biosensors for sequence recognition fabricated by employing CNT [816]

Working electrode Measured DNA Immobilization technique Detection

technique

Experiment parameter Detection limit References

MWNTs–COOH–

DHP/GCE (casting)

Guanine, adenine, calf thymus

DNA

DPV CV Accumulation potential no

(effect), accumulation time

(0–2 min, hybridization signal

increasing) solution pH and

scan rate

7.5 · 10�9 mol L�1

(guanine)

5 · 10�9 mol L�1

(adenine) 100

[831]

MWNTs/GCE(casting)

Fee guanine, single strandedcalf thymus DNA, nucleic acid

segments related BRCAI breast

cancer gene (60 mer)

Probe oligonucleotideimmobilizing onto magnetic

microsphere via

streptavidin–biotin

interaction

PSA CV Adsorption time (1–3 min,hybridization signal

increasing); potential range

0–0.6 V, (hybridization signal

independent); stripping current

(3–20 lA, hybridization signal

increasing); CV scanning rate

(10–100 mV s�1 hybridization

signal dependant)

40 ng mL�1 [832]

SWNTs–COOH/SPE

(casting)

Synthetic oligonucleotide

(21 mer)

Probe oligonucleotide

immobilizing onto

SWNTs–COOH/SPE via

amino carboxyl bond

DPV SSB concentration

(10–50 lg mL�1, hybridization

signal increasing); immobilized

probe concentration(0–10 lg mL�1, hybridization

signal increasing) SWNT

amount (0.1–0.5%, increasing)

0.5 lg mL�1 [834]

MWNTs–COOH/GCE

casting

Synthetic oligonucleotide for

colitoxin (24 mer)


immobilizing onto

MWNTs-COOG/GCE via

amino carboxyl bond

DPV MWNTs-COOH amount

(0.5–0.2 lg, hybridization signal

increasing); hybridization time

(0–4 min, hybridization signal

increasing)

1 · 10�10 mol L�1 [826]

MWNTs–COOH/GCE

electrochemical

polymerization

Polynucleotide target related

to human porphobilinogen

deminase PBGD promoterfrom 170 to 142 (29 mer)


immobilizing onto magnetic

nanoparticles via aminocarboxyl bond

DPV Hybridization temperature

(28–40 �C, hybridization signal

increasing) hybridization ionicstrength (NaCl 0–1.4 mol L�1,

hybridization signal increasing)

2.3 · 10�14 mol L�1 [833]

864S

.V.N

.T.

Ku

chib

ha

tlaet

al.

/P

rog

ressin

Ma

terials

Scien

ce5

2(

20

07

)6

99

–9

13

SWNTs–

COOH/Glass


(20 mer)


immobilizing onto SWNTs-

COOH glass via amino

carboxyl bond

Fluorescence

imaging

SWNT layer number [824]

MWNTs–

COOH/GCE(casting)


(24 mer)


doping into PPy film onMWNTs–COOH/SPE via

electropolymerization

EIS MWNTs-COOH amount (0.05–

0.2 lg, hybridization signalincreasing) hybridization time

metal ion (Zn2+ > Co2+ Ni2+)

3.0 · 10�8 mol L�1,

5.0 · 10–11 MOL�1[829,830]

Aligned CNT

on SiO2

substrate


related to the wild-type allele

of BRCAI gene (18 mer) and

attached with a 10 bp polyG


immobilizing onto aligned

nanotube via amino

carboxyl bond

AC

voltammetry

CV

Array density Lower than a few

attamoles

[827]

Aligned

carbon

nanotube

on a gold

substrate


(20 mer)



carbon nanotube via amino

carboxyl bond

CV [828]

Alignedcarbon

nanotube

on Al2O3

substrate

One DNA sequenceimmobilizing onto aligned

CNT tip via amino-carboxyl

bond

SEM [835]

one DNA sequence


CNT sidewalls via

hydrophobic interaction

Mercury

film/GCE

Probe 1 immobilized onto

96 well microplate via

streptavidin–biotin

interaction

SWV 5 pg mL�1 [825]

Probe 2 immobilized CDS-

SWNTs via streptavidin–

biotin interaction

CNT

modified

electrode

PSA 1 fg mL�1 [836]

S.V

.N.T

.K

uch

ibh

atla

eta

l./

Pro

gress

inM

ateria

lsS

cience

52

(2

00

7)

69

9–

91

3865

Fig. 8.3. SEM images of two 1 · 1 cm2 electrodes fabricated from the forest-like MWNT arrays with (a) highdensity and (b) low density. The average tube–tube spacing is about 280 nm and 1.5 nm, respectively. The scalebars are 500 nm. Nearly half of the CNTs in the high-density sample show dark tips slightly protruding above theSiO2 matrix surface but wrapped with a layer of SiO2 showing brighter contrast. Other CNTs show deep darkdots corresponding to tips shortened below the SiO2 surface by electrochemical etching. Some deep voids betweenthe SiO2 grains are shown in both images (reproduced with permission from [827]).


self-assembled MWCNT which offers enough space for target DNA molecules to accessthe immobilized DNA probe [839]. Single-strand DNA chains were chemically graftedonto aligned carbon nanotube electrodes for probing complementary DNA and targetDNA chains of specific sequences [828]. An acetic acid-plasma treatment on gold-sup-ported aligned carbon nanotubes generated from pyrolysis of iron (II) phthalocyanine,was carried out. This was followed by grafting ssDNA chains with an amino group atthe 5A-phosphate end (Fig. 8.5) [828]. The amperiometric response from such alignednanotube arrays was found to be at least 20 times higher than the conventional flat elec-trodes, Fig. 8.6 [828]. These ssDNA immobilized aligned carbon nanotubes can be repeat-

Fig. 8.4. (a) and (b) CV measurements in 1 mM K4Fe(CN)6 and 1.0 M KCl; (c) and (d) CV measurements of Federivative functionalized MWNT array electrodes in 1.0 M KCl solution; and (e) and (f) ACV measurements at50 Hz and an amplitude of 50 mV on the staircase DC ramp from 0.55 to 1.20 V. (a), (c), and (e) are measuredwith the high-density CNT array electrode and (b), (d), and (f) are measured with the low-density one,respectively. All CV measurements are taken with a scan rate of 20 mV/s. The arrows indicate the position of theredox waves of the Fe derivative functionalized on the CNT array electrode. The dotted lines and dash linesindicate the baselines of the background current (reproduced with permission from [827]).


edly used as a highly-selective electrochemical sensor for sequence-specific DNA diagno-ses. It has been reported by various researchers that the tip of the nanotubes are morereactive than the side walls and can be easily functionalized into carboxyl group. Thus

Fig. 8.5. A schematic illustration of the aligned nanotube–DNA electrochemical sensor. The upper right SEMimage shows the aligned carbon nanotubes after having been transferred onto a gold foil. For reasons of clarity,only one of the many carboxyl groups is shown at the nanotube tip and wall, respectively (reproduced withpermission from [828]).


amide modified DNA could be covalently attached at the tips of the CNT. Thus immobi-lization could be achieved easily and at will along the tips or along the side walls where theDNA is generally adsorbed. These array based CNT sensors present a platform for directintegration on electronic devices such as chips and are hence a promising future for DNAsensing. The sensitivity could be improved further with more research groups workingtowards a specific aim which could provide easier and rapid solutions for molecular diag-nosis, particularly for early cancer detection, point-of-care and field uses.

