A simple method to synthesize NiO fibers

Article abstract of DOI:10.1016/j.jssc.2003.08.033

Nickel oxide fibers with diameters 1-2μm and lengths up to 100μm were synthesized by a simple pyrolytic decomposition method in NaCl flux with NiSO₄ as precursor. X-Ray diffraction, X-ray photoelectron spectrum (XPS) and scanning electron microscopy (SEM) techniques were used to characterize these fibers. They have single-crystal structure, large aspect ratios and macroscopic quantity. Suitable precursor and molten NaCl environment are conjectured to be key factors in this method.

Full text of DOI:10.1016/j.jssc.2003.08.033

ARTICLE IN PRESS
Journal of Solid State Chemistry 177 (2004) 2281–2284
A simple method to synthesize NiOfibers
ꢀ
Yongjie Zhan, Chunrong Yin, Changlin Zheng, Wenzhong Wang, and Guanghou Wang
National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China
Received 29 April 2003; received in revised form 8 August 2003; accepted 19 August 2003
Abstract
Nickel oxide fibers with diameters 1–2 mm and lengths up to 100 mm were synthesized by a simple pyrolytic decomposition method
in NaCl flux with NiSO as precursor. X-Ray diffraction, X-ray photoelectron spectrum (XPS) and scanning electron microscopy
4
(SEM) techniques were used to characterize these fibers. They have single-crystal structure, large aspect ratios and macroscopic
quantity. Suitable precursor and molten NaCl environment are conjectured to be key factors in this method.
r 2003 Elsevier Inc. All rights reserved.
Keywords: A1.Crystal morphology; B1. Oxides
1
. Introduction
fibers were synthesized in NaCl flux with NiSO4 as
precursor. It enriches our knowledge about how to
choose suitable precursors in similar works.
NiO(bunsenite) is an important transition metal
˚
oxide with a cubic rock salt-like structure (a ¼ 4:195 A),
As an antiferromagnetic material (T ¼ 523 K, bulk)
N
and p-type semiconductor, its complex band structure,
magnetic character and conductivity mechanism have
been the focus of many research works [1–5]. It can be
used as catalyst, as fine antiferromagnetic layer in
magnetic multifilms and is important components in
electrochromic devices and gas sensors also [6–12]. NiO
particles [13–19] can be synthesized in different ways,
but many of which have similar pyrolytic processes that
anneal Ni(OH)2 or other compounds of low-decom-
position temperatures such as carbonates, oxalates at
different temperatures, and as-prepared NiOparticles of
different sizes were granular shapes usually. A higher
temperature to decompose the sulfate will increase the
sizes of the obtained NiOparticles, which is unsatisfac-
2. Experimental
All reagents used in the experiment were analytically
pure.
A typical procedure to synthesize NiOfibers is given
as follows: 10 mL NiSO solution (0.2 mol/L) and 20 mL
4
saturated NaCl solution were mixed homogeneously at
first, sufficient absolute alcohol was added in and the
mixed solution was stirred quickly on agitator, mixture
of NiSO and NaCl was then deposited, the deposition
4
was filtered, dried and green precursor was prepared.
The precursor placed in ceramic boat was annealed at
ꢁ
8
20 C in electronic furnace for 1 h. Then the obtained
product was cooled and washed with sufficient distilled
water to clear away the residual NaCl, dark green
sample was obtained after drying the product.
tory in most occasions. So NiSO was never considered
4
before. But our work revealed that NiSO also has its
4
special use in crystal growth. Because of the potential
applications of one-dimensional or quasi-one-dimen-
sional (1D) materials, simple methods to synthesize
them with high yields are valuable. In this paper, we
report such a simple pyrolytic method in which NiO
The as-prepared sample was characterized by X-ray
powder diffraction (XRD) on a Rigaku Rotaflex d/
MAX-RA rotation anode XRD using graphite mono-
˚
chromatized CuKa radiation (l ¼ 1:5418 A). Its scan-
ning electron microscopy (SEM) images were taken with
a HITACHI X-650 SEM and compositions of the
sample were checked by X-ray photoelectron spectrum
(XPS) using a VG ESCALAB MKII spectrometer.
ꢀ
Corresponding author..
E-mail address: yj zhan@sina.com.cn (Y. Zhan).
0022-4596/$ - see front matter r 2003 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2003.08.033

