ZnO nanotriangles: Synthesis, characterization and optical properties

Article abstract of DOI:10.1016/j.jallcom.2008.09.196

Zinc oxalate, [Zn(C₂O₄)·2H₂O] was used as a precursor to prepare zinc oxide nanotriangles by thermal decomposition. The different combinations of triphenylphosphine, and oleylamine were added as surfactants to control the

Full text of DOI:10.1016/j.jallcom.2008.09.196

Journal of Alloys and Compounds 476 (2009) 908–912
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
ZnO nanotriangles: Synthesis, characterization and optical properties
Masoud Salavati-Niasaria,b,∗, Noshin Mirb, Fatemeh Davarb
a Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O.Box 87317-51167, Islamic Republic of Iran
b Department of Chemistry, Faculty of Science, University of Kashan, Kashan, P.O.Box 87317-51167, Islamic Republic of Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 May 2008
Received in revised form
20 September 2008
Accepted 27 September 2008
Available online 20 November 2008
Zinc oxalate, [Zn(C2O4)·2H2O] was used as a precursor to prepare zinc oxide nanotriangles by thermal
decomposition. The different combinations of triphenylphosphine, and oleylamine were added as surfac-
tants to control the particle size. The products were characterized by X-ray diffraction (XRD), transmission
electron microscopy (TEM), photoluminescence spectroscopy (PL), Fourier transform infrared (FT-IR)
spectroscopy, Ultraviolet–visible (UV–vis) spectroscopy and X-ray photoelectron spectroscopy (XPS). The
synthesized ZnO nanoparticles have a hexagonal zincite structure.
© 2008 Elsevier B.V. All rights reserved.
Keywords:
Nanostructured materials
Inorganic materials
Chemical synthesis
1. Introduction
hydrothermal [27], solvothermal [28] and sonochemical methods
[29].
In recent years, there has been an increasing interest in devel-
oping materials with low-dimensional nanostructure such as
nanotubes and nanocrystals due to their potential technological
applications in nanoscale devices [1–4]. It has been obvious that
their properties depend sensitively on their size and shape. There-
fore, the challenges in nanocrystals synthesis are to control not only
the crystal size but also the shape and morphology [5,6].
Zinc oxide is a material of particular interest because of its
unique optical and electronic properties. ZnO is a wide bandgap
semiconductor (3.37 eV) and has a large excition binding energy
of 60 meV, which is also luminescent, and has emerged as a good
candidate for many applications. Nanosized ZnO has great poten-
tiality for being used in preparing solar cell [7,8], gas sensors [9],
chemical absorbent [10] varistors [11], electrical and optical devices
[12], electrostatic dissipative coating [13], catalysts for liquid phase
hydrogenation [14], and catalysts for photo-catalytic degradation
[15] instead of titania nanoparticles [16].
Among various techniques for synthesis of ZnO nanoparticles,
thermal decomposition is one of the most common to produce sta-
ble monodisperse suspensions with the ability of self-assembly.
Nucleation occurs when the metal precursor is added into a heated
solution in the presence of surfactant, while the growth state take
place at a higher reaction temperature [30]. ZnO nanostructures can
also be synthesized from different precursors [31,32]. Choi et al. [33]
reported a method for the large-scale synthesis of uniform-sized
hexagonal pyramid-shaped ZnO nanocrystals by the thermolysis
of Zn–oleate complex. Zinc acetylaceton has been reported to be
a good precursor for ZnO nanoparticles from thermal decompo-
sition. Salavati-Niasari et al. [34] have synthesized uniform ZnO
nanoparticles with size 12 nm by thermal decomposition of zinc
acetylacetonate in oleylamine. And so, monodisperse ZnO nanopar-
ticles were successfully prepared through the decomposition of
zinc acetylacetonate precursor [35]. Recently Ahmad et al. [36]
have synthesized zinc oxide nanoparticles with average size 55 nm
by thermal decomposition of zinc oxalate, which were prepared
with emulsion method involving cetyltrimethyl ammonium bro-
mide (CTAB) as the surfactant. Kanade et al. [37] have investigated
the effect of particle size of ZnO synthesized via thermal decom-
position of zinc oxalate at 450 ◦C. Muruganandham and Wu [38]
have reported synthesis of ZnO nanobundles from zinc oxalate
by decomposition at 400 ◦C for 12 h. We decided to prepare ZnO
nanocrystals from zinc oxalate at the less temperature and shorter
time than the last works done by others. However, an improvement
in the thermal decomposition process should be made in preparing
ZnO nanotriangles with uniform size in order to extend the appli-
In the past decade, various different physical or chemical
synthetic approaches have been developed to produce ZnO, includ-
ing vapor phase oxidation [17], thermal vapor transport and
condensation (TVTC) [18,19], chemical vapor deposition (CVD)
[20], precipitation [21–23], sol–gel [24,25], microemulsion [26],
∗
Corresponding author at: Institute of Nano Science and Nano Technology, Uni-
versity of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran.
Tel.: +98 361 5555333; fax: +98 361 5552935.
E-mail address: salavati@kashanu.ac.ir (M. Salavati-Niasari).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.09.196

