X-ray diffraction patterns show that the nanorods are high-quality crystals growing along [001] direction with a high consistent orientation perpendicular to the substrate while the
Trang 1Growth Of ZnO Nanorods by Hydrothermal
Method Under Different Temperatures
T H Meen, W Water, Y S Chen*, W R Chen, L W Ji and C J Huang
Abstract –In this study, the aqueous solution
method was employed to synthesize one-dimensional
well-aligned ZnO nano-array on ITO glass substrate
We can find that the dimension of ZnO nanorod will
changes with different growth temperature. X-ray
diffraction patterns show that the nanorods are
high-quality crystals growing along [001] direction
with a high consistent orientation perpendicular to the
substrate while the growth temperature is equal to 80
SEM images show that the average diameters of ZnO
nanorods are about 60-90 nm by changing growth
temperature The smallest diameter of ZnO nanorods
is observed while the growth temperature is equal to
75 ℃ The UV/Vis spectra analyses show the
absorption peaks appear at 330nm, 370nm and 390nm
while growth temperature increases from 65 ℃ to 85
℃
I INTRODUCTION ZnO nanostructure has been envisioned to enhance
performance of various technologically important devices
such as short-wavelength lasers[1], Gratzel-type solar cell
[2],[3], and chemical sensors [4],[5] The interest in
synthesis of well-aligned ZnO nanowires or nanorods on
substrates keeps growing ZnO has shown a great deal of
research in DSSCs [6]–[9] due to some of its fascinating
properties Comparing with other semiconductors, ZnO
has unique excellent properties, such as higher binding
energy (60meV), wide band gap (3.37 eV), high
breakdown strength, cohesion, and exciton stability
Moreover, ZnO is one of the hardest materials in the
family of II–VI semiconductors Electron mobility in
ZnO is more than that in TiO2 making the former suitable
for DSSCs Recently, it has become possible to form
vertical nanowires of ZnO [10] Such nanowires
expectedly provide morphology for better electron
transport The vertical geometry also provides a more
open structure for filling with hole-transporting
materials[11]-[14] The preparation of 1D ZnO
nanostructures has been demonstrated by various
methods, including vapor–liquid–solid (VLS) growth
[15],[16], chemical vapor deposition (CVD) [17],[18], hydrothermal process [19], and template-based methods
expensive, and the choice of substrate restricted, complex process controlling and high temperature are unfavorable for an industrialized process Recently, a solution-based approach was developed to achieve highly oriented nanorods film with high surface area on substrate, which has the advantages of mild synthetic conditions, simple manipulation and large scale-up production It opens a door for future optoelectronic devices based on ZnO nanostructure arrays [21]–[25] In this work, we report the hydrothermal growth of high quality ZnO nanorods perpendicularly oriented on ITO substrates, and
investigate them by X-ray diffraction, scanning
electronmicroscopy (SEM) and ultraviolet-visible absorption spectra analyses These high quality ZnO nanorods can be applied on the electrode of dye-sensitized solar cell to increase the contact area between ZnO and dye, resulting in the enhancement of efficiency for dye-sensitized solar cell.