8.2.2.3. CNT based enzyme electrodes. Our body contains several enzymes that catalyzebiologically important chemical reactions which otherwise would take eternity to com-plete. During this process of catalysis, electrochemically active species such as NAD/NADH are produced which, can be detected using powerful electrochemical devices.The specificity of enzymes in catalyzing the reactions works in favor of scientists. Mostof the enzymes used in the detection and sensing systems are oxidases and dehydrogenases.CNTs are extremely attractive for these enzyme based electrodes because of their extre-

Fig. 8.6. (A) Cyclic voltammograms of the aligned carbon nanotube electrode immobilized with ssDNA (I)chains followed by hybridization with the FCA-labeled cDNA (II) probes (a) and an Au electrode immobilizedwith ssDNA (I) chains followed by hybridization with the FCA-labeled cDNA (II) probes under the sameconditions (b). Note, the geometric area of the aligned carbon nanotube electrode is 1.5 mm · 1.0 mm and thearea of the gold is 2.0 mm · 1.5 mm. The electrochemical measurements were carried out in an aqueous solutionof 0.1 M H2SO4 vs. Ag/AgCl at a scan rate of 0.1 V s�1. The concentration of the FCA-labeled cDNA (II) probeis 0.05 mg ml�1. (B) The dependence of redox current at the oxidation potential of FCA (0.29 V) on the cDNA(II) concentration for the aligned carbon nanotube–DNA sensor (reproduced with permission from [828]).


mely low-potential detection of hydrogen peroxide and NADH (byproducts of enzymecatalysis) Fig. 8.7 [813]. As discussed above the successful designing of CNT based elec-trodes requires proper functionalization and immobilization of the sensing species onthe CNT. From the perspective of electrochemical reactivity the side walls of CNT are sug-gested to have properties similar to those of the basal plane of highly oriented pyrolyticgraphite (HOPG), while their open ends resemble the reactivity of the edge planes ofHOPG [813]. Enzyme based CNT electrodes could be fabricated as CNT-biocompositeelectrodes, CNT-coated electrodes, vertically aligned CNT electrodes.

CNT based biocomposite electrodes (CNT/insulator/enzyme) have been reported atlength [841–843]. In biocomposite electrodes CNT are the only conductive species andthe rest of the composite acts as the reservoir of the enzyme. As discussed already inthe article CNT based paste enzyme electrodes (CNTPE) mixed with mineral oil have beensynthesized [843,844]. Bromoform as a binder for synthesizing MWCNT based composite

b

a

-10

-15

-40

-65

-90

Cur

rent

(μA

)

Potential (V)

0.4 1

0

8

16

24b

a

10.5

Cur

rent

(μA

)

Potential (V)

b

a

-10

-15

-40

-65

-90

Cur

rent

(μA

)

Potential (V)

0.4 1

0

8

16

24b

a

10.5

Cur

rent

(μA

)

Potential (V)

A B

Fig. 8.7. Hydrodynamic voltammograms for 1 mM hydrogen peroxide (A) and 1 mM NADH (B) using thegraphite/Teflon (a) and CNT/Teflon (b) composite electrodes. Electrolyte, phosphate buffer (pH 7.4) (trend only,[840]).


electrodes has also been used [845]. Wang et al. reported a binderless biocomposite elec-trode based on mixing of glucose oxidase enzyme within CNT [846]. Antifouling proper-ties of nafion films are combined with catalytic action of CNT in sensing applications.CNT electrodes have been coated widely with nafion films, owing to their ability to solu-bilize CNT, for preparing CNT-based electrode transducers for a wide range of sensingapplications [847]. Platinum nanoparticles have been dispersed in the nafion coatedCNT electrodes to improve the sensing ability of the electrodes [842]. Array based CNTelectrodes can also be used as enzyme electrodes for sensing applications. MWCNT grownon Pt substrates have been shown useful in sensing applications [848]. Glucose biosensorsbased on CNT nanoelectrode ensembles (NEEs) have been developed that contain mil-lions of nanoelectrodes embedded in an epoxy matrix on a chromium coated silicon sub-strate Fig. 8.8 [849]. NEEs eliminate potential interference through the preferentialdetection of hydrogen peroxide. Such development of interference-free transducers shouldsimplify the design and fabrication of conventional and miniaturized sensing probes. Sev-eral other types of glucose biosensors based on conducting polymer [850], SWCNT on aferrocene mediator [851], binderless CNT-glucose oxidase composites [846], alignedMWCNT in a non-enzymatic detection [852], SWCNT transistor [853], etc. have also beenreported.

8.2.3. Metals and derivatives based biochemical sensors

Metallic and semiconductor nanowires could also be used for the detection of biologicalspecies. Semiconductor nanowires have been used as sensors for the detection of biological

Exposed CNT tips

EC treatment CrSi

EDC/ Sulfo-NHSGOx

CO2- CO2

- CO2- CO2

-

HN-GOx HN-GOx HN-GOx HN-GOx

C=O C=O C=O C=O

Exposed CNT tips

EC treatment CrSi

EDC/ Sulfo-NHSGOx

CO2- CO2

- CO2- CO2

-

HN-GOx HN-GOx HN-GOx HN-GOx

C=O C=O C=O C=O

A

B

Fig. 8.8. Fabrication of a glucose biosensor based on CNT nano-electrode ensembles: (A) electrochemicaltreatment of the CNT NEEs for functionalization; (B) coupling of the enzyme (glucose oxidase) to thefunctionalized CNT NEEs [849].


molecules [725]. Si nanowires (SiNW) were functionalized with biotin to study the ligandreceptor binding of biotin–streptavidin, Fig. 8.9. It was shown that the conductance ofbiotin increased rapidly to a constant value upon addition of 250 nM streptavidin solu-tion. The observed increase in conductance was attributed to binding of negativelycharged (streptavidin in this case) species to the p-type SiNW surface. Functionalizationof nanowires with biotin was vital to the sensitivity as no change in conductance wasobserved for non-functionalized Si nanowires. Recently field effect silicon nanowiresdevices have been studied for detection of ssDNA. SiNW were functionalized usingpeptide nucleic acid receptors and lead to an increased conductance on binding with

Fig. 8.9. Real-time detection of protein binding. (A) Schematic illustrating a biotin-modified SiNW (a) andsubsequent binding of streptavidin to the SiNW surface (b). The SiNW and streptavidin are drawn approximatelyto scale. (B) Plot of conductance vs. time for a biotin-modified SiNW, where region 1 corresponds to buffersolution, region 2 corresponds to the addition of 250 nM streptavidin, and region 3 corresponds to pure buffersolution. (C) Conductance vs. time for an unmodified SiNW; regions 1 and 2 are the same as in (B). (D)Conductance vs. time for a biotin-modified SiNW, where region 1 corresponds to buffer solution and region 2 tothe addition of a 250 nM streptavidin solution that was pre-incubated with 4 equivalents d-biotin. (E)Conductance vs. time for a biotin-modified SiNW, where region 1 corresponds to buffer solution, region 2corresponds to the addition of 25 pM streptavidin, and region 3 corresponds to pure buffer solution. Arrowsmark the points when solutions were changed (reproduced with permission from [725]).


polyanionic ssDNA to p-type nanowires surface. Use of gold nanotubes for detection ofproteins have been demonstrated [854]. The sensing paradigm was similar to stochasticsensing. Although research on the use of metal-oxide nanowires as sensors is still in earlystages, encouraging experiments have been reported that are interesting in their own rightand indicative of a promising future [855].

8.3. Drug delivery

Over the years the emergence of nanotechnology had a significant impact on drug deliv-ery sector, affecting just about every route of administration from oral to injectable. Drugdelivery vehicles thus become a common point of interest to materials scientists as well aspharmacologists. For utilizing the inherent advantages of a drug delivery system the car-rier of the drug needs to be amply loaded and should be specific to the site of action. Spec-ificity of action based on changes in pH, etc. is fundamental to the designing of any drugdelivery system. Though there are several drug systems available, the central focus of thisarticle would be to touch upon CNT and ODNS based drug delivery vehicles.

