Synthesis and optical characteristics of ZnO nanocrystals

Abstract

Zinc oxide nanomaterials with an average particle size of 20–30 nm are readily synthesized by the reaction of zinc acetate
and oxalic acid under hydrothermal conditions. The samples are characterized by XRD, SEM, TEM, UV and photoluminescence (PL)
studies. The average crystal size of the as prepared ZnO nanopowder is determined by XRD and the values are in good agreement
with the TEM analysis. UV absorption spectra revealed the absorption at wavelength < 370 nm indicating the smaller size of
ZnO nanoparticles. The quality and purity of ZnO nanomaterial crystalline samples are confirmed by photoluminescence spectra.

Full text

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Bull. Mater. Sci., Vol. 32, No. 2, April 2009, pp. 165–168. © Indian Academy of Sciences.

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Synthesis and optical characteristics of ZnO nanocrystals
D SRIDEVI* and K V RAJENDRAN
Department of Physics, Presidency College, Chennai 600 093, India
MS received 20 June 2008; revised 19 December 2008
Abstract. Zinc oxide nanomaterials with an average particle size of 20–30 nm are readily synthesized by the
reaction of zinc acetate and oxalic acid under hydrothermal conditions. The samples are characterized by
XRD, SEM, TEM, UV and photoluminescence (PL) studies. The average crystal size of the as prepared ZnO
nanopowder is determined by XRD and the values are in good agreement with the TEM analysis. UV absorp-
tion spectra revealed the absorption at wavelength < 370 nm indicating the smaller size of ZnO nanoparticles.
The quality and purity of ZnO nanomaterial crystalline samples are confirmed by photoluminescence spectra.

Keywords. Nanomaterials; optical materials; properties; ZnO.
1. Introduction
ZnO is a wide bandgap semiconductor with optoelec-
tronic properties that make it an attractive candidate for a
variety of device applications. Zinc oxide is a versatile
material that has achieved applications in photocatalysts,
solar cells, chemical sensors, piezoelectric transducers,
transparent electrodes (Bauer et al 2001; Duran et al 2002),
electro luminescent devices, and ultraviolet laser diodes
(Johnson et al 2001; Zhang et al 2002). Compared to
other semiconductors, ZnO has wide bandgap of 3⋅37 eV
and other large excitation binding energy, which makes
excitation stable even at room temperature. There are
various methods in order to synthesize ZnO nanomaterials
viz. solvothermal synthesis, sol–gel (Li and Kanbunde
1992; Mondelaers et al 2002), combustion synthesis
(Vorkakpic and Matsaukas 1998; Hwang et al 2004),
spray analysis (Ko et al 2006), thermal hydrolysis (Kodas
1989), hydrothermal synthesis etc. Hydrothermal synthe-
sis is one of the most extensively used and cost effective
methods for the preparation of nanomaterials. In this study,
we have synthesized ZnO nanomaterials and their optical
and surface morphological properties are investigated.
2. Experimental
Zinc oxide is synthesized in aqueous solutions by taking
zinc acetate and oxalic acid under hydrothermal condi-
tions. 0⋅1 M solution of zinc acetate (AR) is taken in a
beaker containing 0⋅1 M solution of oxalic acid (AR ) and
stirred for about 12 h. The white precipitates thus ob-
tained are filtered and washed with acetone and distilled
water to remove impurities and dried over 120°C for 6 h
in order to remove water molecules.
Zn(CH3COO)2⋅2H2O + H2C2O4⋅2H2O ––––→
ZnC2O4.XH2O + CH3COOH + 2H2O
ZnC2O4⋅XH2O → ZnO + CO + CO2 + XH2O.
The calcination process is carried out over a temperature
of 400–450°C in order to remove CO and CO2 from the
compound.
Thus obtained samples are characterized by powder
X-ray diffractometer XPERT PRO with CuKα X-ray
radiation (λ = 0⋅15496 nm). The crystalline nature of
ZnO samples is confirmed by sharp intense peaks. The
surface morphology of the sample is observed by scan-
ning electron microscopy (SEM, JEOL, JSM-67001) and
high-resolution transmission electron microscopy
(HRTEM, HITACHI model H-800). The composition of
elements like Zn, O is confirmed by energy dispersive X-
ray spectra (EDX). Optical absorption spectrum is taken
with SHIMA DZU UV-310PC, UV scanning spectropho-
tometer. The room temperature photoluminescence
(PL) spectrum of ZnO is recorded with fluorescence
spectrometer (FLS920) using Xe lamp as the excitation
source at excitation wavelength (λex = 325 nm). The
luminescence is dispersed with a monochromator and
recorded.
3. Results and discussion
Figure 1(a) shows the XRD patterns of ZnO nanocrysta-
lline powder. The sharp intense peaks of ZnO confirms
the good crystalline nature of ZnO and the peaks origi-
nated from (100), (002), (101), (102), (110), (103), (200),

