Fabrication of hollow spheres and thin films of nickel hydroxide and nickel oxide with hierarchical structures

Article abstract of DOI:10.1021/jp046797o

Hollow spheres and thin films of Ni(OH)₂ and NiO with unusual form and hierarchical structures have been synthesized by a simple solution chemistry method. First, in situ formed Ni(OH)₂ nanoflakelets organized on the surface of styrene - acrylic acid copolymer (PSA) latex particles to form core/shell structures. Ni(OH)₂ hollow shells built up with nanoflakelets were obtained after subsequent removal of the core latex particles by dissolving PSA latex in toluene; the removal of the cores by calcinations would result in NiO hollow shells, also with hierarchical structures. BET calculation showed the surface area of the NiO hollow spheres was 156 m²/g. The nanoflakelets could also organize themselves into thin films with hierarchical structures. It is anticipated that these novel structures will have some unique applications in Ni-based batteries and other potentials.

Full text of DOI:10.1021/jp046797o

J. Phys. Chem. B 2005, 109, 1125-1129
1125
Fabrication of Hollow Spheres and Thin Films of Nickel Hydroxide and Nickel Oxide with
Hierarchical Structures
Debao Wang,* Caixia Song, Zhengshui Hu, and Xun Fu
College of Chemistry and Molecular Engineering, Qingdao UniVersity of Science & Technology,
Qingdao 266042, PRC
ReceiVed: July 20, 2004; In Final Form: October 3, 2004
Hollow spheres and thin films of Ni(OH)2 and NiO with unusual form and hierarchical structures have been
synthesized by a simple solution chemistry method. First, in situ formed Ni(OH)2 nanoflakelets organized on
the surface of styrene-acrylic acid copolymer (PSA) latex particles to form core/shell structures. Ni(OH)2
hollow shells built up with nanoflakelets were obtained after subsequent removal of the core latex particles
by dissolving PSA latex in toluene; the removal of the cores by calcinations would result in NiO hollow
shells, also with hierarchical structures. BET calculation showed the surface area of the NiO hollow spheres
was 156 m2/g. The nanoflakelets could also organize themselves into thin films with hierarchical structures.
It is anticipated that these novel structures will have some unique applications in Ni-based batteries and other
potentials.
Introduction
required as structure-directing reagents.16 Walsh and Mann
reported the fabrication of hollow shells of skeletal-structured
calcium carbonate from oil-water-surfactant microemulsions
using polystyrene beads as the substrate.17 More recently,
Mirkin’s group investigated the assembly of rodlike building
blocks into curved hollow structures.18 Liu and Zeng elucidated
a two-tiered organizing scheme for the construction of CuO
hollow spheres with CuO nanoribbons.19 Herein, we demonstrate
a scheme for the construction of curved architectures with in
situ formed Ni(OH)2 nanoflakelets as building blocks. Curved
shells built up with building blocks of nanoparticles,15,20
nanorods,18 nanoribbons,19 or skeletal structures17 have been
reported; however, to the best of our knowledge, few reports
have concerned the synthesis of Ni(OH)2 and NiO nanoflakelets
as nanoscale building blocks for the construction of hollow shells
and thin films with hierarchical structures.
Nickel hydroxide (Ni(OH)2), as one of the most important
transition metal hydroxides, has received increasing attention
due to its extensive applications, especially as a positive
electrode active material, in alkaline rechargeable Ni-based
batteries.1 NiO is a very prosperous material, and it has been
used in various fields, such as catalysis,2 battery cathodes,3
electrochromic films,4 and fuel cell electrodes.5 Ni(OH)2 can
be conveniently synthesized in solutions of Ni(II), a process
which usually involves homogeneous precipitation by employing
urea.6,7 NiO can be conveniently prepared by thermal decom-
position of its precursors.8 As the current research is driving
toward nanoscale phenomena and technology, the exploration
of new synthesis methods for Ni(OH)2 and NiO nanocrystals
with different morphologies will find new applications or
improve existing performance. It has been reported that the
capacity of the positive electrode could be significantly increased
when nanophase Ni(OH)2 was added to micrometer-size spheri-
cal Ni(OH)2.9 Therefore, many reports have concerned the
synthesis of Ni(OH)2 and NiO nanocrystals, including nanosized
R-Ni(OH)2,10 single-crystalline Ni(OH)2 nanorods and NiO
nanorings,11 Ni(OH)2 and NiO nanosheets,12a and nanoribbons.12b
However, how to synthesize Ni(OH)2 and NiO spheres with
hierarchical nanostructures is still a big challenge to chemists.
Recently, morphology-controlled synthesis and the large-scale
self-assembly of the nanoscale building blocks into complex
structures have been the focus of significant interests in materials
chemistry and device fabrications. A number of processes have
been reported on the fabrication of common one-, two-, and
three-dimensional architectures.13 Controlled organization of
primary building units into curved structures represents another
challenge for materials synthesis as a result of their high
potential in new technological applications.14,15
Experimental Section
All the chemicals were of analytical purity and were used as
received. Poly(styrene-methyl acrylic acid) (PSA) latex used
as core particles was prepared by the radical copolymerization
method using ammonium persulfate as the initiator.21 Figure
2a shows the TEM image of the as-synthesized PSA latex
particles with uniform size and diameter of about 600 nm.
Typically, an aqueous solution containing 10.0 g/L of PVP,
0.0024 mol/L of NiSO4, 0.048 mol/L of urea, and 1.8 g/L of
PSA latex was aged in capped test tubes at 90 °C for 48 h.
After that, the suspension was cooled to room temperature and
centrifuged. The precipitates were redispersed into the reaction
solution, and the coating procedure was repeated two more
times. Finally, the precipitates were washed several times with
water and ethanol, respectively, and dried under vacuum at 40
°C to obtain core/shell particles. The coating experiments were
conducted under different conditions, and the results are
summarized in Table 1. To prepare Ni(OH)2 hollow spheres,
the obtained composite particles were dispersed in toluene, then
centrifuged and redispersed in toluene. This process was
repeated several times to dissolve the PSA cores completely.
For the construction of hollow spheres with hierarchical wall
structures, surfactants or tri-block copolymers are usually
* Corresponding author. Tel: +86-532-4022787. E-mail: wangdb@
ustc.edu.
10.1021/jp046797o CCC: $30.25 © 2005 American Chemical Society
Published on Web 12/31/2004