M. Salavati-Niasari et al. / Journal of Alloys and Compounds 476 (2009) 908–912
909
Scheme 1. Schematic depiction of the selective formation ZnO nanocrystals with triangle shapes and hexagonal in shapes by thermal decomposition.
cation areas and satisfy the needs of fundamental research. To the
best of our knowledge the used of zinc oxalate as precursor in ther-
mal decomposition at 240 ◦C is the first time. Here, we report a
simple, green, low-cost, and reproducible process for the synthesis
of ZnO nanotriangles from zinc oxalate, [Zn(C2O4)·2H2O], as pre-
cursor. In this process, oleylamine was used as both the medium
and the stabilizing reagent.
with ethanol. These products could easily be re-dispersed in nonpolar organic sol-
vents, such as hexane or toluene (Scheme 1). The synthesized nanocrystals were
characterized by XRD, TEM, FT-IR, UV–vis and PL techniques.
3. Results and discussion
X-ray powder diffraction pattern of ZnO nanotriangles at room
temperature is shown in Fig. 1a. The XRD pattern is consistent with
the spectrum of ZnO, and no peak attributable to possible impu-
rities is observed. All of the reflection peaks of the XRD pattern
can be indexed well to hexagonal phase ZnO (space group: P63mc;
JCPDS No. 79–0205, with calculated cell parameters a = 3.24 Å and
c = 5.18 Å). The sharp diffraction peaks manifested show that the
obtained ZnO nanocrystals have high crystallinity.
2. Experiment
2.1. Materials
All the chemicals reagents used in our experiments were of analytical grade and
were used as received without further purification. Zinc oxalate, [Zn(C2O4)·2H2O],
precursor was synthesized according to this procedure: the 10 mmol of zinc acetate
dihydrate Zn(CH3COO)2·2H2O was dissolved into 10 mL of distilled water to form a
homogeneous solution. A stoichiometric amount of potassium oxalate dissolved in
an equal volume of distilled water was dropwise added into the above solution under
magnetic stirring. The solution was stirred for about 15 min and an orange precipi-
tate was isolated, washed and dried at 50 ◦C. The obtained product was characterized
by FT-IR and elemental analyses.
The crystallite sizes of the as-synthesized ZnO, Dc, was calcu-
lated from the major diffraction peaks of the base of (1 0 1) using
the Scherrer formula (Eq. (1)),
Kꢀ
Dc
=
(1)
ˇ cos ꢁ
2.2. Characterization
where K is a constant (ca. 0.9), ꢀ is the X-ray wavelength used in XRD
(1.5418 Å), ꢁ the Bragg angle; ˇ is the pure diffraction broadening
of a peak at half-height, that is, broadening due to the crystal-
lite dimensions. The size of the crystallite was estimated from
Debye–Scherrer equation [41] is about 30 nm. Chemical purity and
stoichiometry of the samples were tested by EDX. The strong peaks
related to Zn and O is found in the spectrum Fig. 1b.
X-ray diffraction (XRD) patterns were recorded by a Rigaku D-max C III, X-ray
diffractometer using Ni-filtered Cu K˛ radiation. Elemental analyses were obtained
from Carlo ERBA Model EA 1108 analyzer. Transmission electron microscopy
(TEM) image and energy dispersive X-ray (EDX) was obtained on a Philips EM208
transmission electron microscope with an accelerating voltage of 200 kV. Fourier
transform infrared (FT-IR) spectra were recorded on Shimadzu Varian 4300 spec-
trophotometer in KBr pellets. The electronic spectra of the complexes were taken
on a Shimadzu Ultraviolet–visible (UV–vis) scanning spectrometer (Model 2101
PC). Room temperature photoluminescence (PL) was studied on an F-4500 flu-
orescence spectrophotometer. X-ray photoelectron spectroscopies (XPS) of the
as-prepared products were measured on an ESCA-3000 electron spectrometer with
nonmonochromatized Mg K␣ X-ray as the excitation source.
Fig.
2 shows a comparison of FT-IR spectrum of (a)
[Zn(C2O4)·2H2O], and (b) ZnO nanotriangles. The broad absorption
bands at ∼3400 cm−1 encompass the O H stretching vibrations of
adsorbed water [42] on the ZnO surface. The spectrum also con-
tains one strong absorption bands at 425 cm−1 which confirms
the stretching vibration ZnO (ꢂ ZnO) bands [43]. The spectrum
of ZnO nanotriangles, (curve b), shows two weak stretch vibra-
tions at 2921 cm−1 and 2865 cm−1 attributing to the C H stretching
models of the oleylamine carbon chain[35], indicating oleylamine
molecules are absorbed on the surface of ZnO nanocrystals. There
was no evidence of free precursor, [Zn(C2O4)·2H2O] in the sample,
because stretch vibration of C O (ꢃC O) and C O (ꢃC O) oxalate
disappeared [44]. So the oleylamine serves as the capping ligand
that controls growth [40].
2.3. Experimental
The current synthetic procedure is a modified version of the method developed
by Hyeon and others for the synthesis of nanocrystals for metals that employs the
thermal decomposition of transition metal complexes [39,40]. The synthetic path-
way is shown in Scheme 1. In this synthesis ZnO nanotriangles were prepared by
the thermal decomposition of zinc oxalate, [Zn(C2O4)·2H2O]–oleylamine complex as
precursor. First, the [Zn(C2O4)·2H2O]–oleylamine complex was prepared by reaction
0.6 g of [Zn(C2O4)·2H2O] and 5 mL of oleylamine. The mixed solution was placed in a
50 mL three-neck distillation flask and heated up to 140 ◦C for 60 min. The resulting
metal-complex solution was injected into 5 g of triphenylphosphine (TPP) at 240 ◦C.
The color of the solution changed from orange to black, indicating that colloidal
products were generated. The black solution was aged at 240 ◦C for 45 min, and was
then cooled to room temperature. The white nanocrystals products were precipi-
tated by adding excess ethanol to the solution. The ZnO nanocrystals were washed
Fig. 3 shows the TEM images of the ZnO nanocrystals. The TEM
image (Fig. 3) shows ZnO nanotriangles with an average size of
31 nm.

