A simple and novel route for the preparation of ZnO nanorods

Article abstract of DOI:10.1016/S0038-1098(02)00114-X

Synthesis of zinc oxide (ZnO) nanorods was achieved by thermal decomposition of the precursor of ZnC₂O₄ obtained via chemical reaction between Zn(CH₃COO)₂·2H₂O and H₂C₂O₄·2H₂O in the presence of surfactant nonyl phenyl ether (9)/(5) and NaCl flux. Transmission electron microscopy, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, selected area electron diffraction, and high-resolution transmission electron microscopy were used to characterize the structure features and chemical compositions of the as-made nanorods. The results showed that the as-prepared nanorods are composed of ZnO with diameter of 10-60 nm, and lengths ranging from 1 to 3 μm. The mechanism of formation of ZnO nanorods is also discussed.

Full text of DOI:10.1016/S0038-1098(02)00114-X

PERGAMON
Solid State Communications 122 (2002) 175±179
www.elsevier.com/locate/ssc
A simple and novel route for the preparation of ZnO nanorods
*
Congkang Xu, Guoding Xu, Yingkai Liu, Guanghou Wang
National Laboratory of Solid State Microstructure, Department of Physics, Nanjing University, Nanjing 210093, People's Republic of China
Received 30 January 2002; accepted 18 February 2002 by C.N.R. Rao
Abstract
Synthesis of zinc oxide (ZnO) nanorods was achieved by thermal decomposition of the precursor of ZnC2O4 obtained via
chemical reaction between Zn(CH3COO)2´2H2O and H2C2O4´2H2O in the presence of surfactant nonyl phenyl ether (9)/(5) and
NaCl ¯ux. Transmission electron microscopy, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy,
selected area electron diffraction, and high-resolution transmission electron microscopy were used to characterize the structure
features and chemical compositions of the as-made nanorods. The results showed that the as-prepared nanorods are composed
of ZnO with diameter of 10±60 nm, and lengths ranging from 1 to 3 mm. The mechanism of formation of ZnO nanorods is also
discussed. q 2002 Elsevier Science Ltd. All rights reserved.
PACS: 68.37.Lp; 76.67. 2 n; 81.07. 2 b
Keywords: A. Nanostructures; B. Nanofabrications; C. Transmission electron microscopy
1. Introduction
to possess properties having applications in shock resist-
ance, sound insulation, photosensitization, ¯uorescence,
gas sensitization, and catalysis [20±23], its importance is
manifested in many papers that have been issued for its
synthesis. However, up to now, many methods have been
developed to synthesize ZnO nanoparticles, little work for
the preparation of ZnO nanorods has been reported. In this
work, we report a simple and novel approach to the fabrica-
tion of ZnO nanorods by thermal decomposition of the
precursor of ZnC2O4 obtained via chemical reaction
between Zn(CH3COO)2´2H2O and H2C2O4´2H2O in the
presence of surfactant nonyl phenyl ether (9)/(5) (NP-9/5)
and NaCl ¯ux.
In recent years, quasi one-dimensional (1D) solid nano-
structures (nanowires or nanorods) have stimulated con-
siderable interest for scienti®c research due to their
importance in mesoscopic physics studies and their potential
applications. Compared with micrometer diameter whiskers
and ®bers, these nanostructures are expected to have
remarkable optical, electrical, magnetic, and mechanical
properties. Different approaches have been used for the
preparations of quasi 1D solid nanostructures, for example,
the vapor±liquid±solid (VLS) growth [1±11], solution±
liquid±solid (SLS) method [12], template-mediated growth
method [13±15], electron-beam lithography (EBL) [16],
scanning tunneling microscopy (STM) techniques [17],
and other methods [14±18] of nanowire production.
Exploration of novel methods for the large-scale synthesis
of 1D nanostructures is a challenging research area. As n-
type semi-conductive materials [19], zinc oxide (ZnO) has
received a considerable amount of attention over last few
years because of many applications it has found in various
®eld, especially, ZnO nanostructured materials are expected
2. Experimental
Staring materials are zinc acetate Zn(CH3COO)2´2H2O,
oxalic acid H2C2O4´2H2O, NP-9/5, and NaCl. Among
them, Zn(CH3COO)2´2H2O, H2C2O4´2H2O, and NaCl are
of analytical pure, NP-9/5 is up to 99.0%. Zn(CH3COO)2´
2H2O (4.39 g) and H2C2O4´2H2O (2.53g) according to 1:1
molar rate were mixed with 5 ml of NP-9/5 in a mortar,
ground for several minutes and kept in a thermostat oven
at 50±60 8C for 6 h to prepare the precursor, the product was
washed several times with distilled water and acetone to
remove unreacted reactants, NP-9/5 and by-product, and
* Corresponding author. Tel.: 186-25-3595082; fax: 186-25-
3300535.
E-mail address: wangqun@nju.edu.cn (G. Wang).
0038-1098/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0038-1098(02) 00114-X

