Synthesis of well-dispersed ZnO nanomaterials by directly calcining zinc stearate

Article abstract of DOI:10.1016/j.jallcom.2008.04.048

Well-dispersed ZnO nanomaterials were synthesized by direct calcination of zinc stearate. Results from Fourier transform infrared (FT-IR) spectra and X-ray diffraction (XRD) indicated both the decomposition degree of organic ligand and the purity of calci

Full text of DOI:10.1016/j.jallcom.2008.04.048

Journal of Alloys and Compounds 472 (2009) 343–346
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Synthesis of well-dispersed ZnO nanomaterials by directly calcining zinc stearate
Guangsheng Guoa, Chen Shia, Dongliang Taoa, Weizhong Qianb, Dongmei Hana,∗
a State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China
b Department of Chemical Engineering, Tsinghua University, Beijing 100084, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Well-dispersed ZnO nanomaterials were synthesized by direct calcination of zinc stearate. Results from
Fourier transform infrared (FT-IR) spectra and X-ray diffraction (XRD) indicated both the decomposi-
tion degree of organic ligand and the purity of calcined products were increased with the calcination
temperature. The influence of decomposition temperature on the morphology of ZnO nanomaterials
was investigated by field-emission scanning electron microscopy (FE-SEM) and transmission elec-
tron microscopy (TEM). The experimental results revealed the morphology of ZnO transformed from
nanosheets to hexagonal nanopyramids and then to nanoparticles at 573, 673 and 773 K respectively.
Finally, a morphology evolution model of ZnO nanomaterials under different temperatures was proposed.
© 2008 Elsevier B.V. All rights reserved.
Received 3 November 2007
Received in revised form 15 April 2008
Accepted 16 April 2008
Available online 3 June 2008
Keywords:
Nanostructured materials
Semiconductors
Chemical synthesis
1. Introduction
impurities adsorbed on the surface of ZnO products. Thus, wurtzite-
type ZnO materials with far higher purity are obtained. Moreover,
Due to the fundamental size-dependent properties and the
important technological applications, nanoscale oxide materials
have gained considerable attention in recent years [1,2]. ZnO, a
direct-band gap semiconductor material, has a large band gap of
3.37 eV and a large excitation binding energy of 60 meV, which
make it widely used in many fields [3–7]. Up to now, various meth-
ods, such as microemulsion method and hydrothermal method,
have been proposed to prepare nano-sized ZnO materials [8–10].
In addition, the direct thermal decomposition of metal–oleate
complex in octadecene solution provides a simple method to pre-
pare metal oxide nanostructures with uniform morphology [11,12].
However, this route often suffers from both the decomposition of
the combustible reagent at high temperature, especially in large-
scale synthesis, and the difficulties in the removal of undesirable
impurities from the surface of the products. Comparatively, the
direct pyrolysis of the metal complex in air provides a dry envi-
ronment for the controllable preparation of ZnO nanorods [13],
ZnO zeptoliter bowls and troughs [14], NiO nanoparticles [15] and
ZrO2 nano-laminate [16], with relatively high purity. Following the
thermal-decomposing route, herein, we report the fabrication of
different ZnO nanostructures via the formation of zinc stearate and
the subsequent calcination of the precursor of zinc stearate. Com-
pared with the PVP, the ligand of zinc stearate is not only much
easier to decompose, but also more effective to assist the removal of
due to the protection of organic compounds in thermal decom-
position process, the ZnO nanopyramids and nanoparticles are
well-dispersed and size-homogeneous.
2. Experimental procedure
All reagents are analytical purity and used without further purification. To pre-
pare the metal complex, 0.01 mol (2.97 g) Zn(NO3)2·6H2O and 0.02 mol (5.69 g)
stearic acid were dissolved in 100 ml ethanol at 343 K. Then, 0.02 mol (2.8 ml) triethy-
lamine was dropped into the solution with vigorously stirring. White precipitate was
instantly appeared in the solution. After being filtered and washed with hot ethanol,
the precipitate was dried at 353 K for 12 h. Finally, the produced zinc stearate was
calcined in a muffle furnace at 573, 673 and 773 K for 3 h in air, respectively.
The FT-IR spectra of zinc stearate and calcined products were recorded by Fourier
transform infrared spectroscopy (FT-IR, Nicolet, AVATAR-370MCT) in the range from
400 to 4000 cm−1. The morphology of ZnO was characterized by the field-emission
scanning electron microscope (FE-SEM, JEM-6700F) and the transmission electron
microscope (TEM, JEOL-2010). The selected area electron diffraction (SAED) was
also obtained form the transmission electron microscope. The products were also
characterized by X-ray powder diffraction (XRD) on a Rigaku D/Max 2500V B2+/PC
diffractometer.
3. Result and discussion
Fig. 1 is the FT-IR spectra of zinc stearate and calcination prod-
ucts. The main absorption bands at ∼3438, ∼2916, ∼1539 and
∼1464 cm−1 are consistent with the O–H, C–H, symmetric and
asymmetric C O stretching vibrations of zinc stearate, respec-
tively (Fig. 1A). Compared with the bands in Fig. 1A, the band at
∼3438 cm−1 disappears in Fig. 1B for products calcined at 573 K for
3 h. The intensity of the peaks at ∼2916, ∼1539 and ∼1464 cm−1 is
∗
Corresponding author. Tel.: +86 10 64434808.
E-mail address: handm@mail.buct.edu.cn (D. Han).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.04.048

