Preparation of the Article

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JOURNAL OFNANO- AND ELECTRONICPHYSICS -

[email protected]! Reference source not found.Sumy State University

X-ray peak broadening analysis and optical studies of ZnO nanoparticles derived by Surfac-

tant assisted combustion synthesis

V. Sesha Sai Kumar, K. Venkateswara Rao

Centre for Nano Science and Technology, I.S.T, Jawaharlal Nehru Technological University Hyderabad,

Hyderabad-500 085, Andhra Pradesh, India.

In this paper, synthesis of ZnO nanoparticles is done by a simple and facile surfactant assisted combustion synthesis.The synthesis of ZnO nanoparticles has been prepared using Zinc nitrate as a precursor material, glycine as a fuel with the sup-port of non-ionic surfactant TWEEN 80. The obtained ZnO nanoparticles have been studied using characterization techniques likeX-ray diffraction (XRD), Transmission Electron Microscopy (TEM), and UV-Vis Spectroscopy. XRD results reveal that the sampleis crystalline with a hexagonal wurtzite phase. X-ray peak broadening analysis was used to evaluate the crystallite sizes and lat-tice strain by the Williamson-Hall (W-H) analysis. Further appropriate physical parameters such as strain, stress, and energydensity values were also calculated using W-H analysis with different models, viz, uniform deformation model, uniform deforma-tion stress model and uniform deformation energy density model. Transmission electron microscopy (TEM) result reveals that theZnO nanoparticles sample is spherical in shape showing particle sizes less than 40 nm. The optical properties of ZnO nanopar-ticles were studied by UV-Vis spectroscopy.

Keywords: Surfactant Assisted Combustion Synthesis, X-ray diffraction (XRD), UV-Vis Spectroscopy, Transmission ElectronMicroscope (TEM).

DOI: PACS number(s): 81.07.Bc, 81.20.Ka,61.05.C,68.37.Lp.

(Please, input Your PACS and delete this hint)

1. INTRODUCTIONZinc oxide (ZnO) has recently paying attention due

to its various applications such as UV light emittingdiodes, varistors, gas sensors and catalysts. Zinc oxide(ZnO) is an n-type metal oxide semiconductor with awide band-gap energy (~3.36 eV) and large excitonbinding energy (~60meV) at room temperature [1-4].Therefore, many methods, including solgel [5], hydro-

thermal [6], precipitation [7], sonochemical [8], andothers, have been developed to prepare ZnO nanostruc-tures and ceramics. ZnO is the richest family of nano-structures among all semiconducting materials, both instructures and in properties due to its unique proper-ties.

A perfect crystal would extend in all directions toinfinity, so no crystals are perfect due to their finitesize. This deviation from perfect crystallinity leads to abroadening of the diffraction peaks. The two mainproperties extracted from peak width analysis are thecrystallite size and lattice strain. Crystallite size is ameasure of the size of coherently diffracting domain.The crystallite size of the particles is not generally thesame as the particle size due to the presence of poly-

crystalline aggregates [9]. The most common tech-niques used for the measurement of particle size, ra-ther than the crystallite size, are the Brunauer EmmettTeller (BET), light (laser) scattering experiment, scan-ning electron microscopy (SEM) and TEM analysis.Lattice strain is a measure of the distribution of latticeconstants arising from crystal imperfections, such aslattice dislocation. The other sources of strain are thegrain boundary triple junction, contact or sinterstresses, stacking faults, coherency stresses etc. [10]. X-ray line broadening is used for the investigation of dis-

location distribution. Crystallite size and lattice strainaffect the Bragg peak in different ways. Both these ef-fects increase the peak width, the intensity of the peakand shift the 2 peak position accordingly. W-H analy-sis is a simplified integral breadth method where, bothsize induced and strain induced broadening are decon-voluted by considering the peak width as a function of 2 [11]. Although X-ray profile analysis is an averagemethod, they still hold an unavoidable position for

grain size determination, apart from TEM micro-graphs.

