Interconnected β-Ni(OH)2 sheets and their morphology-retained transformation into mesostructured Ni

Article abstract of DOI:10.1016/j.ssc.2006.01.033

A facile route was described for the synthesis of interconnected β-Ni(OH)₂ sheets, which was realized by thermal treatment of Ni(NH3)62+ solution at 95 °C. X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), infrared absorption spectrum (IR) and Raman spectrum were used to characterize the product. The complexation of ammonia and temperature played important roles for the formation of the assembled structure. Thus-prepared β-Ni(OH) ₂ were morphology-retained transformed into mesostructured Ni by further reduction in H₂ current for 1.5 h.

Full text of DOI:10.1016/j.ssc.2006.01.033

Solid State Communications 137 (2006) 585–588
www.elsevier.com/locate/ssc
Interconnected b-Ni(OH) sheets and their morphology-retained
2
transformation into mesostructured Ni
Xiaomin Ni, Qingbiao Zhao, Beibei Li, Jing Cheng, Huagui Zheng
*
Department of Chemistry, University of Science and technology of China, Hefei, Anhui 230036, People’s Republic of China
Received 16 June 2005; received in revised form 15 January 2006; accepted 19 January 2006 by B. Jusserand
Available online 7 February 2006
Abstract
A facile route was described for the synthesis of interconnected b-Ni(OH)
solution at 95 8C. X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM),
2
6
C
2
sheets, which was realized by thermal treatment of NiðNH Þ
3
infrared absorption spectrum (IR) and Raman spectrum were used to characterize the product. The complexation of ammonia and temperature
2
played important roles for the formation of the assembled structure. Thus-prepared b-Ni(OH) were morphology-retained transformed into
mesostructured Ni by further reduction in H current for 1.5 h.
2
q 2006 Elsevier Ltd. All rights reserved.
PACS: 61.66.Fn; 68.55.Jk; 83.80.Pc
Keywords: B. Chemical synthesis; B. Crystal growth; C. Scanning and transmission electron microscopy
1
. Introduction
assembling them into nanodevices. It is still challenging to
develop simple routes for the generation of assembled structures.
Morphology always greatly affected the properties and
b-Ni(OH) is an important positive electrode active material
2
applications of materials [1]. Various fields, such as electron
conduction, diagnosis, catalysis and nanodevices all press for
materials with special shapes. Recently, the construction of
assembled structures has become a major issue for their unique
applications which could not be achieved by their discrete or bulk
counterparts [2–5]. In general, fabrication of assembled structure
depends on the molecule interactions between the surfactants or
polymers which were absorbed on the surface of subunits. For
instances, smecticlike arrays of Te nanorods were driven by the
van der Waals forces among the surfactant molecules on particle
surfaces; spherical aggregates of monolayer polymer-protected
gold nanoparticles were prepared through hydrogen-bonding
interactions; assembly of magnetite nanoparticles were achieved
in the presence of ternary surfactant combinations [6–8].
However, these techniques most often require special instru-
ments, restricted experimental conditions and delicate know-
how. Moreover, the aggregated structures are easy to collapse if
the templates were removed, resulting in the difficulty in
in rechargeable Ni-based alkaline batteries, and metallic nickel
shows various applications in magnetic recording, catalysis,
and conduction [9,10]. Owing to that shape, properties and
applications of materials are intrinsically linked, exploitations
of different shaped materials have been a focus research theme.
To date, a few different shapes of b-Ni(OH)2 other than
isotropic nanoparticles have been fabricated, such as
nanosheets, nanorods, pancakes, microtubes [11–14]. As for
nickel, hollow spheres, nanobelts and nanowires, nanotubes,
nanorods and hexagonal sheets all have been prepared [15–19].
However, to the best of our knowledge, there have not been
seen any reports on the synthesis of assembled structure of
b-Ni(OH) and Ni up to date.
2
Herein, we introduced a simple route for the creation of
interconnected b-Ni(OH)2 sheets by hydrothermal treating
nickel ions in ammonia solution at 95 8C. In addition, by
reduction of thus-prepared b-Ni(OH) in H current at 500 8C
2
2
for 1.5 h, assembled mesostructure of metallic Ni was
produced, inheriting the morphology of b-Ni(OH) precursor.
2
*
Corresponding author. Tel.: C86 551 3606144; fax: C86 551 3601600.
E-mail address: hgzheng@ustc.edu.cn (H. Zheng).
2. Experimental
All the reagents were analytic purity. In a typical
experiment, 0.4 mmol NiCl $6H O was dissolved into 35 ml
0038-1098/$ - see front matter q 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2006.01.033
2
2

