Structural and electrochemical properties of nanostructured nickel silicides by reduction and silicification of high-surface-area nickel oxide

Article abstract of DOI:10.1016/j.materresbull.2011.11.019

Nanostructured nickel silicides have been prepared by reduction and silicification of high-surface-area nickel oxide (145 m² g ^-1) produced via precipitation. The prepared materials were characterized by nitrogen adsorption, X-ray diffraction, thermal analysis, FT-IR spectroscopy, scanning electron microscopy, transmission electron microscopy, magnetic and electrochemical measurements. The nickel silicide formation involves the following sequence: NiO (cubic) → Ni (cubic) → Ni ₂Si (orthorhombic) → NiSi (orthorhombic) → NiSi₂ (cubic), with particles growing from 13.7 to 21.3 nm. The nickel silicides are ferromagnetic at room temperature, and their saturation magnetization values change drastically with the increase of Si content. Nickel silicides have remarkably low electrical resistivity and noble metal-like properties because of a constriction of the Ni d band and an increase of the electronic density of states. The results suggest that such silicides are promising candidates as inexpensive yet functional materials for applications in electrochemistry as well as catalysis.

Full text of DOI:10.1016/j.materresbull.2011.11.019

Materials Research Bulletin 47 (2012) 867–877
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Materials Research Bulletin
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Structural and electrochemical properties of nanostructured nickel silicides
by reduction and silicification of high-surface-area nickel oxide
Xiao Chen a, Bingsen Zhang b, Chuang Li a, Zhengfeng Shao a, Dangsheng Su b,
Christopher T. Williams c, Changhai Liang a,
*
a Laboratory of Advanced Materials and Catalytic Engineering, State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China
b Department of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society, Germany
c Department of Chemical Engineering, Swearingen Engineering Center, University of South Carolina, United States
A R T I C L E I N F O
A B S T R A C T
Article history:
Nanostructured nickel silicides have been prepared by reduction and silicification of high-surface-area
g
nickel oxide (145 m2 ꢀ1) produced via precipitation. The prepared materials were characterized by
Received 25 March 2011
Received in revised form 2 October 2011
Accepted 10 November 2011
Available online 19 November 2011
nitrogen adsorption, X-ray diffraction, thermal analysis, FT-IR spectroscopy, scanning electron
microscopy, transmission electron microscopy, magnetic and electrochemical measurements. The
nickel silicide formation involves the following sequence: NiO (cubic) ! Ni (cubic) ! Ni2Si (orthor-
hombic) ! NiSi (orthorhombic) ! NiSi2 (cubic), with particles growing from 13.7 to 21.3 nm. The nickel
silicides are ferromagnetic at room temperature, and their saturation magnetization values change
drastically with the increase of Si content. Nickel silicides have remarkably low electrical resistivity and
noble metal-like properties because of a constriction of the Ni d band and an increase of the electronic
density of states. The results suggest that such silicides are promising candidates as inexpensive yet
functional materials for applications in electrochemistry as well as catalysis.
Keywords:
A. Intermetallic compounds
A. Nanostructures
B. Chemical synthesis
C. Electrochemical measurements
D. Electrochemical properties
ß 2011 Elsevier Ltd. All rights reserved.
1. Introduction
CoSi nanowires show ferromagnetic properties with low-temper-
ature freezing of surface spins [8]. Fe3Si nanoparticles prepared by
Metal silicides are intermetallic compounds between metals
and silicon [1,2]. The silicon atoms dissolve into the metal lattices,
which results in withdrawal of electron density from the metals by
the electronegative Si element. Such materials have diverse
physical and chemical properties that are both useful and
fundamentally significant [3–12]. SiMox-alloy electrodes have
been successfully fabricated by using co-sputtering on an
electrolytic deposited Cu foil, and showed improved cycle
performance and capacity retention compared with Si electrodes
[4]. Manganese silicide nanowires having the Nowotny Chimney
ladder phase showed excellent thermoelectric performance and
can be used as efficient phonon scatters [5]. CaSi2 has been shown
to be a good replacement anode material for graphite in lithium-
ion batteries [6]. Bulk MnSi has unusual magnetic orderings,
helimagnetism, and skyrmions at ambient pressure, and the
results at high pressure have revealed partial magnetic ordering
and non-Fermi liquid behaviour [7]. Magnetic measurements of
a high temperature reduction method have shown superpara-
magnetism at temperatures of 10–300 K [9]. Meanwhile, the
amorphous Pd81Si29 alloy prepared by melt spinning exhibits a
more than 50 times higher turnover frequency than Pd/SiO2 for the
semihydrogenation of propargylic alcohol [10]. Finally, highly
dispersed CoSi nanoparticles supported on silica and SBA-15 were
synthesized by MOCVD of Co(SiCl3)(CO)4 at atmospheric pressure
and moderate temperature, and could be used as catalysts for
naphthalene selective hydrogenation [11,12].