In this work, we have synthesized ZnO nanopar-ticles by an easy and novel surfactant assisted combus-tion synthesis using oxidizer zinc nitrate with a combi-nation fuels glycine and TWEEN 80. It was found thatthis method is quick, mild, and energy-efficient andeco-friendly route to produce ZnO nano particles.

In addition, a comparative evaluation of themean particle size of the ZnO nanoparticles obtainedfrom direct TEM measurements and from powder XRDprocedures is reported. The strain associated with theas-prepared and calcined ZnO samples at 500C due tolattice deformation was estimated by a modified form ofW-H, namely, uniform deformation model (UDM). The

other modified models, such as uniform deformationstress model (UDSM) and uniform deformation energydensity model (UDEDM), gave an idea of the stressstrain relation and the strain as a function of energydensity u. In UDM, the isotropic nature of the crystalis considered, whereas UDSM and UDEDM assumethat the crystals are of an anisotropic nature. Also opti-cal properties were studied usingUV-Vis spectroscopy.


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V.SESHASAI KUMAR,K.VENKATESWARARAO J.NANO-ELECTRON.PHYS.

2. EXPERIMENTAL

2.1 Synthesis of ZnO Nanoparticles

Zinc Oxide nano powder is synthesized by sur-factant assisted combustion synthesis using Zn(NO3)2.9H2O and Glycine with TWEEN 80 added as acombination fuel to the above metal nitrates in 1:1 mo-lar ratios following the balanced equation based onrocket fuel chemistry. This solution was then heated ona hot plate until all the liquid is evaporated. The solu-tion containing the above redox mixture boils, foams,catches fire and burns with a smoldering flame leavingfinal powdered ZnO nanoparticles at the base of thebeaker.

19.2 Zn (NO3)3+ 7C2H5NO2 + 3C8H11N 19.2 ZnO+38 CO2+ 34H2O +24.2 N2

The above reaction is balanced according tothe rocket fuel chemistry. According to rocket fuel che-mistry, the valencies of the elements Carbon, Hydro-gen, Nitrogen and Oxygen are given as +4, +1, 0 and -2respectively. The valency of nitrogen is taken as zerobecause of its convertibility into molecular nitrogenduring combustion. The valencies of metal depend uponmetal ions in that compound. The valency of the metalZinc is +2 respectively.

2.2. Characterization

XRD and TEM were used to obtain the textural para-meters of the materials, such as size, shape, composi-tion and crystal structure, in order to recognize theimproved properties of as-prepared and calcined ZnOnanoparticles. XRD was performed within the range of25o 2 65o by XRD, Bruker D 8,Advance, Germany

using CuK as radiation (1.5406 ) in configuration.For TEM analysis, Tecnai G2 F20 S-TWINFEI Transmission Electron Microscope operat-ing at 200 kV was used to examine the morphology ofthe calcined nanoparticles. The optical properties of thesamples were characterized by UV-Vis Spectroscopy(Systronics).

3. RESULTS AND DISCUSSION

3.1 XRD Analysis

The XRD patterns of the as-prepared and calcinedsamples of ZnO nanoparticles in the range of 2 = 250650 are shown in Figure 1. All evident peaks could beindexed as the ZnO wurtzite structure (JCPDS DataCard No: 36-1415). Wurtzite lattice parameters such asthe values of d, the distance between adjacent planes inthe Miller indices (h k l) (calculated from the Braggequation, = 2dsin ), lattice constants a, b, and c, in-ter-planar angle and unit cell volumes were calculate-dare calculated from the Lattice Geometry equa-tion[12]. The lattice parameters of the powders calcinedat different temperatures are summarized in Table 1.

=

(1)

0.866 (2)

Fig. 1- XRD pattern of ZnO nanoparticles before and aftercalcinations.

