ZnO Nanoparticles: Growth, Properties, and

CHAPTER 4

ZnO Nanoparticles: Growth, Properties, and Applications Mohammad Vaseem1 , Ahmad Umar2 , Yoon-Bong Hahn1 1

School of Semiconductor and Chemical Engineering and BK21 Centre for Future Energy, Materials and Devices, Chonbuk National University, Chonju 561-756, South Korea 2 Department of Chemistry, Faculty of Science, Advanced Materials and Nano-Engineering Laboratory (AMNEL), Najran University, P. O. Box 1988, Najran 11001, Kingdom of Saudi Arabia CONTENTS 1. 2. 3. 4.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Crystal Structure of ZnO . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanoparticles of ZnO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Application of ZnO Nanoparticles . . . . . . . . . . . . . . . . . . . 4.1. ZnO Nanoparticles: Bio-Friendly Approach . . . . . . . . 4.2. Solar Cells, Photocatalytic, Photoluminescence, and Sensor Application of ZnO Nanoparticles . . . . . . . . . . 4.3. Cosmetic Application of ZnO Nanoparticles . . . . . . . Summary and Future Directions . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. INTRODUCTION Today, nanotechnology (NT) is operating in various fields of science via its operation for materials and devices using different techniques at nanometer scale. Nanoparticles are a part of nanomaterials that are defined as a single particles 1–100 nm in diameter. From last few years, nanoparticles have been a common material for the development of new cutting-edge applications in communications, energy storage, sensing, data storage, optics, transmission, environmental protection, cosmetics, biology, and medicine due to their important optical, electrical, and magnetic properties. In particular, the unique properties and utility of nanoparticles also arise from a variety of attributes, including the similar size of nanoparticles and biomolecules such as proteins and polynucleic acids. [1] Additionally, nanoparticles can be fashioned with a wide range of metals ISBN: 1-58883-170-1 Copyright © 2010 by American Scientific Publishers All rights of reproduction in any form reserved.

Metal Oxide Nanostructures and Their Applications Edited by Ahmad Umar and Yoon-Bong Hahn Volume 5: Pages 1–36

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ZnO Nanoparticles: Growth, Properties, and Applications

and semiconductor core materials that impart useful properties such as fluorescence and magnetic behavior [2]. Moreover, unlike their bulk counterparts, nanoparticles have reduced size associated with high surface/volume ratios that increase as the nanoparticle size decreases. As the particle size decreases to some extent, a large number of constituting atoms can be found around the surface of the particles, which makes the particles highly reactive with prominent physical properties. Nanoparticles of particular materials show unique material properties, hence, manipulation and control of the material properties via mechanistic means is needed. In addition, synthesis of nanoparticles having uniform 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 synthetic routes enabling the precise control of the morphology and size of the nanoparticles. In addition, nanoparticle synthesis can be possible via liquid (chemical method), solid, and gaseous media [3–15], but due to several advantages over the other methods, chemical methods are the most popular methods due to their low cost, reliability, and environmentally friendly synthetic routes, and this method provides rigorous control of the size and shape of the nanoparticles. In general, nanoparticles with high surface-to-volume ratio are needed, but the agglomeration of small particles precipitated in the solution is the main concern in the absence of any stabilizer. In this regard, preparations of stable colloids are important for nanoparticle growth. In addition, nanoparticles are generally stabilized by steric repulsion between particles due to the presence of surfactant, polymer molecules, or any organic molecules bound to the surface of nanoparticles. Sometimes van der Waals repulsion (electrostatic repulsion) also plays important role in nanoparticles stabilization. With all the issues related to nanoparticle synthesis, there are various types of nanoparticles reported in the literature, e.g., metal nanoparticles, metal oxide nanoparticles, and polymer nanoparticles. Among all these, metal oxide nanoparticles stand out as one of the most versatile materials, due to their diverse properties and functionalities. Most preferentially, among different metal oxide nanoparticles, zinc oxide (ZnO) nanoparticles 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, window materials for displays, solar cells, and drug-delivery [16–20]. ZnO is an attractive material 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) optoelectronics, including laser developments. In addition, due to its non-centrosymmetric crystallographic phase, ZnO shows the piezoelectric property, which is highly useful for the fabrication of devices, such as electromagnetic coupled sensors and actuators [21].

2. CRYSTAL STRUCTURE OF ZnO Crystalline ZnO has a wurtzite (B4) crystal structure at ambient conditions. The ZnO wurtzite structure has a hexagonal unit cell with two lattice parameters, a and c, and belongs to the space group of C46V or P63 mc. Figure 1 clearly shows that the structure is composed of two interpenetrating hexagonal closed packed (hcp) sublattices, in which each consist of one type of atom (Zn or O) displaced with respect to each other along the threefold c-axis. It can be simply explained schematically as a number of alternating planes stacked layer-by-layer along the c-axis direction and composed of tetrahedrally coordinated Zn2+ and O2− . The tetrahedral coordination of ZnO gives rise to the noncentrosymmetric structure. In wurtzite hexagonal ZnO, each anion is surrounded by four cations at the corners of the tetrahedron, which shows the tetrahedral coordination and hence exhibits the sp3 covalent-bonding. The detailed properties of ZnO are presented in Table 1.

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

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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 [22–35]. In the present chapter, we mainly focus on ZnO nanoparticles synthesized by either the sol–gel method (solution method) or the hydrothermal method. As the solution method presents a low cost and environmentally friendly synthetic route, most of the literature for ZnO nanoparticles is based on the solution method. In addition, synthesis of ZnO nanoparticles in the solution requires a well defined shape and size of ZnO nanoparticles. In this regards, Monge et al. [36] reported room-temperature organometallic synthesis of ZnO nanoparticles of controlled shape and size in solution. The principle of this experiment was based on the decomposition of organometallic precursor to the oxidized material in air. It was reported [37] that when a solution of dicyclohexylzinc(II) compound [Zn(c-C6 H11 )2 ] in tetrahydrofuron (THF) was left standing at room temperature in open air, the solvent evaporated slowly and left a white luminescent residue, which was further characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) and confirmed Table 1. Physical properties of ZnO. Properties Lattice parameters at 300 K —a0 (nm) —c0 (nm) —c0 /a0 Density (g/cm3 Stable phase at 300 K Melting point ( C) Thermal conductivity (Wcm−1 C−1 ) Linear expansion coefficient ( C) Static dielectric constant Refractive index Band gap (RT) Band gap (4 K) Exciton binding energy (meV) Electron effective mass Electron Hall mobility at 300 K (cm2 /Vs) Hole effective mass Hole Hall mobility at 300 K (cm2 /Vs) ∗

Value for an ideal hexagonal structures.

ZnO 0.32495 0.52069 1.602(1.633∗ ) 5.606 Wurtzite 1975 0.6, 1-1.2 a0 : 6.5 cm3 × 10−6 c0 : 3.0 cm3 × 10−6 8.656 2.008 3.370 eV 3.437 eV 60 0.24 200 0.59 5–50

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as agglomerated ZnO nanoparticles with a zincite structure having lack of defined shape and size. Monge et al. used a modified experimental condition using a ligand of long chain amine, i.e., hexadecylamine (HDA) under an argon atmosphere in addition to the above-mentioned solution, which resulted in well defined ZnO nanoparticles. It was observed that shape, size, and homogeneity of the as-synthesized products depend upon various reactions conditions, i.e., the nature of the ligand, the relative concentration of reagents, the solvent, the overall concentration of reagents, the reaction time, the evaporation time, and the reaction/evaporation temperature. In addition, when a similar reaction is carried out in dry air, it leads to agglomerated ZnO nanoparticles displaying no defined shape or size. In an elaborative manner, they analyzed that if the concentration of reagents in solution increases from 0.042 to 0.125 mol L−1 nano-objects of higher aspect ratio will be formed. Exchanging THF for toluene or heptane produces nanoparticles of isotropic 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 homogenous nanodisks having size 4.1 nm (Fig. 2(b)). Reducing the reaction time under argon to 5 min prior to oxidation leads to shorter nanorods ∼58 × 27 nm in size. Increasing the reaction temperature leads to isotropic disk-shaped nanoparticles. Exchanging HDA for dodecylamine (DDA) or octylamine (OA) also leads to disks with mean diameters of 3.0 for DDA and 4.0 nm for OA (Figs. 2(c and d)). In addition, nuclear magnetic resonance (NMR) studies (Fig. 3) confirmed that throughout the oxidation process, the amine ligand remains coordinated to zinc and suggested that this coordination participates in controlling the growth of ZnO nanoparticles. Kahn et al. [38] reported the detailed experimental procedure based on the same synthetic route with different experimental parameters, i.e., the effects of solvent, ligand, concentration, time, and temperature. They explained that the reaction of organometallic complexes with oxygen or moisture leads exothermally to a hydroxide material, but in this case they did not observe any traces of hydroxide,

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Figure 2. TEM micrographs of ZnO nanoparticles. (a) ZnO nanorods grown under standard conditions. (b) ZnO nanodisks following a slow oxidation/evaporation process in THF (2 weeks), (c) ZnO nanodisks using DDA instead of HDA as the stabilizing ligand under standard conditions. (d) ZnO nanodisks using OA instead of HDA 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.