1126 J. Phys. Chem. B, Vol. 109, No. 3, 2005
Wang et al.
TABLE 1: Summary of Experimental Conditions versus the Products
sample
no.
Ni2+
(mol/L)
alkaline
(mol/L)
PVP
(g/L)
PSA
(g/L)
products
1
2
3
4
5
0.0024
0.0045
0.0024
0.0024
0.0024
urea, 0.048
urea, 0.048
urea, 0.048
urea, 0.048
ammonia, 0.01
10.0
10.0
0.0
10.0
10.0
1.8
1.8
1.8
0.0
1.8
core/shell structures + Ni(OH)2 thin films
isolated Ni(OH)2 particles + partially coated particles + thin films
thin films + bare PSA particle
no reaction
isolated Ni(OH)2 particles + partially coated particles
electron microscope (SEM) images were recorded to reveal the
actual structures of the urchin-shaped core/shell particles shown
in Figure 2c. As shown in Figure 2d, the core/shell particles
are uniform and well-dispersed and have diameters of about
800 nm. Figure 2e shows an SEM image of the core/shell
particles in higher magnification. It is clear that the fiberlike
coatings under TEM observation are, in fact, constructed with
Ni(OH)2 nanoflakelets. The nanoflakelets are found to stand
organizing on the surface of PSA particles, thus making them
appear as fibers while viewed from TEM images. The thickness
and the edge length of the nanoflakelets observed in SEM
images are in accordance with the diameter and the length of
the nanofibers, respectively, in TEM observations.
Figure 1. XRD patterns of sample 1: (a) before and (b) after
calcinations at 600 °C for 2 h.
Ni(OH)2 hollow spheres were obtained after removing the
core latex by dissolving PSA latex in toluene. Figure 3 illustrates
a typical TEM image of Ni(OH)2 hollow spheres obtained under
this procedure. The urchin-like morphology of Ni(OH)2 shells
is sustained very well even after removing the core latex. The
obvious contrast between the dark edge and pale center is the
evidence for its hollow nature. The wall thickness is about 100
nm. An electron diffraction (ED) pattern in the inset of Figure
3 indicates the polycrystalline nature of nickel hydroxide.
Elemental analysis of typical Ni(OH)2 samples gives 2.0% H,
4.4% C, 1.3% N, 0.7% S, and 51.2% Ni, and the formula is
To obtain NiO hollow spheres, the composite particles were
heated to 600 °C at a rate of 1 °C min-1 in air and kept at 600
°C for 2 h.
TEM images and electron diffraction (ED) patterns of the
samples were taken with a JEM-2000EX (JEOL) transmission
electron microscope (160 kV). SEM images were obtained on
a JSM-6700F field emission scanning electron microscope
equipped with an energy-dispersive X-ray analyzer. X-ray
powder diffraction (XRD) patterns were recorded using a Rigaku
D/Max r-A X-ray diffractometer with Cu KR radiation (λ )
0.154 178 nm). Thermogravimetric analysis (TGA) was carried
out on a TA-50 thermal analyzer (Shimadzu) with a heating
rate of 10 °C min-1 in flowing air. N2 adsorption was determined
by BET measurements using a NOVA-1000e surface area
analyzer. Elemental analysis was carried out using a VarioEL
III analyzer.
found to be Ni(OH)1.26(CO3)0.31(OCN)0.11(SO4)0.025(H2O)0.49
.
Figure 4 shows the TEM and SEM images of the obtained
NiO sample. Hollow particles with obvious contrast between
the dark edge and pale center are revealed by the TEM image
in Figure 4a. Ringlike ED patterns in the inset of Figure 4a
indicate the polycrystalline nature of cubic-phased NiO. The
SEM image in Figure 4b indicates that the obtained hollow
spheres are relatively uniform and well-dispersed, and the
morphology of Ni(OH)2 shells is sustained very well after
calcination. The cracked shells in Figure 4b obviously reveal
the hollow structure of the calcined particles. Figure 4c
represents the SEM image in high magnification to reveal the
actual feature of the urchin-like morphology, indicating the
presence of NiO nanoflakelets organizing into a honeycomb-
like microstructure. BET (Brunauer-Emmett-Teller) surface
areas, calculated from nitrogen adsorption isotherms, show that
the surface area of the NiO hollow spheres is 156 m2/g, which
increases significantly compared with that of the PSA/Ni(OH)2
core/shell particles (23 m2/g) and that of the Ni(OH)2 hollow
spheres (62 m2/g). When the core/shell particles were calcined
at 900 °C, NiO hollow shells with hierarchical structures could
also be sustained perfectly (Figure 4d); however, the total BET
surface area decreased to 141 m2/g.
In addition to the core/shell structures, nickel hydroxide thin
films with architectural structures have been identified on the
wall of the test tubes (Figure 5), which can be easily separated
from the core/shell particles by sedimentation of suspension.
Figure 5a shows the TEM image of the sheet-type structures,
and the thin films seem to be transparent and constructed with
Ni(OH)2 nanofibers of ∼10 nm in diameter and ∼150 nm in
length. The SEM image in Figure 5b reveals the actual surface
structure of the Ni(OH)2 thin films, which are also constructed
Results and Discussion
Figure 1, parts a and b, displays the X-ray diffraction (XRD)
patterns of an as-synthesized sample (sample 1) before and after
calcinations, respectively. No obvious peaks of â-nickel hy-
droxide have been observed in Figure 1a, and the main peaks
in Figure 1b can be indexed to cubic-phased NiO,22 suggesting
the formation of R-nickel hydroxide10 and its conversion to
cubic-phased NiO.
The morphology of the obtained core/shell particles and the
hollow shells was first characterized using a transmission
electron microscope (TEM). As can be seen from Figure 2, parts
b and c, the core/shell particles in sample 1 have a uniform
size and show urchin-like morphology. Ni(OH)2 nanofibers are
clearly visible growing on the surface of PSA cores with one
cycle of coating reaction (Figure 2b). The individual nanofibers
have length of ∼40 nm and width of 5-8 nm. To obtain perfect
Ni(OH)2 shells, the depositing reaction was carried out for two
more cycles. As shown in Figure 2c, Ni(OH)2 nanofibers grow
in density, as well as in length (∼100 nm) and diameter (∼10
nm). More and more fiberlike Ni(OH)2 grow on the surface of
the core latex, and a condensed Ni(OH)2 shell is constructed.
Because TEM micrographs, even of shadowed samples, do not
always reveal the true morphologies of the particles, scanning