Nickel silicides are representative of transition metal silicides
with low resistivity and suitable work functions, and have been
applied in several fields [13–16]. Ni-rich silicides, such as Ni2Si
nanowires, are attractive nanoscale building blocks for inter-
connects and fully silicided gate applications in nanoelectronics
[13]. Single crystal nickel silicide (NiSi) nanowires have ideal
resistivity of about 10 mV cm and remarkably high failure-current
densities of above 108 A cmꢀ2. Nickel silicide/silicon (NiSi/Si)
nanowire heterostructures with atomically sharp metal–semicon-
ductor interfaces also have been fabricated [14]. In addition,
nanocrystalline intermetallic NiSi alloy powders with Si as the
active element prepared by high energy ball-milling could be used
as an anode material for lithium-ion batteries [15]. Silicon-rich
* Corresponding author. Tel.: +86 411 84986056; fax: +86 411 84986056.
E-mail address: changhai@dlut.edu.cn (C. Liang).
URL: http://finechem.dlut.edu.cn/liangchanghai
0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2011.11.019

868
X. Chen et al. / Materials Research Bulletin 47 (2012) 867–877
nickel silicide, NiSi2, has been explored for use in low-power
memory devices, and for using an optical absorber in optical
communications applications [16].
2.2. Characterization
Thermogravimetric/derivative thermogravimetric (TG/DTG)
experiments were performed in Mettler Toledo TGA/SDTA851e
thermogravimetric analyzer. The samples were placed in an
atmosphere of 80% Ar and 20% O2 and heated at 10 8C/min to the
final temperature of 800 8C.
Nitrogen adsorption and desorption isotherms using a multi-
point method at ꢀ196 8C were measured on Micromeritics 2020.
Prior to the measurements, all samples were degassed at a
temperature of 200 8C at vacuum level of 10ꢀ3 Torr for at least 4 h.
Surface areas were calculated from the linear part of the BET plot.
X-ray diffraction (XRD) analyses of the samples were carried
Although Nuzzo et al. reported that supported nickel silicides
demonstrated high catalytic activity and selectivity for the
competitive dehydrogenation and hydrogenolysis of cyclohexane
[17], the properties of bulk nickel silicide nanoparticles have not
been researched in great depth. In our previous research, bulk
nickel silicide nanoparticles showed above 92% selectivity to
styrene in the semihydrogenation of phenylacetylene. This
behaviour was attributed to the electronic and geometrical effects
derived from the addition of Si into the Ni lattice [18]. However,
due in part to the multiple stoichiometries and complex phase
behaviour exhibited by nickel silicides, general and rational
chemical synthesis of silicide material is still challenging [2].
In the present study, we present a novel route to prepare nickel
silicides via reduction and silicification of high-surface-area nickel
oxide from precipitation at relatively low temperature and
atmospheric pressure. The phase and atom ratio can be easily
controlled by experimental conditions. The resultant nickel
silicides with different phases were characterized by X-ray
diffraction (XRD) and transmission electron microscopy (TEM),
and their magnetic and electrochemical properties were explored.
It is found that the nickel silicides with different phases are
ferromagnetic at room temperature, with saturation magnetiza-
tion values drastically decreased when Si atoms are added. In
addition, they show remarkably lower electric resistivity and noble
metal-like properties, suggesting possible applications as novel
electrode materials.
out using a Rigaku D/Max-RB diffractometer with a Cu K
a
monochromatized radiation source, operated at 40 kV and 100 mA.
The average size of nickel silicide particles was evaluated by the
Scherrer formula as:
0:9lK
cos umax
ð2uÞ
a1
L ¼
B
˚
where lK is 1.54178 A, B(2 ) is a full width at half-maximum of
a1
u
the diffraction peak in radians.
The FT-IR spectrum of the Ni(OH)2 precursor was recorded by
using
a Nicolet-20DXB spectrometer in the range of 400–
4000 cmꢀ1 with 4 cmꢀ1 resolution.
The morphologies of the as-prepared samples were studied by
scanning electron microscopy (SEM, JEOL JSM-5600LV), operated
at 15 kV.
High-resolution transmission electron microscopy (HRTEM)
and energy-dispersive X-ray spectroscopy (EDX) were performed
by using a Philips CM200 FEG transmission electron microscope,
operated at 200 kV. The composition analysis was performed in the
scanning transmission electron microscopy (STEM) mode in
combination with energy dispersive X-ray spectroscopy using an
analyzer system (EDAX) in the same microscope.