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Figure 3. 13 C{1 H} 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-VCH Verlag GmbH & Co.

indicating that both hydrolysis and condensation take place at room temperature. This can be due to either exothermic oxidation of the organometallic precursor or to the presence of amines, which are bases in solution medium. However, the observation of forming oxide even without amines confirmed that the oxidation reaction of organometallic precursor is exothermic enough to lead the oxide, and during this process the ligands must control the shape of the nanoparticles by kinetic control of the oxidation reaction. In general, the mechanism of nanoparticle synthesis involves three steps, namely nucleation, growth, and ripening. In this case, water molecules could be responsible for the nucleation step by reacting with the molecular precursor and forming nuclei. In the process, most of the precursor remains intact after this step, and the growth of the particles can occur when the solution is exposed to moisture and air. Moreover, as-synthesized ZnO nano-objects dissolved in most of the common organic solvents are luminescent solutions 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 emission at 370 nm and one deep green emission at 585 nm. Interestingly, these two emission bands are not quenched by the solvents and can be observed at room temperature, both in solution and in the solid state. As from the above report, it is confirmed that the solvent has an important effect on the morphology of ZnO nano-objects. Andelman et al. [39] further elaborated the solvent effect using different solvents, i.e., trioctylamine (TOA), 1-hexadecanol (HD), and 1-octadecene (OD). It was found that during synthetic process using TOA solvent yields nanorods, HD solvent yields nanotriangles, and OD solvent yields spherical nanoparticles. Figure 4 shows the typical XRD spectra for nanotriangles, spherical nanoparticles, and nanorods. The relative intensity of the peaks of nanotriangles and spherical nanoparticles matches the bulk, signifying no preferred orientation. Spherical nanoparticles prepared from octadecene have diameters of 12–14 nm. Figure 5 shows the TEM images of ZnO nanotriangles at various degrees of tilt. The degree of tilt is indicated in the top left-hand corner. At all angles, the shape remains triangular. As the different capping agents have varying ability to stabilize certain planes, which leads to different particle morphologies, the case observed here with varying solvents also plays a significant role in stabilizing specific crystallographic planes of the growing nanocrystal. The use of TOA 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 hexadecanol as a solvent is due to its moderate coordinating capacity and its relatively weak ligand

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

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

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


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Another approach was performed by Ayudhya et al. [40] to show the effect of solvent over the morphology of as-synthesized ZnO products. In their work, single crystalline ZnO nanoparticles in different aspect ratios were synthesized by a solvothermal method using various organic solvents. In a typical synthetic process, zinc acetate as a precursor suspended in four various types of organic solvents was heated in an autoclave in the range of 250–300 C, depending upon the solvent, used for a 2 h reaction process. 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 aromatic solvents (i.e., benzene, toluene, o-xylene, and ethylbenzene). The as-synthesized ZnO products were characterized by XRD, SEM, and TEM. The typical XRD pattern synthesized in various groups of organic solvents (Fig. 6) confirmed that the crystalline phase of ZnO was hexagonal without any impurities. The ZnO crystals grow along the same lattice direction, regardless of the solvent used. SEM micrographs of ZnO nanoparticles synthesized in glycols are shown in Figure 6. From the SEM images, it is clearly observed that the products synthesized in glycols produced polyhedral crystals with the lowest aspect ratios, whereas those synthesized in alcohols produced moderate aspect ratios. The products obtained using n-alkanes or aromatic compounds as solvents produced high aspect ratio ZnO nanorods. The morphology of ZnO nanoparticles synthesized in alcohols strongly depends upon the chain length of the alcohol molecules, whereas a lesser effect is shown with chain length of glycols, and for n-alkanes and aromatic solvents, chain length effect is unnoticeable. As for the growth of ZnO nanoparticles, there is concern that the anhydrous zinc acetate precursor can undergo decomposition and form ZnO nuclei. The thermal stability of zinc acetate has been reported [41–42] to depend on its interaction with the solvent. Moreover, the dielectric constant of the used solvent is attributed to the high temperature requirement in the case of n-alkanes and aromatic compounds having low-dielectric constants compared to glycols and alcohols having high-dielectric constant required low temperature (250 C). In addition, negatively charged molecules adsorbed over the positively charged Zn surface of the (0001) facet of the crystal could retard the growth of crystals in the (0001) direction, which leads to nonpreferential growth of the crystals. The same phenomenon occurred when glycols as solvents, having two hydroxyl groups at both ends, could adsorb onto the (0001) surface of the ZnO crystal, which finally led to the formation of ZnO nanoparticles instead of ZnO 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.

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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 TEM micrographs. (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 nanoparticles. Although the dielectric constant of the solvent is the prime reason for the different morphology of ZnO nanoparticles in this solvothermal synthesis, more detailed characterization 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 typical synthetic process, ZnO grains and ZnO rods were obtained with various aspect ratios at 60 C with 2 h reaction in aqueous solution of ZnSO4 via an acidic route (pH 5.6) with addition of NaOH and in a basic solution of NaOH via a basic route (pH 13.6) with addition of ZnSO4 , respectively. The observed aspect ratios were changed by this synthetic route, although the final pH of the solution was the same. The detailed morphological characterizations were performed by XRD, field emission scanning electron microscopy (FESEM), and field emission transmission electron microscopy (FETEM). XRD analysis confirmed the wurtzite ZnO type structures with peak broadening in the case of the acidic route compared to the basic route, which further confirmed the formation of smaller particles via the acidic route. FESEM images also confirmed the formation of ZnO particles and rods via acidic and basic routes, respectively. Further cumulative undersize distribution of precipitated ZnO particles confirmed that the particle shapes were spherical or ellipsoidal 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 Scherrer’s formula using the (100) and (002) diffraction peaks observed in XRD spectra. Although the value of [OH− ]/[Zn2+ ] and the final pH were the same in the acidic and basic routes, the number of ZnO nuclei formed via the acidic route was deduced to be much higher than that obtained via the basic route because the degree of saturation at the initial stage of the acidic route was extremely high due to the low solubility of ZnO. Thus, most of the precursor species steeply precipitated as nanograins. On the other hand, ZnO nanorods formed in the basic route due to limitation of formed ZnO nuclei at the initial stage, and thus particle size increased via subsequent growth in the progressive stage. To check the effect of water addition in the precursor-methanol solution for the morphological evolution of ZnO particles, Wang et al. [44] performed reactions based on hydrolysis of zinc acetate in methanol solvent at 60 C for 24 h and deposited over


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Al2 O3 ceramic plate via the chemical deposition method. As the water/methanol volume ratio increased, the shape of the ZnO particles changed from irregular particles to plates and then from plates to regular cones, including the size change from nanoscale to micro-scale. In addition, if the volume of added water increased, the height of the cones decreased. Addition of water controlled the hydrolysis of zinc acetate and affected the nucleation process of ZnO significantly. Moreover, addition of water can ¯ impede the [0001] growth and accelerate the [1100] growth if the volume ratio of added water/methanol is equal to or greater than 2:15. In this way, the shape and size of ZnO can 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 25–100 nm via esterification of zinc acetate and ethanol under solvothermal reaction conditions. The reaction temperatures were adjusted from 100–200 C for 24–48 h in an autocontrolled oven. In terms of characterization, XRD and TEM analysis confirmed the high crystallinity and uniform nonagglomerated sphericity of as synthesized ZnO nanoparticles, respectively. By the several reaction conditions, it was confirmed that by changing the reaction temperature and time, the nanoparticle size can be easily controlled. As for the reaction mechanism, Fourier transform infrared (FT-IR) analysis confirmed the existence of ethyl acetate during the esterification reaction. In addition, it may be possible that first OH− anions were produced by esterification reaction between CH3 COO− and ethanol and then zinc cation reacted with as-produced OH− to form ZnO under solvothermal conditions. The presence of ethanol and ester could help to improve the dispersibility of the 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 first reaction, 0.01 M zinc acetate dihydrate was added to 100 ml diethylene glycol (DEG), and then the reaction solution was heated at 160 C and maintained for 1 h, which resulted in white colloidal ZnO, treated as the primary solution. In a second reaction process, 0.01 M zinc acetate and various amount of primary supernatant (5–20 ml) was added to 100 ml DEG and heated at 160 C for 1 h aging. The resulting ZnO white colloid produced 50–300 nm ZnO nanoparticles, depending upon the amount of primary supernatant. To check the structural and optical properties, optimal size with 185 nm ZnO nanoparticles were used. As-synthesized ZnO nanoparticles were characterized by various analytical tools, i.e., XRD, TEM, FESEM, energy dispersive spectroscopy (EDAX), Raman spectroscopy, and UV photoluminescence measurement. TEM observation confirmed that spherical 185 nm-diameter ZnO clusters consisted of primary single crystallites ranging from 6–12 nm. XRD analysis confirmed the hexagonal wurtzite crystallites of as-grown zinc oxide colloidal spheres and sample post-annealed at 350 and 500 C in air for 1 h. Raman spectra of as-grown zinc oxide colloidal spheres and post-annealed samples further confirmed the crystallinity of the products. Moreover, highly efficient near-band edge UV luminescence was attributed to defect-bound excitons with high density of states, which was confirmed by using room-temperature PL analyses. This assumption was further proved by the observation of peak broadening and unchanged position in low-temperature PL spectra, which is similar to the behavior observed in the case of ZnO quantum dots. In addition, broad yellow emission and green emission were observed in room-temperature PL and low-temperature PL, respectively. Further, in temperature-dependent PL, defects such as oxygen interstitials Oi and oxygen vacancies V0 dominate the visible emissions of ZnO spheres. Cheng et al. [47] further reported the enhanced resonant Raman scattering and electronphonon coupling from self-assembled secondary ZnO nanoparticles synthesized by the same procedure described in the above report. Figure 8 shows the typical TEM images of ZnO nanoparticles. Figs. 8(a) and (b) show the mean particle size of 185 nm with spherical shape of ZnO nanoparticles, which consisted of agglomerated primary single crystallite ranging from 6–12 nm. The selected area electron diffraction (SAED) spectra shown in inset of Figure 8(a) confirmed the polycrystalline nature of several secondary ZnO nanoparticles, while the SAED spectra shown in Figure 8(b) confirmed the single

