Synthesis of porous nickel oxide nanofiber

Article abstract of DOI:10.1246/cl.2005.428

Porous nickel oxide (NiO) nanofibers, structured with and composed of linearly oriented NiO nanoparticles, were synthesized via a simple method using a solution containing ethanol, H₂O, NiCl₂, and P123 (Pluronic P123, a triblock copolymer H(OCH₂CH₂) ₂₀(OCH(CH₃)CH₂)₇₀(OCH ₂)₂₀OH). By adjusting the concentration of P123, this method allows for the control of the fiber morphology and the nanoparticle size. Copyright

Full text of DOI:10.1246/cl.2005.428

428
Chemistry Letters Vol.34, No.3 (2005)
Synthesis of Porous Nickel Oxide Nanofiber
Akihiko Nakasa, Eiji Suzuki,Ã Hisanao Usami, and Hitoshi Fujimatsu
Department of Fine Material Engineering, Faculty of Textile Science and Technology,
Shinshu University, 3-15-1, Tokida, Ueda 386-8567
(Received November 29, 2004; CL-041452)
Porous nickel oxide (NiO) nanofibers, structured with and
obtained using Cu Kꢀ radiation. The morphology of the NiO
film was examined using a field emission scanning electron mi-
croscope (FE-SEM, Hitachi Co. Ltd., S-5000).
composed of linearly oriented NiO nanoparticles, were synthe-
sized via a simple method using a solution containing ethanol,
H2O, NiCl2, and P123 (Pluronic P123, a triblock copolymer
H(OCH2CH2)20(OCH(CH3)CH2)70(OCH2)20OH). By adjusting
the concentration of P123, this method allows for the control
of the fiber morphology and the nanoparticle size.
The results of DSC measurements are shown in Figure 1.
Ethanol and H2O were removed from the sample under vacuum.
The broad endothermic peak centered at below 100 ꢀC is due to
the rapid removal of ethanol, H2O, and HCl. The endothermic
peak centered at 200 ꢀC corresponds to the decomposition of
P123, as described in ref 13. Some residual organic matter was
removed at between 250 and 340 ꢀC. On the basis of the DSC
measurements, the calcination of the NiO precursor was per-
formed in air at 500 ꢀC for 30 min.
Figure 2 shows the XRD patterns of the NiO film prepared
from solution containing different amounts of P123 (0.12, 0.5,
1.0, 1.5, and 2.0 g). The diffraction patterns were in good agree-
ment with the JCPDS card, confirming the formation of NiO.15
The particle diameters were estimated using Scherrer’s equation
to be 80–50 nm, 38, 33, and 30 nm, respectively, for 0.12, 0.5,
NiO is a desirable material for p-type transparent conducting
films because it is a p-type semiconductor with a band-gap ener-
gy in the range of 3.6 to 4.0 eV.1,2 By enlarging the specific sur-
face area, NiO has been used in electronic capacitors3 and as an-
tiferromagnetic nanoparticles.4 It has also been used as an elec-
trode in dye-sensitized solar cells.5 In order to control the size of
NiO particles, various synthetic approaches have been report-
ed.3,4,6 In recent years, nanotubes of inorganic semiconductors
such as CeO2,7 ZnO8 and CuO9 have been attracting interest be-
cause of their unique properties such as one dimensional con-
tinuity united with a high specific surface area. For example,
TiO2 nanotubes increased the efficiency of dye-sensitized solar
cells when used as n-type electrodes instead of layered TiO2
nanoparticles,10 presumably because the electron conductivity
was increased by the continuity of the nanotubes.
4
2
0
0
100
200
300
400
500
-2
-4
-6
-8
In the present study, we have developed a simple synthetic
approach for NiO nanofibers by calcination of a NiO precursor
film prepared with ethanol, H2O, NiCl2 and P123. P123 is a sur-
factant frequently used as a template for preparing crack-free
TiO2 films.11–13 P123 has been used for the first time as a tem-
plate for preparing p-type inorganic semiconductors. Although
the preparation of single crystal nanorods of Ni(OH)2 under
harsh conditions using an ammonia solution has been previously
reported,14 our method is unique in being able to control the mor-
phology of the nanofibers without complicated synthetic meth-
ods or harsh preparation conditions. Details of the synthesis
are as follows: NiCl2 (1.0 g) was dissolved in 2.0 g of distilled
water, 4.0 g of ethanol was then added, and finally, a specific