Magnetic measurements were preformed on a JDM-13 vibrat-
ing sample magnetometer. M/H measurements were made with
applied fields up to 5000 Oe at room temperature, and the
complete hysteresis loops were recorded. The magnetic size Dmag
can be estimated by the following equation [22,23]:
2. Experimental
2.1. Sample preparation
Nickel oxide was prepared by calcining nickel hydroxide in air
at 300 8C for 2 h. The nickel hydroxide was obtained by
precipitation of nickel acetylacetonate dissolved in ethylene glycol
with sodium carbonate aqueous solution at 120 8C [19–21]. The
NiO samples were reduced with hydrogen at 350 8C for 3 h and
then silicified from 250 to 450 8C by 10 vol.% SiH4/H2 [18]. The
detailed experimental steps and conditions are shown in Scheme 1.
The as-prepared nickel silicide samples were labelled as T-NiSix,
where T is the silicification temperature.
ꢀ
ꢁ
1=3
18kBTðdM=dHÞ
prMs2
Dmag
¼
Scheme 1. Synthesis of nickel silicides by using nickel hydroxide as a precursor.

X. Chen et al. / Materials Research Bulletin 47 (2012) 867–877
869
100
where kB is the Boltzmann constant, dM/dH is the initial slope near
0.0000
-0.0001
-0.0002
-0.0003
-0.0004
-0.0005
-0.0006
zero field, Ms is the saturation magnetization, and
density of the sample.
r is the bulk
DTG
90
2.3. Measurement of electrochemical properties
Ni(OH)2-Air
The electrochemical performance was evaluated using an
electrochemical working station (CHI660C, China) by a three-
electrode system consisting of a working electrode, a platinum
counter electrode and a Hg/HgO reference electrode. All electro-
chemical experiments were carried out at 25 8C in 6 M KOH
aqueous solution [24]. Electrodes were fabricated by mixing the
as-made T-NiSix (about 50 mg), graphite, and polytetrafluoroethy-
lene in a weight ratio of 85:10:5. The mixture with a small amount
of ethanol was rolled into a thin sheet with a uniform thickness
range from 0.2 to 0.4 mm and cut into round disks of 10 mm in
diameter. The electrode sheet was pressed between two-nickel
foams that function as current collectors. Before the electrochemi-
cal measurements, the electrodes were soaked in electrolyte and
evacuated for 24 h to ensure that the active material was contacted
with the electrolyte completely and thoroughly. Cyclic voltammo-
grams (CV) were recorded from ꢀ1.0 V to ꢀ0.2 V at different
scanning rates. The electrochemical impedance spectroscopy (EIS)
experiments were performed with a bias potential of ꢀ0.05 V vs.
Hg/HgO, amplitude of 5 mV, and a frequency range from 100 kHz
to 10 mHz.
80
TG
70
100
200
300
400
500
600
700
800
Temperature (oC)
Fig. 2. TG-DTG profile of the as-prepared Ni(OH)2 precursor.
concluded that Ni(OH)2 is formed, which some ions may
intercalate into its interlayer, as indicated in Fig. 4A.
To obtain a good understanding of the as-prepared Ni(OH)2
combustion process in the synthesis of nickel oxide, the precursor
was characterized for NiO by using TG/DTG. Fig. 2 illustrates the
thermal decomposition of the nickel hydroxide sample. The mass
loss at about 100 8C is attributed to the elimination of molecular
water. The single mass loss at 300 8C represents the simultaneous
dehydroxylation and decomposition/combustion of carbonate and
acetate anions, releasing water and carbon dioxide [26]. At the
same time, the decomposition of nickel hydroxide occurs via
Ni(OH)2 ! NiO + H2O. No further mass loss was detected above
400 8C, since the precursors have already been converted to nickel
oxides.
3. Results and discussion
3.1. High surface area NiO
Fig. 1 shows the typical FT-IR spectra of the Ni(OH)2 precursor.
The narrow band at 3645 cmꢀ1 and the broad band at about
3436 cmꢀ1 represent the free hydroxyl group and the water
molecules, respectively. The peak around 1629 cmꢀ1 is due to the
bending vibration of the interlayer water molecules, which are
typical for turbostratic hydroxide [21]. The band at around
1369 cmꢀ1 is assigned to the stretching vibration of carbonate
The X-ray diffraction patterns of the as-prepared nickel
hydroxide and nickel oxide are shown in curves B and C of
Fig. 3, respectively. Comparing the as-prepared Ni(OH)2 with the
standard
b-Ni(OH)2 (JCPDS card No. 14-0117), their patterns are
consistent each other. However, The (0 0 1) peak at about 198
appears broad and shift to a low scattering angle, indicating that
some species, such as H2O, EG, CO32ꢀ, and acetylacetonate, have
been incorporated within the galleries between Ni(OH)2 slabs in a
random manner. The conclusion is in good accordance with the
observation from the FT-IR of Ni(OH)2 [27]. By contrast, the (1 0 0)
anion (i.e.,
n
(CO32ꢀ)). The intense peaks at 1048 and 1002 cmꢀ1
correspond to stretching vibrations of (C–C–O) in ethylene glycol.