3.2 Crystallite size and strain

3.2.1 Scherer method

XRD can be utilized to evaluate peak broadening withcrystallite size and lattice strain due to dislocation [13].The crystallite size of the ZnO nanoparticles was de-termined by the X-ray line broadening method using

the Scherer equation: D , where D is the crys-tallite size in nanometers, is the wavelength of the

radiation (1.54056 A for CuK radiation), k is a con-stant equal to 0.94, D is the peak width at half-maximum intensity, and is the peak position. Thebreadth of the Bragg peak is a combination of both in-strument- and sample-dependent effects. To decouplethese contributions, it is necessary to collect a diffrac-tion pattern from the line broadening of a standardmaterial (e.g., silicon) to determine the instrumentalbroadening. The instrument-corrected broadening Dcorresponding to the diffraction peak of ZnO was esti-mated using the relation. From the calculations, theaverage crystalline size of the ZnO nanoparticles beforecalcination is 18.9 nm and after calcination is 25.5nm.

(3)


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J.NANO-ELECTRON.PHYS.

3.2.2 Williamson- Hall method

Strain-induced broadening arising from crystal

imperfections and distortions are related to .A significant property of Equation is the dependency onthe diffraction angle . The Williamson-Hall methoddoes not follow a 1/cos dependency as in the Scherrerequation but instead varies with tan .This fundamen-tal difference allows for a separation of reflection broa-dening when both microstructural causes-small crystal-lite size and microstrain-occur together. The distinct dependencies of both effects laid the basis for the sepa-ration of size and strain broadening in the analysis ofWilliamson and Hall [14]. Addition of the Scherrer eq-

uation and results in the following equations:

(4)

4 (5)

4 (6)The above equation represents Uniform De-

formation Model, where the strain was assumed to beuniform in all crystallographic directions, thus consi-dering the isotropic nature of the crystal, where all thematerial properties are independent of the directionalong which they are measured. The term ( cos) wasplotted with respect to (4sin) for the preferred orienta-tion peaks of ZnO nanoparticles with the wurtzite hex-agonal phase. Accordingly, the slope and y-intersect ofthe fitted line represent strain and crystallite size re-

spectively.In the Uniform Stress Deformation Model,

USDM, a generalized Hookes law refers to the strain,keeping only the linear proportionality between thestress and strain as given by ,where is thestress of the crystal and Y is the modulus of elasticityor Youngs modulus. This equation is just an approxi-mation that is valid for a significantly small strain.Assuming a small strain to be present ZnO nanopar-ticles, Hookes law can be used here. With a furtherincrease in the strain, the particles deviate from thislinear proportionality. Applying the Hookes law ap-proximation to the above eq yields:

(7)For a hexagonal crystal, Youngs modulus is given bythe following relation [15]:

Yhkl

(8)

where s11, s13, s33, s44 are the elastic compliances of ZnOwith values of 7.858 X10-12, -2.206 X 10-12, 6.940 X 10-12, 23.57 X 10-12 m2 N-1, respectively [16]. Youngs mod-ulus, Y, for hexagonal ZnO nanoparticles was calcu-lated as ~127 GPa. Plots were drawn with (4 sin )/Yhklon the x-axis and hklcos on the y-axis for the ZnO-nanoparticles before and after calcinations. The USDMplots for ZnO nanoparticles before and after calcina-tions at 5000C are shown in Figure 3.

The stress calculated from the slope of the fit-ted line is greater for the ZnO nanoparticles before cal-cination than for those for after calcination at 5000C.There is another model that can be used to determinethe energy density of a crystal called the Uniform De-formation Energy Density Model, UDEDM.

In Equation (9), the crystals are assumed tohave a homogeneous, isotropic nature. However, inmany cases, the assumption of homogeneity and isotro-py is not justified. Moreover, the constants of propor-tionality associated with the stress -strain relation are

no longer independent when the strain energy densityu is considered. For an elastic system that followsHookes law, the energy density u (energy per unit) canbe calculated from u= (2Yhkl)/2. Then Equation (9) canbe rewritten according the energy and strain relation

4 (9)

Plots ofhkl cos versus 4 sin (2u/Yhkl) 1/2were constructed and the data fitted to lines. The ani-sotropic energy density u was estimated from the slopeof these lines, and the crystallite size D from the y-intercept; see Figure 4.

