ZnO Nanoparticles

CHAPTER 4

ZnO Nanoparticles: Growth,Properties, and Applications

Mohammad Vaseem1, Ahmad Umar2, Yoon-Bong Hahn1

1School of Semiconductor and Chemical Engineering and BK21 Centre for FutureEnergy, Materials and Devices, Chonbuk National University, Chonju 561-756,South Korea2Department of Chemistry, Faculty of Science, Advanced Materials andNano-Engineering Laboratory (AMNEL), Najran University, P. O. Box 1988, Najran11001, Kingdom of Saudi Arabia

CONTENTS

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12. Crystal Structure of ZnO . . . . . . . . . . . . . . . . . . . . . . . . . . . 23. Nanoparticles of ZnO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24. Application of ZnO Nanoparticles . . . . . . . . . . . . . . . . . . . 19

4.1. ZnO Nanoparticles: Bio-Friendly Approach . . . . . . . . 194.2. Solar Cells, Photocatalytic, Photoluminescence, and

Sensor Application of ZnO Nanoparticles . . . . . . . . . . 234.3. Cosmetic Application of ZnO Nanoparticles . . . . . . . 33

5. Summary and Future Directions . . . . . . . . . . . . . . . . . . . . . 34References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

1. INTRODUCTIONToday, nanotechnology (NT) is operating in various elds of science via its operationfor materials and devices using different techniques at nanometer scale. Nanoparticlesare a part of nanomaterials that are dened as a single particles 1100 nm in diameter.From last few years, nanoparticles have been a common material for the development ofnew cutting-edge applications in communications, energy storage, sensing, data storage,optics, transmission, environmental protection, cosmetics, biology, and medicine due totheir important optical, electrical, and magnetic properties. In particular, the uniqueproperties and utility of nanoparticles also arise from a variety of attributes, includ-ing the similar size of nanoparticles and biomolecules such as proteins and polynu-cleic acids. [1] Additionally, nanoparticles can be fashioned with a wide range of metals

ISBN: 1-58883-170-1Copyright 2010 by American Scientic PublishersAll rights of reproduction in any form reserved.

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Metal Oxide Nanostructures and Their ApplicationsEdited by Ahmad Umar and Yoon-Bong Hahn

Volume 5: Pages 136

2 ZnO Nanoparticles: Growth, Properties, and Applications

and semiconductor core materials that impart useful properties such as uorescenceand magnetic behavior [2]. Moreover, unlike their bulk counterparts, nanoparticles havereduced size associated with high surface/volume ratios that increase as the nanoparticlesize decreases. As the particle size decreases to some extent, a large number of consti-tuting atoms can be found around the surface of the particles, which makes the particleshighly reactive with prominent physical properties. Nanoparticles of particular materialsshow unique material properties, hence, manipulation and control of the material prop-erties via mechanistic means is needed. In addition, synthesis of nanoparticles havinguniform shape and size via easy synthetic routes is the main issue in nanoparticle growth.For the past decade, scientists have been involved in the development of new syntheticroutes enabling the precise control of the morphology and size of the nanoparticles. Inaddition, nanoparticle synthesis can be possible via liquid (chemical method), solid, andgaseous media [315], but due to several advantages over the other methods, chemicalmethods are the most popular methods due to their low cost, reliability, and environ-mentally friendly synthetic routes, and this method provides rigorous control of the sizeand shape of the nanoparticles. In general, nanoparticles with high surface-to-volumeratio are needed, but the agglomeration of small particles precipitated in the solution isthe main concern in the absence of any stabilizer. In this regard, preparations of stablecolloids are important for nanoparticle growth. In addition, nanoparticles are generallystabilized by steric repulsion between particles due to the presence of surfactant, polymermolecules, or any organic molecules bound to the surface of nanoparticles. Sometimesvan der Waals repulsion (electrostatic repulsion) also plays important role in nanoparti-cles stabilization.With all the issues related to nanoparticle synthesis, there are various types of nanopar-

ticles reported in the literature, e.g., metal nanoparticles, metal oxide nanoparticles, andpolymer nanoparticles. Among all these, metal oxide nanoparticles stand out as one ofthe most versatile materials, due to their diverse properties and functionalities. Mostpreferentially, among different metal oxide nanoparticles, zinc oxide (ZnO) nanoparti-cles have their own importance due to their vast area of applications, e.g., gas sensor,chemical sensor, bio-sensor, cosmetics, storage, optical and electrical devices, windowmaterials for displays, solar cells, and drug-delivery [1620]. ZnO is an attractive mate-rial for short-wavelength optoelectronic applications owing to its wide band gap 3.37 eV,large bond strength, and large exciton binding energy (60 meV) at room temperature.As a wide band gap material, ZnO is used in solid state blue to ultraviolet (UV) opto-electronics, including laser developments. In addition, due to its non-centrosymmetriccrystallographic phase, ZnO shows the piezoelectric property, which is highly useful forthe fabrication of devices, such as electromagnetic coupled sensors and actuators [21].

2. CRYSTAL STRUCTURE OF ZnOCrystalline ZnO has a wurtzite (B4) crystal structure at ambient conditions. The ZnOwurtzite structure has a hexagonal unit cell with two lattice parameters, a and c, andbelongs to the space group of C46V or P63mc. Figure 1 clearly shows that the structure iscomposed of two interpenetrating hexagonal closed packed (hcp) sublattices, in whicheach consist of one type of atom (Zn or O) displaced with respect to each other alongthe threefold c-axis. It can be simply explained schematically as a number of alternatingplanes stacked layer-by-layer along the c-axis direction and composed of tetrahedrallycoordinated Zn2+ and O2. The tetrahedral coordination of ZnO gives rise to the non-centrosymmetric structure. In wurtzite hexagonal ZnO, each anion is surrounded by fourcations at the corners of the tetrahedron, which shows the tetrahedral coordination andhence exhibits the sp3 covalent-bonding. The detailed properties of ZnO are presented inTable 1.

3. NANOPARTICLES OF ZnODue to its vast areas of application, various synthetic methods have been employed togrow a variety of ZnO nanostructures, including nanoparticles, nanowires, nanorods,

ZnO Nanoparticles: Growth, Properties, and Applications 3

Figure 1. The hexagonal wurtzite structure model of ZnO. The tetrahedral coordination of Zn-O is shown.O atoms are shown as larger white spheres while the Zn atoms are smaller brown spheres.

nanotubes, nanobelts, and other complex morphologies [2235]. In the present chapter,we mainly focus on ZnO nanoparticles synthesized by either the solgel method (solu-tion method) or the hydrothermal method. As the solution method presents a low costand environmentally friendly synthetic route, most of the literature for ZnO nanoparti-cles is based on the solution method. In addition, synthesis of ZnO nanoparticles in thesolution requires a well dened shape and size of ZnO nanoparticles. In this regards,Monge et al. [36] reported room-temperature organometallic synthesis of ZnO nanoparti-cles of controlled shape and size in solution. The principle of this experiment was basedon the decomposition of organometallic precursor to the oxidized material in air. It wasreported [37] that when a solution of dicyclohexylzinc(II) compound [Zn(c-C6H11)2] intetrahydrofuron (THF) was left standing at room temperature in open air, the solventevaporated slowly and left a white luminescent residue, which was further characterizedby X-ray diffraction (XRD) and transmission electron microscopy (TEM) and conrmed

Table 1. Physical properties of ZnO.

Properties ZnO

Lattice parameters at 300 Ka0 (nm) 0.32495c0 (nm) 0.52069c0/a0 1.602(1.633)

Density (g/cm3 5.606Stable phase at 300 K WurtziteMelting point (C) 1975Thermal conductivity (Wcm1 C1) 0.6, 1-1.2Linear expansion coefcient (C) a0: 6.5 cm3 106

c0: 3.0 cm3 106Static dielectric constant 8.656Refractive index 2.008Band gap (RT) 3.370 eVBand gap (4 K) 3.437 eVExciton binding energy (meV) 60Electron effective mass 0.24Electron Hall mobility at 300 K (cm2/Vs) 200Hole effective mass 0.59Hole Hall mobility at 300 K (cm2/Vs) 550

Value for an ideal hexagonal structures.


as agglomerated ZnO nanoparticles with a zincite structure having lack of dened shapeand size. Monge et al. used a modied experimental condition using a ligand of longchain amine, i.e., hexadecylamine (HDA) under an argon atmosphere in addition to theabove-mentioned solution, which resulted in well dened ZnO nanoparticles. It wasobserved that shape, size, and homogeneity of the as-synthesized products depend uponvarious reactions conditions, i.e., the nature of the ligand, the relative concentration ofreagents, the solvent, the overall concentration of reagents, the reaction time, the evapo-ration time, and the reaction/evaporation temperature. In addition, when a similar reac-tion is carried out in dry air, it leads to agglomerated ZnO nanoparticles displaying nodened shape or size. In an elaborative manner, they analyzed that if the concentrationof reagents in solution increases from 0.042 to 0.125 mol L1 nano-objects of higher aspectratio will be formed. Exchanging THF for toluene or heptane produces nanoparticles ofisotropic morphology with mean diameters of 4.6 for toluene and 2.4 nm for heptane.A slow oxidation/evaporation process in THF (2 weeks) produces only very homoge-nous nanodisks having size 4.1 nm (Fig. 2(b)). Reducing the reaction time under argon to5 min prior to oxidation leads to shorter nanorods 58 27 nm in size. Increasing thereaction temperature leads to isotropic disk-shaped nanoparticles. Exchanging HDA fordodecylamine (DDA) or octylamine (OA) also leads to disks with mean diameters of 3.0for DDA and 4.0 nm for OA (Figs. 2(c and d)). In addition, nuclear magnetic resonance(NMR) studies (Fig. 3) conrmed that throughout the oxidation process, the amine ligandremains coordinated to zinc and suggested that this coordination participates in control-ling the growth of ZnO nanoparticles. Kahn et al. [38] reported the detailed experimentalprocedure based on the same synthetic route with different experimental parameters, i.e.,the effects of solvent, ligand, concentration, time, and temperature. They explained thatthe reaction of organometallic complexes with oxygen or moisture leads exothermallyto a hydroxide material, but in this case they did not observe any traces of hydroxide,

(a)

(c) (d)

(b)

Figure 2. TEM micrographs of ZnO nanoparticles. (a) ZnO nanorods grown under standard conditions. (b) ZnOnanodisks following a slow oxidation/evaporation process in THF (2 weeks), (c) ZnO nanodisks using DDAinstead of HDA as the stabilizing ligand under standard conditions. (d) ZnO nanodisks using OA instead ofHDA under standard conditions. Reprinted with permission from [36], M. Monge et al., Angew. Chem. Int. Ed.42, 5321 (2003). 2003, Wiley-VCH Verlag GmbH & Co.