However, the possible presence of acetylacetonate cannot be ruled
out, because the characteristic bands of acetylacetonate also
appear in the 1600–1300 cmꢀ1 range, and would be covered by the
other bands that are present. At low wavenumbers, two bands
appearing at 618 and 466 cmꢀ1 are assigned to the
d OH and n (Ni–
O) vibrations, respectively [25]. From the results of FT-IR, it is
200
111
220
311
222
D
100
Ni(OH)2
NiO 47-1049
80
60
145 m2g-1
C
B
7.16 nm
300-NiO
Ni(OH)2
2- C-C-O
CO3
40
001
δ H2O
101
40
EG
100
102
201
103
110
202
111
A
20
Ni(OH)2 14-0117
ν H2O
ν OH
δ OH
10
20
30
50
60
70 80 90
ν Ni-O
0
2Theta (deg.)
4000 3500 3000 2500 2000 1500 1000
500
Wavenumbers (cm-1)
Fig. 3. XRD patterns of (A) the standard Ni(OH)2 (JCPDS card No. 14-0117), (B) the
synthesized Ni(OH)2, (C) the synthesized NiO, and (D) the standard NiO (JCPDS card
No. 47-1049).
Fig. 1. FT-IR spectrum of the as-prepared Ni(OH)2.

870
X. Chen et al. / Materials Research Bulletin 47 (2012) 867–877
Fig. 4. Schematic illustration of (A) the morphology of the as-prepared Ni(OH)2 and SEM images (B and C) of the synthesized 300-NiO.
and (1 1 0) peaks are sharp, while the (10 L) series of peaks are
broad. These features are consistent with the presence of stacking
faults in some layers, with an expanded layer spacing for structural
stability. The corresponding XRD pattern of the thermally
decomposed product from the as-prepared Ni(OH)2 is shown as
part C in Fig. 3. It can be observed that all the diffraction peaks in
this pattern are consistent with the pure cubic phase of NiO (JCPDS
card No. 47-1049), indicating the complete transformation of
Ni(OH)2 to NiO at 300 8C after 2 h. Moreover, the estimated average
size of the particles by using the Scherrer equation is 7.2 nm. The
specific surface area calculated from the linear part of the BET plot
is 145 m2 gꢀ1, which is larger than the previously reported surface
area of NiO prepared by a carbon template method [28].
The SEM images of the nickel oxide presented in Fig. 4B and C
demonstrate the typical morphology of platelet-like shapes with
an unordered crystallite aggregation. These observations are quite
promising for nickel oxide materials, since the previously reported
nickel oxides often showed spherical shapes and relatively weak
thermal stability. It is also interesting that the shapes of as-
prepared NiO are closely tied to the layering in nickel hydroxide,
which provides numerous porous channels and affords high
surface areas [29].
43-0989). The diffraction peaks are strong and sharp, indicating
the high degree of crystallinity. For comparison, reduced Ni
powder displays an XRD pattern characteristic of fcc nickel (space
group Fm3m (2 2 5), the lattice constant a = 0.3524 nm, JCPDS
Card No. 04-0850). However, the sample Ni powder exposed to air
is easily oxidized to NiO. Most important, the Ni peaks almost
disappeared in the XRD patterns of T-NiSix samples, indicating
that silicon atoms from silane have infiltrated the metal lattice to
form the nickel silicides. The average particle sizes of 250-NiSix,
350-NiSix, 450-NiSix, and Ni are about 13.7, 18.5, 21.3, 15.5 nm,
respectively, calculated using the Scherrer formula. While the
nickel silicides prepared at lower than 350 8C showed poor
crystallinity in general, they showed well-defined crystallinity
when heated above 400 8C. Based on the XRD results, the
formation of nickel silicides involves the following sequence
with increasing silicification temperatures: Ni2Si (orthorhom-
bic) ! NiSi (orthorhombic) ! NiSi2 (cubic).
In order to investigate the structure and composition distribu-
tion in the materials, TEM and STEM were carried out. The typical
TEM images of a series of nickel silicides with 250, 350 and 450 8C
silicification temperatures are shown in Fig. 6. It can be seen that
the T-NiSix nanocrystals are uniformly dispersed and weakly
aggregated to some degree. The particle sizes increased with
increasing silicification temperature as shown in Fig. 6A, C, and D.
