Hydrothermal synthesis of β-nickel hydroxide nanocrystalline thin film and growth of oriented carbon nanofibers

Article abstract of DOI:10.1016/j.materresbull.2009.03.005

Novel well-crystallized β-nickel hydroxide nanocrystalline thin films were successfully synthesized at low temperature on the quartz substrates by hydrothermal method, and the oriented carbon nanofibers (CNFs) were prepared by acetylene cracking at 750 °C

Full text of DOI:10.1016/j.materresbull.2009.03.005

Materials Research Bulletin 44 (2009) 1765–1770
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Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Hydrothermal synthesis of
b-nickel hydroxide nanocrystalline thin film and
growth of oriented carbon nanofibers
a
a,b,
a
a
a
Enlei Zhang , Yuanhong Tang *, Yong Zhang , Chi Guo , Lei Yang
a
College of Materials Science and Engineering, Hunan University, Changsha 410082, People’s Republic of China
b
Powder Metallurgy Research Institute, Central South University, Changsha 410083, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Article history:
Novel well-crystallized
b
-nickel hydroxide nanocrystalline thin films were successfully synthesized at
Received 13 October 2008
Received in revised form 5 March 2009
Accepted 12 March 2009
Available online 25 March 2009
low temperature on the quartz substrates by hydrothermal method, and the oriented carbon nanofibers
CNFs) were prepared by acetylene cracking at 750 8C on thin film as the catalyst precursor. High
resolution transmission electron microscopy (HR-TEM) measurement shows that thin films were
constructed mainly with hexagonal -nickel hydroxide nanosheets. The average diameter of the
(
b
nanosheets was about 80 nm and thickness about 15 nm. Hydrothermal temperature played an
important role in the film growth process, influencing the morphologies and catalytic activity of the Ni
Keywords:
A. Thin films
catalysts. Ni thin films with high catalytic activity were obtained by reduction of these Ni(OH)
2
B. Chemical synthesis
C. X-ray diffraction
D. Catalytic properties
nanocrystalline thin films synthesized at 170 8C for 2 h in hydrothermal condition. The highest carbon
yield was 1182%, and was significantly higher than the value of the catalyst precursor which was
previously reported as the carbon yield (398%) for Ni catalysts. The morphology and growth mechanism
of oriented CNFs were also studied finally.
ß 2009 Elsevier Ltd. All rights reserved.
1
. Introduction
intermediate during the synthesis of nickel oxide and metal nickel.
2
Therefore, nanostructured Ni(OH) nanocrystalline thin films are
Among transition metal hydroxides, nickel hydroxide (Ni(OH)
2
)
highly desired. Now a variety of physical as well as chemical
methods have been applied for the deposition of metal thin films,
namely, pulsed laser deposition, sputtering, dip coating process,
spray pyrolysis, electrodeposition, etc. [15–23]. But these methods
had many drawbacks such as small area of deposition, requirement
of sophisticated instruments, high working cost of system, high
deposition temperature, wastage of depositing material, cleaning
after each deposition, etc. Thus, seeking simple, easily controlled
has been of particular interest for its importance in fundamental
researches and potential applications in catalysis, electronic and
magnetic fields [1–9]. As inspired by both the potential application
2
of Ni(OH) and the novel properties of nanoscale materials,
considerable works have been done to understand the structure
and electrochemical properties of zero- and one-dimensional
nickel hydroxide, but little attention was paid to the usage of two-
dimensional nickel hydroxide nanoscale materials as catalyst
precursors. It is well known that Ni-based catalysts were
extensively studied due to their relatively higher activities
compared with other transition metals [10–12]. Because the
catalytic activity of the Ni catalysts decreased with time on stream
due to the continuous deposition of carbon, the effective catalysts
should exhibit sufficient carbon storage capacity on a per-gram
basis of nickel, in addition to reasonable activity toward carbon
source decomposition at relatively lower temperatures [13,14].
And nickel hydroxide has been regarded as the most important
2
methods to synthesize Ni(OH) nanocrystalline thin film is still a
hotspot of researchers.
By comparing with other methods, the hydrothermal method
not only helps in processing monodispersed and highly homo-
geneous nanoparticles, but also acts as one of the most attractive
techniques for processing thin films. The hydrothermal method is
ideal for the processing of very fine nanocrystalline thin films
having high purity, controlled stoichiometry, high quality, narrow
particle size distribution, controlled morphology, high crystallinity
and so on because of the highly controlled diffusivity in a strong
solvent media in a closed system [24–26].
In this paper, we report a simple method to directly synthesize
b
2
-Ni(OH) nanocrystalline thin films under hydrothermal condi-
*
Corresponding author at: College of Materials Science and Engineering, Hunan
tions. The influence of experimental conditions and structure of as
obtained thin films have been systemically investigated. Moreover,
we found that Ni thin films with high catalytic activity were
University, Changsha, Hunan 410082, P. R. China. Tel.: +86 7318821778;
fax: +86 7318821778.
E-mail address: yhtang2000@163.com (Y.H. Tang).
0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2009.03.005