8.3.1. Carbon nanotubes in drug delivery

Several extraordinary properties of CNT have already been discussed in the appropriatesections. It must be appreciated by now that these CNT needs to be ably functionalized foruse in different applications. Chemical modification of CNT is required to achieve solubil-ity in water to subsequently employ them in several biological applications especially drugdelivery [704,856]. Their insolubility in all solvents had earlier caused several concernsabout their toxicity and hence limited their use in the biological applications [857]. Similarto their functionalization in sensor applications there are two basic approaches to CNTfunctionalization for applications in drug delivery. These can be classified as covalentaddition or adsorption to the tips and side walls of CNT (to achieve solubility in water)and oxidization using strong acids to generate carboxylic group functionalization thatincrease their dispersability in aqueous solutions [858–860]. To be able to act as an efficientdrug delivery carrier CNT should be able to cross the cell membranes. It has been shownthat properly functionalized CNT can cross the cell membranes effectively and the mech-anism of this uptake was endocytosis independent, implies the non-toxic nature of CNTs[861] [713]. CNT has been used as drug delivery vessels for delivering peptides, nucleicacids and drugs. Ammonium functionalized CNT has been shown to form supramolecularcomplexes with nucleic acids and their subsequent delivery to cells [862,863]. The macro-molecular cationic nature of CNT is similar to other family of non-viral vectors [864,865].Both SW and MWCNT can form stable complexes with biological macromolecules. Effi-ciency of DNA transfer has been shown to increase by functionalization of the side wallswith polyethyleneimine [866]. The ability of functionalized CNT to penetrate into the cellsand to form covalent bonds with macromolecules drives their use in delivery of small syn-thetic drug molecules [713,861]. Wu et al. developed a multiple functionalization processof CNT to enable them with high recognition capacity [867]. In this process CNT wasfunctionalized with several different molecules for specific applications as shown inFig. 8.10. SWCNT in boron delivery for use in boron neutron capture therapy (BNCT)have also been explored [868]. Water soluble SWCNT’s can be synthesized and appendedwith substituted C2B9 carborane units to study their boron tissue distributions. SWCNTwere successfully used as boron delivery agents in BNCT. CNT has also been used for

Fig. 8.10. Covalent attachment of amphotericin B and fluorescein isothiocyanate to CNT. Multi-walled carbonnanotubes are treated with acids to generate carboxylic groups, subsequently modified with a mono-protecteddiaminotriethylene glycol. The nanotubes are then subjected to 1,3-dipolar cycloaddition. Selective cleavage ofthe orthogonal protecting groups allows the introduction of the fluorescein moiety and amphotericin B(reproduced with permission from [868]).


the delivery of bioactive peptides to the immune system [869,870]. Thus transporting capa-bilities of CNT in combination with suitable functionalization can open up new andadvanced avenues for efficient drug delivery systems.

8.3.2. Metals, polymers and derivatives in drug delivery

Polymers are more widely used in drug delivery as compared to the metals or theirderivatives. Drug delivery using one dimensional polymeric materials was covered appro-priately in the polymeric ODNS section. Though there has been a recent interest in the useof metallic (oxide or derivatives) nanoparticles [871–873] as drug delivery agents but thepotential of nanotubes is not fully explored. Functionalized mesoporous silica nanorodscan be used as controlled drug release agents [874]. These silica nanorods were capped withsupramagnetic iron oxide nanoparticles. The controlled release mechanism of the system isbased on the reduction of disulfide linkage between iron oxide caps and the linker-silicananorod hosts by reducing agents such as dihydrolipoic acid (DHLA) [874] as shown inFig. 8.11. The chemistry of the disulfide linkages between silica nanorods and iron oxideparticles was formulated such that the linkages could be cleaved with various cell pro-duced antioxidants. The mesoporous silica nanorods were 200 nm long and 80 nm indiameter with an average pore size of 3.0 nm. Fluoroscein was introduced as guest mole-cule to be encapsulated inside the pores. Martin and group have pioneered the use of tem-plate based synthesis of nanotubes and nanowires. Their extensive work in the field ofnanotube biotechnology has made a significant contribution to the ODNS research[875]. Several polymer nanotubes synthesized by template method can be used in drug

Fig. 8.11. Schematic of the stimuli–responsive delivery system (magnet-MSN) based on mesoporous silicananorods capped with superparamagnetic iron oxide nanoparticles. The controlled-release mechanism of thesystem is based on reduction of the disulfide linkage between the Fe3O4 nanoparticle caps and the linker-MSNhosts by reducing agents such as DHLA (reproduced with permission from [874]).


delivery. Synthesis of silica nano-test-tubes as vehicles for drug delivery has been reported[876]. The main motivation for such a work came from the fact that most of the nanotubesused as drug delivery vehicles are open at both ends thereby requiring special cappingmethods for immobilization of biomolecules within these nanotubes. Making the nano-tubes in the form of a test tube would enable one end of the tube to be permanentlyblocked thereby enabling large quantity of biomolecules to be immobilized inside the testtubes before bottling it up. Silica nano-test-tubes were prepared based on the above advan-tages of nano-test-tubes by a slight modification of the membrane process. A schematic ofthe method used to prepare silica nano-test-tubes is illustrated in Fig. 8.12 while Table 8.2lists various parameters for obtaining such nano-test-tubes. Silica nano-test-tubes wereobtained with dimensions in agreement with the membranes. These were 70 ± 14 nm indiameter and 600 nm in length. Fig. 8.13 shows a TEM micrograph of such nano-test-tubes designed as drug delivery vehicles. It should however be appreciated that onlythe synthesis of nanotubes or nanoparticles does not make them eligible for use as drugdelivery agents and there are several other requirements that must be satisfied. Thereare issues related to toxicity and solubility of nanoparticles which must be addressed

Fig. 8.12. Schematic of the template synthesis method used to prepare nano-test-tubes [876].

Table 8.2Parameters used to prepare alumina template membranes for synthesis of silica nano-test-tubes [876]

Anodization voltage (V) Anodization time (min) Pore diameter (nm) Membrane thickness (lm)

50 10 70 0.650 20 70 1.070 5 170 0.6


and the mechanism of drug release into the medium; all must be addressed before a systemcan be completely established.

8.4. Miscellaneous

The field of ODNS driven biotechnology is growing at a rapid pace. Martin et al. havecovered the emerging trends and synthesis in nanotube biotechnology [875].

8.4.1. Peptide nanotubes

In 1993, a new kind of nanotube was developed from cyclic peptide molecules via self-assembling of these moloecules from basic building blocks under mild conditions [877].These were based on self-assembly through hydrogen bonding and contained an evennumber of alternating D- and L-amino acids, Fig. 8.14. The development of peptide nano-tubes was an important development given the fact that peptides are biological species. A

Fig. 8.13. (a) Transmission electron micrograph of a nano-test-tube prepared in the membrane described in row 2of Table 8.2. (b) Transmission electron micrograph of a nano-test-tube prepared in the membrane described inrow 3 of Table 8.2 (reproduced with permission from [876]).


multitude of biochemical and biotechnological applications were established through thedevelopment of peptide nanotubes. It was shown that these peptide nanotubes can mimicthe biological function of natural compounds such as naturally occurring membrane chan-nels or ion channels [877]. Vital biological functions such as control of ion flow, signaltransduction, molecular transport, etc. are performed by a large family of proteins, pep-tides and other organic metabolites that form biological ion channel membranes. Artificialmembrane channels made from peptide nanotubes were based on self-assembled cylindri-cal beta sheet peptide architecture that essentially contained stacks of peptide rings. It wasreported that peptide rings displayed ion transport activity that was comparable to manynaturally occurring counterparts. Cyclic peptides have been used as components in sto-chastic sensors [878,879]. The use of these cyclic peptides has also been studied in ligand

Fig. 8.14. A typical chemical structure for a cyclic peptide and schematic illustrations of the self-assembly of suchpeptides into nanotubes and nanotube arrays (reproduced with permission from [875]).


and voltage gated ion channels that are used in electrical signaling in our brains, nervesand muscles [880]. A general understanding of transport through ionic channel can bedescribed as blocking of the ionic pathway by the sensing species as it enters the proteinor ion channel resulting in a transient decrease in the measured transmembrane ion cur-rent. The species then diffuse out of the ion channel (this random diffusion out of theion channel is termed as random walk and hence the name stochastic sensors) and therebythe transmembrane current reaches its original value. Such short downward current pulsesare generated as species gets transported into the ion channel. These current pulses arerelated to the concentration of the entering species and the duration of the pulse providesthe clue to the identity of the species such as DNA chains, oligonucleotides, metal ions,etc. [875]. A series of publications since then have explored the field of synthesis and appli-cation of peptide nanotubes to its full potential and are still being constantly modified andreviewed [881–889]. Ion channel mimics based on alumina and gold nanotube were inves-tigated for their ion channel activities [890]. The approach was based on ion sensing modelof Umezawa’s group [891,892] which is just the opposite of the one described above. Inthis approach the base transmembrane ion current is initially turned off to zero currentand is turned on by sensing species (analyte). A DNA-nanotube was also reported forion channel applications [893]. These artificial channels were based on conical gold nano-tubes with DNA attached to nanotube walls to modify the electrochemical response. Theextent of rectification was quantified via the ratio, rmax (the absolute value of current at�1 V (on state) divided by the current at +1 V (off state) as shown in Table 8.3). The roleof surface charge in rectification of ion current in ion channels using conical gold nanotubewas also studied [894]. In a similar effort gold nanotube membranes were synthesized usinga template-based electroless plating technique [76]. Multiple parameters that affect the