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Figure 8. TEM images of secondary ZnO nanoparticles recognized of crystalline subcrystals. (a) A typical lowmagnification TEM image and SAED pattern of several uniform ZnO nanoparticles. (b) High-magnification TEM 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. Reprinted with permission from [47], H.-M. Cheng et al., J. Phys. Chem. B 109, 18385 (2005). © 2005, American Chemical Society.

crystalline pattern of only one ZnO nanoparticle. This means that the secondary ZnO nanoparticles are polycrystalline, consisting of much smaller subcrystals of the same crystal orientation. Figures 8(c and d) further provide much evidence in high resolution TEM (HRTEM) images. It may be possible that van der Waals interaction between the 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 the block of diethylene glycol, the solvent may behave as a microemulsion system, causing the individual ZnO subcrystals to grow up separately and finally assemble to form secondary ZnO nanoparticles under the driven force of van der Waals interaction. Figure 8 shows the SEM images of as-synthesized ZnO nanoparticles and samples collected after post-annealing at 350 and 500 C in air for 1 h. SEM images clearly indicate that during the heating process, ZnO subcrystals fused with neighboring crystals and grain size grew accordingly, which was further confirmed by XRD analysis. Moreover, as-grown ZnO nanoparticles exhibited a phonon red shift in a resonant Raman scattering, compared with the samples after post-annealing at 350 and 500 C. In addition, the electron-phonon coupling parameter is clearly extracted from resonant Raman scattering, and an interesting phenomenon of increasing electron-LO phonon coupling was also discovered when the crystal size of ZnO enlarged after heating treatment. In addition, the Fröhlich interaction may certainly play the main role in the coupling of ZnO particles. Finally, blue shift of UV PL and visible emission induced by interstitial oxygen were also investigated from as-grown and post-annealed ZnO samples, respectively.


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Figure 9. SEM micrographs of secondary ZnO nanoparticles (a) as-grown, (b) annealed at 350 C for 1 h, and (c) annealed at 500 C 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 influences of the reactant concentration were reported by Hu et al. [48]. In a typical process, ZnO nanoparticles were synthesized by using zinc acetate and NaOH in 2-propanol solution. As the nucleation and growth were fast in this synthetic process, at longer times the particle size was controlled by coarsening. In addition, coarsening kinetics were independent of the zinc acetate concentration from 0.5–1.25 mM at a fixed [zinc acetate:NaOH] ratio of 0.625. The width of the size distribution increased slightly with aging time. Moreover, if the zinc acetate concentration was fixed at 1 mM, the kinetics were independent of variation in the [zinc acetate:NaOH] ratio from 0.476–0.625. The presence of water in the reaction mixture was checked, and it was found that at low water concentration, the nucleation and growth of ZnO were very slow, which only slightly affected the coarsening kinetics for water content above ∼20 mM. Thus, by this synthesis method, it is confirmed that ZnO nanoparticles are insensitive to the reactant concentration and presence of water. In another report, Hu et al. [49] explained the influence of anions on the coarsening kinetics of ZnO nanoparticles. Solution phase synthesis of nanoparticles possesses coarsening (also known as Ostwald ripening) and epitaxial attachments (or aggregation), which can compete with nucleation and growth. As a result, particle size distribution can be modified in the system. If nucleation and growth are fast, coarsening and aggregation can dominate the time evolution of the particle size distribution. In addition, random aggregation usually leads to the formation of porous clusters of particles, whereas epitaxial attachment of particles leads to the formation of secondary particles with complex shapes and unique morphologies. In this report, ZnO nanoparticles were synthesized from Zn(CH3 COO)2 , ZnBr2 , and Zn(ClO4 )2 in 2-propanol. ZnO nanoparticles synthesized by Zn(CH3 COO)2 , and ZnBr2 in 2-propanol at 55 C for 8.5 h show particles size 65 ± 12 nm and 49 ± 08 nm, respectively, whereas ZnO nanoparticles synthesized by Zn(ClO4 )2 in 2-propanol at 55 C for 40 min show elongated and irregularly shaped particles via epitaxial attachment of several smaller particles. The rate constant for coarsening

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at constant temperature increases in the order Br− < CH3 COO < ClO4 indicating that the rate is dependent on anion adsorption. On the other hand, the temperature dependent rate constant for coarsening is due to the temperature dependence of the solvent viscosity and 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 sol–gel route. As-synthesized ZnO nanoparticle morphology was confirmed by TEM analysis, which shows spherical particles 3–4 nm in diameter. In a typical synthesis process, zinc acetate (ZnAc) was used as a precursor, and triethanolamine (TEA) was used as a surfactant to produce ZnO nanoparticles at 50–60 C. With the help of FT-IR analysis, they proposed that synthesis of ZnO nanoparticles occurred via an intermediate product called zinc monoacetate, which further assisted the formation of a new complex and then, via a polycondensation process, produced ZnO nanoparticles. In addition, three different ratios of both ZnAc and TEA were chosen to determine the best sol, considering their optical properties. The best sol (0.75 M ZnAc) based on its optical properties was subjected to analysis by PL spectroscopy. Different shapes of UV (broad peak at 360 nm with one shoulder at 330 nm) and green peaks (sharp peak at 520 nm) in the PL spectra of ZnO nanoparticles, synthesized using 0.75 M zinc acetate, suggest the possible use in monochromatic excitation applications. To confirm the optimization parameter for the synthesis of zinc oxide nanoparticles, Kim et al. [51] presented the modified sol–gel route using the Taguchi robust design method. In a typical synthetic process, zinc acetate dehydrate, lithium hydroxide monohydrate (LiOH), hydroxypropylcellulose (HPC), and absolute ethanol were used for the synthesis of ZnO nanoparticles. In this presented work, the molar concentration ratio of [LiOH]/[Zn(Ac)2 ] was varied in the range of 1–5, and the concentration of zinc acetate was fixed at 0.05 M. Also, the concentration of HPC dispersant and feed rate of LiOH and HPC solution were changed in the range of 0.1–0.4 g and 0.33–7.0 ml/min, respectively. After implementing the Taguchi robust design method with an L9 orthogonal array to optimize experimental condition for the preparation of ZnO nanoparticles, it was found that the [LiOH]/[Zn(Ac)2 ] molar ratio was the main parameter, showing a prominent effect on particle size and size distribution of the ZnO nanoparticles. By optimizing the conditions, the observed size of ZnO nanoparticles was ∼30 nm with narrow particles size distribution, confirmed 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 any base, 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 for synthesizing ZnO nanoparticles at 60 C in 10 h. In addition, similar experiments were also preformed by using a mixture solvent i.e., dimethylformamide (DMF), toluene, and THF with methanol, to check the effect of the solvent polarity and water miscibility on the growth of ZnO nanoparticles. The growth rate was greatly controlled by the presence of a water-immiscible non-polar solvent, which led to the formation of almost pure ZnO nanoparticles with near UV emission. On the other hand, the water-miscible polar solvent generates fully defected deep-level emissive ZnO nanoparticles, which agglomerate on standing due to the solvent homogeneity in the reaction mixture. As for the growth mechanism, the zinc acetate precursor underwent four stages: it was first solvated in methanol to form [Zn(MeOH)6 ]+ , then hydrolysis after removal of the intercalated acetate ions produced [Zn(OH)n2−n ], which further polymerized into Zn–O–Zn bridges, and finally transformed into ZnO. Moreover, it was observed that formation of water molecules during decomposition of zinc acetate could be responsible for the growth rate of ZnO nanoparticles. Finally, it was concluded that ZnO crystal growth is more sensitive to the mixture of solvents, which depends on the miscibility, polarity, and homogeneity of the precursor in the reaction medium. Ge et al. [53] reported a simple method to prepare monodispersed ZnO nanoparticles with average size of 52 ± 03 nm at low temperature by ultrasonic treatment. In a typical synthetic process, 0.88 gm zinc acetate dihydrate was mixed with 80 ml of absolute ethanol in a beaker under magnetic stirring at 70 C. In another beaker, 0.23 gm of