3.2. Synthesis of nickel silicides
XRD patterns of nickel silicides prepared using different
silicification temperatures are shown in Fig. 5. For comparison,
the XRD pattern of Ni powder and the standard XRD patterns of
face centred cubic (fcc) nickel, Ni2Si, NiSi, and NiSi2 are also shown
in Fig. 5. XRD patterns of the samples indicate that all the
diffraction peaks can be ascribed to nickel silicides. It was reported
that SiH4 decomposed and hydrogen was largely desorbed by
heating above 200 8C on the surface of nickel nanoparticles, and
silicon dissolved into the bulk nickel particles if the temperature
was above 227 8C [30]. Accordingly, XRD patterns of 250-NiSix
standard NiSi2
450oC
400oC
standard NiSi
350oC
300oC
exhibit (1 2 1), (0 0 2), (2 1 1), and (3 1 0) planes at 2u = 45.5, 48.8,
39.5, and 42.4, respectively, indicating formation of Ni2Si phase
(space group Pbnm (62), the lattice constant a = 0.7066, b = 0.5009,
c = 0.3732 nm, JCPDS Card No. 48-1339). The (2 1 1), (1 1 2), (0 1 1),
standard Ni2Si
250oC
200oC
Ni
(2 0 0), and (1 0 2) planes appear at 2u = 47.3, 45.8, 31.7, 34.2, and
36.0 respectively, indicating the presence of NiSi phase (space
group Pnma (62), the lattice constant a = 0.5233, b = 0.3258,
c = 0.5659 nm, JCPDS Card No. 38-0844).
standard Ni
10
20
30
40
50
60
70
80
90
The Ni2Si phase is a transient phase that readily converts into
the NiSi phase [31,32]. Indeed, in the 350-NiSix sample, the
proportion of NiSi increases slightly, and Ni2Si is almost
negligible. The pattern of the 450-NiSix sample indicates that
all the diffraction peaks can be ascribed to NiSi2 (space group
Fm3m (2 2 5), the lattice constant a = 0.5416 nm, JCPDS Card No.
2Theta (deg.)
Fig. 5. XRD patterns for nanoparticles of metallic Ni that have been partially
converted to NiO before being exposed to air and T-NiSix at silicification
temperatures of 200, 250, 300, 350, and 450 8C, respectively. The JCPDS card
files for Ni, Ni2Si, NiSi, and NiSi2 as references are also shown in the patterns.

X. Chen et al. / Materials Research Bulletin 47 (2012) 867–877
871
Fig. 6. Typical low-magnification TEM and HRTEM images of the T-NiSix samples with (A and B) 250 8C, (C and D) 350 8C, and (E and F) 450 8C silicification temperatures.
Fig. 6B shows the HRTEM image of 250-NiSix sample. The measured
d value is about 0.3290 nm, which is identified as the (1 0 1) plane
of the Ni2Si phase (d = 0.3300 nm) [33]. The crystalline phase was
revealed to be the orthorhombic Ni2Si by the fast Fourier transform
(FFT) (inset of Fig. 6B). The HRTEM image of 350-NiSix revealed its
crystalline nature, with a lattice spacings of 0.1997 nm and
0.2027 nm (Fig. 6D), which can be indexed to (1 1 2) and (2 1 0)
crystalline planes of the orthorhombic phase of NiSi [34]. The
crystalline phase was further confirmed by the FFT (inset of
Fig. 6D). Fig. 6F shows the HRTEM image of 450-NiSix sample. The d
values measured from the image are 0.1978 nm, which is readily
identified as NiSi2 (d = 0.1916 nm, 220) [35]. The crystalline phase
was also revealed to be the cubic NiSi2 by the FFT (inset of Fig. 6F).
Overall, from the analysis of HRTEM, it was found that the phase
transition of nickel silicides was in good agreement with XRD
results.
To further confirm the phase composition, a detailed HRTEM
analysis of the 350-NiSix sample is shown in Figs. 7 and 8. The
HRTEM images of an individual nickel silicide particle (Figs. 7B, D,
and 8B) show that it has a single crystalline structure with highly
ordered, continuous fringe patterns. From Fig. 7C, the measured
lattice spacings observed by HRTEM are 0.204 and 0.195 nm with
an angular relationship of 58.398, which corresponds well,
respectively, to the (2 2 0) and (1 2 1) lattice spacing of the

872
X. Chen et al. / Materials Research Bulletin 47 (2012) 867–877
Fig. 7. TEM (A) and HRTEM (B–E) images of 350-NiSix.
orthorhombic Ni2Si structure [13,36]. The FFT of the lattice-
resolved image (inset of Fig. 7C) shows the reciprocal lattice peaks,
which can be indexed to a primitive orthorhombic lattice. The
waviness of the HRTEM images shown in Fig. 7C indicates that the
surface of the sample is composed of a large concentration of Ni2Si
crystal particles with different angles. From Fig. 7E, the measured
interplanar spacings for the lattice fringes are 0.197 and 0.191 nm,
respectively, which correspond to the (1 1 2) and (2 1 1) lattice
planes of orthorhombic NiSi (Pnma 62) [34]. The FFT pattern (inset)
of the HRTEM image further indicates that the NiSi is a single
crystal. From Fig. 8C, the measured interplanar spacing for the
lattice fringes is 0.320 nm, which corresponds to the (1 1 1) lattice
plane of cubic NiSi2 (Fm3m) [16]. In summary, the 350-NiSix
sample is a mixture of phases including Ni2Si, NiSi, and NiSi2,
which is in harmony with the acquired XRD pattern.