Table 8.3Nanotube mouth diameter (d), DNA attached, rmax, radius of gyration of DNA (rg), extended chain length (l)[893]

d (nm) DNA attached rmax rg (nm) l (nm)a

41 12-mer 1.5 1.4 5.746 15-mer 2.2 1.6 6.942 30-mer 3.9 2.9 12.938 45-mer 7.1 4.0 18.998 30-mer 1.1 2.9 12.959 30-mer 2.1 2.9 12.939 30-mer 3.9 2.9 12.927 30-mer 11.5 2.9 12.913 30-mer 4.7 2.9 12.939 30-mer hairpin 1.4 n/a 6.9

a Includes the (CH2)6 spacer.


nanotube length and diameter were reported and their possible bio-application behaviorsuch as the use of HSA (Human Serum Albumin) immobilized gold nanotube membranesfor separation or warfarin enantiomer was explored. A multitude of other syntheticapproaches and application of peptide nanotubes have been covered in a recent review[895] and the references therein.

As a further discussion to ion channel transport, a brief introduction of carbon nano-tube in ionic transport would be rational. Sesti et al. [896] introduced the single walled car-bon nanotube as another new class of ion chain blockers. SWCNTs with diameter between0.9 and 1.3 nm, C60 fullerenes, MWCNTs and hyperfullerenes were applied to diversechannel types. SWCNTs were reported to block the K+ ion channel subunits in a dosedependent manner. It was also reported that the blockage was dependent on shape anddimension of the CNTs and that SWCNTs were more effective than the spherical fulle-renes and that diameter was the determining factor for both. A simulation of blockingusing the crystal structure KcsA (a selectivity filter) is shown in Fig. 8.15(A). In KcsA,a fullerene with a diameter of 0.72 nm can obstruct the mouth of the selectivity filterthereby obstructing the flow of ions (Fig. 8.15B and C). Larger MWCNTs onions withaverage diameters greater than 3 nm are too big to fit on channel crevices. SWCNTs havetwo types of ends closed end and open ended (Fig. 8.15(D) and (E)). Both of these typesare able to block the selectivity filter through open ended SWCNTs were reported to bemore efficient. In another observation the aligned CNT membranes were used in reversibleswitching of ionic transport [897]. A desthiobiotin was used which can bind reversibly withstreptavidin to modify the entrance of the pores of embedded CNT in membranes thusenabling an on/off switch.

8.4.2. Others

Widespread work in the field of bioseparation and biocatalysis using gold and silicabased nanotubes or nanotube membranes has been done. In one such report [256] silicananotubes synthesized from alumina templates (Si diameter – 60 nm; Fig. 8.16) were usedto immobilize the enzyme glucose oxidase (GOD), to separate the racemic mixtures of SRand RS enantiomers of 4-[3-(4-fluorophenyl)-2-hydroxy-1-[1,2,4]-triazoyl-1-yl-propyl]-benzonitrile (FTB) and to separate cyclohexane from water (shown in Fig. 8.17). The rateand the selectivity of the poly(ethylene glycol) derivatized Au NT membranes, as a

Fig. 8.15. Nanotubes block K+ channels through a pore occlusion mechanism. (A) The crystal structure of theKscA K+ channel. This and other images were constructed by RasMol and the file 1BL8 from the Protein DataBank. (B) and (C) docking simulation of a fullerene. A fullerene with an average diameter of 0.7 nm can fit intothe entrance of the selectivity filter and act as a cork in a bottle to stop ion permeation. (D) Docking simulation ofa capped SWNT showing that, because of its spherical end, it can fit into the selectivity filter like a fullerene. Thefigure shows a SWNT with an average diameter of 0.9 nm, similar to those used in this report. (E) and (F),docking simulation of an open-ended SWNT. An open-ended nanotube can sit on top of the selectivity filterestablishing weak bonds. Simulations of SWNTs with average diameters of 0.9 and 1.3 nm are shown in panels(E) and (F), respectively (reproduced with permission from [896]).

Fig. 8.16. (A) Scanning electron micrograph (SEM) of 60-nm diameter silica nanotubes. The thickness of thealumina template membrane used was 350 nm, which determines the nanotube length. (B) SEM of the surface ofa typical alumina template membrane (reproduced with permission from [256]).


Fig. 8.17. Photographs of vials containing cyclohexane (upper) and water (lower) under UV light excitation afteraddition of 10 mg of nanotubes with (A) dansylamide on inner and C18 on outer surfaces and (B)quinineurethane on inner and no silane on outer surfaces; (C) 10 mg of both A and B nanotubes; 200-nmdiameter nanotubes were used (reproduced with permission from [256]).


function of size, was studied by transporting lysozome, bovine serum albumine, and beta-lactoglobuline [898]. Environment dependant transport of hydrophobic and hydrophilicchemicals was observed using different thiol functionalization with in the nanotubes[899]. Excellent molecular-size based selectivity, down to 10�11 M, was reported by detect-ing the trans-membrane current and potential to detect the chemicals [900]. By function-alizing with cysteine, a pH dependant ion transport of the Au nanotubular membraneswas found. While at low pH a preferential anion transport was observed, at a pH of6.0, near the iso-electric point no selectivity was observed [901]. This electrostatic selectiv-ity was attributed to the protonation of amino and carboxylic groups of the cysteine. Sim-ilar applications in this field from inorganic metals or metal oxides are being explored toimprove the properties of those achieved with other biological or natural systems.

From the present trend of ODNS synthesis and applications in various cross disciplines,we believe that it has the potential to offer important technological and scientific advance-ments in biomedical field. This interdisciplinary faculty is in early stages of developmentand considerable efforts have to be put in by various researchers across the globe to bringout the full potential of their atypical properties. The device fabrication from such nano-tube arrays for use in bio-applications is one such field that needs a lot of labor and dili-gence. As inherent with any nanostructures, issues related to their toxicity needs to beclearly addressed. Polymeric/biological nanostructures which are a mimic of naturallyoccurring structures could be one of the alternatives to the growing edginess over the tox-icity of nanomaterials.

9. ODNS nanocomposites

9.1. Overview

One dimensional nanostructures (ODNS) have taken the attention of both researchesand business leaders within the polymer community. Although a range of different 1D


nanostructures are synthesized for last two decades, CNT-based polymer composites havegotten the maximum attention. Because of the unprecedented combination of mechanical,electrical and thermal properties, CNTs opened up a new avenue for commodity plastics,elastomers, adhesives and coating [902]. The main reason behind the CNT-based nano-composites is their superior mechanical performance relative to the micron-size counter-parts. Although, significant amount of research has been carried in the nanocompositesand related fields, few issues are still to be answered [903]: such as (a) lack of adequate evi-dences on wetting of nanostructured materials by polymer, (b) issues regarding the stresstransfer to the nanostructured materials, (c) information regarding the interfacial strengthand adhesion at nanoscale and (d) molecular mechanism on nanotube–polymer adhesion.This part of the article overviews the results on mechanical properties of ODNS compos-ites, specifically CNT reinforced composites.

9.2. CNT-reinforced composite

9.2.1. Nonaligned CNTs in composites

During early stages of development of CNT-based nanocomposites, CNTs were usedrandomly by mixing with polymers [801,904]. Qian et al., prepared [905] MWCNTs-poly-styrene nanocomposites using solution evaporation method assisted by high-energy soni-cation. First, polystyrene was dissolved in toluene at a mass ratio of 10:1 using high energyultrasonication and followed by mixing with the MWCNTs dispersed toluene in sonicator.The solvent was evaporated out to prepare 0.4 mm thick samples. The mechanical prop-erties of such composites are tabulated in Table 9.1. It is worth mention that the lengthand the diameter of ODNS play an important role in the overall performance of the nano-composites. Hence, a keen observation to the attributes of the reinforcement is a must,while comparing the results from various groups.