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Y. Zhan et al. / Journal of Solid State Chemistry 177 (2004) 2281–2284
2282
3
. Results and discussion
Fig. 1 shows the XRD spectrum of the sample. All the
peaks could be indexed to that of the cubic phase NiO
with standard value (JCPDS No 4-835) and no obvious
peaks of impurity were found.
Fig. 2 shows the morphologies of the NiOsample.
Fibers are the major form and some plate-like structures
are also observed. These fibers have diameters about 1–
2
mm and lengths up to 100 mm. The selected area
electron diffraction information (not shown here) shows
kikuchi lines, revealing that these fibers are single
crystals, which can be verified by optic microscope with
higher magnification. The proportion of fibers in sample
can exceed 70%.
(
a)
XPS information shown in Fig. 3 also proves that
these fibers are NiOcrystals. Only the Ni and Oelement
can be detected obviously, Ni(LMM) and O(KLL) are
the auger peaks of these elements. The center of the Ni2p
peak (calibrated with the C1s peak) is 854.3 eV, which is
consistent with the known value of NiO [20].
(
b)
Fig. 3. SEM images of NiOfibers with different magnifications.
Fig. 1. XRD spectrum of an as-prepared NiOsample.
The chemical reaction in present work is
1
1
1
40000
20000
00000
NiSO4-NiO þ SO3mðat high temperatureÞ
Ni2s
A suitable precursor is the key factor in this sort of
synthesis. By similar routes other precursors such as
Ni(OH) , NiCO , Ni(NO ) cannot give satisfying yields
Ni2p
O(KLL)
O1s
2
3
3 2
80000
60000
40000
20000
0
of NiOfibers at all. How to choose suitable precursors
to synthesize corresponding 1D metal oxides is still
unclear and further research is needed. One obvious
Ni(LMM)
difference between NiSO and other compounds men-
4
C1s
tioned is that NiSO4 is much steadier than those
compounds at high temperature. Fig. 4 shows the
DTA–TGA curves of two different precursors. Data
were taken on a SETARAM TG-DTA/DSC, Labsyst,
Ni3p Ni3s
0
200
400
600
800
1000
1200
ꢁ
the heating-up rates are both 10 C/min. Fig. 4(a) is
about NiCO3 and Fig. 4(b) is about NiSO4
Binding energy (eV)
Fig. 2. XPS of NiOfibers.
(NiSO ꢂ4H O; the weight loss at lower temperature
4
2

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Y. Zhan et al. / Journal of Solid State Chemistry 177 (2004) 2281–2284
2283
growth times are longer. So in the same circumstance, a
precursor such as NiCO can form many smaller NiO
3
particles whereas precursor NiSO form relatively bigger
4
NiOparticles. A longer growth time (or a stronger
growth process) will increase the possibility to change
the typical granular shape of NiOcrystal cores in
following growth process, although it is not effective all
the times.
One effect of the NaCl is to control the sizes of NiSO4
particles. When the NaCl tiny particles were deposited
together with the NiSO particles, they might envelop
4
the NiSO particles to weaken their agglomeration and
4
so control the growth of the NiSO particles because of
4
their dominance in quantity, although the aliquation
effect will counteract the result. Another more impor-
tant effect of NaCl is to provide a fluent circumstance
full of ions, which will affect the growth rates of
different NiOcrystal faces and therefore change the final
morphologies of the crystals. Of course only suitable
precursor can create such quasi-one-dimensional struc-
tures even in this occasion. The fluent circumstance is
also in favor of the transmission of the Ni–O
octahedrons growth units in larger extent.
In summary, a simple template-free method to
prepare NiOfibers is reported here. The directional
properties of these are decided by suitable precursor and
molten NaCl circumstance. These fibers have big aspect
ratios, satisfying yield and good mechanic properties
perhaps. This work shows an interesting and seldom-
concerned aspect of pyrolytic route.
Fig. 4. DTA–TGA curves of different precursors (a) NiCO
3
(b)
4
NiSO .
Acknowledgments
corresponds to the loss of its crystal water), the pyrolytic
ꢁ
This work was financially supported by National
Nature Science Foundation of China (No.
decomposition of NiCO mainly happens below 400 C,
3
ꢁ
whereas that of NiSO mainly happens between 750 C
4
10074024,29890210,10023001).
ꢁ
and 800 C, which is near the experimental temperature
ꢁ
820 C. So it is an acceptable conjecture that the NiSO
precursor has a lower decomposition rate than NiCO at
4
3
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particles in a nucleation process or adhere to existing
particles to grow up in a growth process. The different
rates of these two processes might affect the final
morphologies of the NiOcrystals: When compound
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