Fig. 9. The existence of O was due to the formation of passivated
layers on NiSix particles, which protects the sample from violent
oxidation when it was exposed in air. The result was similar to
previous elemental maps of silicide-modified nickel [28]. Four
regions of the NiSix nanoparticles in Fig. 9 were analyzed by EDX.
The result shows the peaks from both Ni and Si and thus indicates
the presence of both elements in the NiSix particle. The elemental
atomic ratio between Ni (K) and Si (K) at different regions is about
1.5:1, indicating the 350-NiSix sample is mainly consisted of the
mixture Ni2Si and NiSi crystal phases. Unfortunately, the peak from
O (K) is also found in the EDX, further indicating that SiOx
overlayers are present on the passivated 350-NiSix sample.
Prior research efforts do not identify the contribution of
saturation magnetization values from nickel silicides in silicide-
modified nickel materials [28]. However, this property can be
measured and at least partially reflects the structure and physical
properties of the nickel silicides. Fig. 10 shows the magnetization
Homogeneous distributions of Ni, Si, and O in the 350-NiSix
sample were confirmed by STEM-EDX elemental maps, as shown in

X. Chen et al. / Materials Research Bulletin 47 (2012) 867–877
873
Fig. 8. TEM (A) and HRTEM (B and C) images of 350-NiSix from a certain region.
curves of the as prepared nickel nanoparticles, and the series of T-
NiSix samples silicified at 250, 350, and 450 8C, respectively. The
data (Table 1) imply a ferromagnetic-like behaviour at room
temperature, indicated by the presence of a remnant magnetiza-
tion (Mr), a certain coercivity (Hc), and the saturation of the
magnetization curve in rather small external magnetic fields. The
variety of the coercivity is attributed to the modification by silicon
atoms and the phase transformations of NiSix. The saturation
magnetization values at 5000 Oe drastically decreased as Si was
added to Ni. With increasing silicification temperatures (i.e., as
Ni ! Ni2Si ! NiSi ! NiSi2), the saturation magnetization values
first decreased, before eventually increasing. This phenomenon
may be ascribed to the electronic structure of Ni2Si and the crystal
structure of NiSi2 [37,38].
The information of the magnetic particle sizes (as shown in
Table 1) can also be deduced from the magnetization studies by
means of the magnetic size equation. The estimated particle sizes
from the magnetic properties are about 5.9, 6.0, 9.9, and 9.4 nm for
the sample of Ni, 250-NiSix, 350-NiSix, and 450-NiSix, respectively.
The magnetic sizes increase with increasing silicification temper-
ature, which is in agreement with the XRD analyses. These results
indicate that the particles aggregated to some degree, perhaps
because the silicification is an exothermic reaction [40].
potential region of ꢀ1.0 to ꢀ0.2 V vs. Hg/HgO. In the case of NiSix,
the CV profiles retain a distorted rectangular shape and do not
significantly change with the increasing scan rates from 5 to
30 mV sꢀ1
, indicating that the Faraday redox reactions are
electrochemically reversible [41]. From Fig. 11A–C, the anodic
peaks are at about ꢀ0.80, ꢀ0.85, and ꢀ0.90 V, respectively, over
250-NiSix, 350-NiSix, and 450-NiSix samples, which can be assigned
to the hydrogen extraction reaction: T-NiSix–Habs/ads + OHꢀ = T-
NiSix + H2O + eꢀ. Interestingly, this reaction is also observed on Pt/
Pd catalyst and Pd82ꢀxNiySi18 alloys (y = 0–32), which is due to the
oxidation of the absorbed and adsorbed hydrogen [42–44]. The
dissolved silicon atoms expand the crystal lattice of nickel atoms,
which induces a constriction of the d band and an increase of the
electronic density of states near the Fermi level, leading to a
similarity of the catalytic activity with noble metals. The
corresponding cathodic peaks are at ꢀ0.30, ꢀ0.35, and ꢀ0.30 V,
respectively, which can be assigned to the reduction of NiO or
NiOOH. The anode peak has been reported at ꢀ0.2 V for the Ni
oxidation: Ni–OH + Ni–OH = Ni–O + H2O [43,45]. The anodic peak
potentials shifted in the positive direction and the cathodic peak
potentials shifted in the negative direction at higher scan rate. This
resulted in an increase of the peak separation between the anodic
and cathodic peaks.