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LiOH was dissolved in 80 ml absolute ethanol under magnetic stirring for 20 min. After this step, the LiOH-ethanol solution was added dropwise into a Zn2+ -containing solution at 0 C under strong stirring for 1 h, which was further ultrasonically treated for 5 min. XRD and HRTEM images confirmed the crystallinity and structural morphology, respectively, of the as-synthesized ZnO nanoparticles. In addition, it was reported that with varying reflux time, ZnO nanoparticles can be converted to various aspect ratio ZnO nanorods via oriented attachment mechanism that were confirmed by the BFDH model (suggested by Bravasis, Freidel, Donnary, and Harker) and the HP model (proposed by Hartman 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 ethylene glycol, 1,3-propanediol, and 1,4-butanediol, which were further treated at temperatures above 308 K. In particular, if ethylene glycol was used as a solution for Zn(OH)2 dispersion, the synthesized ZnO nanoparticles had average particles size less than 20 nm. Moreover, if the reaction temperature was set at 308 K, the spherical secondary particles with ZnO primary nanoparticles were obtained. When Zn(OH)2 was heated in 1,3-propanediol at 308 K for 24 h, the spherical aggregated morphology of the ZnO primary nanoparticles with average diameter of 9 nm was obtained and if heated in 1,4-butanediol at 308 K for 24 h, the same morphology with average primary ZnO nanoparticle size of 11 nm was obtained, having interparticle pores in both cases. By measuring N2 adsorption isotherm at 77 K, it was concluded that ZnO nanoparticles prepared in ethylene glycol at 308 K contain many interparticle pores with less densely packed spherical aggregated morphology, whereas ZnO nanoparticles prepared in 1,3-propanediol and 1,4-butanediol show more densely packed primary ZnO nanoparticles. Thus, the formation of ZnO nanoparticles depends greatly not only on the heating temperature but also on the diol solutions used 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 of addition played an important role in determining the characteristics of the synthesized particles. In a polyol synthetic method, water can induce hydrolysis and condensation reactions of the Zn precursor when injected into a hot precursor solution maintained at 180 C, which induces a short burst of homogenous nucleation and leads to growth of aggregated equiaxial ZnO nanoparticles with average diameter of 24 nm. If a higher amount of polyvinyl pyrrolidone (PVP)—a water-soluble polymer—is used, it will lead to aggregation of free ZnO nanoparticles. In addition, increasing the amount of water added to the precursor solution enlarges the aspect ratio of the rod-shaped particles and increases the particle size of the equiaxial particles due to enhanced hydrolysis and condensation of the Zn ion complex. Moreover, zinc acetate concentration also slightly influences the particles size and aspect ratio when water is injected into the hot precursor solution. Furthermore, the effect of the hydration ratio (ratio of molar concentration of total water, DI water + hydrated water, to zinc acetate) on the particles’ characteristics via the water injection method were also discussed. Varying the hydration ratio from 4 to 8 did not change the particle morphology to a great extent. The particle diameter increased from 24 to 32 nm, and showed a slight deviation from equiaxial growth with increasing hydration ratio. Thus, it was concluded that method of water addition, concentration of zinc acetate, and the hydration ratio play important roles in determining the characteristics of ZnO particles. Ning et al. [56] reported the synthesis of mesoporous ZnO particles using octadecylamine (ODA) and DDA as templates via the sol–gel method. Particle size calculated using Scherrer’s formula with XRD analysis was 32 nm when processed with ODA and 40 nm with DDA. The densities of ZnO processed with ODA, with DDA, and without a template were reported as 5.31, 5.37, and 5.42 cm2 /g; respectively. In addition, it was reported that surface analysis confirmed the porosity of the ZnO particles when processed with ODA and DDA. Moreover, hugely enhanced electroluminescence (EL) was observed from porous ZnO particles when direct current electric field from 2–4.66 V/m was used. Furthermore, emission intensities of the ZnO sample processed with DDA

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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 reported threshold voltage is just 2 V/m. Based on the above analysis, it was confirmed that porous ZnO particles can enhance EL intensity. Cozzoli et al. [57] reported the non-hydrolytic route for the synthesis of nearly spherical ZnO nanocrystals with diameter less than 9 nm via a sequential reduction-oxidation reaction. In a typical synthetic process, ZnO nanocrystals were synthesized in a surfactant mixture of hexadecylamine and oleic acid (OLEA) via a two-step chemical process: first hot reduction (at 180–250 C) of zinc halide by superhydride (LiBEt3 H) and then oxidation of the resulting product. The reported results confirmed that controlled growth of ZnO nanocrystal was dependent on OLEA-assisted generation of intermediate metallic nanoparticles as well as adjustment of oxidation of the metallic nanoparticles using a mild oxidant, triethylamine-N -oxide, rather than molecular oxygen. Furthermore, the reported synthetic approach demonstrates that organic-soluble ZnO nanocrystals of low size dispersion and of stable size can be useful for optoelectronic, catalytic, and sensing purposes. Xie et al. [58] reported the low temperature synthesis of uniform ZnO particles with controllable morphologies. In addition, characteristic luminescence patterns were also presented. In a typical synthesis process, uniform ZnO particles were synthesized in an aqueous solution with the presence of TEA below 80 C assisted via sonication. It was reported that with increasing TEA concentration, one can systematically control the morphology of elongated rugby ball-like ellipsoidal to half-ellipsoidal ZnO particles. FESEM analysis of many rugby ball-like ZnO particles shows that particles have an average length of about 620 nm and mean diameter of about 400 nm. By systematic investigation, it was confirmed that formation of rugby ball-like ZnO particles resulted from the first growth of a half-ellipsoidal particle followed by the germination and growth of a second half at its base. Moreover, it was studied with close relationship between particle characteristics and optical properties with a high spatial resolution cathodoluminescence (CL) and shows that the ellipsoidal particles are intrinsically encoded with characteristic barcode-like UV luminescence patterns. Additionally, luminescence spectra can be tuned by heat treatments at elevated temperatures. By this extensive proof, the authors believe that well-defined uniform ellipsoidal ZnO particles embedded with unique luminescence characteristic can hold great potential for use in bioengineering and photonics, such as biological 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 was dissolved in the oxygen-free solvent acetonitrile, which was transferred into a Teflon autoclave and then heated at 100 C for 2 days. The resulted products were characterized by TEM, SEM, and XRD analysis. The TEM micrograph shows the particles size in the range of 15–85 nm, sometimes with well faceted hexagonal morphology. It is interesting to note that in such a simple reaction, systems like zinc acetylacetonate and acetonitrile are able to induce the formation of complex structures without any additional structuredirecting agent. Even the large number of organic species detected in final products confirmed the complex reaction pathways during the reaction, and these organic components during nanoparticle formation are prerequisite to understanding and controlling the 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 of ZnO nanoparticles, solid Zn(c-C6 H11 )2 was dissolved in a THF solution of lithium precursor and OA used as stabilizer. Two different lithium precursors, i.e., Li[N(CH3 )2 ] and Li[N(Si(CH3 )3 )2 ], and one sodium precursor, namely, Na[N(Si(CH3 )3 )2 ], were used with the proportion varied from 1 to 10 mol% compared to Zn. It was observed that Li precursors induced the synthesis of ZnO nanoparticles; otherwise, without Li or with the use of Na precursor the synthesis of ZnO nanorods was induced. Figure 10 shows the TEM micrograph of ZnO nanoparticles synthesized using the Li[N(CH3 )2 ] precursor with nanoparticle size varied from 37 ± 07 nm to 25 ± 04 nm [series 1]. Figure 11 shows the


