Synthesis of ZnO nanoparticles by hydrothermal method
P. M. Aneesh, K. A.Vanaja, M. K. Jayaraj*
Optoelectronic Devices Laboratory, Cochin University of Science and Technology, Kochi-682 022,
India
ABSTRACT
Stable, OH free zinc oxide (ZnO) nanoparticles were synthesized by hydrothermal method by varying the
growth temperature and concentration of the precursors. The formation of ZnO nanoparticles were confirmed by x-ray
diffraction (XRD), transmission electron microscopy (TEM) and selected area electron diffraction (SAED) studies. The
average particle size have been found to be about 7-24 nm and the compositional analysis is done with inductively
coupled plasma atomic emission spectroscopy (ICP-AES). Diffuse reflectance spectroscopy (DRS) results shows that the
band gap of ZnO nanoparticles is blue shifted with decrease in particle size. Photoluminescence properties of ZnO
nanoparticles at room temperature were studied and the green photoluminescent emission from ZnO nanoparticles can
originate from the oxygen vacancy or ZnO interstitial related defects.
Key words: Hydrothermal, ZnO, band gap, particle size, photoluminescent emission
*Corresponding author
Tel: +91-484-2577404
Fax: +91-484-2577595
Email: mkj@cusat.ac.in
Nanophotonic Materials IV, edited by Zeno Gaburro, Stefano Cabrini, Proc. of SPIE
Vol. 6639, 66390J, (2007) · 0277-786X/07/$18 · doi: 10.1117/12.730364
Proc. of SPIE Vol. 6639 66390J-1
1. INTRODUCTION
Semiconductors with dimensions in the nanometer realm are important because their electrical, optical and
chemical properties can be tuned by changing the size of particles. Optical properties and are of great interest for
application in optoelectronics, photovoltaics and biological sensing. Various chemical synthetic methods have been
developed to prepare such nanoparticles.
Zinc Oxide (ZnO) is a unique material with a direct band gap (3.37eV) and large exciton binding energy of
60meV
1, 2
. It has been widely used in near-UV emission, gas sensors, transparent conductor and piezoelectric
application
3 - 7
. Most of the ZnO crystals have been synthesized by traditional high temperature solid state method which
is energy consuming and difficult to control the particle properties. ZnO
11- 13
nanoparticles can be prepared on a large
scale at low cost by simple solution - based methods, such as chemical precipitation
8, 9
, sol-gel synthesis
10
, and
solvothermal/hydrothermal reaction
11 - 13
. Hydrothermal technique is a promising alternative synthetic method because of
the low process temperature and very easy to control the particle size. The hydrothermal process have several advantage
over other growth processes such as use of simple equipment, catalyst-free growth, low cost, large area uniform
production, environmental friendliness and less hazardous. The low reaction temperatures make this method an attractive
one for microelectronics and plastic electronics
14
. This method has also been successfully employed to prepare nano-
scale ZnO and other luminescent materials. The particle properties such as morphology and size can be controlled via the
hydrothermal process by adjusting the reaction temperature, time and concentration of precursors.
The present study focuses on the hydrothermal synthesis of ZnO nanopowders and the effect of reaction
temperatures, concentration of the precursors and time of growth on its properties. The hydrothermal synthesis of ZnO
powders has four advantages (1) powders with nanometer- size can be obtained by this method (2) the reaction is carried
out under moderate conditions (3) powders with different morphologies by adjusting the reaction conditions and (4) the
as-prepared powders have different properties from that of the bulk.
2. EXPERIMENTAL DETAILS
In order to synthesize the ZnO nanoparticles, stock solutions of Zn(CH
3
COO)
2
.2H
2
O (0.1 M) was prepared in
50ml methanol under stirring. To this stock solution 25ml of NaOH (varying from 0.2 M to 0.5 M) solution prepared in
methanol was added under continuous stirring in order to get the pH value of reactants between 8 and 11. These
solutions was transferred into teflon lined sealed stainless steel autoclaves and maintained at various temperature in the
range of 100 - 200
o
C for 6 and 12 h under autogenous pressure. It was then allowed to cool naturally to room
temperature. After the reaction was complete, the resulting white solid products were washed with methanol, filtered
and then dried in air in a laboratory oven at 60
o
C.
