Hierarchical structured Ni nanoring and hollow sphere arrays by morphology inheritance based on ordered through-pore template and electrodeposition

Article abstract of DOI:10.1021/jp062255q

Fabrication of micro/nano-hierarchical Ni ordered nanostructured arrays is demonstrated by electrochemical deposition on the ordered alumina through-pore template induced by solution-dipping the colloidal monolayer. The morphology of the Ni nanostructured arrays exhibits a ringlike or hollow spherical structure depending on the template geometry and appropriate deposition parameters. The skeletons of the arrays are of floc- or flakelet-like fine structure on the nanoscale. The formation of such morphologies is attributed to the preferential growth along the inner wall of the alumina pores, while the nanoflakelet fine structure originates from a morphology inheritance process or the transitional product Ni(OH)₂ which leads to the final nanostructured Ni crystals. This morphology inherence could be useful in the field of nanofabrication. Such micro/nanohierarchically structured arrays show good magnetic properties and will find applications in the fields of catalysis, magnetics, optoelectrics, surface-enhanced Raman scattering (SERS), and new nanodevices.

Full text of DOI:10.1021/jp062255q

J. Phys. Chem. B 2006, 110, 15729-15733
15729
Hierarchical Structured Ni Nanoring and Hollow Sphere Arrays by Morphology
Inheritance Based on Ordered Through-Pore Template and Electrodeposition
Guotao Duan, Weiping Cai,* Yuanyuan Luo, Zhigang Li, and Yong Lei
Key Lab of Materials Physics, Anhui Key Lab of Nanomaterials and Nanotechnology, Institute of Solid State
Physics, Chinese Academy of Sciences, Hefei, 230031 Anhui, People’s Republic of China
ReceiVed: April 11, 2006; In Final Form: June 20, 2006
Fabrication of micro/nano-hierarchical Ni ordered nanostructured arrays is demonstrated by electrochemical
deposition on the ordered alumina through-pore template induced by solution-dipping the colloidal monolayer.
The morphology of the Ni nanostructured arrays exhibits a ringlike or hollow spherical structure depending
on the template geometry and appropriate deposition parameters. The skeletons of the arrays are of floc- or
flakelet-like fine structure on the nanoscale. The formation of such morphologies is attributed to the preferential
growth along the inner wall of the alumina pores, while the nanoflakelet fine structure originates from a
morphology inheritance process or the transitional product Ni(OH)2 which leads to the final nanostructured
Ni crystals. This morphology inherence could be useful in the field of nanofabrication. Such micro/nano-
hierarchically structured arrays show good magnetic properties and will find applications in the fields of
catalysis, magnetics, optoelectrics, surface-enhanced Raman scattering (SERS), and new nanodevices.
1. Introduction
The nanoring structure has received much attention due to
its importance in the fundamental studies of persistent current,1
the bidomain state in magnetics,2 and novel optical properties3
and also due to its technical application in, for example,
recording media.4 The hollow sphere is another special structure
with applications ranging from versatile microreactors to
advanced functional materials.5 Generally, nanorings are syn-
thesized by electron-beam lithography,2 molecular-beam epit-
axy,6 chemical modification,7 self-assembly,8 and a suitable
structured template technique,9 while hollow spheres are usually
fabricated by templating a colloidal sphere core10 or a self-
assembly strategy.5b Such structures and their patterns could
be the basis of nanodevice design.11 However, morphology-
controlled synthesis of ordered nanoring-like and hollow spheri-
cal-like structured arrays in a simple way is still a challenge.
In recent years, colloidal monolayer lithography has proven
to be a promising strategy for the fabrication of various two-
dimensional (2D) ordered nanostructured arrays,12-17 which have
exhibited potential applications in nanodevices. Generally, such
ordered nanostructured arrays are of compactness in skeleton.