(a)

(b)

Figure 3. 13C{1H} NMR spectra of (a) the free HDA ligand and (b) ZnO nanoparticles coated with HDA.Reprinted with permission from [36], M. Monge et al., Angew. Chem. Int. Ed. 42, 5321 (2003). 2003, Wiley-VCHVerlag GmbH & Co.

indicating that both hydrolysis and condensation take place at room temperature. Thiscan be due to either exothermic oxidation of the organometallic precursor or to the pres-ence of amines, which are bases in solution medium. However, the observation of form-ing oxide even without amines conrmed that the oxidation reaction of organometallicprecursor is exothermic enough to lead the oxide, and during this process the ligandsmust control the shape of the nanoparticles by kinetic control of the oxidation reaction.In general, the mechanism of nanoparticle synthesis involves three steps, namely nucle-ation, growth, and ripening. In this case, water molecules could be responsible for thenucleation step by reacting with the molecular precursor and forming nuclei. In the pro-cess, most of the precursor remains intact after this step, and the growth of the particlescan occur when the solution is exposed to moisture and air. Moreover, as-synthesizedZnO nano-objects dissolved in most of the common organic solvents are luminescentsolutions that can be deposited on various surfaces as a monolayer or as thick layers.This luminescent solution shows two emission bands: one near-band edge UV emissionat 370 nm and one deep green emission at 585 nm. Interestingly, these two emissionbands are not quenched by the solvents and can be observed at room temperature, bothin solution and in the solid state.As from the above report, it is conrmed that the solvent has an important effect

on the morphology of ZnO nano-objects. Andelman et al. [39] further elaborated thesolvent effect using different solvents, i.e., trioctylamine (TOA), 1-hexadecanol (HD), and1-octadecene (OD). It was found that during synthetic process using TOA solvent yieldsnanorods, HD solvent yields nanotriangles, and OD solvent yields spherical nanoparti-cles. Figure 4 shows the typical XRD spectra for nanotriangles, spherical nanoparticles,and nanorods. The relative intensity of the peaks of nanotriangles and spherical nanopar-ticles matches the bulk, signifying no preferred orientation. Spherical nanoparticles pre-pared from octadecene have diameters of 1214 nm. Figure 5 shows the TEM images ofZnO nanotriangles at various degrees of tilt. The degree of tilt is indicated in the topleft-hand corner. At all angles, the shape remains triangular. As the different cappingagents have varying ability to stabilize certain planes, which leads to different parti-cle morphologies, the case observed here with varying solvents also plays a signicantrole in stabilizing specic crystallographic planes of the growing nanocrystal. The use ofTOA as a solvent leads to rod growth, but when the solvent changed from TOA to OD,the formation of spherical particles occurred because OD is not a coordinating solvent,and no crystal favored any growth direction, so the particles grew in a spherical shape.In addition, one possible reason for the formation of nanotriangles using hexadecanolas a solvent is due to its moderate coordinating capacity and its relatively weak ligand


a

b

c

Figure 4. XRD spectra of zinc oxide (a) nanotriangles, (b) spherical nanoparticles, and (c) nanorods. Reprintedwith permission from [39], T. Andelman et al., J. Phys. Chem. B 109, 14314 (2005). 2005, American ChemicalSociety.

capacity. Moreover, as-synthesized ZnO particles analyzed by room temperature photolu-minescence (PL) measurement indicated that the green band emission is associated withsurface defects and shows a strong dependence of morphology, with suppression of thegreen band emission in the case of spherical nanoparticles and nanotriangles (preparedin TOA/hexadecanol).

Figure 5. TEM images of ZnO nanotriangles at various degrees of tilt. The degree of tilt is indicated in the topleft-hand corner. At all angles, the shape remains triangular. Reprinted with permission from [39], T. Andelmanet al., J. Phys. Chem. B 109, 14314 (2005). 2005, American Chemical Society.


Another approach was performed by Ayudhya et al. [40] to show the effect of solventover the morphology of as-synthesized ZnO products. In their work, single crystallineZnO nanoparticles in different aspect ratios were synthesized by a solvothermal methodusing various organic solvents. In a typical synthetic process, zinc acetate as a pre-cursor suspended in four various types of organic solvents was heated in an auto-clave in the range of 250300C, depending upon the solvent, used for a 2 h reactionprocess. The solvents used in the experiment were alcohols (i.e., 1-butanol, 1-hexanol,1-octanol, and 1-decanol), glycols (i.e., 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol,and 1,6-hexanediol), alkanes (i.e., n-hexane, n-octane, and n-decane), and aromaticsolvents (i.e., benzene, toluene, o-xylene, and ethylbenzene). The as-synthesized ZnOproducts were characterized by XRD, SEM, and TEM. The typical XRD pattern synthe-sized in various groups of organic solvents (Fig. 6) conrmed that the crystalline phaseof ZnO was hexagonal without any impurities. The ZnO crystals grow along the samelattice direction, regardless of the solvent used. SEM micrographs of ZnO nanoparticlessynthesized in glycols are shown in Figure 6. From the SEM images, it is clearly observedthat the products synthesized in glycols produced polyhedral crystals with the lowestaspect ratios, whereas those synthesized in alcohols produced moderate aspect ratios.The products obtained using n-alkanes or aromatic compounds as solvents producedhigh aspect ratio ZnO nanorods. The morphology of ZnO nanoparticles synthesizedin alcohols strongly depends upon the chain length of the alcohol molecules, whereasa lesser effect is shown with chain length of glycols, and for n-alkanes and aromaticsolvents, chain length effect is unnoticeable. As for the growth of ZnO nanoparticles,there is concern that the anhydrous zinc acetate precursor can undergo decompositionand form ZnO nuclei. The thermal stability of zinc acetate has been reported [4142] todepend on its interaction with the solvent. Moreover, the dielectric constant of the usedsolvent is attributed to the high temperature requirement in the case of n-alkanes andaromatic compounds having low-dielectric constants compared to glycols and alcoholshaving high-dielectric constant required low temperature (250C). In addition, negativelycharged molecules adsorbed over the positively charged Zn surface of the (0001) facetof the crystal could retard the growth of crystals in the (0001) direction, which leads tononpreferential growth of the crystals. The same phenomenon occurred when glycols assolvents, having two hydroxyl groups at both ends, could adsorb onto the (0001) surfaceof the ZnO crystal, which nally led to the formation of ZnO nanoparticles instead ofZnO nanorods. On the other hand, alcohols having long chains (i.e., octanol, and decanol)

Figure 6. XRD patterns of ZnO powders synthesized in (a) 1-hexanol, (b) 1,6-hexanediol, (c) n-hexane, and(d) benzene. Reprinted with permission from [40], S. K. N. Ayudhya et al., Crystal Growth & Design 6, 2446(2006). 2006, American Chemical Society.


(a) (b)

(e)(c) (d)

Figure 7. SEM micrographs of ZnO particles synthesized via the solvothermal process in (a) 1,3-propanediol,(b) 1,4-butanediol, (c) 1,5-pentanediol, and (d) 1,6-hexanediol. Insets in the images are the corresponding TEMmicrographs. (e) Sample of the SAED pattern of the synthesized ZnO. Reprinted with permission from [40],S. K. N. Ayudhya et al., Crystal Growth & Design 6, 2446 (2006). 2006, American Chemical Society.

show less polarity, which leads to the formation of high-aspect ratio ZnO nanoparti-cles. Although the dielectric constant of the solvent is the prime reason for the differentmorphology of ZnO nanoparticles in this solvothermal synthesis, more detailed charac-terization and actual mechanism are needed.To show the effect of acidic and basic solution routes on the morphology of ZnO,

Kawano et al. [43] synthesized ZnO nanoparticles with various aspect ratios. In a typicalsynthetic process, ZnO grains and ZnO rods were obtained with various aspect ratiosat 60C with 2 h reaction in aqueous solution of ZnSO4 via an acidic route (pH 5.6) withaddition of NaOH and in a basic solution of NaOH via a basic route (pH 13.6) with addi-tion of ZnSO4, respectively. The observed aspect ratios were changed by this syntheticroute, although the nal pH of the solution was the same. The detailed morphologicalcharacterizations were performed by XRD, eld emission scanning electron microscopy(FESEM), and eld emission transmission electron microscopy (FETEM). XRD analysisconrmed the wurtzite ZnO type structures with peak broadening in the case of the acidicroute compared to the basic route, which further conrmed the formation of smaller par-ticles via the acidic route. FESEM images also conrmed the formation of ZnO particlesand rods via acidic and basic routes, respectively. Further cumulative undersize distribu-tion of precipitated ZnO particles conrmed that the particle shapes were spherical orellipsoidal with diameters of 32 and 44 nm, respectively, via the acidic route at pH 12.8,which were consistent with the crystallite size calculated by Scherrers formula usingthe (100) and (002) diffraction peaks observed in XRD spectra. Although the value of[OH]/[Zn2+] and the nal pH were the same in the acidic and basic routes, the numberof ZnO nuclei formed via the acidic route was deduced to be much higher than thatobtained via the basic route because the degree of saturation at the initial stage of theacidic route was extremely high due to the low solubility of ZnO. Thus, most of theprecursor species steeply precipitated as nanograins. On the other hand, ZnO nanorodsformed in the basic route due to limitation of formed ZnO nuclei at the initial stage, andthus particle size increased via subsequent growth in the progressive stage.To check the effect of water addition in the precursor-methanol solution for the mor-

phological evolution of ZnO particles, Wang et al. [44] performed reactions based onhydrolysis of zinc acetate in methanol solvent at 60C for 24 h and deposited over