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Figure 10. TEM images of series 1 nanoparticles: (a) 1%, (b) 2%, (c) 5%, and (d) 10% Li. Reprinted with permission 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 ] precursor with nanoparticle size varied from 43 ± 10 nm to 31 ± 08 nm [series 2]. Figures 10(a–d) and Figures 11(a–d) show that as the Li amount increases, the size of the nanoparticles decreases, whatever the Li precursor. The insets of Figures 11(c and d) show the HRTEM image and confirm the monocrystalline nature of the ZnO nanoparticles. XRD analysis confirmed the presence of the hexagonal zincite phase, space group P63 mc in all samples. In addition, the optical properties of these nanoparticles were measured by dissolving solid samples in distilled THF. The absorption spectrum for all the samples shows a strong absorption between 300 and 350 nm followed by a sharp decrease. Furthermore, the luminescence properties of these samples were also investigated, which shows one broad emission band in the visible range for an excitation wavelength of 320 nm. This shows that presence of Li ions leads to a blue shift of the emission band of ZnO nanoparticles. The observed emission maxima vary from 582 to 535 nm for the Li[N(CH3 )2 ] precursor and from 581 to 534 nm for the Li[N(Si(CH3 )3 )2 ] precursor. This blue shift increases as the concentration of precursor increases, and consequently, as the size of the nanoparticles decreases. Moreover, the observed emission intensity is very strong, which can be clearly seen by the human eye as illustrated in Figure 12, which opens the perspective for the preparation of LEDs. Bardhan et al. [61] synthesized sub-micrometer ZnO particles with controlled morphology, i.e., rings, bowls, hemispheres, and disks, via a simple wet-chemistry approach using zinc acetate as a precursor, ammonium hydroxide as a base, and ethanol as a solvent. The reported morphologies were varied with the concentration of zinc acetate, i.e., at 0.05 M

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Figure 11. TEM images of series 2 nanoparticles: (a) 1%, (b) 2%, (c) 5%, and (d) 10% Li. Reprinted with permission 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, reaction temperature, pH, and concentration of ammonium hydroxide also played an important role for the formation of various ZnO morphologies. In addition, these synthesized ZnO particles show strong white-light emission via UV excitation, which is due to the presence of surface defects resulting from the method of fabrication and synthesis conditions. As a result, the authors believe that based on the properties of these ZnO particles, it may lead to the development of economical, white light-emitting materials for solid-state lighting applications. Hong et al. [62] reported the synthesis of quasi-spherical ZnO nanoparticles with diameters of ∼20 nm using zinc acetate as a precursor. In a typical synthetic process, 5% PEG surfactant solution was transferred into a three-neck flask, and then zinc acetate and (NH4 )2 CO3 solutions were added dropwise with vigorous stirring. The resulting suspension was kept for 2 h at room temperature under stirring. After the completion of the reaction, the product was filtered and washed with ammonia solution and ethanol, dried under vacuum for 12 h, and calcinated at 450 C for 3 h. The as-synthesized ZnO nanoparticle surfaces were further grafted by polystyrene (PSt) in a non-aqueous suspension via free-radical polymerization to reduce the aggregation among nanoparticles and to improve the compatibility between the nanoparticles and the organic matter, which made a stable suspension in organic solvents. The resulting ZnO nanoparticles and PStgrafted ZnO nanoparticles were characterized by TEM, XRD, FT-IR analysis, zeta potential measurement, lipophilic degree (LD) test, photocatalytic analysis, sedimentation test, and contact angle measurement. It is reported that bare ZnO nanoparticles have high


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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 photocatalytic activity. Moreover, the LD of the composite particles after high temperature was stable, and the photoluminescence of the PSt-grafted ZnO nanoparticles was observed by the naked eye. In addition, ZnO nanoparticles can also be used to reinforce the electrical conductivity of poly(vinylidene fluoride) (PVDF) films. 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, zinc acetate dihydrate was dissolved in distilled water, and then NaOH was added to make a transparent Zn(OH)2− 4 solution, followed by the addition of an ionic liquid, 1-hexyl-3methylimidazolium bis (trifluoromethylsulfonyl) imide, liquid [hmim][NTf2 ]. After this step, the resulting solution was ultrasonically irradiated for 1 h with 40 kHz frequency of ultrasound waves. The total acoustic power injected into the sample solution was found to 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 ultrasound irradiation 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 ZnO products. As for the growth mechanism, RTIL consists of cations [C6 mim]+ and anions [NTf2 ]− . Cationic species combined with Zn(OH)2− 4 species present in the solution though electrostatic attraction, and these cation-anion couples led to the formation of ZnO nuclei via dehydration due to strong polarization of [C6 mim]+ . Moreover, the newly generated ZnO surface was greatly inhibited by [C6 mim]+ ions, so the anisotropic growth of ZnO crystals were markedly modified. As the method is very effective for the synthesis of ZnO nanoparticles in a green media, it could be useful for synthesizing ZnO nanoparticles with high yields. Another approach to synthesize ZnO nanoparticles via one-step mechanochemical process was reported by Lu et al. [64]. In a typical synthesis process, matrix salts were prepared by mixing zinc sulphate heptahydrate, potassium hydroxide, and potassium

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chloride. The mixing reaction was carried out in a paste state at room temperature with short grinding time without any external energy input. The as-produced ZnO nanoparticles had a mean diameter of 22.1 nm, which exhibited excellent UV-blocking properties (UV absorption maxima at 358 nm) for cosmetic application, confirmed by UV-visible (UV-vis) spectrometry. In addition, authors compared the raw material costs with other mechanochemical processes and found that this process is more favorable than 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, which produced ZnO nanoparticles via dehydration, have been reported. There was some more literature related to direct conversion of inorganic or organic precursor to zinc oxide nanoparticles, i.e., Rataboul et al. [65] reported the synthesis of ZnO nanoparticles from thermal oxidation of Zn particles, which were produced by the decomposition of organometallic precursor [Zn(C6 H11 )2 ] in a wet anisole. Zn particles can be prepared in the presence or absence of polymer. In addition, whatever the synthesis and stabilization modes, the particles display a uniform size and narrow size distribution. As-synthesized products were characterized by TEM, XRD, and XPS, which were fully consistent with the 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 polar basic aprotic solvent i.e., dimethyl sulfoxide (DMSO) or DMF at room temperature. The reproducibility of as-synthesized products depends upon the control over water content and reaction temperature. As the hydrolysis of zinc carboxylates produced ZnO nanoparticles with different sizes, solvent basicity and the interaction of DMSO and water play an important 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 confirmed by powder X-ray diffraction spectroscopy. By HRTEM analysis, it was confirmed that low concentration (2 × 10−4 M) of zinc cyclohexanebutyrate produced 2.12 nm average size ZnO nanocrystallites, and zinc acetate produced 3 nm average size ZnO nanocrystallites. In addition, if zinc cyclohexanebutyrate was used as a starting material, ZnO nanocrystals with rock salt coexisting with a wurtzite structure were produced. The presence of rock salt ZnO nanoparticles might be due to the phase transformation induced by particle size and/or by the interaction of cyclohexanebutyrateZnO nanoparticles. Moreover, dynamic light backscattering size measurements of ZnO nanoparticles were also performed in the DMSO colloidal dispersion to detect small individual 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 × 10−4 M) ZnO dispersions in DMSO did not show any flocculation or red shifts in 2 months, probably due to the concentrated dynamic stabilizing action of carboxylate ions and solvent molecules, ZnO colloids in DMF were not stable and readily formed precipitates, which can adhere to glass walls, and produced ZnO films. This synthesis method is reported as a new, direct, clean, and very easy pathway to obtain ZnO nanoparticles and can be applied to other metallic carboxylate salts 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 the preparation of zinc alginate gels by ionic gelation between zinc solution and sodium alginate. The resulting wet beads were heated at 800 and 450 C for 24 h. The structural morphologies and crystallinity of the as-synthesized ZnO nanoparticles were characterized by SEM, TEM, XRD, and micro-Raman spectroscopy. In more detailed observation, it was reported that ZnO nanoparticles possessed wurtzite structures with single crystalline hexagonal phase confirmed by XRD analysis and SAED. In addition, it was reported that heating temperature and the kind of zinc agent (i.e., zinc nitrate or zinc acetate) influence the size of ZnO nanoparticles. Furthermore, Raman scattering confirmed the existence of defects in the nanoparticles.