Hollow Spheres and Thin Films of Ni(OH)2 and NiO
J. Phys. Chem. B, Vol. 109, No. 3, 2005 1127
Figure 2. (a) TEM image of PSA latex particles, (b) TEM image of sample 1 with one cycle of reaction, and (c) TEM and (d,e) SEM images of
sample 1 with three cycles of reaction.
It has been reported that negatively charged polystyrene latex
can be covered with a smooth inorganic layer by the hydrolysis
of metal ions.24 It is of interest, in our study, that Ni(OH)2
nanoflakelets instead of nanoparticles grow on the surface of
the core latex. Although the exact formation mechanism is still
unclear, we may just suppose that the formation of Ni(OH)2
nanoflakelets might be related to the nature of its lamellar 2-D
structures; as is well-known, Ni(OH)2 is a layered compound
of CdI2 type. Urea has been widely used as a Bro¨nsted base25
for the synthesis of R-nickel hydroxide6 since the pH value of
the medium remains relatively low throughout the process.
While aging the reaction solution, urea slightly decomposes25
and reacts with nickel ions to form Ni(OH)2 nuclei on the surface
of PSA templates. The nucleation stage is then followed by
diffusion of nickel ions and hydroxide ions to the surface of
Figure 3. TEM image and ED pattern (inset) of the Ni(OH)2 hollow
PSA particles to feed the preferential growth, and Ni(OH)2
nuclei grow into Ni(OH)2 nanoflakelets. At the same time, the
nucleation takes place on the wall of the test tube, which leads
to the growth of Ni(OH)2 nanoflakelets to construct Ni(OH)2
thin films. In the next cycle of reaction, new nuclei continue
generating on the surface of PSA latex and growing into
flakelets. As a result, the in situ formed Ni(OH)2 nanoflakelets
plant themselves and organize into honeycomb-structured shells
or thin films with irregularly shaped pores.
spheres obtained from sample 1.
with Ni(OH)2 nanoflakelets. NiO thin films were obtained by
thermal decomposition method at 600 °C for 2 h using the as-
synthesized Ni(OH)2 thin films as the precursor. It can be seen
from the TEM and SEM images (Figure 5, parts c and d) that
the morphology and the characteristics of Ni(OH)2 thin films
are sustained perfectly after thermal decomposition to NiO.
Although many reports have concerned the deposition of Ni-
(OH)2 and NiO thin films,23 few of them have concerned the
deposition of nanoflakelets as building units to assemble into
films with architectural morphologies.
To obtain NiO hollow shells, the removal of core particles
and the decomposition of Ni(OH)2 to NiO were performed by
calcining the core/shell particles in a stove in air. The thermal