Recently, Koerner et al. reported [906] that addition of small amount of (0.5–10 vol%)of MWCNTs to thermoplastic elastomer morthane produced polymer nanocomposites.

Table 9.1Mechanical properties of few nanocomposites

Composites Loading of carbonnanomaterials

Composites tensile properties References

Strength Modulus

Polystyrenea and MWCNTs (1 wt%) 1 wt% 16 ± 2 MPa 1620 ± 130 MPa [905]Polystyrenea and MWCNTs (1 wt%) 1 wt% 16 ± 2 MPa 1620 ± 130 MPa [905]Polypropyleneb and carbon fiber �28 MPa – [907]PAN and MWCNTs 1.8 vol% 31% increase 36% increase [908]Polystyrene and MWCNTs 0.1% 24.4 MPa 2.47 GPa [909]Polystyrene and MWCNTs 0.5% 17.9 MPa 2.01 GPa [909]Polystyrene and MWCNTs 1.0% 21.6 MPa 2.35 GPa [909]Polystyrene and MWCNTs 2.0% 18.8 MPa 2.18 GPa [909]PVA and SWCNTs 60 wt% 1.8 GPa 80 GPa [910]UHMWPE and MWCNTs 1 wt% 25% increase 25% increase [911]PMMA and carbon fiber 5 wt% 200% increase 50% increase [912]

a Tensile strength of polystyrene: 12.8 ± 1 MPa and modulus: 1190 ± 130 MPa.b Tensile strength of polypropylene 26 MPa and modulus: 1250 MPa.


These novel structures were characterized with high electrical conductivity (r � 1–10 S cm�1), low electrical percolation (/ � 0.005) and enhancement of mechanical proper-ties including increased modulus and yield stress. The ability to stretch elastomer above1000% before rupture is another added advantage. Such nanocomposites were preparedby mixing the short and light grinding CNTs with small amount of polymers in tetrahy-drofuran. After keeping several hours under stirring, the mixture was transferred to moldsto allow slow solvent evaporation followed by drying in vacuum oven at 50 �C until a con-stant weight is achieved [906].

Silica-coated MWCNTs reinforced PMMA nanocomposites were found to haveenhanced mechanical properties than the composites with bare MWCNT and PMMA[913]. Such nanocomposites with 4% of CNT-coated with silica were calculated to havea maximum hardness of 120 ± 20 MPa and Young’s modulus of 9 ± 1 GPa, measuredby nanoindentation technique [913].

9.2.2. Aligned CNTs in composites

Aligning CNTs to form nanofiber composites combining the benefits and several oftheir merits was a real challenge until last few years [914]. Among the several approaches,the electrospinning technique was recently used to incorporate CNTs in a polymericmatrix [914–918]. As described in the discussion on CNTs, CNTs possess better mechan-ical, electrical, electronic and other characteristics compared to any other material. As forexample, SWCNTs have a modulus as high as TPa and tensile strength up to GPa. Thespinning process is expected to align CNTs or their bundles along the fiber directionsdue to combination of dielectrophoretic forces caused by dielectric or conductivity mis-match between CNTs and polymer solutions and high shear forces induced by spinning[917]. Both SWCNTs [914,915,918] and MWCNTs [919] were used to develop such nano-composites. In few early approaches [914,918], dispersed CNTs in polymer solutions like,polyacrylonitrile, polyimide were electrospun in order to achieve nanofibers. In otherapproaches [915,919], CNTs were dispersed in water using amphiphiles. For MWCNT-poly(ethylene oxide) (PEO) nanocomposites [919], small molecules like sodium dodecylsulfate (SDS), large molecular weight highly branched polymer like gum arabic were usedas the amphiphiles, where as for SWCNTs [915], alternating copolymer of styrene andsodium maleate were used. For conductive CNT-based composites [916], CNTs wereincorporated through electrospinning technique into poly(vinylidene fluoride) (PVDF)in dimethyl-formamide (DMF) solutions. Fig. 9.1 shows of such electrospun fibers withdifferent concentrations (wt%) of CNTs. But none of these above studies reported anymechanical property of the fibers.

Continuum mechanics approach has been used to characterize the CNTs as well asnanocomposites for last few years [920–923]. Applying traditional textile-mechanicsapproach and anisotropic elasticity theory [920], the stress distribution and effective elasticproperties of aligned CNT composite in which fibers are layered cylinders with arrays ofCNTs arranged to form a hexagonal cross-section is studied. Finite element method(FEM) and boundary element methods (BEM) [924,925] were also used to understandthe mechanical behavior of CNT nanocomposites in recent times. It has also been revealed[921] that the stress gradient across the interface of CNT and matrix is very high and thenumber of elements can become prohibitively large for FEM in large-scale modeling ofCNT nanocomposites if the continuum models can be used. Recently, a fast multipoleboundary element method [921] was applied in the modeling of CNT-reinforced nanocom-

Fig. 9.1. Scanning electron micrograph (SEM) of electrospun fibers from 20% PVDF/DMF solutions withdifferent CNTs concentrations (a) and (b) 0 wt%, (c) 0.002 wt%, (d) 0.0025 wt%, (e) 0.004 wt% and (f) 0.005 wt%(reproduced with permission from [916]).


posites. In a recent study [909], molecular mechanics and elasticity calculations were usedto quantify some of the interfacial characteristics that critically control the performance ofa composite material. They also reported [909] that in absence of chemical bondingbetween CNTs and matrix, a few non-bond interactions, such as, electrostatic and vander Waals forces, exist. Such non-bond interactions resulted in CNT-polymer interfacialshear stresses about 138 and 186 MPa at 0 K for CNT-epoxy and CNT-polystyrene com-posites, respectively.


The addition of minor amounts of one dimensional filler to a polymer matrix hasattracted a significant amount of attention. A high aspect ratio of nanostructured materi-als and its extraordinary mechanical properties provide the best reinforcement to thenanocomposites for the several applications requiring extremely light weight but highlyelastic and very strong materials.

10. Toxicity of nanomaterials

10.1. Introduction

As described in the previous sections, nanoscience and technology can dramaticallychange the properties and applications of the current industrial and research materials.Such extraordinary physicochemical properties bring along a concern about the adverseeffects of nanomaterials on biological systems. For example, carbon nanotubes, CNT,have been shown to be selectively separated by DNA. This is an exceptional finding fromthe scientific point of view for selective separation of CNT and designing a biosensor [926]based on the selectivity. However it may be instantaneously evident that CNT have thepotential to interact with the DNA which can lead to undesirable results when presentinside a living human body. Proliferation of nanotechnology at an alarming rate couldpresent new challenges to the environment by producing a range of materials that couldpotentially be toxic. There are several collaborative works pursued internationally dedi-cated specifically towards exploring the unknowns of nanotechnology. The results havebeen mixed and outcry of banning the nanotechnology has so far been merely a concernthan threat. Current discussion is aimed at providing an insight into the toxicity of nanom-aterials with few results from the present studies. An approach has been discussed on thebasis of current research trends. Due to limited amount of data for toxicity of ODNS, thissection is generalized to all nanomaterials with a mention of ODNS toxicity whereverapplicable.

10.2. Classification

Human beings are exposed to airborne nanosized particles throughout their life.Whether natural or industrial, the exposure to nanomaterials has been consistent buthas raised manifolds with the advent and proliferation of industrial revolution. Thus itis necessary to classify the materials in order to treat them exclusively. Nanosized particlesare variably called ultra-fine particles by toxicologists {US Environmental ProtectionAgency}, Aitken mode and nucleation mode particles by atmospheric scientists {Kulmala2004; National Research Council (NRC) 1983} and engineered nano-structured materialsby the material scientists {National Nanotechnology Initiative 2004} [927].