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Wahab et al. [68] reported the synthesis of ZnO nanoparticles from the conversion of hydrozincite [Zn5 (CO3 )2 (OH)6 ]. In a typical synthetic process, hydrozincite was prepared by reaction between zinc acetate dihydrate with urea in deionized water at 70 C for 2 h via the sol–gel method. The quality and composition of the as-grown hydrozincite were confirmed by XRD analysis with all the characteristic peaks for hydrozincite as well as by FT-IR analysis. Furthermore, as-synthesized plate-like hydrozincite was converted to ZnO nanoparticles with calcination at different temperatures, i.e., at 300, 500, 700, and 900 C. The morphological characterization was done by FE-SEM and TEM analysis, which show that as the calcination temperature increased, particles size also increased in the range of 20–300 nm. As-synthesized zinc oxide nanoparticles were further characterized by HRTEM equipped with SAED, and the distance between lattice fringes was confirmed as ∼0.52 nm corresponding to the (0001) crystal plane. Thermogravimetric analysis (TGA) of as-grown hydrozincite from room temperature to 700 C revealed that the primary weight loss, which starts at ∼130 C was due to solvent evaporation and secondary weight loss observed at ∼290 C was due to phase transformation from hydrated zinc oxide to zinc oxide. Niasari et al. [69] presented the synthesis of ZnO nanoparticles by thermal decomposition of [bis(acetylacetonato)zinc(II)]-oleylamine complex. First, zinc acetate-oleylamine complex was prepared by the reaction between zinc acetate and oleylamine (C18 H37 N) at 100 C for 90 min in a high purity oxygen atmosphere followed by injection of metalcomplex solution into triphenylphosphine (C18 H15 P) at 220 C, resulting in a black color solution of [bis(acetylacetonato)zinc(II)]-oleylamine complex. Further, the black solution was aged at 210 C for 45 min, resulting in a white nanoparticle product that was precipitated by adding excess ethanol to the solution. The resulted white products were characterized by XRD, PL spectroscopy, and FT-IR spectroscopy. Morphological characterization shows zinc oxide nanoparticles with an average size of 12–20 nm, which was confirmed by SEM and TEM analysis. PL analysis shows the important strong blue-shift emission 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 general need to synthesize metal oxide semiconducting nanoparticles for possible applications in biological sensing, biological labeling, drug and gene delivery, and nanomedicines [70–73]. In particular, due to their easy fabrication, environmentally friendly nature, and non-toxic synthesis route, ZnO nanoparticles can provide a better option for various biological applications. However, water solubility and biocompatibility of ZnO nanoparticles are 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 nearneutral pH and low temperature (37 C). In a detailed synthesis process, a specific volume of zinc nitrate hexahydrate was added into the buffer tris(hydroxymethyl)aminomethane at pH 8 in an incubator for 4 h at 37 C. The as-synthesized ZnO nanoparticles were characterized by SEM, TEM, XRD analysis, FT-IR spectroscopy, and thermogravimetry/mass spectrometry (TG/MS). Figure 13(a) shows the TEM image of as-synthesized ZnO nanoparticles with a mean diameter of 20 nm, and Figure 13(b) shows the SEM image of ZnO nanoparticles calcined at 1,000 C with a mean diameter of 300 nm. Moreover, Figure 14 represents the XRD pattern of as-synthesized ZnO nanoparticles as well as samples calcined at different temperatures, i.e., 80, 600, and 1,000 C, which confirmed the crystallinity and stable wurtzite phase of ZnO nanoparticles. XRD crystal planes peaks became sharper as the samples were heated at higher temperatures with increasing crystal size. Furthermore, in terms of application, the buffer tris(hydroxymethyl)aminomethane represented a standard nontoxic buffer that is inert to a wide variety of chemicals and biomolecules and can be satisfactorily used for a variety of biological reactions. In addition, this buffer has an important role for the sphericity

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Figure 13. (a) Bright-field TEM image of as-obtained ZnO nanoparticles precipitated in aqueous solution at pH 8 and 37 C and (b) SEM image of ZnO particles after heat treatment in air at 1,000 C for 2 h. Reprinted with permission from [74], L. P. Bauermann et al., J. Phys. Chem. B 110, 5182 (2006). © 2006, American Chemical Society.

of the synthesized ZnO nanoparticles, it acts as a polydentade ligand, which adsorb strongly on one or more surfaces of ZnO, inhibiting crystal growth, and as a result, nearly spherical ZnO nanoparticles are produced. Moreover, the buffer also increases the rate of hydrolysis of the zinc-water complex by consuming protons during the reaction and produced ZnO with some trapped protons in the interstitial sites of ZnO crystals, which after further heating at about 180 C caused a decrease in the unit cell volume of ZnO due to the removal of interstitial protons from the crystalline structure of ZnO. The authors believe that during crystallization, new hybrids of ZnO can be produced by introducing biomolecules. 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 modification with water-soluble ligands, silanization, or encapsulation within block-copolymer micelles. In this regard, Wang et al. [75] reported the synthesis of a water-soluble ZnOAu nanocomposite having dual functionality, i.e., ZnO provides fluorescence, and Au is used for organic functionality for bioconjugation. In a typical synthetic process, first, ZnO nanocrystals were prepared using zinc acetate dihydrate in ethanol via refluxing with stirring at 80 C for 3 h. Furthermore, the as-synthesized ZnO nanocrystals were employed as a seeding surface for the nucleation and growth of reduced gold by citrate to produce ZnO-Au nanocrystals having water-soluble characteristics. As-synthesized ZnO-Au nanocrystals were characterized by TEM and XRD and confirmed as dumbbell-shaped ZnO-Au nanocrystals having wurtzite ZnO and fcc Au with diameters of 4.9 and 7.1 nm,

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


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respectively. It was reported that surface plasmon absorption band of ZnO-Au NCs was broadened and red shifted relative to monometallic Au nanoparticles. In addition, the UV emission intensity of ZnO-Au nanocrystals was 1 order of magnitude higher than in 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 electromagnetic 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 primary human immune cells. ZnO nanoparticles were synthesized by forced hydrolysis of zinc acetate at 160 C in diethylene glycol media. As-synthesized ZnO nanoparticles with ∼13 nm diameter were characterized by TEM, XRD, and UV-vis spectrophotometery. The ZnO nanoparticles showed complete inhibition of E. coli growth at concentrations ≥3.4 mM, whereas growth of S. aureus was completely inhibited for concentrations ≥1 mM. In a more detailed observation, flow cytometry viability assays using a two color live/dead Backlight kit, demonstrated that growth-inhibiting properties of ZnO nanoparticles corresponded to the loss of cell viability, but identical particles have minimal effects on primary human T cell viability at concentrations toxic to gram-negative and grampositive bacteria. These observations confirmed the toxic nature of ZnO nanoparticles for different 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 killing of cancer cells and activated human T cells using ZnO nanoparticles. For the synthesis of 8–13 nm ZnO nanoparticles, the authors adopted the forced hydrolysis of zinc acetate at 160 C in DEG. Then, ZnO nanoparticles were reconstituted in phosphate buffered saline (PBS) solution. After reconstitution, nanoparticles were sonicated for 10 min and immediately vortexed before being added to cell culture. The response of normal human cells to ZnO nanoparticles under different signaling environments was examined and compared to the response of cancerous cells. In addition, ZnO nanoparticles exhibited a strong ability to kill cancerous T cells (∼28–35×) compared to normal cells. Moreover, it was observed that activation state of the cell contributes to the nanoparticle toxicity, as resting T cells display a relative resistance while cells stimulated through the T cell receptor and CD28 costimulatory pathway show greater toxicity, which results in a direct relation to the level of activation. It was reported that appearance of toxicity was due to the involvement of generated reactive oxygen species, as it was found that cancerous T cells produced higher inducible levels than normal T cells. Furthermore, ZnO nanoparticles induced apoptosis in Jurkat T cells, which is shown in Figure 15. To analyze the induced apoptosis, two types of samples were prepared: (1) untreated cells and (2) cells treated with 0.3 Mm nanoparticles for 20 h or treated with 100 nM okadaic acid for 20 h (positive control) and then stained with green fluorescent annexin V antibody to detect apoptotic membrane and stained with red fluorescent dye PI to detect permeable membranes using the Vybrant apoptosis assay kit #2 (Molecular Probes). In terms of characterization, cells were visualized by confocal microscopy which is shown in Figures 15(A–C) for control cells not treated with nanoparticles. Figure 15(A) shows control differential interference contrast (DIC), Figure 15(B) shows a control DIC image with green and red fluorescence overlay, and Figure 15(C) shows a control green and red fluorescence image. Cells treated with ZnO nanoparticles are shown in Figures 15(D–G), in which Figure 15(D) shows treated nanoparticles in the DIC image, Figure 15(E) shows a DIC image with green and red flurorescence overly, Figure 15(F) shows a green and red flurorescence image, and Figure 15(G) shows an additional green and red flurorescence image of nanoparticletreated cells of lower magnification. To further clarify naonparticle-induced apoptosis, cells were left untreated (Fig. 16(A)), cells were treated with 100 nM okadaic acid for 20 h as a positive control for apoptosis (Fig. 16(B)), and cells were treated with 0.3 mM ZnO NP for 20 h (Fig. 16(C)) and then stained with DNA dye, acridine orange, and visualized by fluorescent microscopy. In Figures 16(B and C), arrows indicate the typical apoptotic cells characterized by shrunken appearance and condensed or fragmented nuclei. Collectively, these results indicate that ZnO NPs induce apoptosis in Jurkat T cells. These