Recently, there was a report that showed that when the skeleton
is composed of nanosized building blocks or porous structures,
the ordered arrays will combine the merits of patterned arrays
and nanostructures and show the possibility of application in,
for example, surface-enhanced Raman scattering (SERS), su-
perhydrophobicity, and new nanodevices.18 In this paper, we
demonstrate the fabrication of hierarchical Ni nanostructured
arrays by electrodeposition based on an ordered Al2O3 through-
pore template induced by solution-dipping the colloidal mono-
layer. The Ni arrays show ringlike or hollow spherical structure,
depending on the template geometry. Importantly, the skeleton
of the arrays is of floc- or flakelet-like fine structure on the
nanoscale due to morphology inheritance in the electrochemical
Figure 1. Schematic cross-section illustration of the fabrication
procedures for ringlike and hollow spherical-like nanostructured
arrays: (A) PS colloidal monolayer on ITO substrate by heating; (B)
ordered alumina through-pore array with pore depth (H) smaller (left)
or larger (right) than the radius of the PS (R); (C) ordered Ni ringlike
(left) or hollow spherical-like (right) arrays.
process. Such hierarchically structured arrays show good
magnetic properties and will find applications in the fields of
catalysis, magnetics, optoelectrics, SERS, and new nanodevices.
The details are reported in this article.
2. Experimental Section
The fabrication strategy is illustrated in Figure 1. Some glass
slides as substrates were cleaned according to Dyune’s
procedures.12a The suspensions of polystyrene sphere (PS) of
1000 nm in diameter were bought from Alfa Aesar Corporation.
Large-area ordered PS colloidal monolayers (larger than 1 cm2)
were synthesized on the glass substrates by spin-coating on a
custom-built spin-coater.
Several pieces of ITO-glass substrates were cleaned in
acetone, ethanol, and distilled water for 30 min each. The PS
monolayer on the glass substrate was integrally lifted off on
* To whom all correspondence should be addressed. E-mail: wpcai@
issp.ac.cn.
10.1021/jp062255q CCC: $33.50 © 2006 American Chemical Society
Published on Web 07/22/2006

15730 J. Phys. Chem. B, Vol. 110, No. 32, 2006
Duan et al.
the surface of the distilled water in a cup and then picked up
with a piece of ITO glass, as previously illustrated,13d followed
by heating at 120 °C for 10 min (see step A in Figure 1). In
succession, a droplet (∼20 µL) of Al(NO3)3 aqueous solution
(with concentrations of 0.2, 0.5, or 1.0 M in this study) was
put on the heated monolayer on the ITO-glass substrate,
followed by spinning at 300 round per minute to disperse the
solution into the interstitials of the PS monolayer, drying at 120
°C for 1 h. Finally, we immersed it into the methylene chloride
(CH2Cl2) solution to remove the PS monolayer before heating
at 150 °C for 8 h to decompose the Al(NO3)3 into Al2O3. After
being ultrasonically cleaned in acetone, ethanol, and distilled
water for 10 min, respectively, the Al2O3 ordered through-pore
arrays were thus obtained (see step B in Figure 1).
The substrate coated with the Al2O3 through-pore array was
used as a working electrode and set into a three-electrode
electrolytic cell. An aluminum frame and insulating tape were
used to cover the margins of the substrate. A solution composed
of 0.01 M NiCl2 and 0.03 M (NH4)2SO4 was used as the
electrolyte; the pH value was controlled to 8.5 with ammonia.
A polycrystalline clean nickel sheet and a saturated calomel
electrode (SCE) were used as the auxiliary and reference
electrodes, respectively. The distance between the working
electrode and the auxiliary electrode was about 6 cm. The
electrodeposition was carried out at 60 °C and different
deposition potentials versus SCE for a certain time. After
deposition, the Al2O3 ordered pore skeleton was removed by
immersion in 5 wt % NaOH aqueous solution and Ni ordered
nanostructured arrays were thus obtained (see step C in Figure
1).
The morphologies of all the samples were examined on a
field emission scanning electron microscope (FE-SEM) (Sirion
200). X-ray diffraction (XRD) was performed on a Philips
X’Pert instrument using the Cu KR line (0.154 19 nm). The
hysteresis loop measurements of the samples were conducted
at 300 and 150 K on a superconducting quantum interference
device (SQUID) magnetometer with the applied field parallel
to the films.
Figure 2. FE-SEM images of ordered alumina through-pore arrays:
(A) the array formed with 0.2 M Al(NO3)3 (H < R); (B) the array
formed with 0.5 M Al(NO3)3 (H > R). All scale bars are 1 µm.
