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Chemistry Letters Vol.34, No.2 (2005)
Conversion of Conventional NiO Powders into Nanostructures by a Simple Chemical Method
Jian Quan Qi,ꢀy;yy Ting Zhang,y Mei Lu,yyy Yu Wang,y Wan Ping Chen,y Long Tu Li,yy and Helen Lai Wah Chany
yDepartment of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University, Hong Kong
yyDepartment of Materials Science & Engineering, Tsinghua University, Beijing 100084, P. R. China
yyyDepartment of Chemistry, Lanzhou University, Lanzhou 730000, P. R. China
(Received October 14, 2004; CL-041215)
We report the synthesis of nanostructured nickel oxide and
nickel hydroxide from conventional NiO powders onto different
substrates by a chemical method which simply involves dissolv-
ing and decomposition steps. Scanning electron microscope
(SEM) observations have revealed various morphologies of the
nanostructures on different substrates. With the described meth-
od, no impurity residue will remain on the substrates, hence re-
quiring no additional washing and separation processing.
structures (about 10-nm thick and 200-nm long for each fiber).
An enlarged picture of such structure is shown in the bottom-left
inset of Figure 1. In the other areas (the brighter ones) are the
coralloid-ball-like structures with ꢃ500 nm in diameter. An en-
larged picture of such structures is shown in the upper-left inset
of Figure 1. We believe the coralloid balls are actually the aggre-
gations of the grass-fiber-like nanostructures.
Nanostructured materials have attracted much attention
mainly due to their novel properties that may not be present in
bulk materials.1–4 As important functional materials that can
be widely used as catalyst,5–8 battery electrode materials,9–11
and chemical sensors,12 nickel oxide (NiO) and nickel hydroxide
(Ni(OH)2) have been extensively investigated. Several forms of
nanostructures of NiO and Ni(OH)2, including nanoparticles,13
nanorods,14 and nanorings,15 have been successfully synthesized
under high pressures at high temperatures in autoclaves through
hydrothermal method. There are still extensive efforts to develop
these nanostructures via simple and low-cost processing. In this
communication we report a simple and convenient chemical
method by which nanostructured NiO and Ni(OH)2 can be ob-
tained from conventional NiO powders. These nanomaterials
were intentionally synthesized on substrates because we believe
that, compared with free standing ones (such as nanoparticles),
the nanostructures on appropriate substrates are one step closer
for making practical devices.
In our work, nanostructures of Ni(OH)2 and NiO were con-
verted from conventional NiO powders by a process as follows:
0.8-g nickel oxide black (BDH Chem. Ltd., UK) was dissolved
in 100 mL of 25% aqueous ammonia (International Laboratory,
USA). After 24 h of dissolving, the solution turned to a light blue
color. The solution was then dip coated on fused quartz and con-
ventional sodium glass substrate, respectively, followed by a
baking at 100 ꢁC for 10 h in an oven. Finally, the samples were
annealed at different temperatures.
Figure 1. SEM image of the as-prepared Ni(OH)2 on glass sub-
strate. The upper-left inset shows the coralloid-ball-like nano-
structures and the bottom-left inset shows the grass-fiber-like
nanostructures.
Figure 2 shows the XRD pattern of the as-prepared nickel
hydroxide on glass substrate. With reference to JCPDS Card #
74-2075, all the peaks in Figure 2 are identified to be from hex-
agonal Ni(OH)2. It is interesting to note that the (001) peak is
much broader than the (100) peak. For nanostructures, such
broadening reflects the decrease in crystallite size according to
Scherer’s equation,
Kꢀ
L ¼
ð2Þ
ꢁcos ꢂ
The dissolving of the raw NiO powders in ammonia is ex-
pessed in Eq 1. The evaporation process at 100 ꢁC produces
Ni(OH)2 by decomposing the products in Eq 1. Upon annealing
at higher temperatures, Ni(OH)2 further decomposes into NiO.
where L is the crystallite size, ꢀ is the wavelength of the X-ray
radiation (Cu Kꢃ ¼ 0:15418 nm), K is usually taken as 0.89, ꢁ is
the line width at half-maximum height after subtraction of equip-
ment broadening, and ꢂ is diffraction angle. In our experiments,
the equipment broadening is about 0.3 degree. The line width at
half-maximum of peak (001) is 6.05 degree after we separate the
left shoulder peak of an unknown phase, and that of peak (100) is
0.47 degree. Thus, it can be estimated that along the 001-direc-
tion, the crystallite size is ꢃ2 nm while along 100-direction, it is
ꢃ50 nm. Noticing such sizes are much smaller than the geomet-
ric sizes of the ‘‘fiber,’’ we may reasonably assume that each
‘‘fiber’’ itself is polycrystalline. Moreover, the intensity ratio of
(001) over (100) in Figure 2 is ꢃ100:76, while the ‘‘standard’’
NiO þ 6NH3 þ H2O ! Ni(NH3)26þ þ 2OHꢂ
ð1Þ
X-ray diffraction (XRD) and scanning electron microscope
(SEM) were employed to pursue the microstructure and phase
information of the reaction products. Figure 1 shows a represen-
tative SEM image of Ni(OH)2 grown on a piece of sodium glass
substrate. Two obviously different morphologies are observed.
Dispersing all over the substrate (the dark areas in Figure 1) is
a two-dimensional network, consisting of grass-fiber-like nano-
Copyright Ó 2005 The Chemical Society of Japan
Qi, Jian Quan