To further explain the electronic properties of nickel silicides,
CV curves of the series of T-NiSix samples at scan rates 5 mV sꢀ1
are shown in Fig. 11D. With the increasing silicification
temperature, the electrochemical surface areas for T-NiSix were
increased. The anodic peak potentials shifted in the negative
3.3. Electrochemical properties of nickel silicides
Fig. 11 shows cyclic voltammograms of the nickel silicides in
6 M KOH aqueous solution at different scan rates over the main
Table 1
Magnetic parameters at room temperature for the synthesized Ni and T-NiSix.
Samples
Hc (Oe)
Ms (emu/g)
Mr (emu/g)
ra (g/cm3)
Dmag (nm)
250-NiSix
350-NiSix
450-NiSix
Ni
151
124
150
140
19.71
9.37
4.98
1.94
5.00
5.48
6.09
4.11
2.53
9.17
6.0
9.9
9.4
5.9
18.95
23.19
a
Ni atomic density [39].

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X. Chen et al. / Materials Research Bulletin 47 (2012) 867–877
Fig. 9. STEM X-ray energy-dispersive spectrometry (EDS) high resolution mapping for Ni, Si, and O in the sample of 350-NiSix and TEM-EDX patterns of stabilized sample in
different regions.
direction and the cathodic peak potentials shifted in the positive
direction. This suggests that the phases transform from Ni2Si to
NiSi, and then change to NiSi2. In order to compare the function of
T-NiSix and Ni in the electrocatalytic process, the CV behaviour of
Ni in KOH solution was also monitored and the obtained curve is
shown in Fig. 11D (curve Ni). It is clear from the comparison that T-
NiSix were responsible for the performance of electrocatalytic
properties.
Nickel silicides consisting of Si and Ni have a very high melting
point and excellent electrochemical properties. The resistivity of

X. Chen et al. / Materials Research Bulletin 47 (2012) 867–877
875
exhibit noble metal-like behaviour. Thus, the resistance value of
the total cell is reduced because of the lower electric resistivity.
The Nyquist plot (Fig. 12A) is composed of a small semi-circle in
the high-frequency range (as clearly shown in Fig. 12B) attributed
to the charge-transfer reaction at the electrolyte/electrode
interface. In addition, the sloping line in the low-frequency range
corresponds to the Warburg impedance within the electrode
attributed to the hydrogen desorption in the bulk of the electrode.
The EIS spectra of T-NiSix samples have significant difference from
that of metallic Ni (Fig. 12C), indicating that the formation of nickel
silicides have remarkably decreased electric resistivity and can
greatly influenced the electrocatalytic activity. A similar phenom-
enon has been observed for surface-nitrided nickel foil [48].
Depending upon the shape of the EIS spectrum of T-NiSix, an
equivalent circuit of Rs(Q(RctZw)) mode assumed and output by the
ZsimpWin software is shown in Fig. 12D, where Rs is the solution
resistance, CPE is constant phase element, Rct is charge-transfer
resistance, and Zw is Nernst diffusion impedance. The EIS
parameters of Ni and T-NiSix are listed in Table 2. The Rs of all
of electrodes is low, as would be expected for measurements under
identical conditions of electrolyte concentration and temperature
[49]. Obviously, the surface charge-transfer resistance (Rct) of the
T-NiSix electrode is much lower than that of the metallic Ni
electrode, indicating that nickel silicide provides an outstanding
electrocatalytic activity and results in the increased conversion
efficiency.
Fig. 10. Hysteresis M/H loops at room temperature corresponding to Ni and T-NiSix
with 250, 350 and 450 8C silicification temperature.
Ni2Si, NiSi, and NiSi2 thin films are 24, 14, and 35 mV cm [46],
respectively, which has a general resemblance to 18.4 mV cm for
the metallic Ni film but a much lower electric resistivity than
1 ꢁ 105 mV cm for silicon [47], indicating that the nickel silicides
1.0
0.8
250-NiSix
350-NiSix
0.6
A
0.5
B
0.4
0.2
0.0
0.0
-0.5
-0.2
-0.4
-1
-1
30 mV s
-1.0
-1.5
30 mV s
-0.6
-0.8
-1.0
-1
-1
20 mV s
20 mV s
-1
-1
10 mV s
10 mV s
-1
-1
5 mV s
5 mV s
-1.0
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-1.0
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
0.6
0.3
1.5
1.0
450-NiSix
C
D
0.5
0.0
0.0
-0.3
-0.6
-0.9
-1.2
scan rate: 5 mV s-1
-0.5
-1.0
-1.5
-2.0
-2.5
-1
250-NiSix
350-NiSix
450-NiSix
Ni
30 mV s
-1
20 mV s
-1
10 mV s
-1
5 mV s
-1.0
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3 -0.2
-1.0
-0.8
-0.6
-0.4
-0.2
Potential ( V )
Potential ( V )
Fig. 11. Cyclic voltammetry (CV) curves of nickel silicides at different scan rates within a potential window of ꢀ1.0 to ꢀ0.2 V vs. Hg/HgO. (A) 250-NiSix; (B) 350-NiSix; (C) 450-
NiSix; (D) the series of T-NiSix samples and Ni at a scan rate 5 mV sꢀ1
.