1128 J. Phys. Chem. B, Vol. 109, No. 3, 2005
Wang et al.
Figure 4. (a) TEM image, ED pattern (inset), and (b,c) SEM images of NiO hollow spheres obtained from sample 1 at 600 °C, and (d) SEM
images of NiO hollow spheres obtained at 900 °C.
Figure 5. TEM and SEM images of (a,b) Ni(OH)2 thin films and (c,d) NiO thin films obtained from sample 1 obtained at 600 °C.
behavior of PSA/Ni(OH)2 core/shell particles (sample 1) was
investigated with TGA measurements (Figure 6). It has been
reported that the decomposition of Ni(OH)2 to NiO occurs
between 298 and 342 °C.12a The TGA curve in Figure 6 indicates
that there is no obvious weight loss when the temperature is
higher than about 600 °C. The calcinations in air result in the

Hollow Spheres and Thin Films of Ni(OH)2 and NiO
J. Phys. Chem. B, Vol. 109, No. 3, 2005 1129
hierarchical structures. The honeycomb-like structured Ni(OH)2
and NiO would open up the possibility of finding their new
applications or improving existing performance, for example,
as candidates for studying the nanoarchitecture-dependent
performance as cathodes in micro-rechargeable lithium batteries.
To this purpose, we are currently investigating the structure of
these hollow shells in more detail, as well as the incorporation
of functional additives into the shells.
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The coating experiments were carried out under different
conditions. Table 1 lists the obtained experimental results. We
found that urea is irreplaceable for the formation of Ni(OH)2
nanoflakelets as coatings on the core latex. It was believed that
hydroxide ions could be released at a suitable rate to feed the
growth of Ni(OH)2 nanoflakelets on the core particles. When
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particles (sample 5).
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nickel ions and more cycles of reaction were necessary to the
formation of condensed shells organized with nickel hydroxide
nanoflakelets. When the concentration of nickel ions was higher
(for example 0.0045 mol/L), isolated nickel hydroxide particles
together with the aggregation of partially coated core particles
would appear in the products (sample 2). PVP is another factor
that has influence on the growth of Ni(OH)2 nanoflakelets as
coatings on the surface of the latex particles. For comparison,
we performed the experiment with no addition of PVP (sample
3). Ni(OH)2 nanoflakelets were deposited mostly to construct
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Ni(OH)2 thin films in the absence of PSA latex particles.
However, the reaction did not take place while keeping other
conditions constant (sample 4). Further studies are needed to
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In summary, the work presented here constitutes a simple
method for the fabrication of hollow spheres and thin films of
Ni(OH)2 and NiO with unusual form and hierarchical structures.
The simplicity is that the aging procedure results in the growth
of Ni(OH)2 nanoflakelets as well as their assembly into
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