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Figure 15. ZnO nanoparticle-induced apoptosis in Jurkat T cells. Cells were left untreated, treated with 0.3 mM ZnO NP for 20 h, or treated with 100 nM okadaic acid for 20 h (positive control) and stained with a green fluorescent annexin V antibody to detect apoptotic membranes and with the red fluorescent dye PI to detect permeable membranes using the Vybrant apoptosis assay kit #2 (Molecular Probes). Cells were visualized by confocal microscopy and representative images are shown. (A)–(C) control cells not treated with nanoparticles, (A) control DIC image, (B) control DIC image with green and red fluorescence overlay, (C) control green and red fluorescence image. (D)–(G) cells treated with nanoparticles; (D) nanoparticle-treated DIC image, (E) nanoparticle-treated DIC image with green and red fluorescence overly, (F) nanoparticle-treated green and red fluorescence image, (G) an additional green and red fluorescence image of nanoparticle-treated cells of lower magnification. 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 to protect against NP toxicity or enhance the destruction of disease-causing cell types such as 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 nanoparticles using a standard microbial method. In a typical synthetic process, two methods were employed. In the first method, zinc nitrate and sodium hydroxide were mixed at room temperature with stirring for 2 h, and the resulting zinc hydroxide precipitate was washed with distilled water until pH became neutral, followed by dropwise addition of H2 O2 to produce zinc peroxide translucent solution, which was then heated at 350 C to produce ZnO with active surface oxygen species. In another method, surfacemodified ZnO nanocrystals were prepared by dissolving zinc acetate in 2-propanol at 80 C with stirring, followed by the addition of 2-mercaptoethanol, which was continuously stirred for 2 h. The resulting mixture was then hydrolyzed by adding NaOH in 2-propanol, followed by ultrasonic agitation for 2 h, and then the synthesized products were 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 ZnO nanoparticles formed by the precipitation method (Fig. 17(a)) and formed by base hydrolysis in propanol medium (Fig. 17(b)). It was observed that when capping molecules were used, the kinetics of nucleation and accumulation were affected in such a way


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Figure 16. Detection of apoptotic morphological changes in Jurkat cells treated with ZnO NP. Cells were left untreated (A), or treated with 100 nM okadaic acid for 20 h as a positive control for apoptosis (B), or treated with 0.3 mM ZnO nanoparticles for 20 h (C) and stained with acridine orange and visualized by fluorescent microscopy. Arrows indicate typical apoptotic cells characterized by a shrunken appearance and condensed or fragmented 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 remained the same, which in turn produced particles with narrow size distribution as compared to particles synthesized without a capping molecule. Furthermore, as-synthesized ZnO nanoparticles with different concentrations underwent bacteriological tests by standard microbial method in terms of minimum inhibitory concentration (MIC) and disk diffusion, which were performed in Luria-Bertani and nutrient agar media on solid agar plates and in liquid. Figure 18 shows the bacterial efficacies with ZnO suspension with thee different nanoparticle sizes after 24 h incubation of aliquots in the lowest concentration range (0.01–1 mM) and the highest concentration range (5–100 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 resulted in the generation of a large number of active oxygen species, which kill bacteria more effectively. As a result, it was observed that ZnO nanoparticles were more abrasive than bulk ZnO and thus contributed greater mechanical damage of the cell membrane and enhanced the anti-bacterial effect of ZnO nanoparticles. Reported observations and results confirmed that ZnO nanoparticles may be applicable to medical devices that are coated with nanoparticles against microbes.

4.2. Solar Cells, Photocatalytic, Photoluminescence, and Sensor Application of ZnO Nanoparticles Regarding ZnO nanoparticle application in solar cells, Suliman et al. [79] reported the synthesis of ZnO nanoparticles with average diameter of 30 nm by using zinc chloride as a precursor and NaOH as a base in a PVP solution of water at 160 C for 8 h via the hydrothermal method. The as-synthesized structures were characterized by TEM, SEM, and XRD analyses. Absorption spectrum was measured using a UV-vis spectrophotometer. To make a ZnO film over transparent conducting glass (TCO), ZnO nanoparticles were dissolved in ethanol and then applied over the TCO surface using the doctor blade technique, which resulted in a 6 m thick film of ZnO nanoparticles over the TCO, and finally it was annealed for 30 min at 450 C. To make dye-sensitized ZnO thin films, the film 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). The TCO acted as a counter electrode on which 340 nm thick layer of Pt was deposited by sputtering. 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 resulting films of 0.4 cm2 were illuminated through the conducting glass support with an Oriel

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

91192 AM 1.5 solar simulator as the light source. ZnO nanoparticles films were then measured by photocurrent–voltage (I–V ) which gives a fill factor of 0.513, short-circuit current of 1.2 mA/cm2 , open-circuit voltage of 573 mV, and an overall light-to-electricity conversion efficiency 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 process, ZnO aggregates were synthesized via polyol-mediated precipitation by using zinc acetate in diethylene glycol at 160 C with refluxing. It was reported that with adjustment in zinc acetate concentration, rate of heating, and the amount of stock solution that is added, one can readily control the size of individual ZnO aggregates. To make a photoelectrode films, ZnO aggregates were deposited by drop-cast method on a fluorinedoped tin oxide (FTO) glass substrate, and thickness of films depended on the number of drops. Finally, the ZnO films were heated at 350 C in air for 1 h to remove residual organic chemicals. The as-synthesized product morphologies were characterized by SEM

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ZnO Nanoparticles: Growth, Properties, and Applications (a)

(b)

Figure 18. (a) and (b) Bactericidal efficiency of samples 1 and 2 and bulk ZnO suspensions at different concentrations. 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 films with polydisperse aggregates of ZnO with a size distribution of 120–360 nm (Fig. 19(a)) and 120–310 nm (Fig. 19(b)), films consisting of monodisperse aggregates having average sizes of 350, 300, 250, and 210 nm (Figs. 19(c–g)), magnified SEM image (Fig. 19(h)), and a schematic illustration to show the structure of the ZnO aggregates formed by closely packed nanocrystallites ∼12 nm in size. To make the ZnO film sensitized, it was immersed into N3 dye for 20 min and then rinsed with ethanol to get rid of the excess dye. Then, cells were constructed using a platinum-coated silicon wafer as the counter electrode and the ZnO film as a working electrode. These two electrodes were placed side by side with 20 m separating space, where the I/I− 3 electrolyte was injected with capillarity. The solar cell’s performance was measured by the irradiation of air mass (AM) 1.5 simulated sunlight at 100 mW cm−2 . Figure 20 shows the typical I–V curve

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Figure 19. SEM images of hierarchically-structured ZnO films with submicrometer-sized aggregates. SEM images of the films consisting of polydisperse aggregates with a size distribution of (a) 120–360 nm and (b) 120–310 nm. SEM images of the films consisting of monodisperse aggregates with average sizes of (c) 350 nm, (d) 300 nm, (e) 250 nm, and (f) 210 nm. (h) is a magnified SEM image, and (i) is a schematic illustration to show the structure of ZnO aggregates formed by closely packed nanocrystallites. Reprinted with permission from [80], Q. Zhang et al., Adv. Funct. Mater. 18, 1 (2008). © 2008, Wiley-VCH Verlag GmbH & Co.