*Author for correspondence (mspr1972@yahoo.com)
H2O

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D Sridevi and K V Rajendran

166
(112) and (201) reflections of hexagonal ZnO (Yang et al
2005). The size of the particles is calculated by Debye
Scherrer’s formula. The average crystal size is calculated
by the formula
D = k(λ/βcosθ),
K is a constant equal to 0⋅89, λ the X-ray wavelength
(0⋅154095 nm), β the full wavelength at half maximum
and θ the half diffraction angle.

The crystal size of the ZnO nanoparticles calculated from
FWHM was tabulated in table 1.
Energy dispersive X-ray spectra displayed in figure
1(b) and table 2 furnish the composition of various ele-
ments in the prepared sample.
Figure 2 represents the scanning electron micrographs
of ZnO nanomaterials synthesized under aqueous me-




Figure 1. (a) XRD pattern of ZnO nanoparticles after calcina-
tion at 450°C and (b) EDX spectrum of ZnO nanoparticles.
dium. The orientation growth of ZnO crystal in water is
higher (Kwon et al 2002). And the spherical shaped
morphology is observed in the micrograph as shown in
figure 2. The SEM pictures show distinguished spherical
morphology with a self-aligned prismatic nanoparticles.
The transmission electron micrograph of the ZnO sample
is given in figure 3. The figure clearly indicates the
morphology of the particles to be roughly spherical and
homogeneous. Some of the particles are agglomerates.
The distribution of particles obtained from the TEM
image is given in figure 4.
Figure 5 shows the optical absorption spectrum of ZnO
nanopowder prepared under aqueous conditions. The UV
visible spectra for ZnO nanoparticles synthesized in
aqueous media displayed excitonic absorption peak at
363 nm which implies the lower particle size ZnO. The


Table 1. Particle size estimated from the diffraction spectrum
in figure 1 by using half maximum widths.
Particle
Phase Width 2θ (deg) (hkl) size (nm)

ZnO 0⋅42 31⋅750 100 21
0⋅37 34⋅380 002 24
0⋅39 36⋅180 101 25
0⋅39 47⋅500 102 24
0⋅40 56⋅540 110 24
0⋅35 62⋅780 103 32
0⋅30 66⋅310 200 29
0⋅41 67⋅840 112 26
0⋅29 68⋅910 201 35


Table 2. Composition of elements in ZnO sample.
Element Wt% At%

OK 24⋅44 56⋅93
ZnK 75⋅56 43⋅07
Total 100⋅00 100⋅00



Figure 2. SEM image of ZnO nanocrystals.

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Synthesis and optical characteristics of ZnO nanocrystals

167
bandgap calculated from the UV cut-off is found to be
3⋅41 eV ZnO nanoparticle. These band gap values blue
shifted relative to the bulk zinc oxide of 3⋅37 eV (Huang
et al 2001).
Figure 6 depicts the photoluminescence spectrum of
nanosize ZnO synthesized in aqueous medium. Strong
emission peak centred at 397 nm was observed in ZnO.
The ZnO sample exhibits only UV bandgap lumines-
cence but no oxygen defects luminescence was observed.
(In general, it is observed around 530 nm, i.e. green–
yellow emission). Generally, a green–yellow emission is



Figure 3. TEM image of ZnO nanocrystals.



Figure 4. Particle distribution obtained from TEM image.
observed in PL spectra, due to recombination of photo
generated holes with singly ionized charge state of spe-
cific defect (Hsieh et al 2003). However, absence of the
green yellow emission in our samples indicates the poten-
tial of our synthetic strategy to produce a low concentra-
tion of oxygen defects and high optical quality of single
crystal ZnO (Zu et al 1997; Gao et al 2005).
4. Conclusions
ZnO nanomaterials have been successfully synthesized by
hydrothermal method using zinc acetate and oxalic acid
as the reactants. The XRD pattern reveals that the synthe-
sized materials are crystalline in nature. The size of the
particles determined by XRD is in good agreement with
TEM analysis. The optical properties of ZnO nanomaterials
are studied by absorption and photoluminescence spectra.



Figure 5. UV absorption spectrum of obtained sample.



Figure 6. PL spectrum of obtained sample.

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D Sridevi and K V Rajendran

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The high crystalline and purity of the ZnO nanomaterials
are confirmed by photoluminescence spectra.
Acknowledgement
The authors are grateful to the University Grants Com-
mission for financial support.
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