Preparation of ZnO Nanostructures by Solvothermail Method

1. Journal of Microscopy Society of Thailand 2009, 23(1): 75-78
75
Preparation of ZnO Nanostructures by
Solvothermal Method
Doungporn Yiamsawas, Kanittha Boonpavanitchakul, and Wiyong Kangwansupamonkon*
National Nanotechnology Center, 111 Thailand Science Park, Paholyothin Road, Klong 1, Klong Luang,
Phathumthani, 12120 Thailand
*Corresponding author, e-mail: wiyong@nanotec.or.th
Abstract
The solvothermal method was employed to synthesize ZnO nanostructure. This technique is based on
thermal decomposition of organometallic compound in organic solvent and has been successfully applied for the
synthesis of various types of nanosized metal oxide with large surface area, high crystallinity and high thermal
stability. The influences of type of on shape of the synthesized nanostructures as well as the mechanism were
investigated. An interesting correlation between aspect ratio of the ZnO products and physical properties of the
solvent was observed and presented in this work. The size and morphology from different environmental were
investigated. The different environments during ZnO preparation led to the different morphology with wide and
length in the range of 8.9-69 nm and 68-108 nm, respectively and 0.17-0.93 in the term of the aspect-ratio. The
TEM image clearly reveals the lattice spacing is about 0.52 corresponding to the (0 0 1) plane of hexagonal-
structured ZnO.
Background
Zinc oxide is an important basic material due
to its low cost, large band gap (3.37 eV), large
exciton binding energy (60 MeV), and
luminescent proper-ties [1]. It is widely used in
many applications, such as catalyst, gas sensor,
filtering materials for ultraviolet light, and also
antimicrobial material. Currently many interesting
ZnO nanostructures, including nanorods,
nanobridges and nanonails have been fabricated.
These structures are expected to have potential
applications in building functional nanoelectronic
devices. Control of the particle shape is of interest
for nanostructured material synthesis because
electrical and optical properties of nanomaterials
depend sensitively on both size and shape of the
particles. Different methods have been reported
for ZnO nanostructure, therein, a vapor-phase
transport process with the assistance of metal
catalysts. However, expensive raw materials,
complex process control and sophisticated
equipment are often needed and this is
unfavorable for large-scale synthesis. As a better
alternative route, soft chemical methods have
proven to be appealing because of their low
growth temperature and good potential for scale-
up [2,3]. Various chemical method are applied for
the prepa-ration of metal oxide nanoparticles,
such as the gas phase methods, sol-gel methods,
evaporative decomposition of solution and wet
chemical synthesis [4]. To promote the formation
of ZnO nanostructures, poly(vinyl pyrrolidone)
(PVP) is frequently used as a templating molecule
[5]. In this research, we report a solvothermal-
assisted heat treatment method to synthesize ZnO
nano-structure. This synthesis involved precursors
of Zn(CH3COO)2.2H2O and carried out at low
temperature (80°C). Different solvents were used
and investigated to have an effect on crystals
morphologies.
Materials and Methods
In typical, synthesis of ZnO was carried out by
solvothermal process at 80°C. Poly(vinyl
pyrrolidone) PVP 30K was dissolved in absolute
ethanol under stirring at room temperature, then
zinc acetate dihydrate was slowly added to the
solution. Consequently, the solid NaOH was put
into the reaction mixture. The resulting solution
was stirred for several minutes. The solution was
then transferred to polypropylene vessel, then
sealed and heated in temperature-controlled
autoclave at 80°C for 24 h. After cooling to room
temperature, the white powder was precipitated and
then washed with absolute ethanol several times to
dissolve other impurities. Finally, the powder was
dried under vacuum at 60°C overnight and
determined in terms of their structural, morphology
and optical properties. The mixed solvent of
ethylene glycol and absolute ethanol without
polymer template was also investigated.
The crystalline structures of the products
were characterized by X-ray diffraction (XRD)
analysis using a JEOL JDX-3530 diffractometer.
Dry powder of samples which were re-dispersed
in ethanol and consequently deposited on a glass
slide were used for the structural measurement.
The X-ray diffraction (XRD) patterns were
recorded from 20°C to 80°C in 2θ with a
scanning rate of 0.2°/s. The absorbance spectra
were recorded on a PerkinElmer Lambda 650
UV-vis spectrophotometer. Size and morphology
of the products were observed by transmission

2. Journal of Microscopy Society of Thailand 2009, 23(1): 75-78
76
electron microscopy (TEM) which was taken on
a JEOL JEM-2010 electron microscopy using an
accelerating voltage of 200 kV in bright field and
electron diffraction (ED) modes. A small drop of
the ZnO powder re-dispersed by ethanol was
dropped on a carbon film-coated copper grid. The
sizes of ZnO were measured and averaged by
several TEM images (n = 20).