3. Results and Discussion
3.1. Alumina Ordered Through-Pore Template. On the
basis of the PS colloidal monolayer and Al(NO3)3 solution-
dipping,13c we can obtain alumina ordered through-pore arrays
after removing the PS monolayer. The morphologies can be
controlled just by the concentration (C) of Al(NO3)3. Figure 2
shows typical FE-SEM images of alumina ordered pore arrays
on ITO glass with different pore depths (H) corresponding to
the colloidal monolayer of 1000 nm PSs. When C ) 0.2 M,
the pores in the arrays are of bowl-like morphology with a pore
depth less than the radius (R) of PSs due to the geometry of
PSs and show circular openings from the top view (see Figure
2A). When C is up to 0.5 M, H > R. A hole in the wall exists
between two adjacent pores, resulting from area contact of PSs
due to heating the colloidal monolayer before solution-dipping,
as seen in Figure 2B. Further characterization shows that the
pore wall or skeleton is amorphous Al2O3. The formation of
alumina ordered through-pore arrays is easily understood. The
colloidal monolayer was sintered on the ITO substrate by
heating, which led to area contact with the substrate. The ordered
through-pore array was thus formed, due to the interstitial
geometry in the template, after the Al(NO3)3 solution-dipping
technique and removal of PSs, as previously reported.13c
3.2. Ni Nanoring and Hollow Sphere Arrays. On the basis
of such Al2O3 ordered pore arrays and electrodeposition in the
Figure 3. FE-SEM images of ordered Ni nanoring arrays using the
template shown in Figure 2A. The electrodeposition potential vs SCE
and the deposition time are, respectively, (A) -0.8 V and 20 min and
(C) -1.0 V and 16 min. (B) The local enlarged image of sample A.
Inset in part C: local enlarged image. All scale bars are 500 nm.
electrolyte, Ni ordered nanostructured patterns were obtained.
Figure 3 shows the morphologies of the samples on ITO glass,
based on the template shown in Figure 2A, after electrodepo-
sition with different potentials versus SCE and removal of
alumina in 5% NaOH solution. At a low negative deposition
potential (-0.8 V vs SCE), Ni grows preferentially along the
inner wall of the pores, forming a ringlike structure (see Figure
3A,B). The thickness of the film or H value should be close to

Ni Nanoring and Hollow Sphere Arrays
J. Phys. Chem. B, Vol. 110, No. 32, 2006 15731
Figure 5. XRD spectra for ITO (A) and the as-deposited samples (B-
D), corresponding to Figures 3A,C and 4A, respectively. Bottom:
standard diffraction of Ni powers.
obtained after deposition at -0.8 V versus SCE (see Figure
6C). The high-magnification images show that the shell layer
of hollow spheres is composed of a large quantity of flakelet-
like nanocrystals, which grew along the inner wall of alumina
pores, and the thickness of the nanoflakelets is estimated to be
nearly 20 nm (see Figure 6B).
Further experiments show that the pH values in the electrolyte
and alumina template are important to the morphology of the
deposited products, in addition to the deposition potential. A
low pH value (say, 4.5) leads to the spherical particle array
without flakelet-like fine structure (see Figure 7A). Without the
ordered alumina pore array, at low deposition potential (-0.8
V vs SCE), Ni cannot be deposited onto the bare ITO substrate.
At -1.0 to -2.0 V versus SCE, the Ni is deposited onto the
ITO substrate, forming particle chainlike film without any
flakelet-like fine structure (see Figure 7B). As for choice of
the alumina as the template, it is crucial to fabricate the Ni arrays
with hierachical structure in this work because the transitional
product Ni(OH)2 can grow along the pore wall of the alumina
(see the following subsection). Alumina is insulating and can
be removed in alkaline solution, which is important, since ITO
and Ni can be dissolved in acidic solution. Our experiments
have demonstrated that ordered pore arrays of many other
materials, such as Fe2O3, ZnO, TiO2, CeO2, and In2O3, induced
by solution-dipping the colloidal monolayer,13c cannot be used
as the template for the Ni arrays.