1
766
E. Zhang et al. / Materials Research Bulletin 44 (2009) 1765–1770
obtained by reduction of these Ni(OH)
2
nanocrystalline thin films
synthesized with the hydrothermal method, and the oriented
carbon nanofibers were formed on the Ni thin films by cracking
acetylene.
2
. Experimental
All of the chemicals were of analytical grade and used without
further purification. In a typical procedure, 0.1 M solution of
NiSO O was taken in the 1:2 stoichiometric molar ratio along
ꢀ6H
with 20% solution of NaOH. NiSO O immediately reacted with
ꢀ6H
the NaOH in the solution to form Ni(OH) . The mixed solution
250 ml) was transferred into a stainless steel autoclave of capacity
4
2
4
2
2
(
(
1000 ml). Then, a quartz substrate (5 mm ꢁ 5 mm ꢁ 2 mm) was
put into the autoclave. After the autoclave was sealed, then heated
to 100–300 8C, 1.8–11 MPa of pressure were maintained for several
hours. Finally, the autoclave was cooled naturally. The light green
film was firmly formed on the surface of the substrate. The product
was washed by distilled water for several times to remove excess
salts, and then dried at 80 8C overnight in air.
Fig. 1. XRD patterns of the as-received samples with different temperatures.
˚
˚
14-0117) displaying a hexagonal lattice (a = 3.12 A and c = 4.62 A)
The catalytic activity of
acetylene conversion, C
powder were placed in a quartz boat and were then introduced
b
-Ni(OH)
2
was tested by the degree of
. The -Ni(OH) thin film and
with the typical peaks at 2
respectively [30,31]. It proves that the thin film with
deposited on the substrate. Crystalline sizes presented in Table 1
were calculated using the Scherrer equation.
u
= 19.258, 33.068, 38.548, and 52.18,
-Ni(OH) has
2
H
2
! 2C + H
2
b
2
b
2
into a chamber of a tube furnace. The quartz boat was heated to
ꢂ1
4
4
00 8C at a rate of 10 8C min under vacuum and then reduced at
00 8C for 2 h in a flow of 9% hydrogen in argon under atmospheric
K
l
Dh k l
¼
(2)
pressure. Subsequently, a flow of 9% acetylene in argon was
introduced into the chamber at a flow rate of 100 sccm, oriented
CNFs were formed on the thin film by deposition of carbon atoms
from decomposition of acetylene at 700 8C for 1 h. The percentage
of carbon yield is defined as follows [27–29]:
bhkl cos u
where Dhkl is the crystalline size along the [h k l] direction, bhkl is
the fwhm of a given [h k l] peak, and K is the structure factor. The
diffraction peaks are gradually becoming narrower and crystal size
larger with increasing hydrothermal temperature to 170 8C,
weight of carbon deposited on the catalyst
carbon yieldð%Þ ¼ 100ꢁ
indicating the growth of
2
b-Ni(OH) crystalline, as shown in
weight of the catalyst
Table 1. The width of peaks also indicates that the particles size is
of the nanometer scale. With increasing the hydrothermal
temperature from 170 to 300 8C, the diffraction peaks and average
(
1)
The synthesized samples were characterized by X-ray diffrac-
tion (XRD) using the Semens D 5000 diffractometer with Cu Ka
˚
size of
Fig. 2 clearly reveals the morphologies of the nanostructure
Ni(OH) thin films deposited on the quartz substrates under
different hydrothermal temperatures and time. The thickness of
the films was about 100 nm. At room temperature (Fig. 2a), a few
sheets were obtained with poorly crystallized structure and more
irregular shape. When the temperature increased to 170 8C
(Fig. 2b), well-crystalline hexagonal structure nanosheets were
observed, and the nanosheet had the small face diameter of about
80 nm. Then further increasing the hydrothermal temperature and
2
b-Ni(OH) crystallites are not varied obviously.
radiation (
l
= 1.54178 A). Morphology observation of
b
-Ni(OH)
2
b-
thin film and oriented CNFs was performed with field emission
transmission electron microscopy (SEM, JEOJ JSM-5600LV), high
resolution transmission electron microscopy (HR-TEM, JEOL JEM-
2
3
010). Thermogravimetric (TG) data and differential scanning
calorimetry (DSC) curves were obtained at a heating rate of
ꢂ1
1
0 8C min in nitrogen by thermal analyzer (Netzsch STA449C).
3
. Results and discussion
time (Fig. 2c and d), structure and crystalline size of
b-Ni(OH)
2
Hydrothermal temperature is one of the most important factors
nanosheets were not varied obviously, indicating that the
formation process was almost complete. It is in a good agreement
to influence the film features, since the nucleation and growth
processes of Ni(OH) are determined by the hydrothermal
2
with the result of XRD. It is seen from Fig. 2e that the b-Ni(OH)
2
temperature. The crystal phase and structure information of the
thin films were obtained by XRD measurements. Fig. 1 shows the
nanosheets obviously exhibit the preferred orientation along the
direction perpendicular to the substrate, and the nanosheets are
homogeneous, displaying a hexagonal structure. It is well known
XRD pattern of the Ni(OH)
hydrothermal temperatures. One of the phase is indexed to the
0 0 1), (1 0 0), (1 0 1), and (1 0 2) planes of -Ni(OH) (JCPDS no.
2
thin films deposited with various
that the
2 2
b-Ni(OH) possesses the typical CdI structure [32], which
(
b
2
is layered structure, so formed M(OH)
2
nanosheets are stable
Table 1
Effect of different hydrothermal conditions on the catalytic activity of
2
b-Ni(OH) samples.
Sample
Temperature (8C)
Time (h)
Pressure (MPa)
Crystalline size (nm)
Face diameter
Yield of CNFs (wt.%)
Thickness
S1
S2
S3
S4
S5
Room temperature
2
2
2
2
8
Atmosphere
15
75
80
85
80
10
15
16
15
16
177
494
100
170
300
170
0.3
2
1822
1611
1258
9.4
2