Nanoparticles can originate from several sources and are listed in Table 10.1 as inten-tional and unintentional sources [927]. Researchers have classified these particles in threecategories namely (a) nanosized particle (NSP) which includes all engineered and ambientnanosized spherical particles <100 nm, (b) engineered nanoparticles (NP) which includeonly spherical nanoparticles engineered in the laboratory, and (c) ultra-fine particles(UFP) which include ambient and laboratory generated NSPs that are not produced ina controlled, engineered way. In addition to these spherical nanoparticles are the particles

Table 10.1UFPs/NPs (<100 nm), natural and anthropogenic sources [927]

Natural Anthropogenic

Unintentional Intentional (NPs)

• Gas to particle conversions• Forest fires• Volcanoes (hot lava)• Viruses• Biogenic magnetite: magnetotactic bacte-

ria proctoctists, mollusks, arthropods,fish, birds human brain, meteorite

• Ferritin (12.5 nm)• Microparticles (<100 nm; activated cells)

• Internalcombustionengines

• Power plants• Incinerators• Jet engines• Metal fumes

(smelting,welding etc)

• Polymer fumes• Other fumes• Heated surfaces• Frying broiling,

grilling• Electric motors

• Controlled size and shape designed forfunctionality

• Metals, semiconductors, metal oxides,carbon, polymers

• Nano-spheres, -wires, -needles, -tubes,-shells, -rings, -platelets, -belts, -spirals

• Untreated coated (nanotechnologyapplied to many products; cosmetics,medical, fabrics, electronics, optics,displays, etc.)


with various shapes and morphology like rods, tubes, fibers, rings and wires which can beclassified separately as their behavior is a little different from the spherical particles.

10.3. Routes of entering human cycle

Toxicology studies are relevant if there is a way in which the NSPs can enter the humanbody system. Airborne inhalation and entry through the respiratory tract seems to be themost likely route of exposure to nanoparticles. It was shown by Maynard et al. [928] in amodel workplace that a very low concentration of <50 lg/m3 can enter the human bodyupon significant exposures to airborne single engineered carbon nanotubes or C60 fulle-renes. Exposure via other routes is not much studied in detail and is less plausible unlessit is by direct ingestion through food or drug delivery, dermal contact through applicationof oils, skin creams or as a contaminant in water through nanoparticles treated membranesystem.

10.3.1. Respiratory

Inhalation of the airborne nanoparticles through the respiratory tract is the most com-mon way to enter the human cycle. Although inhalation of engineered nanoparticles seemsto be less likely a route of entering the human body in laboratory or industrial measuresbecause of the safety procedures followed but is still plausible. However degradation ofengineered nanomaterials that are available commercially can be one such route. It wasshown by various studies that the inhaled nanoparticles are efficiently deposited by diffu-sional mechanisms in all regions of the lungs. Hoet et al. [929] summarized that mostnanosized spherical solid materials easily enter the lungs. In a preliminary study by twoindependent groups [930,931] pulmonary effects of single walled CNTs were demonstrated.


It was concluded that CNTs can reach the lungs and can be even more dangerous thancarbon black or quartz. Recently Zhang et al. [932] compared the deposition of nano-and micro-sized particle in human upper airway system. It was proposed as with micro-size particles, deposition of nanoparticles occurs at greater concentration around thecarinal ridges when compared to the straight segments in the bronchial airways; however,deposition distributions are much more uniform along the airway branches. The deposi-tion enhancement factors vary with bifurcation, particle size, and inhalation flow rate.Specifically, the local deposition is more uniformly distributed for relatively large-sizenanoparticles (average diameter 100 nm) than for small-size nanoparticles (average diam-eter 1 nm). The authors hypothesized that larger uniform distribution of nanoparticles canlead to greater toxicity as it presents a greater area to react with the cell membranes andgreater ability to absorb and transport toxic substances. Once deposited, NSPs in contrastto larger sized particles can translocate easily to reach other target organs by varioustransfer routes and mechanisms.

It has also been demonstrated that NSPs are not cleared effectively by alveolar macro-phages. Fig. 10.1 displays the results of several studies of exposure of rats to different sizedparticles of polystyrene beads [927]. It was found that as opposed to 80% retention of themicron sized particle in the macrophages only 20% of NSPs were retrieved by the macro-

Fig. 10.1. In vivo retention of inhaled nanosized and larger particles in alveolar macrophages (A) and inexhaustively lavaged lungs (epithelial and interstitial retention); (B) 24 h post-exposure. The alveolar macrophageis the most important defense mechanism in the alveolar region for fine and coarse particles, yet inhaled singletNSPs are not efficiently phagocytized by alveolar macrophages [927].


phages. The more the tendency of the particles to stay in pulmonary region the more arethe chances of increased toxicity to the specified or target organs via translocation.

It was reported earlier that long fibers will not be effectively cleared from the respiratorytract thereby increasing their bio-persistence [929]. It is known that in the alveoli the rateat which fibers are physically cleared depends on the ability of alveolar macrophages tophagocytose them. Thus fibers having a diameter that is longer than the macrophagesdiameter will not be effectively cleared from the lungs. Stanton et al. [933,934] carried acareful study of glass fibers and reported for mesothelioma induction in rats, the peakactivity of fibers was greatest for fibers longer than 8 lm and less than 1.5 lm in diameter.The hypothesis commonly known as the Stanton hypothesis though valid for micro sizeparticles; can be very much applicable on the aspect ratios of nanofibers also. The bio-durability of fibers less than 100 nm is not expected to be largely different from theirmicron long counter parts especially in case of CNT and polymeric fibers where thelengths generally run in micron scale. It has been pointed out by various studies that CNTshave shown signs of toxicity [928,930,931,935–937]. The aspect ratio plays an importantrole atleast in case of CNT. It has been shown [936] that MWCNTs induce more stressthan Multi-Wall Carbon Nano Onions (MWCNOs). It was suggested that there is a qual-itative difference in response to low dose and high dose. While CNT at high dose produceinnate immune response, carbon nano-onions do not. Thus in general it can be summa-rized that fibrous nanomaterials may be more toxic as compared to their spherical coun-terparts. However, this does not mean that all the ODNS with high aspect ratio will betoxic and those with low aspect ratio will be non-toxic.

10.3.2. Intestinal

NSPs cleared from the respiratory tract can enter the gastrointestinal tract (GI). Theycan also be ingested directly, if present in food or water systems or from drug delivery sys-tems. There is considerable literature on the intestinal uptake of materials but the cellularuptake of nanomaterials is less studied and most of them have shown that NSP’s passthrough GI tract and are eliminated rapidly.

10.3.3. Dermal

Skin is an important barrier structure in three layers of epidermis, dermis and the sub-cutaneous layer. Dermal exposure is another important uptake source for NSPs especiallybecause of the increased interest in the use of TiO2 and other nanoparticles for protectionagainst UV in various dermal creams and lotions. The main types of particulates mostcommonly studied for dermal exposure are liposomes, solid poorly soluble TiO2, polymerparticulates and submicron emulsion particles. Cracks or cuts in skins can be a site ofentry from the surface of a skin. Tinkle et al. [938] have proposed the entry of NSP’sthrough the unbroken skin during the flexing of the wrist. They have also demonstratedthat flexing of the skin can lead to uptake of micron long fluorescent beads. TiO2 particles(5–20 nm) have been argued to penetrate into the skin cells and interfere with the immunesystem. Gurr et al. [939] reported that anatase TiO2 nanoparticles approximately (10 and20 nm) induced oxidative DNA damage, lipid peroxidation, and micronuclei formation,and increased hydrogen peroxide and nitric oxide production in BEAS-2B cells, a humanbronchial epithelial cell line in the absence of photo activation. However, the treatment


with anatase particles (P200 nm) did not induce oxidative stress in the absence of lightirradiation. Thus it is easy to hypothesize in present case that the shorter the size of thenanoparticles easier it becomes for the NSPs to permeate through skin and induce dam-age. It was reported that contrary to popular belief, the photocatalytic activity of the ana-tase is higher than that of the rutile, that 200 nm sized rutile titania can induce H2O2 andoxidative DNA damage in the absence of light while the anatase titania did not catalyzethe reaction. Hostynek et al. [940] stated that the uptake of materials via the skin is a com-plex process and there are both exogenous and endogenous factors involved.

10.4. Factors controlling the toxicity of nanomaterials

NSPs can penetrate inside the human body via various routes and as discussed abovecould persist in the system because of the incapability of the macrophages to phagocytosethem. Whether these persisting nanomaterials react with the body, stay inert or react withthe system will govern their toxic properties.