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Fig. 1. FT-IR spectra of zinc stearate (A) and the products calcined at different tem-
peratures (B) 573 K, (C) 673 K and (D) 773 K.
Fig. 2. XRD patterns of ZnO products at different temperatures.
significantly decreased in Fig. 1B. These results indicate that zinc
stearate is partly decomposed at 573 K. The process include the
Fig. 2 is the XRD patterns of ZnO products calcined at different
temperatures. The product calcined at 573 K has a broad diffraction
peak at around 22◦, which is resulted from the amorphous car-
bon produced in the decomposition process of zinc stearate. Except
this broad peak, other peaks are all attributed to the characteris-
tic ones of wurtzite-type ZnO (JCPDS, 36-1451). Comparatively, all
diffraction peaks of product at 673 and 773 K can be indexed to
the hexagonal ZnO with wurtzite structure, and no any peaks of
impurities can be detected in the XRD pattern. The XRD result is
dehydration and the subsequent destroying of the C–H and C
O
bonds. Further increasing the temperature to 673 and 773 K, the
bands at ∼2916, ∼1539 and ∼1464 cm−1 nearly totally disappear
and the new bands appear at 460 and 473 cm−1 (Fig. 1C and D).
The new bands at 460 and 473 cm−1 are the typically stretching
vibrations of ZnO. These indicate the nearly complete transforma-
tion of complex precursors to the final inorganic ZnO product if the
calcination temperature is higher than 673 K.
Fig. 3. SEM images of zinc stearate (A) and the products calcined at different temperatures (B) 573 K, (C) 673 K and (D) 773 K.

G. Guo et al. / Journal of Alloys and Compounds 472 (2009) 343–346
345
Fig. 4. TEM and SAED images of ZnO products at different temperatures (A) 573 K, (B) 673 K and (C) 773 K.
in agreement with that of FT-IR spectra that zinc stearate is nearly
completely decomposed if the temperature is higher than 673 K.
Fig. 3 shows the SEM images of zinc stearate and the products
calcined at different temperatures. Zinc stearate presents a sheet-
like structure with average area of 1.4 ␮m × 1.4 ␮m (Fig. 3A). The
products calcined at 573 K formed the nanosheets with a thick-
ness of around 25 nm (Fig. 3B). It is clear that there are some
powders filling the vacancy of the sheet-like products. Accord-
ing to the XRD results, the nanosheets are the mixture of zinc
stearate and ZnO. Due to the similarity of product morphology
in Fig. 3A and B, it can be concluded that the decomposition at
573 K is uniform in each position of the nanosheets. However, the
direct thermal decomposition at 673 K would result in the forma-
tion of ZnO nanopyramids with average size of 230 nm (Fig. 3C).
The relatively uniform size of ZnO nanopyramids indicates the
quick collapse of the former nanosheet-like products and the for-