Al2O3 ceramic plate via the chemical deposition method. As the water/methanol volumeratio increased, the shape of the ZnO particles changed from irregular particles toplates and then from plates to regular cones, including the size change from nano-scale to micro-scale. In addition, if the volume of added water increased, the height ofthe cones decreased. Addition of water controlled the hydrolysis of zinc acetate andaffected the nucleation process of ZnO signicantly. Moreover, addition of water canimpede the [0001] growth and accelerate the [1100] growth if the volume ratio of addedwater/methanol is equal to or greater than 2:15. In this way, the shape and size of ZnOcan be tailored by adjusting the volume ratio.Du et al. [45] have given a new reaction to synthesized ZnO nanoparticles with nearly

uniform, spherical morphologies and controlled the size range from 25100 nm via ester-ication of zinc acetate and ethanol under solvothermal reaction conditions. The reac-tion temperatures were adjusted from 100200C for 2448 h in an autocontrolled oven.In terms of characterization, XRD and TEM analysis conrmed the high crystallinityand uniform nonagglomerated sphericity of as synthesized ZnO nanoparticles, respec-tively. By the several reaction conditions, it was conrmed that by changing the reactiontemperature and time, the nanoparticle size can be easily controlled. As for the reac-tion mechanism, Fourier transform infrared (FT-IR) analysis conrmed the existence ofethyl acetate during the esterication reaction. In addition, it may be possible that rstOH anions were produced by esterication reaction between CH3COO and ethanoland then zinc cation reacted with as-produced OH to form ZnO under solvothermalconditions. The presence of ethanol and ester could help to improve the dispersibility ofthe as-synthesized ZnO nanoparticles.Cheng et al. [46] demonstrated the synthesis of ZnO colloidal spheres by the sol

gel method. In a typical synthetic process, they used two types of reaction processes.In the rst reaction, 0.01 M zinc acetate dihydrate was added to 100 ml diethylene gly-col (DEG), and then the reaction solution was heated at 160C and maintained for 1 h,which resulted in white colloidal ZnO, treated as the primary solution. In a second reac-tion process, 0.01 M zinc acetate and various amount of primary supernatant (520 ml)was added to 100 ml DEG and heated at 160C for 1 h aging. The resulting ZnO whitecolloid produced 50300 nm ZnO nanoparticles, depending upon the amount of primarysupernatant. To check the structural and optical properties, optimal size with 185 nmZnO nanoparticles were used. As-synthesized ZnO nanoparticles were characterized byvarious analytical tools, i.e., XRD, TEM, FESEM, energy dispersive spectroscopy (EDAX),Raman spectroscopy, and UV photoluminescence measurement. TEM observation con-rmed that spherical 185 nm-diameter ZnO clusters consisted of primary single crystal-lites ranging from 612 nm. XRD analysis conrmed the hexagonal wurtzite crystallitesof as-grown zinc oxide colloidal spheres and sample post-annealed at 350 and 500C inair for 1 h. Raman spectra of as-grown zinc oxide colloidal spheres and post-annealedsamples further conrmed the crystallinity of the products. Moreover, highly efcientnear-band edge UV luminescence was attributed to defect-bound excitons with highdensity of states, which was conrmed by using room-temperature PL analyses. Thisassumption was further proved by the observation of peak broadening and unchangedposition in low-temperature PL spectra, which is similar to the behavior observed inthe case of ZnO quantum dots. In addition, broad yellow emission and green emissionwere observed in room-temperature PL and low-temperature PL, respectively. Further, intemperature-dependent PL, defects such as oxygen interstitials Oi and oxygen vacanciesV0 dominate the visible emissions of ZnO spheres.Cheng et al. [47] further reported the enhanced resonant Raman scattering and electron-

phonon coupling from self-assembled secondary ZnO nanoparticles synthesized by thesame procedure described in the above report. Figure 8 shows the typical TEM imagesof ZnO nanoparticles. Figs. 8(a) and (b) show the mean particle size of 185 nm withspherical shape of ZnO nanoparticles, which consisted of agglomerated primary singlecrystallite ranging from 612 nm. The selected area electron diffraction (SAED) spectrashown in inset of Figure 8(a) conrmed the polycrystalline nature of several secondaryZnO nanoparticles, while the SAED spectra shown in Figure 8(b) conrmed the single


(a) (b)

(d)(c)

Figure 8. TEM images of secondary ZnO nanoparticles recognized of crystalline subcrystals. (a) A typical low-magnication TEM image and SAED pattern of several uniform ZnO nanoparticles. (b) High-magnicationTEM image of one individual ZnO nanoparticle and its corresponding single-crystal-like SAED spots. (c) and (d)HRTEM images of the central area and boundary, respectively of one individual ZnO nanoparticle. Reprintedwith permission from [47], H.-M. Cheng et al., J. Phys. Chem. B 109, 18385 (2005). 2005, American ChemicalSociety.

crystalline pattern of only one ZnO nanoparticle. This means that the secondary ZnOnanoparticles are polycrystalline, consisting of much smaller subcrystals of the samecrystal orientation. Figures 8(c and d) further provide much evidence in high resolu-tion TEM (HRTEM) images. It may be possible that van der Waals interaction betweenthe surface molecules of the nanocrystallites forms the driving force for self-assembly,and then colloidal nanocrystal can be assembled to form solids. In addition, due to theblock of diethylene glycol, the solvent may behave as a microemulsion system, causingthe individual ZnO subcrystals to grow up separately and nally assemble to form sec-ondary ZnO nanoparticles under the driven force of van der Waals interaction. Figure 8shows the SEM images of as-synthesized ZnO nanoparticles and samples collected afterpost-annealing at 350 and 500C in air for 1 h. SEM images clearly indicate that duringthe heating process, ZnO subcrystals fused with neighboring crystals and grain size grewaccordingly, which was further conrmed by XRD analysis. Moreover, as-grown ZnOnanoparticles exhibited a phonon red shift in a resonant Raman scattering, comparedwith the samples after post-annealing at 350 and 500C. In addition, the electron-phononcoupling parameter is clearly extracted from resonant Raman scattering, and an interest-ing phenomenon of increasing electron-LO phonon coupling was also discovered whenthe crystal size of ZnO enlarged after heating treatment. In addition, the Frhlich inter-action may certainly play the main role in the coupling of ZnO particles. Finally, blueshift of UV PL and visible emission induced by interstitial oxygen were also investigatedfrom as-grown and post-annealed ZnO samples, respectively.


(a) (b)

(c)

Figure 9. SEM micrographs of secondary ZnO nanoparticles (a) as-grown, (b) annealed at 350C for 1 h, and(c) annealed at 500C for 1 h. Reprinted with permission from [47], H.-M. Cheng et al., J. Phys. Chem. B 109,18385 (2005). 2005, American Chemical Society.

During the synthesis of ZnO nanoparticles, the inuences of the reactant concentrationwere reported by Hu et al. [48]. In a typical process, ZnO nanoparticles were synthesizedby using zinc acetate and NaOH in 2-propanol solution. As the nucleation and growthwere fast in this synthetic process, at longer times the particle size was controlled bycoarsening. In addition, coarsening kinetics were independent of the zinc acetate con-centration from 0.51.25 mM at a xed [zinc acetate:NaOH] ratio of 0.625. The widthof the size distribution increased slightly with aging time. Moreover, if the zinc acetateconcentration was xed at 1 mM, the kinetics were independent of variation in the[zinc acetate:NaOH] ratio from 0.4760.625. The presence of water in the reaction mix-ture was checked, and it was found that at low water concentration, the nucleation andgrowth of ZnO were very slow, which only slightly affected the coarsening kinetics forwater content above 20 mM. Thus, by this synthesis method, it is conrmed that ZnOnanoparticles are insensitive to the reactant concentration and presence of water.In another report, Hu et al. [49] explained the inuence of anions on the coarsen-

ing kinetics of ZnO nanoparticles. Solution phase synthesis of nanoparticles possessescoarsening (also known as Ostwald ripening) and epitaxial attachments (or aggregation),which can compete with nucleation and growth. As a result, particle size distribution canbe modied in the system. If nucleation and growth are fast, coarsening and aggregationcan dominate the time evolution of the particle size distribution. In addition, randomaggregation usually leads to the formation of porous clusters of particles, whereas epi-taxial attachment of particles leads to the formation of secondary particles with complexshapes and unique morphologies. In this report, ZnO nanoparticles were synthesizedfrom Zn(CH3COO)2, ZnBr2, and Zn(ClO4)2 in 2-propanol. ZnO nanoparticles synthe-sized by Zn(CH3COO)2, and ZnBr2 in 2-propanol at 55C for 8.5 h show particles size65 12 nm and 49 08 nm, respectively, whereas ZnO nanoparticles synthesized byZn(ClO4)2 in 2-propanol at 55C for 40 min show elongated and irregularly shaped parti-cles via epitaxial attachment of several smaller particles. The rate constant for coarsening


at constant temperature increases in the order Br < CH3COO < ClO4 indicating that therate is dependent on anion adsorption. On the other hand, the temperature dependentrate constant for coarsening is due to the temperature dependence of the solvent viscosityand the temperature dependence of the bulk solubility of ZnO.Vafaee et al. [50] reported the preparation and characterization of ZnO nanoparticles

based on a novel solgel route. As-synthesized ZnO nanoparticle morphology was con-rmed by TEM analysis, which shows spherical particles 34 nm in diameter. In a typicalsynthesis process, zinc acetate (ZnAc) was used as a precursor, and triethanolamine (TEA)was used as a surfactant to produce ZnO nanoparticles at 5060C. With the help of FT-IRanalysis, they proposed that synthesis of ZnO nanoparticles occurred via an intermediateproduct called zinc monoacetate, which further assisted the formation of a new complexand then, via a polycondensation process, produced ZnO nanoparticles. In addition, threedifferent ratios of both ZnAc and TEA were chosen to determine the best sol, consideringtheir optical properties. The best sol (0.75 M ZnAc) based on its optical properties wassubjected to analysis by PL spectroscopy. Different shapes of UV (broad peak at 360 nmwith one shoulder at 330 nm) and green peaks (sharp peak at 520 nm) in the PL spectraof ZnO nanoparticles, synthesized using 0.75 M zinc acetate, suggest the possible use inmonochromatic excitation applications.To conrm the optimization parameter for the synthesis of zinc oxide nanoparticles,