Figure 4. FE-SEM images of ordered Ni arrays using the template
shown in Figure 2A. The electrodeposition potential vs SCE and the
deposition time are, respectively, (A) -2.0 V and 8 min and (C) -1.2
V and 14 min. The scale bars are 500 nm for part A and 2 µm for part
B.
but less than R. The high-magnification FE-SEM image shows
that there are innumerable floc-like (or flakelet-like) nanocrystals
on the ring surface. The morphology of such ordered Ni ringlike
arrays can be controlled by the deposition potential. When the
deposition potential was -1.0 V versus SCE, the floc-like Ni
nanocrystal grew on both the ITO substrate and the inner walls
of the pores (see Figure 3C). With further increase of the
potential to -2.0 V versus SCE, the floc (or flakelet) vanished,
while spherical particles with a rough surface were formed
within the ordered alumina pores (see Figure 4A). A middle
deposition potential of -1.2 V versus SCE leads to a transitional
morphology (see Figure 4B). Both spherical particles and
nanorings with flakelet-like structure are formed in or along
the pores. The corresponding XRD spectra, shown in Figure 5,
indicate that all the as-deposited samples are nickel crystal with
a face-centered structure.
As mentioned above, the low negative deposition potential
induces the preferred growth of Ni along the pore wall. Thus,
if we use the template shown in Figure 2B, truncated hollow
spherelike Ni arrays should be obtained by using a low negative
deposition potential, which has been confirmed by our further
experiments. Figure 6A,B shows the morphology of the sample
electrodeposited at -0.8 V versus SCE based on the template
shown in Figure 2B, after removal of the template. The circular
openings on the top of the hollow spheres can be adjusted by
the alumina template morphology which depends on the
concentration of the precursor solution Al(NO3)3. Further, if
using the template prepared by 1.0 M Al(NO3)3, ordered Ni
hollow sphere arrays with much smaller top openings were
3.3. Formation of Ni Nanostrauctured Arrays. Ni deposi-
tion can be described by the following reactions:
[Ni(NH3)2]2+ ) Ni2+ + 2NH3
Ni2+ + 2e- f Ni
(1)
(2)
(3)
(4)
(E1° ) -0.23 V)
NH3 + H2O ) NH4+ + OH-
Ni2+ + 2OH- f Ni(OH)2
Ni(OH)2 + 2e- f Ni + 2OH-
(E2° ) -0.66 V) (5)
where E° is the standard electrode potential. Ni2+ ions in the
electrolyte exist in the form of [Ni(NH3)2]2+ 19
and there are
,
two reaction paths along which deposition of Ni will occur [path
I, (1) f (2); path II, (1) f (3) f (4) f (5)]. In an electrolyte
with a low pH value, the reaction will go along path I. When
the pH is high, the reactions along path II should occur, in

15732 J. Phys. Chem. B, Vol. 110, No. 32, 2006
Duan et al.
Figure 7. FE-SEM images of sample A electrodeposited at -0.8 V
vs SCE for 20 min and at pH 4.5 using the template shown in Figure
2A and Ni particle film (B) electrodeposited at -1.0 V vs SCE for 16
min and at pH 8.5 without template. Both scale bars are 1 µm.
flakelet-like nanocrystals.20 Thus, deposited Ni(OH)2 would be
of flakelet-like fine structure and then decompose into Ni during
the reaction, but the flakelet- or floc-like morphology should
be kept. We call it a morphology inheritance process.
Then, the formation of ring- or hollow spherelike structure
can be understood. Al2O3 used here is an insulating material
and can be removed easily, which is the main reason that we
chose it as the secondary template. When the potential is low
negative, corresponding to the reactions along path II, the
transitional product Ni(OH)2 preferentially grows along the
alumina pore wall because of the low barrier in energy, leading
to ringlike or hollow spherelike structured patterns due to the
geometry of the Al2O3 template. A small increase of the
deposition potential can promote growth, resulting in growth
along both the pore wall and the ITO substrate after nucleation
at the bottom of the pore wall (see Figure 3C).
As mentioned above, a low pH value or high negative
deposition potential will lead to the deposition reactions along
path I, which results in disappearance of the preferential growth
along the pore wall and the flakelet-like fine structure, and
spherical particle arrays are formed (see Figures 7A and 4A).