Results and Discussion
XRD pattern of synthesized ZnO nanostructures
by solvothermal reaction at 80°C is shown in Fig. 1.
All peaks of the obtained product are
corresponding to the hexagonal wurtzite structure
of ZnO reported in JCDDS card (NO. 36-1451, a =
3.249 Å, c = 5.206 Å). No characteristic peaks
were observed for the other impurities. According
to the intensity and half width of the characteristic
peaks in the XRD pattern, the ZnO nanostructures
still crystallized well even though the preparation
was carried out at low temperatures. This result
confirmed that ZnO was successfully synthesized
by the solvothermal reaction. However, it was
found that the signal intensity due to the 0 0 2 plane
in Fig. 1 (a) and (b) is quite low. This may be
resulted from the inadequate ZnO film coating on
the glass slide, causing some of the crystal plane
hard to be detected.
The two common reaction media (ethanol and
ethylene glycol) were used for studying the
morphology control of the ZnO nanostructure. It
was found that the difference of solvent leads to the
different morphology of ZnO nano-structures as
shown in TEM images (Fig. 2).
The nanoparticles shown in Fig. 2 (a.1, a.2)
which were prepared in ethylene glycol have the
diameters of 68.1±7 nm. Moreover, the prepared-
ZnO consists of a large quantity of well dispersed
and uniform ZnO nanostructure with spherical
shape. Fig 2 (b.1, b.2) shows rod shape synthesized
in absolute ethanol. The diameter and length of
these nanorods are 8.2±2 nm and 54.3±11 nm,
respectively. Note that the short ZnO nanorods
could be observed when the ultrasonication was
used in the sample preparation for TEM analysis.
The high resolution TEM images of nanostructures
show that the lattice fringes between two adjacent
planes about 0.52 nm which is equal to the lattice
constant of the ZnO. This further indicates that the
obtained structures have a wurtzite hexagonal
phase and preferentially grew along the c-axis [0 0
1] direction. The selected area electron diffractions
(SAED) in Fig. 2 (a.3, b.3, c.3) indicate that all the
ZnO nanostructures are crystalline.
In addition, the semi-sphere ZnO nano-crystals,
which were obtained in the mixed alcohol solution
without using PVP at the same condition in the
presence of the polymer template molecules were
shown in Fig. 2 (c1, c2). TEM images show
approximately uniform morphology with diameter
and length of 68.9±9 nm and 108.4±9 nm,
respectively.
The results demonstrate that the PVP plays an
important role in controlling the ZnO size and
shape. PVP act as a template, which can form chain
structures. With the polymer template, ZnO can
grow up along these chains to form nanostructure.
On the other hand, PVP can form a shell
surrounding the particles to prevent them from
aggregating to larger particles and grain growth as
a result of its steric effect. It has been reported that
the presence of a capping molecule (such as
Fig. 1 XRD patterns of synthesized ZnO sample from solvothermal process 24 h at 80°C with (a) PVP K30 and ethylene
glycol, (b) PVP K 30 and ethanol and (c) no polymer template and 50:50 of ethylene glycol:ethanol.

3. Journal of Microscopy Society of Thailand 2009, 23(1): 75-78
77
poly(vinyl pyrrolidone)) can alter the surface
energy of crystallographic surfaces, in order to
promote the anisotropic growth of the nanocrystals
[6,7]. The PVP adsorbs on crystal nuclei to serve as
growth director. With this condition, the structure
of the solvent led to the morphology control
depending on its interaction with the growing
crystal. Ethylene glycol with its structure and
bulkier functional groups (OH) comparing to
ethanol limits the growth of all crystal planes, thus
producing a more uniform dimensional shape like
sphere. In the other hand, ethanol which has less
complicated structure allows the growth of some
crystal planes, thus giving the nanorod structure. In
deed, when the PVP template was not used the
growth of the crystal planes in the mixed solvent
become no orientation. This led to the semi-
spherical structure of the synthesized ZnO.