Previously, we had and /2 wherethe stress s was calculated as 2 .The resultsof these plots show a slight change in energy density ofthe ZnO nanoparticles with calcination temperature.Table 2 reviews the geometric parameters of ZnO na-noparticles before and after calcinations at 5000C ob-tained from Scherrers formula, various modified formsof W-H analysis.


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Fig. 2 (a) & (b) - The W-H analysis of ZnO nanoparticlesbefore and after calcinations assuming UDM.

Fig. 3(a) & (b)- Modified form of W-H analysis assumingUSDM for ZnO nanoparticles before calcinations and aftercalcinations.

Fig. 4(a) & (b)- The modified form of W-H analysis assumingUDEDM ZnO nanoparticles before and after calcinations.


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Table1- The structure parameters of ZnO nanoparticles before and after calcination at 5000C

Sample 2 hkl dhkl

(Ao)

Structure Lattice

Parameter(Ao)

V (Ao3)

ZnO

(as-made)

(100)(002)

2.81912.6073

Hexagonal 3.25525.2145c/a

1.6019

47.83

ZnO

(5000C)

(100)(002)

2.82242.6112

Hexagonal 3.25905.2222c/a

1.6023

48.03

Table2- The geometric parameters of ZnO nanoparticles before and after calcination at 5000C

3.3 Transmission Electron Microscopy:

In TEM, electron beams focused by electro-magnetic lines are transmitted through a thin sampleof ZnO nanopowders. Figures 5 and 6 display the TEMimage and selected area electron diffraction (SAED)pattern of ZnO nanoparticles before calcination andafter calcination. The average size of the ZnO nanopar-ticles is observed to be less than 40 nm in TEM analy-sis and clearly indicates that the ZnO nanoparticles arecrystalline with a wurtzite structure, and no other im-purities were observed. This is in close agreement with

the results obtained from powder XRD data. In addi-tion, the rings with a dotted pattern in SAED confirm

the wide size distribution of ZnO nanoparticles.

Sample Scherer

method

Williamson- method

UDM USDM USDM

D nm D nm x 10-3 D nm x 10-3

(MPa)

D nm x 10-3 (MPa) u (KJm-3)

ZnO

(as-

made)

18.90863 41.87 2.75 35.72 2.925 371.51 39.75 2.072 263.18 27.27

ZnO

(5000C)

25.55177 44.71 1.59 44 1.538 195.34 44.565 1.255 159.381 100.01


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Fig.5 - TEM pictures of as-prepared ZnO nanoparticles

Fig.6 - SAED pattern of as-prepared ZnO nanoparticles

3.4 UV-Vis Spectroscopy:

The electronic absorption spectrum of ZnO samples inthe UVvis range enables to characterize the absorp-tion edge related to semiconductor band structure. Fig-ure 7 shows the UVvisible spectra of ZnO nanopar-ticles synthesized by surfactant assisted combustionsynthesis. The optical absorption coefficient () hasbeen evaluated [17] from the measured spectral extinc-tion coefficient, k(), using the following expression:

4 (10)where is the wavelength of the absorbed photon.

The direct band gap energy (Eg) for the ZnOnanoparticles is determined by fitting the reflectiondata to the direct transition equation h= A (h- Eg)n,where is the optical absorption coefficient, h is thephoton energy, Eg is the direct band gap and A is a con-stant and n depends on the kind of optical transitionthat prevails. Specifically, with n =1/2, a good linearityhas been observed for the direct allowed transition, themost preferable one in the system studied here. Theexact value of the band gap is determined by extrapo-lating the straight line portion of (h) 2Vs h to the x-axis. The direct band gap is found to be 3.3 eV beforecalcination and 3.46 eV after calcinations for ZnO na-noparticles which are shown in Figure 8 and Figure 9.An increase in the band gap is observed due to thequantum confinement effects in the ZnO nano particles.A typical exciton absorption at 374 nm and 377 nm isalso observed in the absorption spectrum correspondsto ZnO nanoparticles before and after calcinations re-spectively.