Kim et al. [51] presented the modied solgel route using the Taguchi robust designmethod. In a typical synthetic process, zinc acetate dehydrate, lithium hydroxide mono-hydrate (LiOH), hydroxypropylcellulose (HPC), and absolute ethanol were used for thesynthesis of ZnO nanoparticles. In this presented work, the molar concentration ratio of[LiOH]/[Zn(Ac)2] was varied in the range of 15, and the concentration of zinc acetatewas xed at 0.05 M. Also, the concentration of HPC dispersant and feed rate of LiOH andHPC solution were changed in the range of 0.10.4 g and 0.337.0 ml/min, respectively.After implementing the Taguchi robust design method with an L9 orthogonal array tooptimize experimental condition for the preparation of ZnO nanoparticles, it was foundthat the [LiOH]/[Zn(Ac)2] molar ratio was the main parameter, showing a prominenteffect on particle size and size distribution of the ZnO nanoparticles. By optimizing theconditions, the observed size of ZnO nanoparticles was 30 nm with narrow particlessize distribution, conrmed by TEM analysis.Uthirakumar et al. [52] reported the low temperature solution approach to synthesis

nanocrystalline ZnO nanoparticles from a single molecular precursor without using anybase, surfactant, template etc. via a single step process. In a typical synthetic process,zinc acetate dihydrate was used as a precursor and methanol was used as a solvent forsynthesizing ZnO nanoparticles at 60C in 10 h. In addition, similar experiments werealso preformed by using a mixture solvent i.e., dimethylformamide (DMF), toluene, andTHF with methanol, to check the effect of the solvent polarity and water miscibility onthe growth of ZnO nanoparticles. The growth rate was greatly controlled by the pres-ence of a water-immiscible non-polar solvent, which led to the formation of almost pureZnO nanoparticles with near UV emission. On the other hand, the water-miscible polarsolvent generates fully defected deep-level emissive ZnO nanoparticles, which agglom-erate on standing due to the solvent homogeneity in the reaction mixture. As for thegrowth mechanism, the zinc acetate precursor underwent four stages: it was rst solvatedin methanol to form [Zn(MeOH)6]+, then hydrolysis after removal of the intercalatedacetate ions produced [Zn(OH)n2n], which further polymerized into ZnOZn bridges,and nally transformed into ZnO. Moreover, it was observed that formation of watermolecules during decomposition of zinc acetate could be responsible for the growth rateof ZnO nanoparticles. Finally, it was concluded that ZnO crystal growth is more sensitiveto the mixture of solvents, which depends on the miscibility, polarity, and homogeneityof the precursor in the reaction medium.Ge et al. [53] reported a simple method to prepare monodispersed ZnO nanoparti-

cles with average size of 52 03 nm at low temperature by ultrasonic treatment. In atypical synthetic process, 0.88 gm zinc acetate dihydrate was mixed with 80 ml of abso-lute ethanol in a beaker under magnetic stirring at 70C. In another beaker, 0.23 gm of


LiOH was dissolved in 80 ml absolute ethanol under magnetic stirring for 20 min. Afterthis step, the LiOH-ethanol solution was added dropwise into a Zn2+-containing solutionat 0C under strong stirring for 1 h, which was further ultrasonically treated for 5 min.XRD and HRTEM images conrmed the crystallinity and structural morphology, respec-tively, of the as-synthesized ZnO nanoparticles. In addition, it was reported that withvarying reux time, ZnO nanoparticles can be converted to various aspect ratio ZnOnanorods via oriented attachment mechanism that were conrmed by the BFDH model(suggested by Bravasis, Freidel, Donnary, and Harker) and the HP model (proposed byHartman and Predok).Uekawa et al. [54] reported synthesis of ZnO nanoparticles by heating Zn(OH)2 in a

diol solution. ZnO nanoparticles were obtained when Zn(OH)2 was dispersed in ethyleneglycol, 1,3-propanediol, and 1,4-butanediol, which were further treated at temperaturesabove 308 K. In particular, if ethylene glycol was used as a solution for Zn(OH)2 disper-sion, the synthesized ZnO nanoparticles had average particles size less than 20 nm. More-over, if the reaction temperature was set at 308 K, the spherical secondary particles withZnO primary nanoparticles were obtained. When Zn(OH)2 was heated in 1,3-propanediolat 308 K for 24 h, the spherical aggregated morphology of the ZnO primary nanoparticleswith average diameter of 9 nm was obtained and if heated in 1,4-butanediol at 308 K for24 h, the same morphology with average primary ZnO nanoparticle size of 11 nm wasobtained, having interparticle pores in both cases. By measuring N2 adsorption isothermat 77 K, it was concluded that ZnO nanoparticles prepared in ethylene glycol at 308 Kcontain many interparticle pores with less densely packed spherical aggregated morphol-ogy, whereas ZnO nanoparticles prepared in 1,3-propanediol and 1,4-butanediol showmore densely packed primary ZnO nanoparticles. Thus, the formation of ZnO nanopar-ticles depends greatly not only on the heating temperature but also on the diol solutionsused for preparation.Lee et al. [55] synthesized ZnO nanoparticles with controlled shapes and sizes by using

a simple polyol method. It was reported that the amount of water and the method ofaddition played an important role in determining the characteristics of the synthesizedparticles. In a polyol synthetic method, water can induce hydrolysis and condensationreactions of the Zn precursor when injected into a hot precursor solution maintainedat 180C, which induces a short burst of homogenous nucleation and leads to growthof aggregated equiaxial ZnO nanoparticles with average diameter of 24 nm. If a higheramount of polyvinyl pyrrolidone (PVP)a water-soluble polymeris used, it will leadto aggregation of free ZnO nanoparticles. In addition, increasing the amount of wateradded to the precursor solution enlarges the aspect ratio of the rod-shaped particlesand increases the particle size of the equiaxial particles due to enhanced hydrolysis andcondensation of the Zn ion complex. Moreover, zinc acetate concentration also slightlyinuences the particles size and aspect ratio when water is injected into the hot precursorsolution. Furthermore, the effect of the hydration ratio (ratio of molar concentration oftotal water, DI water + hydrated water, to zinc acetate) on the particles characteristicsvia the water injection method were also discussed. Varying the hydration ratio from4 to 8 did not change the particle morphology to a great extent. The particle diameterincreased from 24 to 32 nm, and showed a slight deviation from equiaxial growth withincreasing hydration ratio. Thus, it was concluded that method of water addition, con-centration of zinc acetate, and the hydration ratio play important roles in determiningthe characteristics of ZnO particles.Ning et al. [56] reported the synthesis of mesoporous ZnO particles using octadecy-

lamine (ODA) and DDA as templates via the solgel method. Particle size calculatedusing Scherrers formula with XRD analysis was 32 nm when processed with ODA and40 nm with DDA. The densities of ZnO processed with ODA, with DDA, and withouta template were reported as 5.31, 5.37, and 5.42 cm2/g; respectively. In addition, it wasreported that surface analysis conrmed the porosity of the ZnO particles when pro-cessed with ODA and DDA. Moreover, hugely enhanced electroluminescence (EL) wasobserved from porous ZnO particles when direct current electric eld from 24.66 V/mwas used. Furthermore, emission intensities of the ZnO sample processed with DDA


and ODA were enhanced 12 times and 20 times, respectively, at a voltage of 4.66 V/m.The observed EL spectrum shows mainly broad emission peak at 556 nm. The reportedthreshold voltage is just 2 V/m. Based on the above analysis, it was conrmed thatporous ZnO particles can enhance EL intensity.Cozzoli et al. [57] reported the non-hydrolytic route for the synthesis of nearly spheri-

cal ZnO nanocrystals with diameter less than 9 nm via a sequential reduction-oxidationreaction. In a typical synthetic process, ZnO nanocrystals were synthesized in a surfac-tant mixture of hexadecylamine and oleic acid (OLEA) via a two-step chemical process:rst hot reduction (at 180250C) of zinc halide by superhydride (LiBEt3H) and then oxi-dation of the resulting product. The reported results conrmed that controlled growth ofZnO nanocrystal was dependent on OLEA-assisted generation of intermediate metallicnanoparticles as well as adjustment of oxidation of the metallic nanoparticles using amild oxidant, triethylamine-N -oxide, rather than molecular oxygen. Furthermore, thereported synthetic approach demonstrates that organic-soluble ZnO nanocrystals of lowsize dispersion and of stable size can be useful for optoelectronic, catalytic, and sensingpurposes.Xie et al. [58] reported the low temperature synthesis of uniform ZnO particles with

controllable morphologies. In addition, characteristic luminescence patterns were alsopresented. In a typical synthesis process, uniform ZnO particles were synthesized in anaqueous solution with the presence of TEA below 80C assisted via sonication. It wasreported that with increasing TEA concentration, one can systematically control the mor-phology of elongated rugby ball-like ellipsoidal to half-ellipsoidal ZnO particles. FESEManalysis of many rugby ball-like ZnO particles shows that particles have an averagelength of about 620 nm and mean diameter of about 400 nm. By systematic investiga-tion, it was conrmed that formation of rugby ball-like ZnO particles resulted from therst growth of a half-ellipsoidal particle followed by the germination and growth of asecond half at its base. Moreover, it was studied with close relationship between particlecharacteristics and optical properties with a high spatial resolution cathodoluminescence(CL) and shows that the ellipsoidal particles are intrinsically encoded with characteristicbarcode-like UV luminescence patterns. Additionally, luminescence spectra can be tunedby heat treatments at elevated temperatures. By this extensive proof, the authors believethat well-dened uniform ellipsoidal ZnO particles embedded with unique luminescencecharacteristic can hold great potential for use in bioengineering and photonics, such asbiological labeling, multiplexed bioassays, and optical probes inside photonic crystals.Buha et al. [59] reported the nonaqueous synthesis of nanocrystalline zinc oxide

nanoparticles. In a typical synthesis process, zinc(II) acetylacetonate, as a precursor wasdissolved in the oxygen-free solvent acetonitrile, which was transferred into a Teonautoclave and then heated at 100C for 2 days. The resulted products were characterizedby TEM, SEM, and XRD analysis. The TEM micrograph shows the particles size in therange of 1585 nm, sometimes with well faceted hexagonal morphology. It is interestingto note that in such a simple reaction, systems like zinc acetylacetonate and acetonitrileare able to induce the formation of complex structures without any additional structure-directing agent. Even the large number of organic species detected in nal productsconrmed the complex reaction pathways during the reaction, and these organic com-ponents during nanoparticle formation are prerequisite to understanding and controllingthe nonaqueous synthesis of metal oxide materials.Glaria et al. [60] reported synthesis of ZnO nanoparticles via an organometallic route

and explained that lithium ions act as growth-controlling agent. For the synthesis ofZnO nanoparticles, solid Zn(c-C6H11)2 was dissolved in a THF solution of lithium pre-cursor and OA used as stabilizer. Two different lithium precursors, i.e., Li[N(CH3)2] andLi[N(Si(CH3)3)2], and one sodium precursor, namely, Na[N(Si(CH3)3)2], were used withthe proportion varied from 1 to 10 mol% compared to Zn. It was observed that Li pre-cursors induced the synthesis of ZnO nanoparticles; otherwise, without Li or with theuse of Na precursor the synthesis of ZnO nanorods was induced. Figure 10 shows theTEM micrograph of ZnO nanoparticles synthesized using the Li[N(CH3)2] precursor withnanoparticle size varied from 37 07 nm to 25 04 nm [series 1]. Figure 11 shows the


(b)(a)

(d)(c)

Figure 10. TEM images of series 1 nanoparticles: (a) 1%, (b) 2%, (c) 5%, and (d) 10% Li. Reprinted with per-mission from [59], A. Glaria et al., New J. Chem. 32, 662 (2008). 2008, The Royal Society of Chemistry.

