ZnO Nanoparticles: Growth, Properties, and Applications

Full text

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CHAPTER 4
ZnO Nanoparticles: Growth,
Properties, and Applications
Mohammad Vaseem1, Ahmad Umar2, Yoon-Bong Hahn1
1School of Semiconductor and Chemical Engineering and BK21 Centre for Future
Energy, Materials and Devices, Chonbuk National University, Chonju 561-756,
South Korea
2Department 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.Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.Crystal Structure of ZnO . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.Nanoparticles of ZnO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.Application of ZnO Nanoparticles . . . . . . . . . . . . . . . . . . .
4.1.ZnO Nanoparticles: Bio-Friendly Approach . . . . . . . .
4.2.Solar Cells, Photocatalytic, Photoluminescence, and
Sensor Application of ZnO Nanoparticles . . . . . . . . . . 23
4.3.Cosmetic Application of ZnO Nanoparticles . . . . . . .
5.Summary and Future Directions . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
2
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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, includ-
ing the similar size of nanoparticles and biomolecules such as proteins and polynu-
cleic acids. [1] Additionally, nanoparticles can be fashioned with a wide range of metals
ISBN: 1-58883-170-1
Copyright © 2010 by American Scientific Publishers
All rights of reproduction in any form reserved.
1
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 consti-
tuting 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 prop-
erties 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 environ-
mentally 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 nanoparti-
cles stabilization.
With all the issues related to nanoparticle synthesis, there are various types of nanopar-
ticles reported in the literature, e.g., metal nanoparticles, metal oxide nanoparticles, 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) nanoparti-
cles have their own importance due to their vast area of applications, e.g., gas sensor,
chemical sensor, bio-sensor, cosmetics, storage, optical and electrical devices, window
materials for displays, solar cells, and drug-delivery [16–20]. ZnO is an attractive mate-
rial for short-wavelength optoelectronic applications owing to its wide band gap 3.37 eV,
large bond strength, and large exciton binding energy (60 meV) at room temperature.
As a wide band gap material, ZnO is used in solid state blue to ultraviolet (UV) opto-
electronics, including laser developments. In addition, due to its non-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 C4
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 non-
centrosymmetric 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 sp3covalent-bonding. The detailed properties of ZnO are presented in
Table 1.
6Vor P63mc. Figure 1 clearly shows that the structure is
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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ZnO Nanoparticles: Growth, Properties, and Applications
3
Figure 1. The hexagonal wurtzite structure model of ZnO. The tetrahedral coordination of Zn-O is shown.
O atoms are shown as larger white spheres while the Zn atoms are smaller brown spheres.
nanotubes, nanobelts, and other complex morphologies [22–35]. In the present chapter,
we mainly focus on ZnO nanoparticles synthesized by either the sol–gel method (solu-
tion method) or the hydrothermal method. As the solution method presents a low cost
and environmentally friendly synthetic route, most of the literature for ZnO nanoparti-
cles is based on the solution method. In addition, synthesis of ZnO nanoparticles in 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 nanoparti-
cles 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-C6H11)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 ZnO
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)
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
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.