E. Zhang et al. / Materials Research Bulletin 44 (2009) 1765–1770
1767
Fig. 2. SEM images of samples synthesized by different hydrothermal conditions. (a) S1; (b) S3; (c) S4; (d) S5. (e) The magnified SEM image of S3.
geometrical morphologies in the surface chemistry context,
because with sheet shapes M(OH) have a low system energy.
which is significantly smaller than that of the reported [3]. It is seen
from Fig. 3b that -Ni(OH) nanosheet shows well hexagonal
2
b
2
These sheet-like nuclei aggregate together under hydrothermal
condition, and then they grow up, and the slurry dissolves
gradually. At the same time, the nucleation takes place on the
structure and a single-crystalline nature of the nanosheet (fast
Fourier transform (FFT), inset of Fig. 3b). The enlarged detail of a
stack side (Fig. 3c) enables the mean thickness of a layer to be
estimated at about 15 nm, such as Fig. 3c. The FFT image (inset of
substrate, which leads to the growth of Ni(OH)
construct Ni(OH) thin film.
The TEM images of -Ni(OH)
2
nanosheets to
2
2
Fig. 3c) indicates that it consisted of b-Ni(OH) nanosheets with an
b
2
nanosheets prepared at 170 8C
oriented crystallographic axis along the [0 0 1] direction. The
possible chemical composition of the sample was analyzed
through the EDS (Fig. 3d). The presence of peaks demonstrates
that the nanosheets are composed of Ni, O, small amount of Cu and
for 2 h are shown in Fig. 3. TEM image (Fig. 3a) underlines not only
the homogeneous shape of the sample, but also the lamellar
feature of the particles, and the mean face diameter is about 80 nm,

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E. Zhang et al. / Materials Research Bulletin 44 (2009) 1765–1770
Fig. 3. (a) TEM image of sample S3; (b) HR-TEM image of sample S3. The inset is the corresponding FFT. (c) HR-TEM image of sample S3 stack side. The inset is the
corresponding FFT. (d) EDS spectrum of sample S3.
C. The copper and carbon signals were due to the supporting
copper grid.
All of the film samples deposited with various temperature
exhibit similar TG-DSC curves. Fig. 4 shows TG-DSC curves of
2 2
b-Ni(OH) thin film synthesized at 170 8C for 2 h. The b-Ni(OH)
underwent multi-step weight losses due to dehydration and
decomposition. The initial weight loss of about 1% represented the
removal of the physically adsorbed water molecules. The followed
weight loss of about 17% is in good agreement with the theoretical
2
weight loss value (19.4%) caused by the decomposition of Ni(OH) .
The DSC curve shows only one large endothermic peak. The
endothermic peak with the maximums peak located at around
3
40 8C corresponds to the endothermic behavior during the
decomposition from Ni(OH) to NiO. It has been reported that
the decomposition of Ni(OH) to NiO occurs between 298 and
42 8C [31,33]. The temperature ranges of the endothermic peak in
2
2
3
the DSC curve fits very well with those of the weight loss in the TG
curve.
The Ni thin films were obtained by reduction of
b-Ni(OH)
2
nanocrystalline thin films. The catalytic activity of Ni thin films can
be test through the absorption of carbon by catalyst from the gas
phase and precipitation of carbon to form CNFs. In order to
examine the effect of hydrothermal condition catalytic reaction,
five catalysts pretreatment with different conditions were studied
Fig. 4. TG and DSC of sample S3.