TEM micrograph of ZnO nanoparticles synthesized using the Li[N(Si(CH3)3)2] precursorwith nanoparticle size varied from 43 10 nm to 31 08 nm [series 2]. Figures 10(ad)and Figures 11(ad) show that as the Li amount increases, the size of the nanoparti-cles decreases, whatever the Li precursor. The insets of Figures 11(c and d) show theHRTEM image and conrm the monocrystalline nature of the ZnO nanoparticles. XRDanalysis conrmed the presence of the hexagonal zincite phase, space group P63mc inall samples. In addition, the optical properties of these nanoparticles were measured bydissolving solid samples in distilled THF. The absorption spectrum for all the samplesshows a strong absorption between 300 and 350 nm followed by a sharp decrease. Fur-thermore, the luminescence properties of these samples were also investigated, whichshows one broad emission band in the visible range for an excitation wavelength of320 nm. This shows that presence of Li ions leads to a blue shift of the emission bandof ZnO nanoparticles. The observed emission maxima vary from 582 to 535 nm for theLi[N(CH3)2] precursor and from 581 to 534 nm for the Li[N(Si(CH3)3)2] precursor. Thisblue shift increases as the concentration of precursor increases, and consequently, as thesize of the nanoparticles decreases. Moreover, the observed emission intensity is verystrong, which can be clearly seen by the human eye as illustrated in Figure 12, whichopens the perspective for the preparation of LEDs.Bardhan et al. [61] synthesized sub-micrometer ZnO particles with controlled morphol-

ogy, i.e., rings, bowls, hemispheres, and disks, via a simple wet-chemistry approach usingzinc acetate as a precursor, ammonium hydroxide as a base, and ethanol as a solvent. Thereported morphologies were varied with the concentration of zinc acetate, i.e., at 0.05 M


(b)(a)

(d)(c)

Figure 11. TEM images of series 2 nanoparticles: (a) 1%, (b) 2%, (c) 5%, and (d) 10% Li. Reprinted with per-mission from [59], A. Glaria et al., New J. Chem. 32, 662 (2008). 2008, The Royal Society of Chemistry.

(rings), 0.01 M (bowls), 0.02 M (hemispheres), and 0.025 M (disks). Moreover, reactiontemperature, pH, and concentration of ammonium hydroxide also played an importantrole for the formation of various ZnO morphologies. In addition, these synthesized ZnOparticles show strong white-light emission via UV excitation, which is due to the presenceof surface defects resulting from the method of fabrication and synthesis conditions. As aresult, the authors believe that based on the properties of these ZnO particles, it may leadto the development of economical, white light-emitting materials for solid-state lightingapplications.Hong et al. [62] reported the synthesis of quasi-spherical ZnO nanoparticles with diam-

eters of 20 nm using zinc acetate as a precursor. In a typical synthetic process, 5%PEG surfactant solution was transferred into a three-neck ask, and then zinc acetateand (NH4)2CO3 solutions were added dropwise with vigorous stirring. The resulting sus-pension was kept for 2 h at room temperature under stirring. After the completion ofthe reaction, the product was ltered and washed with ammonia solution and ethanol,dried under vacuum for 12 h, and calcinated at 450C for 3 h. The as-synthesized ZnOnanoparticle surfaces were further grafted by polystyrene (PSt) in a non-aqueous suspen-sion via free-radical polymerization to reduce the aggregation among nanoparticles andto improve the compatibility between the nanoparticles and the organic matter, whichmade a stable suspension in organic solvents. The resulting ZnO nanoparticles and PSt-grafted ZnO nanoparticles were characterized by TEM, XRD, FT-IR analysis, zeta poten-tial measurement, lipophilic degree (LD) test, photocatalytic analysis, sedimentation test,and contact angle measurement. It is reported that bare ZnO nanoparticles have high


Figure 12. Evolution of the emission at room temperature of the ZnO nanoparticles for series 2: (a) 1%, (b) 2%,(c) 5%, and (d) 10%. Reprinted with permission from [59], A. Glaria et al., New J. Chem. 32, 662 (2008). 2008,The Royal Society of Chemistry.

photocatalytic activity, although PSt-grafted ZnO nanoparticles have no photocatalyticactivity. Moreover, the LD of the composite particles after high temperature was stable,and the photoluminescence of the PSt-grafted ZnO nanoparticles was observed by thenaked eye. In addition, ZnO nanoparticles can also be used to reinforce the electricalconductivity of poly(vinylidene uoride) (PVDF) lms.Ultrasound-assisted green synthesis of ZnO nanoparticles in room-temperature ionic

liquids (RTILs) was reported by Goharshadi et al. [63]. In a typical synthesis process, zincacetate dihydrate was dissolved in distilled water, and then NaOH was added to makea transparent Zn(OH)24 solution, followed by the addition of an ionic liquid, 1-hexyl-3-methylimidazolium bis (triuoromethylsulfonyl) imide, liquid [hmim][NTf2]. After thisstep, the resulting solution was ultrasonically irradiated for 1 h with 40 kHz frequency ofultrasound waves. The total acoustic power injected into the sample solution was foundto be 50 W. The as-synthesized ZnO nanoparticles were characterized by XRD, SEM,and TEM. Various experiments were done to check the effects of RTILs and ultrasoundirradiation on the morphology of ZnO nanoparticles. On the basis of this observation,it was found that RTIL and ultrasound have a critical role in the formation of ZnOproducts. As for the growth mechanism, RTIL consists of cations [C6mim]+ and anions[NTf2]. Cationic species combined with Zn(OH)24 species present in the solution thoughelectrostatic attraction, and these cation-anion couples led to the formation of ZnO nucleivia dehydration due to strong polarization of [C6mim]+. Moreover, the newly generatedZnO surface was greatly inhibited by [C6mim]+ ions, so the anisotropic growth of ZnOcrystals were markedly modied. As the method is very effective for the synthesis of ZnOnanoparticles in a green media, it could be useful for synthesizing ZnO nanoparticleswith high yields.Another approach to synthesize ZnO nanoparticles via one-step mechanochemical pro-

cess was reported by Lu et al. [64]. In a typical synthesis process, matrix salts wereprepared by mixing zinc sulphate heptahydrate, potassium hydroxide, and potassium


chloride. The mixing reaction was carried out in a paste state at room temperaturewith short grinding time without any external energy input. The as-produced ZnOnanoparticles had a mean diameter of 22.1 nm, which exhibited excellent UV-blockingproperties (UV absorption maxima at 358 nm) for cosmetic application, conrmed byUV-visible (UV-vis) spectrometry. In addition, authors compared the raw material costswith other mechanochemical processes and found that this process is more favorablethan others.Up to now, ZnO nanoparticles synthesized by using zinc precursors with either base

or solvent for the combination of OH ions to produce zinc hydroxide moieties, whichproduced ZnO nanoparticles via dehydration, have been reported. There was somemore literature related to direct conversion of inorganic or organic precursor to zincoxide nanoparticles, i.e., Rataboul et al. [65] reported the synthesis of ZnO nanoparticlesfrom thermal oxidation of Zn particles, which were produced by the decomposition oforganometallic precursor [Zn(C6H11)2] in a wet anisole. Zn particles can be prepared inthe presence or absence of polymer. In addition, whatever the synthesis and stabilizationmodes, the particles display a uniform size and narrow size distribution. As-synthesizedproducts were characterized by TEM, XRD, and XPS, which were fully consistent withthe results.Gattorno et al. [66] reported a novel synthesis pathway of ZnO nanoparticles with

narrow size distribution from spontaneous hydrolysis of zinc carboxylate salts in a polarbasic aprotic solvent i.e., dimethyl sulfoxide (DMSO) or DMF at room temperature. Thereproducibility of as-synthesized products depends upon the control over water contentand reaction temperature. As the hydrolysis of zinc carboxylates produced ZnO nanopar-ticles with different sizes, solvent basicity and the interaction of DMSO and water play animportant role in the hydrolysis mechanism. To check the stability and optical properties,ZnO colloids were analyzed by UV-vis electronic absorption and emission spectroscopy,and crystallinity was conrmed by powder X-ray diffraction spectroscopy. By HRTEManalysis, it was conrmed that low concentration (2104 M) of zinc cyclohexanebutyrateproduced 2.12 nm average size ZnO nanocrystallites, and zinc acetate produced 3 nmaverage size ZnO nanocrystallites. In addition, if zinc cyclohexanebutyrate was used asa starting material, ZnO nanocrystals with rock salt coexisting with a wurtzite structurewere produced. The presence of rock salt ZnO nanoparticles might be due to the phasetransformation induced by particle size and/or by the interaction of cyclohexanebutyrate-ZnO nanoparticles. Moreover, dynamic light backscattering size measurements of ZnOnanoparticles were also performed in the DMSO colloidal dispersion to detect small indi-vidual nanoparticles and assemblies of ZnO nanoparticles. By more extensive research,it was found that cyclohexanebutyrate acts as a more effective capping agent than acetate.Moreover, although low colloidal (2 104 M) ZnO dispersions in DMSO did not showany occulation or red shifts in 2 months, probably due to the concentrated dynamicstabilizing action of carboxylate ions and solvent molecules, ZnO colloids in DMF werenot stable and readily formed precipitates, which can adhere to glass walls, and pro-duced ZnO lms. This synthesis method is reported as a new, direct, clean, and very easypathway to obtain ZnO nanoparticles and can be applied to other metallic carboxylatesalts to form the corresponding nanostructures metal oxides.A new method to produce zinc oxide nanoparticles by thermal decomposition of zinc

alginate was reported by Baskoutas et al. [67]. The reported method is based on thepreparation of zinc alginate gels by ionic gelation between zinc solution and sodiumalginate. The resulting wet beads were heated at 800 and 450C for 24 h. The structuralmorphologies and crystallinity of the as-synthesized ZnO nanoparticles were character-ized by SEM, TEM, XRD, and micro-Raman spectroscopy. In more detailed observation, itwas reported that ZnO nanoparticles possessed wurtzite structures with single crystallinehexagonal phase conrmed by XRD analysis and SAED. In addition, it was reported thatheating temperature and the kind of zinc agent (i.e., zinc nitrate or zinc acetate) inuencethe size of ZnO nanoparticles. Furthermore, Raman scattering conrmed the existence ofdefects in the nanoparticles.