The synthesized samples were characterized for their structure by x-ray diffraction (Rigaku D max-C) with Cu
Kα radiation. Transmission electron microscopy (TEM) selected area electron diffraction (SAED) and high resolution
transmission electron microscopy (HRTEM) were performed with a JEOL JEM-3100F transmission electron microscope
operating at 200 kV. The sample for TEM was prepared by placing a drop of the ZnO suspension in methanol onto a
standard carbon coated copper grid. The grids were dried before recording the micrographs. The optical band gap E
g
was
estimated from the UV-Vis-NIR diffuse reflectance spectroscopic (UV-Vis-NIR DRS) studies in a wavelength range
from 190nm to 1200nm with JASCO V-570 spectrophotometer. The samples for this study were used in the form of
powder and pure BaSO
4
used as the reference. The elemental composition of the ZnO nanoparticles were determined by
using Thermo Electron IRIS INTREPID II XSP DUO inductively coupled plasma atomic emission spectrometer (ICP-
AES). Room temperature photoluminescence (PL) of the samples was measured on Horiba Jobin Yuon Fluoromax-3
spectrofluorimeter using Xe arc lamp as the excitation source.
3. RESULTS AND DISCUSSION
The x-ray diffraction data were recorded by using Cu Kα
radiation (1.5406 A
o
). The intensity data were
collected over a 2θ range of 20-80
o
. The average grain size of the samples was estimated with the help of Scherrer
equation using the diffraction intensity of (101) peak.
Proc. of SPIE Vol. 6639 66390J-2
x-ray diffraction studies confirmed that the synthesized materials were ZnO with wurtzite phase and all the
diffraction peaks agreed with the reported JCPDS data
15
and no characteristic peaks were observed other than ZnO.
The mean grain size (D) of the particles was determined from the XRD line broadening measurement using
Scherrer equation
16
.
D=0.89λ / (βCosθ)
Where λ is the wavelength (Cu Kα), β is the full width at the half- maximum (FWHM) of the ZnO (101) line
and θ is the diffraction angle.
A definite line broadening of the diffraction peaks is an indication that the synthesized materials are in
nanometer range. The grain size was found to be in the range of 7-24 nm depending on the growth condition. The lattice
parameters calculated were also in agreement with the reported values.
The reaction temperature greatly influences the particle morphology of as-prepared ZnO powders. Figure 1
shows that the XRD patterns of ZnO nanoparticles synthesized at various temperature with 0.3 M NaOH for 6 h. As the
reaction temperature increases, FWHM decreases. Thus the size of ZnO nanoparticles increases as the temperature for
the hydrothermal synthesis increases. This is due to the change of growth rate between the different crystallographic
planes.
Fig. 1. XRD patterns of ZnO nanoparticles synthesized from 0.3 M NaOH at various temperatures for 6 h.
Figure 2 shows the variation of FWHM and grain size of ZnO nanoparticles synthesized from 0.3 M NaOH at
different temperatures for a growth time of 6 h. The average grain size calculated by Scherrer equation is observed to
increases from 7 nm to 16 nm as the temperature increases from 100
o
C to 200
o
C
17
.
Proc. of SPIE Vol. 6639 66390J-3
Fig. 2. Variation of FWHM and grain size of ZnO nano particles
synthesized from 0.3 M NaOH with temperature
for a growth time of 6 h.
ZnO structures with different grain sizes can be obtained by controlling the concentration of the precursors.
ZnO nanoparticles were synthesized by keeping the concentration of Zn(CH
3
COO)
2
.2H
2
O as 0.1 M in all reactions, the
concentration of NaOH was varied from 0.2 M to 0.5 M at 200
o
C for 12 h. Figure 3 shows the XRD pattern of ZnO
nanoparticles synthesized by varying the concentration of precursors. All the peaks match well with the standard wurtzite
structure
15
and the FWHM of the (101) diffraction peak increases with the decreasing concentration of the NaOH. These
results reveal that the molar ratio of OH
-
to Zn
2+
is a dominant factor for the formation of the ZnO nanoparticles.
Fig. 3. XRD patterns of ZnO nanoparticles synthesized at various concentration of NaOH (0.2 M, 0.3 M, 0.4 M, and 0.5 M)
at 200
o
C for 12 h.
Proc. of SPIE Vol. 6639 66390J-4
Figure 4 shows the variation of FWHM and grain size of ZnO nanoparticles with concentration of NaOH
precursor for the samples grown at 200
0
C for 12 h. It has a linear variation with the concentration of NaOH precursor.
The grain size increases from 12 nm to 24 nm as the concentration of NaOH precursors increases from 0.2 M to 0.5 M.
Fig. 4. Variation of FWHM and grain size of ZnO nano particle with various concentration of NaOH grown at 200
o
C
for a growth time of 12 h.
Fig. 5. ) TEM image of the ZnO nanoparticles synthesized from 0.5 M NaOH
at 150
o
C for 6 h . Inset shows the SAED image
Proc. of SPIE Vol. 6639 66390J-5