Fig. 7 - UV-vis absorption spectra of ZnO nanoparticles beforeand after calcinations.

Fig.8 - Band gap energy of ZnO nanoparticles beforecalcination


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Fig.9 -Band gap energy of ZnO nanoparticles aftercalcination

4. CONCLUSIONS

ZnO nanoparticles were synthesized by surfac-tant assisted combustion synthesis process and charac-terized by XRD, TEM and UV-Vis Spectroscopy. Theline broadening of ZnO nanoparticles due to the smallcrystallite size and strain was analyzed by Scherrersformula. The size and strain contributions to line broa-dening were analyzed by the method of Williamson andHall using uniform deformation, uniform deformationstress, and uniform deformation energy density models.From the results, it was observed that the strain valuedecreased but the particle size increased as calcinationstemperature was increased.TEM image of calcined ZnOnanoparticles reveals the nanocrystalline nature, andtheir particle size is found to be less than 40 nm. Thevalue of crystallite size calculated from the W-H analy-

sis is in agreement with that of the average crystallitesize measured from TEM.

In UV Vis spectroscopy, typical exciton absorp-tion at 374 nm and 377nm is observed in the absorptionspectrum corresponds to ZnO nanoparticles before andafter calcinations respectively, also observed the bandgaps for ZnO nanoparticles as 3.3 eV before calcinationand as 3.46 eV after calcinations.

REFERENCES

1. Yoshitake Masuda, Naoto Kinoshita,FuyutoshiSato, and Kunihito Koumoto, Crystal Growth& Design, 76, 6 (2006).

2. M.Singhai, V.Chhabra, P.Kang,D.O.Shah, Ma-terials Research Bulletin,32,2, (1997).

3. Hongyan Xu, Xiulin Liu, Deliang Cui, Mei Li,Minhua Jiang, Sensors and Actuators B, 114,301 (2006).

4. Miriam S. Tokumoto,Sandra H. Pulcinel-li,Celso V. Santilli,and Valerie Briois , J.Phys. Chem. B, 107, 568, (2003).

5. A. Muthuvinayagam, Boben Thomas, P. Den-nis Christy, R. Jerald Vijay, T. Manovah Da-vid, and P. Sagayaraj, Archives of AppliedScience Research, 3,4 (2011).

6. E. Sanatgar-Delshade, A. Habibi-Yangjeh, M.Khodadadi-Moghaddam, Monatsh Chem142,119, (2011).

7. Ruby Chauhan, Ashavani Kumar, Ram PalChaudhary, Journal of Optoelectronics andBiomedical Materials, 3, 1, (2011).

8. Priya Mishra, Raghvendra S Yadav, Avinash CPandey, Digest Journal of Nanomaterials andBiostructures, 4, 1, (2009).

9. K. Ramakanth, Basics of X-ray Diffraction andits Application, I.K. International PublishingHouse Pvt. Ltd., New Delhi, (2007).

10. T. Ungar, J. Mater. Sci, 42, 1584(2007).11. C. Suryanarayana, M. Grant Norton, X-ray

Diffraction: A Practical Approach, New York,(1998).

12. B.D. Cullity, Elements of X-ray Diffraction,Addison-Wesley Publishing Company Inc.,California, (1956).

13. R. Yogamalar, R. Srinivasan, A. Vinu, K. Ari-ga, A.C. Bose, Solid State Commun, 149, 1919(2009).

14. M. Birkholz, Thin Film Analysis by X-rayScattering, Wiley-VCH Verlag GmbH and Co.KGaA, Weinheim, (2006).

15. J. Zhang, Y. Zhang, K.W. Xu, V. Ji, Solid StateCommun, 139, 87, (2006).

16. J.F. Nye, Physical Properties of Crystals: TheirRepresentation by Tensors and Matrices. Ox-ford, New York, (1985).

17. J. Tauc, Amorphous and Liquid Semiconduc-tors, Plenum, New York, 1974; ibid, Phys.

Status Solidi, 15, 62 (1996).

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