586
X. Ni et al. / Solid State Communications 137 (2006) 585–588
distilled water to give a green transparent solution. Then 5 ml
ammonia solution (28 wt%) was added under magnet stirring
and the solution turned blue, indicating the complexation of
2
C
Ni and ammonia. The mixture was transformed into a 50 ml
Teflon-lined autoclave, sealed and maintained at 95 8C for 4 h.
After the heating treatment was completed, green powder was
found deposited on the wall of the clave. The product was
collected and rinsed, and finally dried in a vacuum at 105 8C for
4 h (sample 1). Part of the dried powder was heated in H2
K1
current (30 ml min ) at 450 8C for 1.5 h in a fixed-bed
reactor, and further kept in H current until the temperature was
2
decreased to room temperature (sample 2).
X-ray diffraction (XRD) patterns of the samples were
recorded on a Philips X’pert diffractometer with Cu Ka
˚
radiation (lZ1.54178 A). Morphology of the samples were
studied with transmission electron microscope (TEM, Hitachi,
H-800) with an accelerating voltage of 200 kV, field emission
scanning electron microscopy (FSEM, JEOL JSM-6300F).
Infrared absorption (IR) spectrum was performed with a
Nicolet FT-IR-170 SX spectrometer at room temperature. The
Raman spectrum was collected at ambient temperature on a
SPEX 1403 spectrometer with an argon-ion laser at an
excitation wavelength of 514.5 nm.
2
Fig. 2. FE-SEM image of the b-Ni(OH) sample: (a) a low-magnification
image; (b) a high-magnification image; (c) FE-SEM image of a section of the
self-assembled structure; (d) a typical TEM image and corresponding and the
corresponding ED pattern of two discrete hexagonal nickel sheets.
sheets were aligned vertically with a height of about 3.0 mm, as
seen in Fig. 2(c). Thus-prepared structure was found very firm
and long-time ultrasonication could not destroy it. During the
TEM studies, only a few detached sheets were observed. Two
typical hexagonal sheets with a diameter of 0.4–1.0 mm were
shown in the image inset in Fig. 2(d) and the corresponding ED
pattern demonstrated that the sheets were single crystalline.
According to the diffraction dots, the basal surface of the sheets
could be indexed as the {0001} planes. ED patterns recorded
on the different individual sheet were essentially the same,
indicating that all the sheet-like subunits possessed the same
crystallographic character. Considering their special
assembling manner, the intensified (001) peak of Fig. 1(a)
could be ascribed to the non-random average orientation of the
platelet subunits.
3
. Results and discussion
Fig. 1(a) gave the XRD patterns of the sample 1, which was
assigned to the obtained b-Ni(OH) . No peaks of a-Ni(OH) or
2
2
nickel oxides were detected, indicating pure product was
generated under current experimental conditions. Compared
with the standard pattern of b-Ni(OH) (JCPDS 74-2705), the
2
peak intensity of (00l) was sharply strengthened.
Morphology of the sample was studied with FE-SEM and
TEM. A typical low-magnification image in Fig. 2(a) shown
that as-prepared b-Ni(OH) was film-like. The corresponding
2
Fig. 3 gave a typical IR spectrum of the resulted b-Ni(OH) .
2
magnified image in Fig. 2(b) shown that the film comprised
sheet-like crystals with a size of 50–100 nm in thickness and
K1
A narrow and sharp peak at 3650 cm
(
phase of the product [20]. The strong band at 527 cm
was owing to the n
OH) stretching vibration, which confirmed the b-Ni(OH)2
0
.3–1.0 mm in side length. A section image detailed that the
K1
a
b
2
0
30
40
50
60
70
80
5
00
1000
1500
2000
2500
3000
3500
4000
Wavenumber (cm–1
)
Fig. 1. XRD patterns of the two samples. (a) The obtained b-Ni(OH)
sample of Ni.
2
; (b) the
Fig. 3. IR spectrum of as-prepared b-Ni(OH)
2
sample.