10.4.1. Surface area

It plays a major role in deciding the interaction of the materials with the humanbody. One must appreciate by now that nanomaterials have high surface area to volumeratio and thus are more reactive than their coarse counter parts. Thus surface area ismore important and appropriate for expressing the response of NSPs to human bodyor environment. One such example was presented by Oberdorster et al. [941] in a preli-minary study where pulmonary inflammatory response to anatase and rutile TiO2 wascharacterized. When the dose to neutrophil response was expressed as a function ofmass, the two forms of titania showed entirely different response [Fig. 10.2(A) and(C)]. While when expressed as a function of surface area the response was same for boththe particles [Fig. 10.2(B) and (D)]. It can thus be assumed safely that particle surfacearea for particles of different sizes but same chemistry is a better dosemetric than particlemass or number.

10.4.2. Surface chemistryIt seems that the role of surface chemistry had been underemphasized in the present

research of nanotoxicity. Whether the particle remains suspended as individual particleor as an aggregate depends on its surface chemistry. A small aggregate or single particleis presumed to be more toxic than an aggregate of NSPs as the relative surface area pre-sented would be reduced due to agglomeration or aggregation. A thorough study of thesurface characteristics could reveal the possible behavior of the material inside the humanbody. For e.g. whether the material has good wetting characteristic, has surface area thatcatalyses specific chemical reactions or remain passive and allow fibrous tissue to grow onits surface. It was shown in a study [942] that the rats when treated to polymeric vapors ofPTFE having a diameter of 18 nm were highly toxic, causing severe lung injury with highmortality rate of within 4 h after a 15 min inhalation exposure to 50 lg/m3. It was alsoshown in a separate study [943] that the gas phase alone was not the acutely toxic and thataging of PTFE fume particles for 3 min increased their size to >100 nm due to agglomer-

Fig. 10.2. Percentage of neutrophils in lung lavage of rats (A, B) and mice (C, D) as indicators of inflammation24 h after intratracheal instillation of different mass doses of 20-nm and 250-nm TiO2 particles in rats and mice.(A, C) The steeper dose response of nanosized TiO2 is obvious when the dose is expressed as mass. (B, D) Thesame dose response relationship as in (A, C) but with dose expressed as particle surface area; this indicates thatparticle surface area seems to be a more appropriate dosemetric for comparing effects of different-sized particles,provided they are of the same chemical structure (anatase TiO2 in this case). Data show mean ± SD (reproducedwith permission from [927]).


ation, which resulted in loss of toxicity. These findings emphasize the importance of sur-face chemistry in deciding a fate of the NSPs.

10.4.3. Oxidative stress and reactive oxygen species

It is established that there is a direct correlation between the ROS generating capability,the surface area of NSPs and the inflammatory response in the lung [927,944–947]. Nelet al. [947] summarized from various resources [927,944–948] that ROS generation isthe best-developed paradigm for the explanation of toxic effects of inhaled nanoparticles.As opposed to normal coupling conditions in mitochondrion (low ROS generation), underexcess ROS generation due to nanoparticles exposure the natural antioxidant defensemechanisms may be inundated [949]. The role of the surface groups is paramount tothe behavior of nanomaterials in vivo. Surface groups as pointed out earlier can makethe material hydrophilic, hydrophobic, lipophillic or lipophobic or catalytically active orpassive. An illustration of the various possibilities of reaction of surface groups with

Fig. 10.3. Possible mechanisms by which nanomaterials interact with biological tissue. Examples illustrate theimportance of material composition, electronic structure, bonded surface species (e.g., metal-containing), surfacecoatings (active or passive), and solubility, including the contribution of surface species and coatings andinteractions with other environmental factors (e.g., UV activation) (reproduced with permission from [947]).


biological tissue is presented in Fig. 10.3 [947]. The figure emphasizes on the importance ofvarious factors contributing to the interaction with biological system. Oxidative stress dueto exposure to NSPs of TiO2, quartz, carbon black, etc. results in airway inflammation andinterstitial fibrosis [927,944–947,950]. Oxidative stress is considered to be a potential ele-ment for screening the toxicity of NSP’s. Responses at each level of oxidative stress suchas large antioxidant enzyme expression or cytokines in the lungs of animals have beenincorporated as screening assays in vivo [947].

Though the above discussion raises the concerns about the ROS generating capabilitiesof nanomaterials our research group is working on ceria nanoparticles/rods which haveshown exceptional radical scavenging properties and promote cell longevity owing to theirpowerful antioxidant properties [32]. The radical scavenging properties of ceria are pres-ently being explored for developing a radical sensor. Thus a careful consideration of allthe properties and parameters must be made before a statement is passed for possiblebehavior of nanomaterials. In our belief all the NSP’s presently used should be categorizedin groups based on their reactivity and properties and a model should be developed forstudying the toxicity.


10.5. Future research and trends

No clinically relevant toxicity has yet been reported for NSP’s but there is a growingconcern among the researchers worldwide about the possible outcomes of the ill effects(if any). A definitive pathological mechanism applicable to particular system is yet to bedetermined and accepted internationally. Most of the data have reported the toxicity interms of the blockage of airways leading to asphyxiation from unrealistic high dosageof materials. It has been suggested that the granulomatous inflammation observed duringthe bio-persistence of ODNS particle could be a similar affect to asbestos fibers and themetal impurities present could account for the toxicity [930,947]. There are issues that needto be addressed as to whether there could be substantial uptake of nanomaterials from theworkplace including both laboratory and the industrial scale. There are several questionsthat are unanswered and a proper strategy to evaluate the toxic effect of nanomaterialsthus needs to be formulated. The scope of funded research on nanomaterials is increasingand there are efforts made towards screening the toxicity of materials also at the researchfront [951]. A list of useful organizations and websites directly or indirectly involved inresearch or funding of nanotechnology are listed in Appendix A. National Nanotechnol-ogy Initiative (NNI), a US Federal research and development program for nanotechnol-ogy, has a section devoted to identify the potential exposure, possible toxicity and need forpersonal protective equipment when working with nanoscale materials. Several other USagencies (NSET (The Nanoscale Science and Engineering and Technology), NEHI (Nano-technology Environment and Health Implications), EPA (Environmental ProtectionAgency), NIOSH (National Institute for Occupational Safety and Health), etc. are work-ing to identify support and monitor the impact of nanotechnology on environment andhealth. The National Toxicology Program (NTP) funds research on the toxicity ofnanomaterials.

All these effort are in a right direction to critically analyze the possible toxicologicaloutcomes of nanoscale materials. An entire science discipline is emerging out of the effortsof various groups actively involved and equally concerned about the behavior of nanom-aterials when in contact with human body. A protocol needs to be developed to concate-nate all the research and work internationally at a scale of highest magnitude. A very goodexample of systematic approach towards identifying the possible effects of nanotoxicitycan be taken from a recent article on relative risk analysis of manufactured nanomaterialsfrom an insurance industry point of view [952]. A systematic approach was adopted to firstidentify the processes currently in industrial use and a risk score was determined based onthe toxicity, water solubility, flammability and emissions. A similar systematic approachtowards identification and proliferation of nanotechnology is needed when scrutinizedfor toxicity. A model based on our understanding of the need for monitoring the toxicitybehavior is presented as a flow-chart in Fig. 10.4.

It is indeed true that sufficient information is not available about the toxicity of nanom-aterials for developing a full life cycle analysis. The preliminary information has somemodest and alarming concerns about the possible toxic effects. The mechanism beingdependent on too many parameters; is hard to visualize and characterize. There arechances of discovering entirely new routes of interaction of materials with the humanbody. The aim of nanotechnology is to provide better and technologically feasible solu-tions to mankind but not at the cost of human lives. Chances cannot be taken by refuting

Category based on surface properties and

reactions

Nanoparticles

Category based on shape,size and metallic, ceramic or

polymeric or composite

Category based on industrial or

Laboratory frame

Identification of systems showing signs of toxicityin actual conditions

Category based on applications of nanomaterials

Mechanism of toxicityin actual dosages

Rating for severity

of toxicity

Curing Protocolsin case of

Emergency

Prevention and awarenessmodules for Laboratory and

industrial scale

Industrial Environment

Categorize

Model

Identification

Details

Laboratory Environment Industrial products failure/exposure

Models for actual uptake of materials via various

routes in a practical environment

Prevention

Measures

Fig. 10.4. Possible protocol flow chart for avoiding the toxic effects at various scales.


the preliminary findings as too diminutive to threat as human lives will be at the receivingend. Collaborative and exhaustive efforts are the key to achieve a significant breakthroughalong with well designed and meticulous plan of action.