E. Zhang et al. / Materials Research Bulletin 44 (2009) 1765–1770
1769
on the catalytic performance of the reduced Ni catalysts. Previous
reports has revealed that temperature has influence on the storage
capacity, nickel content and weight loss for precipitated Ni(OH)
32–34]. For example, on increasing the temperature of hydro-
2
[
thermal treatment from 60 to 160 8C, the tap density increase from
ꢂ3
0
.4 to 2.0 g cm , and wt.% Ni increase from 48.7 to 58.7, and
weight loss at 800 8C decreases from 25.4% to 19.7% [33]. The
increase in catalytic activity with increase in treatment tempera-
ture could be due to increase of Ni content. If the Ni content is
increased, carbon storage capacity of Ni would be enhanced after
being reduced. Thus the CNFs yield increase remarkably. It is a
good agreement with our experiment results.
Ni thin films with the high catalytic activity were obtained by
2
reduction of these Ni(OH) nanocrystalline thin films synthesized
at 170 8C for 2 h in hydrothermal condition. XRD measurement
was then conducted to follow the evolution of the Ni thin film.
Fig. 5 shows that one of the phase is indexed to the (1 1 1), (0 0 4),
and (2 2 0) planes of Ni (JCPDS no. 04-0850) with the typical peaks
Fig. 5. XRD pattern of the Ni thin film reduction of sample S3.
at 2u
= 44.58, 51.858 and 76.378, respectively. It is revealed that the
nanocrystalline thin films were reduced into Ni thin film
Ni(OH)
2
(as listed in Table 1). As shown in Table 1, the carbon yield slightly
increased from 177% to 494% over S1 to S2, respectively. The
highest carbon yield, being 1822%, was obtained for S3. Never-
theless, the carbon yield declined to 1611% for S4, and decreased to
1
1
258% over S5. The catalytic activity passes through a maximum at
70 8C for 2 h. From the reaction results of acetylene decomposi-
tion, it seems that the hydrothermal temperature of the
synthesized Ni(OH) thin film precursors has an important effect
2
Fig. 6. (a) SEM image of as-grown oriented CNFs on a quartz substrate. The inset is
Fig. 7. (a) TEM image of the orienteded CNFs; (b) HR-TEM image of the single
the expanded views of the tip. (b) SEM image of the middle section.
nanofiber.

1
770
E. Zhang et al. / Materials Research Bulletin 44 (2009) 1765–1770
on the quartz substrate. For the reduced Ni thin film, the carbons
were all in oriented nanofibers form. Fig. 6a displays a SEM image
of oriented CNFs growing normal to the quartz substrate. Most of
CNFs are approximately perpendicular to the substrate. The
magnified SEM image (inset of Fig. 6a) reveals a top view of a
bundle surface. Fig. 6b shows section SEM image of the highly
oriented CNFs with quite uniform diameter. TEM observations
were performed to investigate the morphology of the oriented
CNFs sample in detail. Fig. 7a depicts that the nanofibers are
generally oriented in one direction to form an array, which are
several microns long. The Ni cluster was all located at the tips of
CNFs, which is in good agreement with that shown from SEM
images. It can clearly be seen that Ni cluster was located at the tips
of oriented CNFs. CNFs remained well oriented arrays even after
drastic ultrasonic agitation. Fig. 7b shows the single nanofiber has
the diameter of about 50 nm. Additionally, the graphitic sheets are
waved over a shot range and thus are the most defective structures,
and the outer sheets are less crystalline compared with the inner
ones. In our experiment, based on the TG-DSC experiment, the
calcinations at 400 8C in argon resulted in the decomposition of
Acknowledgements
This research work is supported by the Creative Research Group
of National Science Foundation of China (Grant No. 50721003) and
the Foundation of the Ministry of Education of China for Returned
Scholars (Grant No. 2005383).
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[
condition of
2
b-Ni(OH) thin film as the catalyst precursor in
3
synthesizing oriented CNFs has been found to be 170 8C for 2 h. A
yield up to 1822 wt.% was obtained by acetylene cracking on the
(
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