X. Ni et al. / Solid State Communications 137 (2006) 585–588
587
Fig. 5. SEM images of the sample obtained at different temperature: (a) 80 8C.
b) 110 8C.
/
/
(
ammonia, in the following growth stage, these nuclei gradually
developed into interconnected sheets with the newly produced
Ni(OH) continuously feeding [25]. Of course, the nucleation
/
/
3
00
400
500 3500
3550
3600
3650
2
wavenumber (cm–1
)
and growth of particles in solution was inevitable which
emerged as flowerlike microparticles [13]. As the perfectness
of thin films is related to the reaction rate, at different
temperatures the obtained films exhibited different details [26].
Keeping other reaction conditions same, lowing the tempera-
ture to 80 8C resulted in much larger flakes with several thin
hexagonal sheets lying on the surface (Fig. 5(a)). While at a
higher temperature of 110 8C, films with thicker hexagonal
sheets protrudent emerged, as seen in Fig. 5(b). It indicated that
the temperature affected the growth rates of the basal and edge
2
Fig. 4. Raman spectrum of as-prepared b-Ni(OH) sample.
corresponded to the lattice vibration d (OH) of hydroxyl groups
K1
[
stretching vibration and d (H O) bending vibration of water
20]. Peaks at 3440 and 1641 cm resulted from the n (H O)
2
2
molecules, which were absorbed on the product. A small weak
K1
peak centered around 460 cm
stretching mode [21].
Raman spectrum of the product was given in Fig. 4. Two
was assigned to the Ni–O
K1
small peaks located at 317.0 and 446.8 cm , together with a
K1
strong sharp peak at 3581.1 cm
faces of the Ni(OH) plates. A higher temperature facilitated
2
emerged, which were
the growth of edge surface, while the lower temperature
favored the development of the basal surfaces. However,
thermal dependence of the surface energies of the basal and
edge faces of the sheets was still not unknown and a more
deeply research was underway. As for the disordered
aggregation of the sheets in Fig. 5(b), it was possibly attributed
to the faster reaction rate at higher temperature which was not
favorable for their ordered assembly.
consistent with the reported values of crystalline b-Ni(OH)2
K1
[
22]. The absence of the peak centered around 510 cm
indicated the nice crystallization of the product with few proton
vacancies and stacking faults [23].
Interestingly, in the experiment we found that most of the
green product was deposited on the wall of the claves and little
powder was collected from the solution. It was shown that the
product was practically the film of b-Ni(OH) which grew
2
directly on the Teflon matrix of the clave. However, in previous
reports dispersive nanorods and nanosheets of b-Ni(OH) were
We further tried to transform the assembled structure of b-
Ni(OH) into metallic nickel by hydrogen reduction. XRD
2
2
pattern in Fig. 1(b) revealed that the obtained black powder
was face-centered cubic (fcc) nickel (JCPDS 4-850). No
characteristic peaks of hydroxides or oxides were detected. FE-
SEM images in Fig. 6 revealed the resulted nickel powder
possessed a mesoporous skeleton consisted of interconnected
particles, approximately maintaining the size and the
assembled shape the precursor. However, the former compact
sheets became porous, which may be resulted from the release
of H and O atoms during the thermal reduction. New properties
including catalysis or conduction are expected to be found in
this mesostructured nickel.
fabricated by a similar reaction using ammonia as complexing
agents and alkali source [11,12]. But we noted that the
temperature in their experiments was much higher to 180 and
2
00 8C. Meanwhile, the concentrations of nickel source were
about 8 and 17 times higher than that of our system,
respectively. Based on investigations on the effect of the
experimental conditions, we found that the lower temperature
was of great importance for the formation of our self-
assembled structure, while the concentration of nickel source
did not show obvious influence. In the current system,
generation of Ni(OH) could be could be formulated as the
2
2
6
C
C
reaction: NiðNH Þ C2H O/NiðOHÞ YC2NH C4NH .
3
2
2
4
3
When heating, the ammonia decomplexation occurred for the
sharp decrease of the complexation constant, resulting in the
generation of Ni(OH) particles. The presence of ammonia and
2
the low temperature made the reaction process at a relatively
slow rate [24]. As the surface of the wall of the clave was not
absolutely smooth and due to temperature gradient from the
wall to the inside, the generated nickel hydroxide would
preferentially nucleate on the active sites of the wall. Due to its
intrinsic brucite-type structure and the complexation of
Fig. 6. FE-SEM image of the Ni sample: (a) image of the top surface of the self-
assembled structure; (b) image of a section of the self-assembled structure.

5
88
X. Ni et al. / Solid State Communications 137 (2006) 585–588
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4
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1
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2
(
2004) 214.
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nickel chloride using ammonia as complexing agent. The
complexation of ammonia and the low temperature were found
play important roles for the formation of this special
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2
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2
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1
Acknowledgements
[
1
The authors are grateful to the transmission electron
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