11. Summary, challenges and future scope

Bio-technology, information technology and nanotechnology can be termed as thegreatest inventions of this century. However, the inherent dependence of the other twotechnologies on the nanoscience/technology has made it much more significant acrossthe globe. Among the various branches in the nanotechnology, one dimensional nano-structures (ODNS) have paved the way for numerous advances in both fundamentaland applied sciences. Nanowires, nanotubes, nanofibers etc; with at least one dimensionin nanometer size, fall under the category of ODNS. Almost all classes of materials i.e.,metals, semiconductors, ceramics and organic materials have been used to produceODNS. However, carbon nanotubes have occupied the most significant place and are


the most widely studied ODNS. Optical, electronic and thermo photovoltaic applicationsare a few to name among the numerous applications attracted by the unique physical andchemical properties of ODNS. Starting with the economic wet chemical synthesis to themost sophisticated and controlled atomic layer deposition techniques have been used bythe scientists for synthesis of ODNS. Various laboratories and research initiativesthroughout the globe have been established to exclusively study the fundamental aspectsof the ODNS. Researchers have made a significant progress in studying the growth kinet-ics using theoretical methods and correlated with the experiments. Industries have comeup with special equipments to grow ODNS.

In spite of the significant amount of research being carried there is still a long way torealize the use of ODNS in large scale practical applications. Although the preliminaryexperiments at the laboratory level are successful, scaling up the production and retain-ing the properties is not trivial. Evaluation of the properties of a single ODNS is often abottleneck in obtaining a real estimation of the material properties. In the nano-bio-research, functionalizing the CNTs and other nanotube membranes with proper activatedtips that can selectively allow the ions to pass through the channel is a major challenge.Rapid shrinking in the size of electronic devices, increased demands for efficient materialshave brought physicists, chemists, material scientists and the biologists together inachieving these challenging tasks. As expressed by various researchers, methods to cir-cumvent the problems are gaining more importance. Much more efficient bottom-upapproaches have to be investigated in order to realize the excellent feature of the mate-rials. Need for applications like selective sensors, catalysts, alternative energy conceptsand the biological research initiatives have been the ambassadors across the globe andwill bring the scientific community more closer in near future through the field of ODNS.While the aforementioned are the technological challenges, a completely different per-spective is gaining much more attention in view of the possible ill effects from the nano-structured materials to the environment and the people. Keeping this in mind, a briefdiscussion of the toxicity effects are covered in the previous section. While there are arti-cles that dealt with high dosage and reported the harmful affects, our experience withceria nanoparticles is contrasting and proves their biocompatible nature. Hence, cautionshould be exercised before making any conclusions, other than the obvious features suchas pyrophoric nature.

Acknowledgements

Prof. Seal’s nanotechnology research is supported by National Science Foundation,Office of Naval Research Young Investigator Award (ONR-YIP), ONR-DURIP (DefenseUniversity Research Incentive Program), National Institute of Health, Missile DefenseAgency, NASA, Florida Space Grant, State of Florida, Siemens Westinghouse, LockheedMartin, Materials Interface Inc., MD Andersen Cancer Center, DOE, NASA SBIR Ph Iand II. Authors would like to acknowledge Dr. Jeff De Hosson, Dr. T.B. Massalski andDr. Carl C. Koch for their valuable comments and suggestions on the article. Authorswould also like to acknowledge their collaborators and co-researchers.

Appendix A

A.1. Global funding status in nanotechnology

Year 1997 $M 1998 $M 1999 $M 2000 $M 2001 $M 2002 $M 2003 $M 2004 $M 2005 $M

W Europe 126 151 179 200 225 400 650 950 1050Japan 120 135 157 245 465 720 800 900 950USA 116 190 255 270 465 697 863 989 1083Others 70 83 96 110 380 550 800 900 1000

Total 432 559 687 825 1535 2367 3113 3739 4083100% 129% 159% 191% 355% 547% 720% 866% 945%

894S

.V.N

.T.

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terials

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A.2. Funding strategy in USA towards nanotechnology from various organizations

Year 2000 $M 2001 $M 2002 $M 2003 $M 2004 $M 2005 $M 2006 $M

NSF 97 150 204 221 256 338 344DOD 70 125 224 322 291 257 230DOE 58 88 89 134 202 210 207NIH 32 40 59 78 106 142 144NIST 8 33 77 64 77 75 75NASA 5 22 35 36 47 45 32NIOSH 3 3EPA 5 6 5 5 5 5TSA 2 1 1 1 1USDA 1 2 3 11DOJ 1 1 1 2 2 2

Total 270 464 697 863 989 1083 1054

A.3. List of organizations and useful websites related to nanotoxicity

Air Force Multi-disciplinary University Research Initiative (MURI)Center for Biological and Environmental Nanotechnology (CBEN) at RICE University(http://cben.rice.edu/index.cfm)Center for Environmental and Human Toxicology at the University of Florida (http://www.floridatox.org)Environmental Protection Agency (EPA) (http://www.epa.gov/)Food and Drug Administration (FDA)Health and Environmental Sciences Institute (HESI) (http://www.hesiglobal.org/)http://es.epa.gov/ncer/rfa/2004/2004_manufactured_nano.htmlhttp://foundry.lbl.gov/http://icon.rice.edu/research.cfmhttp://www.azonano.com/news_old.asphttp://www.cdc.gov/niosh/topics/nanotech/nano_exchange.htmlhttp://www.nanotoxicology.ufl.edu/Links.htmlhttp://www.nist.gov/public_affairs/nanotech.htmhttp://www2.envmed.rochester.edu/envmed/tox/faculty/oberdoerster.htmlInternational Council on Nanotechnology (ICON) (http://cohesion.rice.edu/centersan-dinst/cben/industry.cfm?doc_id=5023)International Life Sciences Institute (ILSI)International Life Sciences Institute’s Risk Science Institute (ILSI RSI)National Nanotechnology Initiative (NNI) (http://www.nano.gov/)Nanoscale Science and Engineering Centers (NSEC) (http://www.nsf.gov/crssprgm/nano/info/centers.jsp)Nanoscale Science and Engineering Technology (NSET)Nanotechnology Characterization Laboratory (http://ncl.cancer.gov/)

100% 172% 258% 320% 366% 400% 390%

http://cben.rice.edu/index.cfm

http://www.floridatox.org

http://www.floridatox.org

http://www.epa.gov/

http://www.hesiglobal.org/

http://es.epa.gov/ncer/rfa/2004/2004_manufactured_nano.html

http://foundry.lbl.gov/

http://icon.rice.edu/research.cfm

http://www.azonano.com/news_old.asp

http://www.cdc.gov/niosh/topics/nanotech/nano_exchange.html

http://www.nanotoxicology.ufl.edu/Links.html

http://www.nist.gov/public_affairs/nanotech.htm

http://www2.envmed.rochester.edu/envmed/tox/faculty/oberdoerster.html

http://cohesion.rice.edu/centersandinst/cben/industry.cfm?doc_id=5023

http://cohesion.rice.edu/centersandinst/cben/industry.cfm?doc_id=5023

http://www.nano.gov/

http://www.nsf.gov/crssprgm/nano/info/centers.jsp

http://www.nsf.gov/crssprgm/nano/info/centers.jsp

http://ncl.cancer.gov/


Nanotechnology Environment and Health Implications (NEHI)National Institute for Occupational Safety and Health (NIOSH)National Institute of Standards and Technology (http://ncl.cancer.gov/)National Institutes of Health (http://www.nih.gov/)National Science Foundation (NSF) (http://www.nsf.gov/funding/pgm_list.jsp?type=xcut)National Toxicology Program (NTP) (www.niehs.nih.gov/oc/factsheets/nano.htm)

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http://ncl.cancer.gov/

http://www.nih.gov/

http://www.nsf.gov/funding/pgm_list.jsp?type=xcut

http://www.nsf.gov/funding/pgm_list.jsp?type=xcut

http://www.niehs.nih.gov/oc/factsheets/nano.htm


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