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ZnO Nanoparticles: Growth, Properties, and Applications
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 evapo-
ration time, and the reaction/evaporation temperature. In addition, when a similar reac-
tion is carried out in dry air, it leads to agglomerated ZnO nanoparticles displaying 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 molL−1nano-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 homoge-
nous 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 ∼5?8 × 2?7 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 control-
ling 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,
(a)
(c)(d)
(b)
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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(a)
(b)
Figure 3.
Reprinted with permission from [36], M. Monge et al., Angew. Chem. Int. Ed. 42, 5321 (2003). © 2003, Wiley-VCH
Verlag GmbH & Co.
13C{1H} NMR spectra of (a) the free HDA ligand and (b) ZnO nanoparticles coated with HDA.
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 pres-
ence of amines, which are bases in solution medium. However, the observation of form-
ing 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 nucle-
ation, 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 pro-
cess, 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 nanoparti-
cles. Figure 4 shows the typical XRD spectra for nanotriangles, spherical nanoparticles,
and nanorods. The relative intensity of the peaks of nanotriangles and spherical nanopar-
ticles matches the bulk, signifying no preferred orientation. Spherical nanoparticles pre-
pared from octadecene have diameters of 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 parti-
cle 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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ZnO Nanoparticles: Growth, Properties, and Applications
a
b
c
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 photolu-
minescence (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 pre-
cursor suspended in four various types of organic solvents was heated in an auto-
clave 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 synthe-
sized 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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ZnO Nanoparticles: Growth, Properties, and Applications
(a) (b)
(e)
(c)(d)
Figure 7. SEM micrographs of ZnO particles synthesized via the solvothermal process in (a) 1,3-propanediol,
(b) 1,4-butanediol, (c) 1,5-pentanediol, and (d) 1,6-hexanediol. Insets in the images are the corresponding 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 nanoparti-
cles. Although the dielectric constant of the solvent is the prime reason for the different
morphology of ZnO nanoparticles in this solvothermal synthesis, more detailed charac-
terization and actual mechanism are needed.
To show the effect of acidic and basic solution routes on the morphology of ZnO,
Kawano et al. [43] synthesized ZnO nanoparticles with various aspect ratios. In a 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 ZnSO4via 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 addi-
tion 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 par-
ticles via the acidic route. FESEM images also confirmed the formation of ZnO particles
and rods via acidic and basic routes, respectively. Further cumulative undersize distribu-
tion 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 mor-
phological 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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Al2O3ceramic 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 nano-
scale 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 [1¯100] 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 ester-
ification of zinc acetate and ethanol under solvothermal reaction conditions. The reac-
tion 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, respec-
tively. 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 reac-
tion 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 CH3COO−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 gly-
col (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 reac-
tion 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 con-
firmed that spherical 185 nm-diameter ZnO clusters consisted of primary single crystal-
lites 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
V0dominate the visible emissions of ZnO spheres.
Cheng et al. [47] further reported the enhanced resonant Raman scattering and electron-
phonon coupling from self-assembled secondary ZnO nanoparticles synthesized by 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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ZnO Nanoparticles: Growth, Properties, and Applications
(a)(b)
(d) (c)
Figure 8. TEM images of secondary ZnO nanoparticles recognized of crystalline subcrystals. (a) A typical low-
magnification 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 resolu-
tion 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 sec-
ondary 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 interest-
ing phenomenon of increasing electron-LO phonon coupling was also discovered when
the crystal size of ZnO enlarged after heating treatment. In addition, the Fröhlich inter-
action 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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ZnO Nanoparticles: Growth, Properties, and Applications
11
(a)(b)
(c)
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 con-
centration 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 mix-
ture 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 coarsen-
ing 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 epi-
taxial 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(CH3COO)2, ZnBr2, and Zn(ClO4)2in 2-propanol. ZnO nanoparticles synthe-
sized by Zn(CH3COO)2, and ZnBr2in 2-propanol at 55?C for 8.5 h show particles size
6?5 ± 1?2 nm and 4?9 ± 0?8 nm, respectively, whereas ZnO nanoparticles synthesized by
Zn(ClO4)2in 2-propanol at 55?C for 40 min show elongated and irregularly shaped parti-
cles via epitaxial attachment of several smaller particles. The rate constant for coarsening

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ZnO Nanoparticles: Growth, Properties, and Applications
at constant temperature increases in the order Br−< CH3COO < ClO4indicating 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 con-
firmed 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 mono-
hydrate (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 L9orthogonal 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 pres-
ence 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 agglom-
erate 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 nanoparti-
cles with average size of 5?2 ± 0?3 nm at low temperature by ultrasonic treatment. In a
typical synthetic process, 0.88 gm zinc acetate dihydrate was mixed with 80 ml of abso-
lute ethanol in a beaker under magnetic stirring at 70?C. In another beaker, 0.23 gm of

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ZnO Nanoparticles: Growth, Properties, and Applications
13
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, respec-
tively, 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)2in a
diol solution. ZnO nanoparticles were obtained when Zn(OH)2was 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)2disper-
sion, the synthesized ZnO nanoparticles had average particles size less than 20 nm. More-
over, if the reaction temperature was set at 308 K, the spherical secondary particles with
ZnO primary nanoparticles were obtained. When Zn(OH)2was 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 N2adsorption 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 morphol-
ogy, whereas ZnO nanoparticles prepared in 1,3-propanediol and 1,4-butanediol show
more densely packed primary ZnO nanoparticles. Thus, the formation of ZnO nanopar-
ticles depends greatly not only on the heating temperature but also on the diol 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, con-
centration 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 octadecy-
lamine (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 pro-
cessed 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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ZnO Nanoparticles: Growth, Properties, and Applications
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 spheri-
cal 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 surfac-
tant 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 (LiBEt3H) and then oxi-
dation 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 mor-
phology 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 investiga-
tion, 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 structure-
directing agent. Even the large number of organic species detected in final products
confirmed the complex reaction pathways during the reaction, and these organic com-
ponents 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-C6H11)2was dissolved in a THF solution of lithium pre-
cursor 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 pre-
cursors 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 3?7 ±0?7 nm to 2?5±0?4 nm [series 1]. Figure 11 shows the

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ZnO Nanoparticles: Growth, Properties, and Applications
15
(b) (a)
(d) (c)
Figure 10. TEM images of series 1 nanoparticles: (a) 1%, (b) 2%, (c) 5%, and (d) 10% Li. Reprinted with per-
mission from [59], A. Glaria et al., New J. Chem. 32, 662 (2008). © 2008, The Royal Society of Chemistry.
TEM micrograph of ZnO nanoparticles synthesized using the Li[N(Si(CH3)3)2] precursor
with nanoparticle size varied from 4?3±1?0 nm to 3?1±0?8 nm [series 2]. Figures 10(a–d)
and Figures 11(a–d) show that as the Li amount increases, the size of the nanoparti-
cles decreases, whatever the Li precursor. The insets of Figures 11(c and d) show the
HRTEM image and confirm the monocrystalline nature of the ZnO nanoparticles. XRD
analysis confirmed the presence of the hexagonal zincite phase, space group P63mc 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. Fur-
thermore, 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 morphol-
ogy, 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