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M. Salavati-Niasari et al. / Journal of Alloys and Compounds 476 (2009) 908–912
Fig. 2. FT-IR spectra of (a) Zn(oxalate)·2H2O and (b) ZnO nanotriangles.
energy of some crystallographic faces. The growth of the ZnO
nanocrystals with triangle shapes and hexagonal in shapes can be
explained on the basis of the schematic view presented in Scheme 1
[46].
Optical properties of the ZnO nanotriangles were investigated by
UV–vis spectra and photoluminescence (PL) spectroscopy at room
temperature (Fig. 4). Fig. 4a illustrates the UV–vis absorption spec-
trum of the nanotriangles which has a red-shift (379 nm) compared
to that of bulk ZnO (∼370 nm). This red-shift can be explained by
the formation of shallow levels inside the bandgap due to impu-
rity atoms present in the lattice [47]. The PL spectra Fig. 4b shows a
near band-edge emission observed at about 383 nm and a relatively
broad blue-green emission band at about 472 nm. The UV emission
at 379 nm was similar to the bulk bandgap at 383 nm of the ZnO,
which originated from the recombination of free excitons [33,48]. A
broad blue-green emission band above 472 can be related to a singly
charged oxygen vacancy, which results from the recombination of
a photo generated hole with a charge state of the specific defect,
such as oxygen vacancies, or resulted from the surface deep-level
[49,50].
Fig. 1. (a) XRD pattern of the ZnO nanotriangles (the bottom bars shows the standard
ZnO from JCPDS No. 79–0205) and (b) EDX spectrum of the ZnO nanotriangles.
The growth mechanism could be well understood on the basis
of the following reactions and the crystal habits of triangular ZnO.
In the presence of oleylamine as solvent, [Zn(C2O4)]–oleylamine
were synthesized. After addition of triphenylphosphine (TPP) to
these solutions at high temperature, [Zn(C2O4)]–oleylamine were
decomposed. TPP was widely used in the synthesis of phosphine-
stabilized gold or other metal nanoparticles. The phenyl groups
in TPP can provide greater steric hindrance than straight-chained
alkyl groups such as tributyl and trioctyl to control the size of
metal nanoparticles [45]. The addition of TPP into the mixture of
oleylamine and complex as an additional surfactant reduced the
particle size much further and resulted in nanocrystals with very
thin arrays. Oleylamine is known as a ligand that binds tightly
to the metal nanoparticles surface. The combined effects of TPP
and oleylamine were much more profound than those of individ-
ual contributions. When the triphenylphosphine (TPP) was used
as the complexing agent, the circumference of produced nuclei
was capped by this surfactant, leading to the increase of surface
Fig. 3. TEM image of ZnO nanotriangles.

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Fig. 4. (a) UV–vis absorption and (b) room temperature PL spectra of the ZnO nanotriangles.
It is known that the large specific surface nanoparticles are
important in catalysis. Therefore, the products were characterized
by X-ray photoelectron spectroscopy (XPS) to analyze the surface
of the products. Fig. 5 shows the XPS for the as-synthesized sam-
ples. The XPS gives the binding energies of O 1s, Zn 2p1/2 and Zn
2p3/2 for the as-synthesized ZnO to be 530.5, 1044.5 and 1044.6 eV
respectively. A full survey scan did not reveal other element peaks.
These results indicate that ZnO nanotriangles have formed. Com-
pared to those for other methods for synthesizing ZnO powders,
the conditions for the reaction are very moderate.
4. Conclusion
In summary, ZnO nanotriangles were synthesized via thermal
decomposition of [Zn(C2O4)]–oleylamine complex. This method
employed an inexpensive, reproducible process for the large-scale
synthesis of ZnO nanocrystals.
Acknowledgement
Authors are grateful to council of University of Kashan for pro-
viding financial support to undertake this work.
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