II EXPERIMENTAL
The ZnO nanorods were prepared from zinc nitrate in
a neutral aqueous solution under hydrothermal conditions The procedure consists of two steps: (1) deposition of ITO substrates with densely and uniformly ZnO films by
RF sputter as the buffer layer, and (2) hydrothermal growth of ZnO nanorods in aqueous solution In detail, the aqueous solutions of zinc nitrate (0.5g) and methenamine (0.5g) were stirred uniformly An 80 nm thick ZnO layer was first deposited on ITO glass using a
RF sputter deposition system under an Ar and O2 pressure
of 5x10-2torr The hydrothermal growth was carried out at
65 ℃ ~ 85 ℃ in a sealed beaker by immersing the modified substrates in the aqueous solution (100ml) containing Zn(NO3)2 (0.5 M) and methenamine (0.35 M) for 10 hours The morphology, structure, and optical properties of ZnO nanorods were studied by X-ray diffraction (XRD), scanning electron microscope (SEM), and Ultraviolet-Visible spectrophotometer (UV/Vis spectrophotometer)
III RESUATS AND DISCUSSION The crystal structure of as-prepared ZnO nanorods was analyzed by XRD X-ray diffraction patterns of ZnO nanorods with different growth temperature are shown in Fig.1 All diffraction peaks well indexed to the standard diffraction pattern of hexagonal ZnO phase except for 2θ=36o
and 37o In comparison with the standard XRD
T H Meen, W Water, Y S Chen W R Chen and L W Ji
are with the Institute of Electro-Optical and Materials Science,
and Department of Electronic Engineering, National Formosa
University, Yunlin 632, Taiwan, R.O.C C J Huang is with
the Department of Applied Physics, National University
of Kaohsiung, Nan-Tzu 811, Kaohsiung, R O C
E-mail: inshen1017@hotmail.com
Trang 2pattern of ZnO, the much higher relative intensity of the
(002) diffraction peak provides further evidence that the
nanorods are preferentially oriented in the c-axis
direction The strongest (002) peak of diffraction pattern
appears while the growth temperature is equal to 80 ℃
Fig 1 X-ray diffraction patterns of ZnO nanorods with
different growth temperature.
SEM was used to investigate the nanostructure of
ZnO nanorods Figures 2 show the SEM images of ZnO
nanorods obtained under different growth temperatures
They show that a dense array of hexagonal ZnO nanorods
having a diameter of from 30nm to 150nm are formed
under different growth temperatures, and the average
diameters of ZnO nanorods are listed in Table I It is
noted that Fig 2(d) shows the best nanostructure of ZnO
nanorods From the results of Fig 1 and Figs 2, the best
growth temperature of ZnO nanorods is 80 ℃. The
cross-section image of ZnO nanorods arrays grown at 80
℃ is shown in Fig 3 It is found that all ZnO nanorods
grow almost vertically from the substrate, and the length
of nanorods is about 1.3um
TABLE I THE AVERAGE DIAMETER OF ZNO NANORODS
WITH DIFFERENT GROWTH TEMPERATURES
Fig 2 SEM images of ZnO nanorods with different growth temperatures：（a）65 ℃（b）70 ℃（c）75 ℃
（d）80 ℃（e）85 ℃
Trang 3Fig 3 A cross-section view of SEM image of ZnO
nanorods with growth temperature equal to 80 ℃.
Figure 4 shows the UV-Vis absorption spectra of ZnO
nanorods under different growth temperatures The
absorption peaks appear at 330nm, 370nm and 390nm
while the growth temperature increases from 65 ℃ to 85
℃, and the strongest absorption peak at 390nm is
observed while the growth temperature is equal to 75 ℃
It is indicated that the smallest average diameter of ZnO
nanorods has the best absorption for UV light From the
results of XRD, SEM and UV-Vis analyses for ZnO
nanorods, we can apply these high quality ZnO nanorods
on the electrode of dye-sensitized solar cell to increase
the contact area between ZnO and dye, resulting in the
enhancement of efficiency for dye-sensitized solar cell.
Fig.4 The UV-Vis absorption spectra of ZnO under
different growth temperature from 65 ℃ to 85 ℃
IV CONCLUSION
In this study, we have successfully synthesized ZnO
nanorods on ITO glass substrate From the results of
XRD and SEM, the best growth temperature of ZnO
nanorods is 80 ℃, at which the average diameter and
length of ZnO nanorods are about 70.4 nm and 1.3um
The absorption peaks appear at 330nm, 370nm and
390nm while the growth temperature increases from 65
℃ to 85 ℃, and the strongest absorption peak at 390nm is
observed while the growth temperature is equal to 75 ℃
These high quality ZnO nanorods can be applied on the
electrode of dye-sensitized solar cell to increase the contact area between ZnO and dye, resulting in the enhancement of efficiency for dye-sensitized solar cell
ACKNOWLEDGEMENT
The research is supported by National Science Council, R.O.C under contract Nos NSC 96-2622- E-150-027-CC3 and NSC 96-2221-E-150-028
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