quantity of P123 (0.12, 0.5, 1.0, 1.5, and 2.0 g) was dissolved
in the mixed solution under room temperature and atmospheric
pressure conditions. The solution was stirred for 1–2 h. The
NiO film precursor was obtained by spin coating a 4-cm2 ITO
glass (Nippon Soda Co. Ltd., E-020) surface with the solution
at 300 rpm for 3 s and then at 2000 rpm for 10 s, at room temper-
ature. The ITO was immersed for 2 h in an alkali bath for obtain-
ing a hydrophilic surface prior to spin coating. Differential scan-
ning calorimetric analysis (DSC, Rigaku, DSC8230) of the pre-
cursor films was carried out in air at a heating rate of 10 ꢀC/min
in order to determine suitable calcining conditions. Finally, the
precursor film was calcined to obtain NiO film. Characterization
of the NiO film was then conducted. X-ray diffraction (XRD,
Rigaku, CN4148B2, 40 kV, 30 mA) patterns of the samples were
Temperature [oC]
Figure 1. DSC curve of porous NiO nanofiber precursors. The
amount of P123 is 1.0 g.
(101)
(012)
(e)
(d)
(c)
(b)
(a)
30
35
40
45
50
2 theta [degree]
Figure 2. XRD patterns of porous NiO nanofiber on glass. The
amount of P123 is (a) 0.12 g, (b) 0.5 g, (c) 1.0 g, (d) 1.5 g, and (e)
2.0 g. Calcination temperature is 500 ꢀC for 30 min.
Copyright ꢀ 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.3 (2005)
429
These results imply that P123 in the sample solution formed mi-
celles and functioned as a template,12–14 and the size and shape
of the micelle changed with an increase in the concentration of
P123. Interestingly, fibers with a fairly high aspect ratio, with
lengths of 20–50 mm and diameters of 300–500 nm, were
formed. Furthermore, it was confirmed using the SEM micro-
graph that a single fiber consisted of many nanoparticles with di-
ameters of 30–50 nm, as shown in Figure 4. The diameters of the
nanoparticles are consistent with the particle diameters obtained
from the XRD data. The nanoparticles were seen to be linearly
connected and shaped into fibers. When the nanoparticles diam-
eter is 30 nm, the calculated surface area is 28.7 m2/g for the
nanofibers constituted with nanoparticles, our nanofibers with
nanoparticle, therefore, should have a higher specific surface
area than the calculated nanofibers without nanoparticle
(1.9 m2/g), which is advantageous for dye-molecular adsorption
or surface electronic charging. In spite of being porous, good
electrical conductivity is expected for the porous NiO nanofiber
film since it has better continuity than an unconnected packing of
nanoparticles.
(b)
(a)
(c)
(d)
Figure 3. FE-SEM micrograph of porous NiO nanofiber on
ITO. The amount of P123 is (a) 0.12 g, (b) 0.5 g, (c) 1.0 g, (d)
2.0 g. Scale bar and magnification is 24 mm and  1000. Calci-
nation temperature is 500 ꢀC for 30 min.
In conclusion, we obtained porous NiO nanofibers consist-
ing of NiO nanoparticles using a simple synthetic procedure,
the spin coating of a P123–NiCl2–H2O–ethanol solution
followed by calcination.
(a)
This work was supported by the Cooperative Link for
Unique Science and Technology for Economy Revitalization
(CLUSTER) of Japan’s Ministry of Education, Culture, Sports,
Science and Technology. Dr. A. Nakasa was supported by 21st
Century COE program.
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Figure 4. FE-SEM micrograph of porous NiO nanofiber on
ITO. The amount of P123 is 1.0 g, Scale bar and magnification
is (a) 500 nm and  30000, (b) 250 nm and  60000. Calcination
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1.5, and 2.0 g of P123. This result indicates that the diameter of
NiO nanoparticles can be controlled by adjusting the amount of
P123 in the precursor solution.
The FE-SEM micrographs of the NiO films (Figures 3 and
4) revealed a unique NiO morphology, namely a porous nanofib-
er structure. The morphology of NiO was dramatically changed
with increasing concentration of P123. The morphology shown
in Figure 3d indicates fiber growth in random directions, while
Figures 3a and 3b indicate growth from a certain central point.
Published on the web (Advance View) February 22, 2005; DOI 10.1246/cl.2005.428