176
C. Xu et al. / Solid State Communications 122 (2002) 175±179
Fig. 1. TEM image of the as-prepared ZnO nanorods: (a) morphologies of the as-prepared ZnO nanorods; (b) morphologies of the as-prepared
single ZnO nanorod.
then dried in an oven at 70±80 8C for 12 h. The obtained
product was collected for the fabrication of ZnO nanorods.
ZnC2O4 (2 g) was simultaneously mixed with 8 g of NaCl
powder and then ground for several minutes. The ground
mixture was annealed at 910 8C for 2 h in a porcelain crucible
that was placed in the middle of the alumina tube; the reac-
tion chamber is of alumina tube with a length of 1500 mm
and a diameter of 60 mm. After heat treatment, it was gradu-
ally cooled to room temperature (5 8C/min), the ®nal pro-
duct was washed with distilled water one time, ethanol three
times and then ethyl ether one time using an ultrasonic bath
and a centrifuge.
carried out on an ESCALAB M KII X-ray photoelectron
spectrometer, using Mg Ka X-ray as the excitation
source; Raman spectroscopy was obtained using a RFS
100 spectrometer from 350 to 700 cm21 at room tem-
perature. The 1064 nm line of the laser was used as
the excitation source, with the capability of supplying
200 mW. For high-resolution transmission electron micro-
scopy (HRTEM) and TEM observation, the as-prepared
product was ground in a mortar and suspended in ethanol
using an ultrasonic bath for half an hour. A drop was then
placed on a holey-carbon copper grid.
The as-prepared product was identi®ed by X-ray
powder diffraction (XRD) employing a scanning rate of
0.028/s in a 2u range from 25 to 758, using a Japanese
Rigaku D/max-gA KII X-ray diffractometer equipped
with graphite monochromatized Cu Ka radiation. The
morphologies and dimensions of the products were
observed by transmission electron microscopy (TEM),
which were taken on a JEOL-2010 transmission electron
microscope using an accelerating voltage of 200 kV.
Meanwhile, the as-made products were also characterized
by X-ray photoelectron spectroscopy (XPS), which was
3. Results and discussion
The general TEM morphologies of the as-synthesized
product are shown in Fig. 1. It can be seen that the product
mainly consists of solid rod-like structures. The length of the
rods, as shown in Fig. 1(a), can be up to several micro-
meters. The typical nanorod, as can be seen in Fig. 1(b),
are around 30 nm.
Fig. 2 shows the XRD of the as-prepared products
obtained by thermal decomposition of the precursor of