Wahab et al. [68] reported the synthesis of ZnO nanoparticles from the conversion ofhydrozincite [Zn5(CO3)2(OH)6]. In a typical synthetic process, hydrozincite was preparedby reaction between zinc acetate dihydrate with urea in deionized water at 70C for 2 hvia the solgel method. The quality and composition of the as-grown hydrozincite wereconrmed by XRD analysis with all the characteristic peaks for hydrozincite as well as byFT-IR analysis. Furthermore, as-synthesized plate-like hydrozincite was converted to ZnOnanoparticles with calcination at different temperatures, i.e., at 300, 500, 700, and 900C.The morphological characterization was done by FE-SEM and TEM analysis, which showthat as the calcination temperature increased, particles size also increased in the range of20300 nm. As-synthesized zinc oxide nanoparticles were further characterized by HR-TEM equipped with SAED, and the distance between lattice fringes was conrmed as0.52 nm corresponding to the (0001) crystal plane. Thermogravimetric analysis (TGA) ofas-grown hydrozincite from room temperature to 700C revealed that the primary weightloss, which starts at 130C was due to solvent evaporation and secondary weight lossobserved at 290C was due to phase transformation from hydrated zinc oxide to zincoxide.Niasari et al. [69] presented the synthesis of ZnO nanoparticles by thermal decompo-

sition of [bis(acetylacetonato)zinc(II)]-oleylamine complex. First, zinc acetate-oleylaminecomplex was prepared by the reaction between zinc acetate and oleylamine (C18H37N)at 100C for 90 min in a high purity oxygen atmosphere followed by injection of metal-complex solution into triphenylphosphine (C18H15P) at 220C, resulting in a black colorsolution of [bis(acetylacetonato)zinc(II)]-oleylamine complex. Further, the black solutionwas aged at 210C for 45 min, resulting in a white nanoparticle product that was pre-cipitated by adding excess ethanol to the solution. The resulted white products werecharacterized by XRD, PL spectroscopy, and FT-IR spectroscopy. Morphological charac-terization shows zinc oxide nanoparticles with an average size of 1220 nm, which wasconrmed by SEM and TEM analysis. PL analysis shows the important strong blue-shiftemission band of ZnO nanoparticles, which was attributed to quantum size effect.

4. APPLICATION OF ZnO NANOPARTICLES

4.1. ZnO Nanoparticles: Bio-Friendly Approach

As biomolecules are very sensitive to the solution pH and temperature, there is a generalneed to synthesize metal oxide semiconducting nanoparticles for possible applicationsin biological sensing, biological labeling, drug and gene delivery, and nanomedicines[7073]. In particular, due to their easy fabrication, environmentally friendly nature, andnon-toxic synthesis route, ZnO nanoparticles can provide a better option for various bio-logical applications. However, water solubility and biocompatibility of ZnO nanoparticlesare the main requisites for biological applications. In this regard, Bauermann et al. [74]reported the bio-friendly synthesis of ZnO nanoparticles in aqueous solution at near-neutral pH and low temperature (37C). In a detailed synthesis process, a specic volumeof zinc nitrate hexahydrate was added into the buffer tris(hydroxymethyl)aminomethaneat pH 8 in an incubator for 4 h at 37C. The as-synthesized ZnO nanoparticles werecharacterized by SEM, TEM, XRD analysis, FT-IR spectroscopy, and thermogravime-try/mass spectrometry (TG/MS). Figure 13(a) shows the TEM image of as-synthesizedZnO nanoparticles with a mean diameter of 20 nm, and Figure 13(b) shows the SEMimage of ZnO nanoparticles calcined at 1,000C with a mean diameter of 300 nm.Moreover, Figure 14 represents the XRD pattern of as-synthesized ZnO nanoparti-cles as well as samples calcined at different temperatures, i.e., 80, 600, and 1,000C,which conrmed the crystallinity and stable wurtzite phase of ZnO nanoparticles. XRDcrystal planes peaks became sharper as the samples were heated at higher temper-atures with increasing crystal size. Furthermore, in terms of application, the buffertris(hydroxymethyl)aminomethane represented a standard nontoxic buffer that is inert toa wide variety of chemicals and biomolecules and can be satisfactorily used for a varietyof biological reactions. In addition, this buffer has an important role for the sphericity


(a) (b)

Figure 13. (a) Bright-eld TEM image of as-obtained ZnO nanoparticles precipitated in aqueous solution at pH8 and 37C and (b) SEM image of ZnO particles after heat treatment in air at 1,000C for 2 h. Reprinted withpermission from [74], L. P. Bauermann et al., J. Phys. Chem. B 110, 5182 (2006). 2006, American ChemicalSociety.

of the synthesized ZnO nanoparticles, it acts as a polydentade ligand, which adsorbstrongly on one or more surfaces of ZnO, inhibiting crystal growth, and as a result, nearlyspherical ZnO nanoparticles are produced. Moreover, the buffer also increases the rateof hydrolysis of the zinc-water complex by consuming protons during the reaction andproduced ZnO with some trapped protons in the interstitial sites of ZnO crystals, whichafter further heating at about 180C caused a decrease in the unit cell volume of ZnO dueto the removal of interstitial protons from the crystalline structure of ZnO. The authorsbelieve that during crystallization, new hybrids of ZnO can be produced by introducingbiomolecules.It is well reported that for biological applications the water solubility of a nanomaterial

is the main concern, and generally water solubility is achieved by surface modica-tion with water-soluble ligands, silanization, or encapsulation within block-copolymermicelles. In this regard, Wang et al. [75] reported the synthesis of a water-soluble ZnO-Au nanocomposite having dual functionality, i.e., ZnO provides uorescence, and Au isused for organic functionality for bioconjugation. In a typical synthetic process, rst, ZnOnanocrystals were prepared using zinc acetate dihydrate in ethanol via reuxing with stir-ring at 80C for 3 h. Furthermore, the as-synthesized ZnO nanocrystals were employedas a seeding surface for the nucleation and growth of reduced gold by citrate to pro-duce ZnO-Au nanocrystals having water-soluble characteristics. As-synthesized ZnO-Aunanocrystals were characterized by TEM and XRD and conrmed as dumbbell-shapedZnO-Au nanocrystals having wurtzite ZnO and fcc Au with diameters of 4.9 and 7.1 nm,

a

b

c

d

Figure 14. X-ray powder diffraction patterns of ZnO nanoparticles precipitated in aqueous solution at pH 8and 37C thermally treated in air at (a) 37C, (b) 80C, (c) 600C, and (d) 1,000C. Reprinted with permissionfrom [74], L. P. Bauermann et al., J. Phys. Chem. B 110, 5182 (2006). 2006. American Chemical Society.


respectively. It was reported that surface plasmon absorption band of ZnO-Au NCs wasbroadened and red shifted relative to monometallic Au nanoparticles. In addition, theUV emission intensity of ZnO-Au nanocrystals was 1 order of magnitude higher thanin pure ZnO nanocrystals due to the strong interfacial between ZnO and Au. Moreover,multiphonon Raman scattering of ZnO-Au NCs was enhanced by strong localized elec-tromagnetic of the Au surface plasmon.Reddy et al. [76] reported the toxicity of ZnO nanoparticles to gram-negative [Escherichia

coli (E. coli)], gram-positive [Staphylococcus aureus (S. aureus)] bacterial systems, and pri-mary human immune cells. ZnO nanoparticles were synthesized by forced hydrolysisof zinc acetate at 160C in diethylene glycol media. As-synthesized ZnO nanoparticleswith 13 nm diameter were characterized by TEM, XRD, and UV-vis spectrophotome-tery. The ZnO nanoparticles showed complete inhibition of E. coli growth at concentra-tions 3.4 mM, whereas growth of S. aureus was completely inhibited for concentrations1 mM. In a more detailed observation, ow cytometry viability assays using a two colorlive/dead Backlight kit, demonstrated that growth-inhibiting properties of ZnO nanopar-ticles corresponded to the loss of cell viability, but identical particles have minimal effectson primary human T cell viability at concentrations toxic to gram-negative and gram-positive bacteria. These observations conrmed the toxic nature of ZnO nanoparticles fordifferent bacterial systems, which could lead to biomedical and antibacterial applications.Another approach regarding the use of ZnO nanoparticles in biological applications

was recently reported by Hanley et al. [77]. The authors reported the preferential killingof cancer cells and activated human T cells using ZnO nanoparticles. For the synthesis of813 nm ZnO nanoparticles, the authors adopted the forced hydrolysis of zinc acetate at160C in DEG. Then, ZnO nanoparticles were reconstituted in phosphate buffered saline(PBS) solution. After reconstitution, nanoparticles were sonicated for 10 min and immedi-ately vortexed before being added to cell culture. The response of normal human cells toZnO nanoparticles under different signaling environments was examined and comparedto the response of cancerous cells. In addition, ZnO nanoparticles exhibited a strongability to kill cancerous T cells (2835) compared to normal cells. Moreover, it wasobserved that activation state of the cell contributes to the nanoparticle toxicity, as rest-ing T cells display a relative resistance while cells stimulated through the T cell receptorand CD28 costimulatory pathway show greater toxicity, which results in a direct rela-tion to the level of activation. It was reported that appearance of toxicity was due to theinvolvement of generated reactive oxygen species, as it was found that cancerous T cellsproduced higher inducible levels than normal T cells. Furthermore, ZnO nanoparticlesinduced apoptosis in Jurkat T cells, which is shown in Figure 15. To analyze the inducedapoptosis, two types of samples were prepared: (1) untreated cells and (2) cells treatedwith 0.3 Mm nanoparticles for 20 h or treated with 100 nM okadaic acid for 20 h (positivecontrol) and then stained with green uorescent annexin V antibody to detect apoptoticmembrane and stained with red uorescent dye PI to detect permeable membranes usingthe Vybrant apoptosis assay kit #2 (Molecular Probes). In terms of characterization, cellswere visualized by confocal microscopy which is shown in Figures 15(AC) for controlcells not treated with nanoparticles. Figure 15(A) shows control differential interferencecontrast (DIC), Figure 15(B) shows a control DIC image with green and red uorescenceoverlay, and Figure 15(C) shows a control green and red uorescence image. Cells treatedwith ZnO nanoparticles are shown in Figures 15(DG), in which Figure 15(D) showstreated nanoparticles in the DIC image, Figure 15(E) shows a DIC image with green andred urorescence overly, Figure 15(F) shows a green and red urorescence image, andFigure 15(G) shows an additional green and red urorescence image of nanoparticle-treated cells of lower magnication. To further clarify naonparticle-induced apoptosis,cells were left untreated (Fig. 16(A)), cells were treated with 100 nM okadaic acid for 20 has a positive control for apoptosis (Fig. 16(B)), and cells were treated with 0.3 mM ZnONP for 20 h (Fig. 16(C)) and then stained with DNA dye, acridine orange, and visualizedby uorescent microscopy. In Figures 16(B and C), arrows indicate the typical apoptoticcells characterized by shrunken appearance and condensed or fragmented nuclei. Col-lectively, these results indicate that ZnO NPs induce apoptosis in Jurkat T cells. These