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Figure 20. An example of an I–V curve for dye-sensitized ZnO solar cells under AM 1.5 irradiation. The square 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 Verlag GmbH & Co.

for the polydisperse aggregated sample, which demonstrated the derivation of the opencircuit voltage Voc , the short-circuit current density Isc , and the maximum output power density Pmax . The overall energy conversion efficiency and fill factor FF can be calculated sequentially by = Pmax /Pin and F F = Pmax /Voc × Isc ). As a result, it was found that overall energy-conversion efficiency of the cells could be affected by either average size or size distribution of the ZnO aggregates. The highest overall energy-conversion efficiency of 4.4% was achieved by using films formed by polydisperse ZnO aggregates with broad size distribution of 120–360 nm. In addition, variation in solar cell efficiency was observed due to light scattering, which was generated by submicrometer sized aggregates with a size distribution comparable to the wavelength of incident light, which could extend the travelling distance of light within the photoelectrode film. Moreover, the observed high efficiency with polydisperse aggregates films also due to its ability to provide the film with a closely packed structure, which was beneficial to the transport of electrons in the photoelectrode film. Regarding the application of ZnO nanoparticles in photocatalytic activity, Houskova et al. [81] reported the synthesis of zinc sulfide (ZnS) nanoparticles by homogenous hydrolysis of zinc sulfate and thioacetadmide (TAA) at 80 C and then its conversion to ZnO nanoparticles with annealing at temperature above 400 C in an oxygen atmosphere. The as-synthesized ZnO nanoparticles were characterized by XRD and SEM, HRTEM and SAED. The Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods were used to determine surface area and porosity, which showed pore sizes in the range of 2–6 nm. Further, photocatalytic activities of as-synthesized ZnO nanoparticles were determined by decomposition of Orange II dye in aqueous solution under UV irradiation of 365 nm wavelength. Samples heated at 700 C exhibited a good photocatalytic activity, k = 00379 min−1 (k for P25 Degussa is 0.0222 min−1 . Moreover, it was reported that assynthesized ZnO nanoparticles were evaluated for their non-photochemical degradation of chemical warfare agents to non-toxic products, which established a good decomposition of the mustard gas. Xu et al. [82] reported the synthesis of hierarchically assembled porous ZnO nanoparticles through a self-assembled pathway using surface-modified colloidal ZnO nanocrystals as building blocks and P-123 copolymers as the template in aqueous solution. In a typical synthetic process, first, colloidal ZnO nanocrystallites were prepared by hydrolysis of zinc acetate in a LiOH-ethanol solution, and then ZnO colloids were mixed with taurine in deionized (DI) water with taurine/ZnO molar ratio as 1:1.4. The solution pH was adjusted to 5.0 by adding 1 M HCL followed by vigorous stirring at 25 C for 24 h. The resulting mixture was marked as sample A, and then in another experiment P-123 was mixed with DI water at pH 5 and stirred at 25 C for 24 h (sample B). Finally, a solution


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

was added dropwise into sample B with stirring, and after 3 h stirring, the resulting mixture was heated in an autoclave at 70 C for 3 days, followed by washing and drying. and then the product was calcined at 400 C for 5–7 h. The as-synthesized ZnO products were then characterized by XRD, SEM, TEM, HRTEM, and PL spectroscopy. Figure 21 shows the TEM image of larger ZnO particles, which confirmed that these larger particles may contain many smaller ZnO nanoparticles with uniform size. The HRTEM (Fig. 21(b)) image clearly indicates the contrast difference in each individual nanoparticle having pores of ∼3 nm with average overall particle size ∼17 nm. Figure 21(b) (inset) shows another 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 illustrates the detailed self-assembly processes involving the functionalization of individual ZnO nanoparticles. Furthermore, on the basis of calorimetric measurements, the surface enthalpy of the hydrated porous ZnO is 142 ± 021 J/m2 , which is in good agreement with that of ZnO nanoparticles, which again supported the presence of self-assembled ZnO nanocrystals in nanoporous ZnO. Finally, the photocatalytic activity of porous ZnO nanoparticles was tested on the photodegradation of phenol under ambient conditions. Figure 23 shows the emission spectra of residual phenol in aqueous solution under exposure to UV light for various times in the presence of porous ZnO nanoparticles, TiO2 nanoparticles (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 ZnO nanoparticles. Reprinted with permission from [82], F. Xu et al., Chem. Mater. 19, 5680 (2007). © 2007, American Chemical Society.

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Figure 23. PL emission spectra of the residual phenol under exposure to UV light in the presence of (a) porous ZnO 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 () porous ZnO 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 Chemical Society.

nanoparticles show superior activity to TiO2 nanoparticles, because ZnO absorbs over a large fraction of UV light and the corresponding threshold of ZnO is 425 nm, while other ZnO (commercial and nanopowder ZnO) had less activity than the porous ZnO but 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 the decomposition 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 nanocrystals (3–7 nm) were prepared by alkaline hydrolysis of zinc acetate in the presence of LiOH · H2 O in absolute ethanol for 4–5 days. As-prepared ZnO colloid was precipitated by adding excess of heptane, followed by centrifugation and drying at


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room temperature. Surface modification of ZnO nanoparticles with charged organosilane [(CH3 O)3 Si(CH2 )3 N+ (CH3 )(C10 H21 )Cl− ] was carried out in an alkaline environment. The Cl− counter anions in the nanosalts could be readily exchanged by C9 H19 -C6 H4 (OCH2 CH2 )20 O(CH2 )3 SO− 3 ions, yielding the corresponding sulfonate nanosalt as a waxy solid that melts at 30 C, resulting in a fluid with considerably higher viscosity than the corresponding potassium sulfonate salt. The as-synthesized products were characterized by XRD, TEM, FT-IR, and TGA/DTA analysis. Furthermore, the PL quantum yield of the ZnO sulfonate nanosalt (0.065 mgmL−1 in acetonitrile), corresponding to the green-yellow emission band, was measured relative to that rhodamine 6G (R6G, 30 m in methanol). Moreover, it was suggested by the authors that this property profile could lead to new innovative applications in the areas of optics and photonics, and tuning the emission towards the UV (by doping or different chemical processing) may activate lasing in the ZnO nanoparticles, thus leading to the development of fluid/flexible 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 90 C for 2 days to precipitate out. Then the precipitate was washed with distilled water and further calcinated at 200–900 C for 3 h in air to synthesize ZnO nanoparticles dispersed in an amorphous matrix. The morphologies of the as-synthesized products were characterized by SEM, chemical composition was determined by inductively coupled plasma, ZnO nanoparticles in an amorphous matrix were observed by TEM, crystallinity was observed by XRD, PL images of ZnO nanoparticles were excited by visible light, UV light at 312 nm, or UV light at 254 nm, PL spectra were evaluated by a fluorescence spectrometer using excitation light at 287 nm, and the PL at low temperature was evaluated with a cryostat using liquid helium. Figure 24 shows the TEM images of ZnO nanoparticles in the amorphous matrix prepared with Al addition at 42% (Figs. 24(a1 and a2)) or 23% (Figs. 24(b1 and b2)) after calcination at 250 C for 3 h in air. It was confirmed that ZnO nanoparticles 4–7 nm in diameter were prepared by addition of 23% Al (Fig. 24(b)), and crystallization of ZnO was not suppressed compared to 2–3 nm diameter with 42% Al addition (Fig. 24(a)). Furthermore, control of ZnO nanoparticle size by addition of Al was confirmed by XRD measurement. Figure 25 shows the PL images of ZnO nanoparticles (14, 5.5, 4, or 2.5 nm in diameter) with Al addition excited by visible light, UV light at 312 nm, or UV light at 254 nm. It was confirmed that ZnO with no Al addition (0%) emitted only slightly under UV light 312 nm or UV light 254 nm. As a result, the PL intensity of the ZnO nanoparticles 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 addition decreased the PL intensity due to the suppressed crystallization of ZnO nanoparticles, which is essential for PL. It is shown in Figure 25 that the same ZnO nanoparticles can show different color emissions with different excitation light in this system. In addition, improvement of PL intensity of ZnO nanoparticles was evaluated by fluorescence spectrometer and is shown in Figure 26(a), which confirmed that as the nanoparticle size decreases, a blue shift is observed with increasing intensity due to the quantum size effect. Figure 26(b) shows the absorption spectra of various sizes of ZnO nanoparticles. The absorption spectrum shows the absorption edge as 3.15 eV of ZnO nanoparticles without Al addition, whereas the adsorption edge shifted to a higher energy of 3.45 eV as the size of nanoparticles decreased to 2.5 nm due to quantum size effect. Figures 26(c and d) show PL spectra at low temperature for 2.5 nm ZnO nanoparticles and confirm that ZnO nanoparticles were too small to show temperature dependence of intensity and center wavelength of PL spectra. In conclusion, factors needed for controlling the crystallization of ZnO nanoparticles—which is useful for increasing the PL properties—are Al concentration, calcination temperature, calcination period, and precursor, i.e., hydrotalcite. The authors believed that this novel process can be useful for an advanced technology with strong growth potential for PL devices. Recently, Jin et al. [85] reported the solution-processed UV photodetectors based on colloidal ZnO nanoparticles. Solution-processed UV photodetectors were conveniently fabricated by using films of ZnO nanoparticles. The devices show low dark currents with

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ZnO Nanoparticles: Growth, Properties, and Applications (a) Zn : Al = 58 : 42 a1

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Figure 24. TEM images of ZnO nanoparticles in the amorphous matrix prepared with Al addition at (a1) and (a2) 42% or (b1) and (b2) 23% after calcination at 250 C for 3 h in air. Reprinted with permission from [84], Y. Masuda et al., Crystal Growth & Design 5, 1503 (2008). © 2008, American Chemical Society.

a resistance > 1 T at room temperature as a consequence of low free carrier density in the films in the absence of illumination. In addition, at wavelength below 400 nm, a strong photocurrent was seen with a responsivity of 61 A/W at an average intensity of 1.06 mW/cm2 illumination at 370 nm. The characteristic times for rise and fall of the photocurrent are