C. Xu et al. / Solid State Communications 122 (2002) 175±179
177
Fig. 2. The XRD pattern of the as-prepared ZnO nanorods.
ZnC2O4. The reaction is as follows:
branches, nine optical and three acoustic [24]. As shown
in Fig. 3, the peak at 433 cm21 is attributed to ZnO non-
polar optical phonons E2 mode, which is typical of one of
Raman active branches. The peak at 376 cm21 corresponds
to A1 mode; the peak at 581 cm21 is attributed to ZnO E1
(LO) mode. Clearly, it also demonstrates that the sample is
composed of ZnO with hexagonal structure.
C2O422 1 Zn21 ! ZnC2O4
ꢀ1†
ꢀ2†
ZnC2O4 ! ZnO 1 CO 1 CO2
Its diffraction peaks were quite similar to those of a bulk
ZnO, which can be indexed as the hexagonal wurtzite struc-
ꢀ
ꢀ
Fig. 4(a) and (b) shows XPS spectra taken from the Zn
and O regions of the sample. The peaks at about 1020.9 eV
are attributed to Zn2p3/2 (Fig. 4(a)), which are close to the
data for Zn (2p) in ZnO; on the other hand, we ®nd that Zn2p
peak is sharp, as demonstrated that there is Zn21. From Fig.
4(b), it is seen that the O1s XPS is asymmetric, indicating
that at least two oxygen species are present in the nearby
region. The peak at about 530 eV is due to oxygen in the
ZnO crystal lattice, which corresponds to O±Zn bonds,
ture ZnO (a ˆ 3:249 A; c ˆ 5:206 A) and diffraction data
were in agreement with JCPDS card of ZnO (JCPDS 36-
1451). No characteristic peaks of impurities, such as NaCl,
ZnC2O4, and other precursor compounds were observed.
Thus, the results showed that the as-prepared product is
single phase hexagonal ZnO.
Raman spectra of the as-made ZnO are illustrated in Fig.
3. ZnO has wurtzite symmetry with C6v or 6 mm symmetry.
There are four atoms per unit cell leading to 12 phonon
Fig. 3. The Raman spectra of the as-prepared ZnO nanorods.

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C. Xu et al. / Solid State Communications 122 (2002) 175±179
Fig. 4. The XPS analysis of the as-prepared ZnO nanorods: (a) Zn
region; (b) O region.
whereas the peak at about 532 eV is due to chemisorbed
oxygen caused by surface hydroxyl, which corresponds to
O±H bonds [25]. The atomic composition of Zn and O was
calculated by using the integrated peak area and sensitivity
factors, the atomic ratio of Zn/O is nearly 1:1.
The atomic structure detail of the nanorods was revealed
using HRTEM. Fig. 5(a) is an HRTEM image of a repre-
sentative nanorod with diameter of about 30 nm. The
selected area electron diffraction (SAED) patterns (Fig.
5(b)), taken from the same nanorod shown in Fig. 1(b),
can be indexed to the re¯ection of wurtzite ZnO structure,
which is in consistent with the above XRD results. HRTEM
image shows that the nanorod is structurally uniform, and
the 2D lattice fringes illustrate that the nanorod is a single
crystalline. The interplanar spacing (Fig. 5(a)) is about
0.245 nm, which corresponds to the (101) plane of hexag-
onal ZnO, indicating that one of the growth planes of the
nanorods is along the (101) plane.
Fig. 5. HRTEM micrograph and SAED of one single crystalline
ZnO nanorods: (a) HRTEM micrograph of a 30 nm ZnO nanorod;
(b) SAED of a 30 nm ZnO nanorod.
understanding of the formation mechanism of nanorods.
There usually exist two models that explain the mechanism
of growth for nanorods [18]. One is the VLS growth
mechanism; the other is the conventional spiral mechanism.
In our product, there is no evidence that the ZnO nanorod
growth matches the former. There are no droplets observed
on any end of the nanorods, which is the most remarkable
sign of the VLS mechanism. There exists a conical tip at the
end of nanorod (Fig. 1(b)), which is evidence of the spiral
growth mechanism.
As a rule, the chemical route use direct, often very simple,
chemical reactions to fabricate a product in powder form.
These techniques have in common the fact that they do not
offer underlying of physical or chemical principles. Thus,
the generalization of the processes involved must require the
Apparently, the formation of the nanorods must be
affected by the character of the starting material, such as
the particle size and/or chemical activity in that the

C. Xu et al. / Solid State Communications 122 (2002) 175±179
179
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