(A) (D)

(B) (E)

(C) (F)

(G)

Figure 15. ZnO nanoparticle-induced apoptosis in Jurkat T cells. Cells were left untreated, treated with 0.3 mMZnO NP for 20 h, or treated with 100 nM okadaic acid for 20 h (positive control) and stained with a greenuorescent annexin V antibody to detect apoptotic membranes and with the red uorescent dye PI to detectpermeable membranes using the Vybrant apoptosis assay kit #2 (Molecular Probes). Cells were visualized byconfocal microscopy and representative images are shown. (A)(C) control cells not treated with nanopar-ticles, (A) control DIC image, (B) control DIC image with green and red uorescence overlay, (C) controlgreen and red uorescence image. (D)(G) cells treated with nanoparticles; (D) nanoparticle-treated DIC image,(E) nanoparticle-treated DIC image with green and red uorescence overly, (F) nanoparticle-treated green andred uorescence image, (G) an additional green and red uorescence image of nanoparticle-treated cells oflower magnication. Reprinted with permission from [77], C. Hanley et al., Nanotechnology 19, 295103 (2008). 2008, Institute of Physics Publishing Ltd.

observations may provide the basis for the development of new rational strategies toprotect against NP toxicity or enhance the destruction of disease-causing cell types suchas cancer cells.Padmavathy et al. [78] reported the synthesis of ZnO nanoparticles with various sizes

and then investigated the antibacterial activity of the as-synthesized ZnO nanoparti-cles using a standard microbial method. In a typical synthetic process, two methodswere employed. In the rst method, zinc nitrate and sodium hydroxide were mixedat room temperature with stirring for 2 h, and the resulting zinc hydroxide precipitatewas washed with distilled water until pH became neutral, followed by dropwise addi-tion of H2O2 to produce zinc peroxide translucent solution, which was then heated at350C to produce ZnO with active surface oxygen species. In another method, surface-modied ZnO nanocrystals were prepared by dissolving zinc acetate in 2-propanol at80C with stirring, followed by the addition of 2-mercaptoethanol, which was continu-ously stirred for 2 h. The resulting mixture was then hydrolyzed by adding NaOH in2-propanol, followed by ultrasonic agitation for 2 h, and then the synthesized productswere washed and dried. The as-synthesized products were characterized by XRD, TEM,and PL spectroscopy. Figure 17 shows the TEM micrograph and SAED pattern of ZnOnanoparticles formed by the precipitation method (Fig. 17(a)) and formed by base hydrol-ysis in propanol medium (Fig. 17(b)). It was observed that when capping moleculeswere used, the kinetics of nucleation and accumulation were affected in such a way


(A) (B)

(C)

Figure 16. Detection of apoptotic morphological changes in Jurkat cells treated with ZnO NP. Cells were leftuntreated (A), or treated with 100 nM okadaic acid for 20 h as a positive control for apoptosis (B), or treatedwith 0.3 mM ZnO nanoparticles for 20 h (C) and stained with acridine orange and visualized by uorescentmicroscopy. Arrows indicate typical apoptotic cells characterized by a shrunken appearance and condensed orfragmented nuclei. Reprinted with permission from [77], C. Hanley et al., Nanotechnology, 19, 295103 (2008). 2008, Institute of Physics Publishing Ltd.

that the growth rate of large particles decreased while that of small particles remainedthe same, which in turn produced particles with narrow size distribution as comparedto particles synthesized without a capping molecule. Furthermore, as-synthesized ZnOnanoparticles with different concentrations underwent bacteriological tests by standardmicrobial method in terms of minimum inhibitory concentration (MIC) and disk dif-fusion, which were performed in Luria-Bertani and nutrient agar media on solid agarplates and in liquid. Figure 18 shows the bacterial efcacies with ZnO suspension withthee different nanoparticle sizes after 24 h incubation of aliquots in the lowest concen-tration range (0.011 mM) and the highest concentration range (5100 mM). As a result,enhanced bioactivity of smaller particles was recorded. For smaller ZnO nanoparticles,more particles were needed to cover a bacterial colony (2 m), and more particles resultedin the generation of a large number of active oxygen species, which kill bacteria moreeffectively. As a result, it was observed that ZnO nanoparticles were more abrasive thanbulk ZnO and thus contributed greater mechanical damage of the cell membrane andenhanced the anti-bacterial effect of ZnO nanoparticles. Reported observations and resultsconrmed that ZnO nanoparticles may be applicable to medical devices that are coatedwith nanoparticles against microbes.

4.2. Solar Cells, Photocatalytic, Photoluminescence, and SensorApplication of ZnO Nanoparticles

Regarding ZnO nanoparticle application in solar cells, Suliman et al. [79] reported thesynthesis of ZnO nanoparticles with average diameter of 30 nm by using zinc chlorideas a precursor and NaOH as a base in a PVP solution of water at 160C for 8 h viathe hydrothermal method. The as-synthesized structures were characterized by TEM,SEM, and XRD analyses. Absorption spectrum was measured using a UV-vis spectropho-tometer. To make a ZnO lm over transparent conducting glass (TCO), ZnO nanoparti-cles were dissolved in ethanol and then applied over the TCO surface using the doctorblade technique, which resulted in a 6 m thick lm of ZnO nanoparticles over theTCO, and nally it was annealed for 30 min at 450C. To make dye-sensitized ZnOthin lms, the lm was soaked in 0.5 mM ethanol solution of ruthenium complex,cis bis(isothiocyanato)bis(2,2-bipyridyl-4,4-dicarboxylato)ruthenium (II) (N3 dye). TheTCO acted as a counter electrode on which 340 nm thick layer of Pt was deposited bysputtering. Electrolyte was made by 0.03 M I2/0.3 M LiI in propylene carbonate (PC)which was attracted into the interelectrode space by capillary forces, and then the result-ing lms of 0.4 cm2 were illuminated through the conducting glass support with an Oriel


(a)

(b)

Figure 17. (a) TEM micrograph and SAED pattern (inset) of ZnO formed by precipitation method. (b) TEMmicrograph and SAED pattern (inset) of ZnO formed by base hydrolysis in propanol medium. Reprinted withpermission from [78], N. Padmavathy et al., Sci. Technol. Adv. Mater. 9, 35004 (2008). 2008, Institute of PhysicsPublishing Ltd.

91192 AM 1.5 solar simulator as the light source. ZnO nanoparticles lms were thenmeasured by photocurrentvoltage (IV ) which gives a ll factor of 0.513, short-circuitcurrent of 1.2 mA/cm2, open-circuit voltage of 573 mV, and an overall light-to-electricityconversion efciency of 0.75%.Recently, Zhang et al. [80] reported the synthesis of polydisperse aggregated ZnO

nanocrystals and their application in dye-sensitized solar cells. In a typical synthetic pro-cess, ZnO aggregates were synthesized via polyol-mediated precipitation by using zincacetate in diethylene glycol at 160C with reuxing. It was reported that with adjust-ment in zinc acetate concentration, rate of heating, and the amount of stock solutionthat is added, one can readily control the size of individual ZnO aggregates. To make aphotoelectrode lms, ZnO aggregates were deposited by drop-cast method on a uorine-doped tin oxide (FTO) glass substrate, and thickness of lms depended on the numberof drops. Finally, the ZnO lms were heated at 350C in air for 1 h to remove residualorganic chemicals. The as-synthesized product morphologies were characterized by SEM


(a) (b)

Figure 18. (a) and (b) Bactericidal efciency of samples 1 and 2 and bulk ZnO suspensions at different concen-trations. Reprinted with permission from [78], N. Padmavathy et al., Sci. Technol. Adv. Mater. 9, 35004 (2008). 2008, Institute of Physics Publishing Ltd.

and XRD. Figure 19 shows the SEM images of hierarchically-structured ZnO lms withpolydisperse aggregates of ZnO with a size distribution of 120360 nm (Fig. 19(a)) and120310 nm (Fig. 19(b)), lms consisting of monodisperse aggregates having averagesizes of 350, 300, 250, and 210 nm (Figs. 19(cg)), magnied SEM image (Fig. 19(h)),and a schematic illustration to show the structure of the ZnO aggregates formed byclosely packed nanocrystallites 12 nm in size. To make the ZnO lm sensitized, it wasimmersed into N3 dye for 20 min and then rinsed with ethanol to get rid of the excessdye. Then, cells were constructed using a platinum-coated silicon wafer as the counterelectrode and the ZnO lm as a working electrode. These two electrodes were placedside by side with 20 m separating space, where the I/I3 electrolyte was injected withcapillarity. The solar cells performance was measured by the irradiation of air mass(AM) 1.5 simulated sunlight at 100 mW cm2. Figure 20 shows the typical IV curve

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

Figure 19. SEM images of hierarchically-structured ZnO lms with submicrometer-sized aggregates. SEMimages of the lms consisting of polydisperse aggregates with a size distribution of (a) 120360 nm and(b) 120310 nm. SEM images of the lms consisting of monodisperse aggregates with average sizes of (c) 350 nm,(d) 300 nm, (e) 250 nm, and (f) 210 nm. (h) is a magnied SEM image, and (i) is a schematic illustration toshow the structure of ZnO aggregates formed by closely packed nanocrystallites. Reprinted with permissionfrom [80], Q. Zhang et al., Adv. Funct. Mater. 18, 1 (2008). 2008, Wiley-VCH Verlag GmbH & Co.