In addition, without the Al2O3 template, reaction 4 or the
reactions along path II will be suppressed and only island-shaped
particles without flakelet-like fine structure are formed on the
substrate (see Figure 7B). It means that the Ni(OH)2 can nucleate
and grow only in the existence of the Al2O3 template. Al2O3 is
crucial and indispensable to get such hierarchically structured
patterns.
Figure 6. FE-SEM images of the samples electrodeposited at -0.8 V
vs SCE based on the ordered alumina templates with H > R, prepared
by the precursor Al(NO3)3 with (A) 0.5 M and (C) 1.0 M. (B) The
local magnification of part A. The deposition time is 40 min for part
A and 50 min for part C. Inset in part B: thickness measurement of
the nanoflakelet. The scale bars are 2 µm for parts A and C and 200
nm for part B.
addition to path I. During the initial electrodeposition, the
concentration of Ni2+ close to the substrate will be increased
due to the movement of Ni2+ ions to the cathode, which leads
to the formation of Ni(OH)2 (see reaction 4) because of high
[OH-]. At a low potential, reaction 2 will be restricted and the
reaction along path II will thus be dominant. However, when
the deposition potential is very high, reaction 2 will be
accelerated and the reaction along path I will be dominant again.
In our cases (high pH value and low negative deposition
potential), the reaction along path II is dominant. Ni(OH)2 was
first formed. As known, Ni(OH)2 is a layered compound of CdI2
type. Its lamellar 2D structure easily leads to the formation of
On these bases, the reactions along path II occur only at a
low negative deposition potential, at a high pH value in the

Ni Nanoring and Hollow Sphere Arrays
J. Phys. Chem. B, Vol. 110, No. 32, 2006 15733
reaction. The ringlike or hollow spherical-like Ni ordered
nanostructured arrays with the fine structure are thus obtained,
depending on the pore geometry of the alumina template. Such
hierarchical nanostructured Ni arrays are expected to combine
the merits of both patterned arrays and nanocrystals. They could
exhibit some new properties and potential applications in, for
example, (1) magnetics or magnetic nanodevices; (2) SERS (Ni
has proven to be an effective SERS active material with the
enhancement factor correlated to its surface geometry,22 and
the hierarchical structures in our case could be of high
enhancement factor due to their periodic geometrical charac-
teristics and nanosized flakelets which both are beneficial to
the SERS effect18a,23); (3) catalysis, hydrogen-store materials,
or films due to the ultrathin nanoflakelets with high surface area;
and (4) new functional nanodevices.
Figure 8. Hysteresis loops for the sample shown in Figure 3A. The
applied field is parallel to the film (array) surface.
Acknowledgment. This work was supported by the National
Natural Science Foundation of China (Grant No. 10504035) and
the Major State research program of China “Fundamental
Investigation on Micro-Nano Sensors and Systems based on BNI
Fusion” (Grant No. 2006CB300402).
electrolyte, and with Al2O3 template, resulting in the flakelet-
like fine structure by morphology inheritance from Ni(OH)2.
Otherwise, the reactions along path I will appear, leading to
the final morphology of spherical particles. Thus, in this case,
the reactions along paths I and II should coexist at high pH
value in the electrolyte and with Al2O3 template but at a
moderate deposition potential. This has been confirmed by our
experiment, as shown in Figure 4B, which corresponds to the
potential -1.2 V versus SCE and the template (H < R) shown
in Figure 2A. Both spherical particles and nanorings with
flakelet-like structure are formed, which should originate from
the reactions along paths I and II, respectively.
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4. Summary and Remarks
In conclusion, we have demonstrated the fabrication of
hierarchical Ni ordered nanostructured arrays by morphology
inheritance in an electrochemical process based on an ordered
Al2O3 through-pore template induced by solution-dipping the
colloidal monolayer. In the electrolyte with a high pH value,
deposition on the substrate with Al2O3 template at a low negative
potential will lead to the formation of the transitional product
Ni(OH)2, which is of flakelet-like fine structure and which will
then decompose into Ni with similar morphology during the
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