It has been report that the interaction between
zinc acetate and alcohol under the solvothermal
conditions involved in esterification reaction,
which proceeded to form ZnO, ester and water,
according to the following reaction [8]:
Zn(CH3COO)2 + 2R-OH
ZnO + 2CH3-CO-R + H2O
The room temperature UV-visible spectra were
also measured. The UV-visible spectra of the
prepared ZnO colloidal suspensions are shown in
Fig 3. The spectra exhibit a strong absorption with
an on set around 355 nm except the semi-spherical
shape which shows absorption peak around 366
nm. It is known that the bulk ZnO has absorption at
375 nm in the UV-visible spectrum [9] which is
obviously larger than the prepared ZnO
nanostructures, corresponding to the fact that the
excitonic peak shifts to blue results from
decreasing particle size.
(a.1)
(b.1)
(c.1)
(a.2) (a.3)
(b.2) (b.3)
(c.2) (c.3)
0. 52
0.52
Fig. 2 TEM images and the corresponding selected area diffraction pattern of the synthesized ZnO sample from
solvothermal process 24 h at 80°C with PVP K30 as polymer template and different solvent; (a) PVP K30 and ethylene
glycol, (b) PVP K 30 and ethanol and (c) no polymer template and 50:50 of ethylene glycol:ethanol.

4. Journal of Microscopy Society of Thailand 2009, 23(1): 75-78
78
Conclusion
In summary, ZnO nanocrystals were success-
fully obtained by using solvothermal process at
80°C. Zinc acetate dihydrate was used as the zinc
source. The synthesized ZnO nanoparticles
prepared in ethylene glycol have the diameters of
68.1±7 nm. The ZnO nanorod was obtained by
using absolute ethanol as solvent, giving the
diameter and length of 8.2±2 nm and 54.3±11 nm,
respectively. The absence of PVP with the use of
mixed solvent gave the ZnO semi-sphere shape
with its diameter and length of 68.1±9 nm and
108.4±9 nm, respectively.
Acknowledgements
The authors are grateful for the research funding
from National Nanotechnology Center and Institute
of Solar Energy Technology Development, Thailand
(Grant number: B22 AR010110RDAR01).
References
1. Zhong J, Kitai AH, Mascher P and Puff W.
The influence of processing conditions on
point defects and luminescence centers in
ZnO. J. Electrochem. Soc. 1993, 140: 3644-
3649.
2. Zhang H, Yang D, Ji Y, Ma X, Xu J and Que
D. Low temperature synthesis of flowerlike
ZnO nanostructures by cetyltrimethyl-
ammonium bromide-assisted hydrothermal
process. J. Phys. Chem. B. 2004, 108: 3955-
3958.
3. Liu B, Yu S-H, Zhang F, Li L, Zhang Q, Ren
L and Jiang K. Ring-like nanosheets standing
on spindle-like rods: Unusual ZnO
superstructures synthesized from a flakelike
precursor Zn5(OH)8Cl2·H2O. J. Phys. Chem.
B. 2004, 108: 4338-4341.
4. Moballegh A, Shahverdi HR,
Aghababazadehn R and Mirhabibi AR. ZnO
nanoparticles obtained by mechanochemical
technique and the optical properties. Surf. Sci.
2007, 601: 2850-2854.
5. Lepot N, Van Bael MK, Van den Rul H,
D’Haen J, Peeters R, Franco D and Mullens J.
Synthesis of ZnO nanorods from aqueous
solution. Mater. Lett. 2007, 61: 2624-2627.
6. Park WI, Kim DH, Jung SW and Yi G-C.
Metalorganic vapor-phase epitaxial growth of
vertically well-aligned ZnO nano-rods. Appl.
Phys. Lett. 2002, 80: 4232-4234.
7. Jimin D, Zhimin L, Ying H, Yanan G, Buxing
H, Wenjing L and Guanying Y. Control of
ZnO morphologies via surfactants assisted
route in the subcritical water. J. Cryst Growth.
2005, 280: 126-134.
8. Tonto P, Mekasuwandumrong O, Phatanasri
S, Pavarajarn V and Piyasan P. Preparation of
ZnO nanorod by solvothermal reaction of zinc
acetate in various alcohols. Ceram. Int. 2008,
34: 57-62.
9. Srikant V and Clarke DR. On the optical band
gap of zinc oxide. J. Appl. Phys. 1998, 83:
5447-5451.
Fig. 3 UV-vis spectra of the prepared ZnO (a) PVP K30 and ethylene glycol (nanoparticles), (b) PVP K 30 and ethanol
(nanorods) and (c) no polymer template and 50:50 of ethylene glycol:ethanol (semi-nanoparticles).