Figure 20. An example of an IV curve for dye-sensitized ZnO solar cells under AM 1.5 irradiation. Thesquare shadow is plotted to illustrate the determination of the maximal power output of the solar cells.Reprinted with permission from [80], Q. Zhang et al., Adv. Funct. Mater. 18, 1 (2008). 2008, Wiley-VCH VerlagGmbH & Co.

for the polydisperse aggregated sample, which demonstrated the derivation of the open-circuit voltage Voc, the short-circuit current density Isc, and the maximum output powerdensity Pmax. The overall energy conversion efciency and ll factor FF can be calcu-lated sequentially by = Pmax/Pin and F F = Pmax/Voc Isc). As a result, it was found thatoverall energy-conversion efciency of the cells could be affected by either average size orsize distribution of the ZnO aggregates. The highest overall energy-conversion efciencyof 4.4% was achieved by using lms formed by polydisperse ZnO aggregates with broadsize distribution of 120360 nm. In addition, variation in solar cell efciency was observeddue to light scattering, which was generated by submicrometer sized aggregates with asize distribution comparable to the wavelength of incident light, which could extend thetravelling distance of light within the photoelectrode lm. Moreover, the observed highefciency with polydisperse aggregates lms also due to its ability to provide the lmwith a closely packed structure, which was benecial to the transport of electrons in thephotoelectrode lm.Regarding the application of ZnO nanoparticles in photocatalytic activity, Houskova

et al. [81] reported the synthesis of zinc sulde (ZnS) nanoparticles by homogenoushydrolysis of zinc sulfate and thioacetadmide (TAA) at 80C and then its conversion toZnO nanoparticles with annealing at temperature above 400C in an oxygen atmosphere.The as-synthesized ZnO nanoparticles were characterized by XRD and SEM, HRTEM andSAED. The Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methodswere used to determine surface area and porosity, which showed pore sizes in the rangeof 26 nm. Further, photocatalytic activities of as-synthesized ZnO nanoparticles weredetermined by decomposition of Orange II dye in aqueous solution under UV irradiationof 365 nm wavelength. Samples heated at 700C exhibited a good photocatalytic activity,k = 00379 min1 (k for P25 Degussa is 0.0222 min1. Moreover, it was reported that as-synthesized ZnO nanoparticles were evaluated for their non-photochemical degradationof chemical warfare agents to non-toxic products, which established a good decomposi-tion of the mustard gas.Xu et al. [82] reported the synthesis of hierarchically assembled porous ZnO nanoparti-

cles through a self-assembled pathway using surface-modied colloidal ZnO nanocrystalsas building blocks and P-123 copolymers as the template in aqueous solution. In a typicalsynthetic process, rst, colloidal ZnO nanocrystallites were prepared by hydrolysis ofzinc acetate in a LiOH-ethanol solution, and then ZnO colloids were mixed with taurinein deionized (DI) water with taurine/ZnO molar ratio as 1:1.4. The solution pH wasadjusted to 5.0 by adding 1 M HCL followed by vigorous stirring at 25C for 24 h. Theresulting mixture was marked as sample A, and then in another experiment P-123 wasmixed with DI water at pH 5 and stirred at 25C for 24 h (sample B). Finally, a solution


(a) (b)

Figure 21. (a) Low-magnication cross-sectional TEM image and (b) HRTEM image of the as-obtained porousZnO nanoparticles. The inset in (b) clearly shows the lattice fringes. Reprinted with permission from [82], F. Xuet al., Chem. Mater. 19, 5680 (2007). 2007, American Chemical Society.

was added dropwise into sample B with stirring, and after 3 h stirring, the resultingmixture was heated in an autoclave at 70C for 3 days, followed by washing and drying.and then the product was calcined at 400C for 57 h. The as-synthesized ZnO productswere then characterized by XRD, SEM, TEM, HRTEM, and PL spectroscopy. Figure 21shows the TEM image of larger ZnO particles, which conrmed that these larger particlesmay contain many smaller ZnO nanoparticles with uniform size. The HRTEM (Fig. 21(b))image clearly indicates the contrast difference in each individual nanoparticle havingpores of 3 nm with average overall particle size 17 nm. Figure 21(b) (inset) showsanother HRTEM image of the lattice fringes of the nanocrystal with a spacing of 0.26 nm,correspond to the interplanar distance of (002) plane of hexagonal ZnO. Figure 22 illus-trates the detailed self-assembly processes involving the functionalization of individualZnO nanoparticles. Furthermore, on the basis of calorimetric measurements, the surfaceenthalpy of the hydrated porous ZnO is 142 021 J/m2, which is in good agreementwith that of ZnO nanoparticles, which again supported the presence of self-assembledZnO nanocrystals in nanoporous ZnO. Finally, the photocatalytic activity of porous ZnOnanoparticles was tested on the photodegradation of phenol under ambient conditions.Figure 23 shows the emission spectra of residual phenol in aqueous solution under expo-sure to UV light for various times in the presence of porous ZnO nanoparticles, TiO2nanoparticles (PC-500), commercial ZnO powder, and ZnO nanopowder. The porous ZnO

Figure 22. Schematic illustration of the self-assembly process, involving the functionalization of individual ZnOnanoparticles. Reprinted with permission from [82], F. Xu et al., Chem. Mater. 19, 5680 (2007). 2007, AmericanChemical Society.


(a) (b)

(c) (d)

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Figure 23. PL emission spectra of the residual phenol under exposure to UV light in the presence of (a) porousZnO nanoparticles, (b) TiO2 nanoparticles (PC-500), (c) commercial ZnO powder, and (d) ZnO nanopowder.(e) Curves of the residual fraction of the phenol as a function of UV irradiation time when using () porousZnO nanoparticles, () TiO2 nanoparticles (PC-500), () commercial ZnO powder, and () ZnO nanopowder.Reprinted with permission from [82], F. Xu et al., Chem. Mater. 19, 5680 (2007). 2007, American ChemicalSociety.

nanoparticles show superior activity to TiO2 nanoparticles, because ZnO absorbs overa large fraction of UV light and the corresponding threshold of ZnO is 425 nm, whileother ZnO (commercial and nanopowder ZnO) had less activity than the porous ZnObut higher activity than TiO2 due to the unique surface features and higher surface area.These results indicate that porous ZnO nanoparticles had good photoreactivity in thedecomposition of phenol in waste water.Functionalized ZnO nanoparticles that show liquid-like behavior were synthesized

and their PL properties were reported by Bourlinos et al. [83]. First, ZnO nanocrys-tals (37 nm) were prepared by alkaline hydrolysis of zinc acetate in the presenceof LiOH H2O in absolute ethanol for 45 days. As-prepared ZnO colloid was pre-cipitated by adding excess of heptane, followed by centrifugation and drying at


room temperature. Surface modication of ZnO nanoparticles with charged organosi-lane [(CH3O)3Si(CH2)3N+(CH3)(C10H21)Cl] was carried out in an alkaline environment.The Cl counter anions in the nanosalts could be readily exchanged by C9H19-C6H4-(OCH2CH2)20O(CH2)3SO3 ions, yielding the corresponding sulfonate nanosalt as a waxysolid that melts at 30C, resulting in a uid with considerably higher viscosity than thecorresponding potassium sulfonate salt. The as-synthesized products were characterizedby XRD, TEM, FT-IR, and TGA/DTA analysis. Furthermore, the PL quantum yield of theZnO sulfonate nanosalt (0.065 mgmL1 in acetonitrile), corresponding to the green-yellowemission band, was measured relative to that rhodamine 6G (R6G, 30 m in methanol).Moreover, it was suggested by the authors that this property prole could lead to newinnovative applications in the areas of optics and photonics, and tuning the emissiontowards the UV (by doping or different chemical processing) may activate lasing in theZnO nanoparticles, thus leading to the development of uid/exible laser sources.Masuda et al. [84] reported PL from ZnO nanoparticles embedded in an amorphous

matrix. In a typical synthetic process, Zn(NO3)2 [10 x M], Al(NO3)3 (x M), and urea(3.3 M) were dissolved in distilled water and kept at 90C for 2 days to precipitate out.Then the precipitate was washed with distilled water and further calcinated at 200900Cfor 3 h in air to synthesize ZnO nanoparticles dispersed in an amorphous matrix. Themorphologies of the as-synthesized products were characterized by SEM, chemical com-position was determined by inductively coupled plasma, ZnO nanoparticles in an amor-phous matrix were observed by TEM, crystallinity was observed by XRD, PL imagesof ZnO nanoparticles were excited by visible light, UV light at 312 nm, or UV light at254 nm, PL spectra were evaluated by a uorescence spectrometer using excitation lightat 287 nm, and the PL at low temperature was evaluated with a cryostat using liquidhelium. Figure 24 shows the TEM images of ZnO nanoparticles in the amorphous matrixprepared with Al addition at 42% (Figs. 24(a1 and a2)) or 23% (Figs. 24(b1 and b2)) aftercalcination at 250C for 3 h in air. It was conrmed that ZnO nanoparticles 47 nm indiameter were prepared by addition of 23% Al (Fig. 24(b)), and crystallization of ZnOwas not suppressed compared to 23 nm diameter with 42% Al addition (Fig. 24(a)).Furthermore, control of ZnO nanoparticle size by addition of Al was conrmed by XRDmeasurement. Figure 25 shows the PL images of ZnO nanoparticles (14, 5.5, 4, or 2.5 nmin diameter) with Al addition excited by visible light, UV light at 312 nm, or UV light at254 nm. It was conrmed that ZnO with no Al addition (0%) emitted only slightly underUV light 312 nm or UV light 254 nm. As a result, the PL intensity of the ZnO nanopar-ticles drastically increased with Al addition, and the 2.5 nm ZnO nanoparticles (42% Al)glowed brightly under both UV 312 nm and UV 254 nm. However, excessive Al additiondecreased the PL intensity due to the suppressed crystallization of ZnO nanoparticles,which is essential for PL. It is shown in Figure 25 that

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