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G. Guo et al. / Journal of Alloys and Compounds 472 (2009) 343–346
ZnO nanoparticles by gases in fluidized bed) to avoid the contact
and aggregation of ZnO nanoparticles. The related investigation is
currently being undertaken in our lab.
Scheme 1. Morphology evolution model of ZnO nanomaterials at different temper-
atures.
4. Conclusion
In summary, well-dispersed ZnO nanomaterials were prepared
via the formation of zinc stearate precursor and its subsequent ther-
mal decomposition in air. Controlling the calcination temperature
is effective to tailor the morphology of as-prepared ZnO nano-
materials. The uniform decomposition of nanosheet-like precursor
results in the formation of well-dispersed ZnO nanomaterials. FT-IR
and XRD results show the gradual decomposition of precursor to
produce ZnO with relatively high purity. A morphology evolution
model is proposed to explain the formations of products at different
temperatures.
Apparently, the synthesis route of ZnO nanoparticles in the
present work is relatively simple. The results can be easily repeated
and are useful for the large-scale controllable production at low
cost. It is also expected other metal oxide nanomaterials could be
synthesized by the same method.
mation of small nanopyramids in large quantity. Similarly, the
decomposition at 773 K leads to the formation of uniform ZnO
nanoparticles with diameter of about 150 nm (Fig. 3D). The mor-
phology and size of ZnO powders were further investigated by
TEM. Fig. 4A clearly shows that the nanosheets consist of nanorods
with diameters of 25–65 nm and lengths of 300–900 nm. There
are many holes around the nanorods, indicating the in situ ther-
mal decomposition and the release of gases (i.e. steam, CO2 or
CO, etc.) from the nanosheets in the decomposition process. After
ultrasonic treatment in ethanol, the nanorods were separated from
the nanosheets. This phenomenon indicates the weak interaction
between nanorods and further proves our deduction that the for-
mation of ZnO is resulted from the in situ decomposition of zinc
stearate. Probably, the low temperature provides the opportunity
for the new ZnO species self-assembly to nanorods. Comparatively,
the calcination at 673 or 773 K is intense and the release of gases
would be quickly. Therefore, the sheet-like texture is unable to
maintain but collapse quickly to form nanopyramids or nanoparti-
cles in large quantity. SAED pattern (inset in Fig. 4B and C) indicates
that as-prepared ZnO nanopyramids and nanoparticles are single
crystal.
Acknowledgments
The authors gratefully acknowledge the financial support of Bei-
jing Natural Science Foundation (No. 2073029) and Foundation of
Guangdong Key Laboratory of Green product (GC200604).
ZnO nanopyramids and nanoparticles are well-dispersed, due
to the above quick formation of many small seeds simultane-
ously. It is the in situ protection of organic compounds around the
ZnO species which prevents the agglomeration of ZnO particles.
Although our experiment is carried out at different temperatures
separately, the product after higher temperature decomposition
is due to the further transform of mediate product formed at
low temperature. Therefore, the nanoparticles at 773 K are formed
due to the further transformation of nanopyramids, as shown in
Scheme 1. Obviously, the temperature of 773 K makes the melt-
ing of the rigid surface of nanopyramids (at 673 K) become much
smooth and round surface of ZnO particles. The fast elevating to
773 K for 3 h is different from that of elevating to 673 K for 3 h and
the subsequent elevating to 773 K. Much smaller seeds in high con-
centration may be simultaneously produced in the former mode as
compared with that in the latter one. Thus, the size of product at
773 K for 3 h (Fig. 4C) is smaller than that at 673 K for 3 h (Fig. 4B).
Moreover, temperature of 773 K seems somewhat higher for the
thermal decomposition of such materials and leads to the further
agglomeration of ZnO nanoparticles after the total evaporation or
decomposition of organic compounds (Fig. 4C). From this view-
point, it is necessary to provide a much large space (i.e. suspending
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