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X. Chen et al. / Materials Research Bulletin 47 (2012) 867–877
Fig. 12. The AC impedance spectra of (A) T-NiSix electrode materials and (C) the Ni electrode material; (B) the enlargement of (A) at high frequency region; (D) the equivalent
circuit of EIS.
Table 2
Simulated values of the equivalent circuit elements for the synthesized Ni and T-NiSix electrodes.
Samples
Rs
Q
n
RCT
W
(
V
cm2)
(S-secn/cm2)
(
V
cm2)
(S-sec5/cm2)
250-NiSix
350-NiSix
450-NiSix
Ni
1.64
0.89
0.65
0.55
0.15
0.08
0.08
0.002
0.84
0.81
0.80
0.88
400.40
3.20 ꢁ 1012
0.1906
168.70
569.00
1.60 ꢁ 104
6.21 ꢁ 1011
3.64 ꢁ 109
4. Conclusions
Acknowledgments
In summary, nickel silicide nanoparticles were successfully
synthesized by reduction and silicification of high-surface-area
nickel oxide at relatively low temperature and atmospheric
pressure. As-prepared nickel silicides exhibited phase transition
with increased silicification temperatures, and were ferromagnetic
at room temperature, with saturation magnetization values
drastically decreased when Si was present. The nickel silicides
exhibited electrochemical behaviour that greatly resembled those
of noble metals, because of a constriction of the nickel d band and
the increase of electronic density of states. Overall the results
indicate that the nickel silicides have the potential to be
inexpensive yet efficient materials for applications as catalysts
and electrocatalysts for a variety of applications.
We gratefully acknowledge the financial support provided by
the National Natural Science Foundation of China (No. 20973029),
and the Program for New Century Excellent Talents in Universities
of China (No. NCET-07-0133). C.T.W. thanks Dalian University of
Technology for a ‘‘Seasky’’ professorship.
References
[1] H. Thomas, I.I. Maugh, Science 225 (1984) 403.
[2] A.L. Schmitt, J.M. Higgins, J.R. Szczech, S. Jin, J. Mater. Chem. 20 (2010) 223–235.
[3] L.J. Chen, Silicide Technology for Integrated Circuits, The Institution of Electrical
Engineers, London, UK, 2004.
[4] C.M. Hwang, C.H. Lim, J.H. Yang, J.W. Park, J. Power Sources 194 (2009) 1061–
1067.

X. Chen et al. / Materials Research Bulletin 47 (2012) 867–877
877
[5] J.M. Higgins, A.L. Schmitt, I.A. Guzei, S. Jin, J. Am. Chem. Soc. 130 (2008)
[25] X.F. Song, L. Gao, J. Phys. Chem. C 112 (2008) 15299–15305.
[26] T.N. Ramesh, J. Phys. Chem. B 113 (2009) 13014–13017.
16086–21094.
[6] J. Wolfenstine, J. Power Sources 124 (2003) 241–245.
[7] K. Seo, H. Yoon, S.W. Ryu, S. Lee, Y. Jo, M.H. Jung, J. Kim, Y.K. Choi, B. Kim, ACS Nano
4 (2010) 2569–2576.
[8] K. Seo, K.S.K. Varadwaj, P. Mohanty, S. Lee, Y. Jo, M.H. Jung, J. Kim, B. Kim, Nano
Lett. 7 (2007) 1240–1245.
[9] N. Dahal, V. Chikan, Chem. Mater. 22 (2010) 2892–2897.
[10] R. Tschan, R. Wandeler, M.S. Schneider, M. Burgener, M.M. Schubert, A. Baiker,
Appl. Catal. A 223 (2002) 173–185.
[11] C.H. Liang, A.Q. Zhao, X.F. Zhang, Z.Q. Ma, R. Prins, Chem. Commun. (2009)
2047–2049.
[27] F. Zhou, X.M. Zhao, A. van Bommel, A.W. Rowe, J.R. Dahn, Chem. Mater. 22 (2010)
1015–1021.
[28] X. Chen, A.Q. Zhao, Z.F. Shao, C. Li, C.T. Williams, C.H. Liang, J. Phys. Chem. C 114
(2010) 16525–16533.
[29] D.B. Kuang, B.X. Lei, Y.P. Pan, X.Y. Yu, C.Y. Su, J. Phys. Chem. C 113 (2009)
5508–5513.
[30] Y. Wang, S.J. Sibener, J. Phys. Chem. B 106 (2002) 12856–12859.
[31] F.F. Zhao, J.Z. Zheng, Z.X. Shen, T. Osipowicz, W.Z. Gao, L.H. Chan, Microelectron.
Eng. 71 (2004) 104–111.
[32] D. Mangelinck, K. Hoummada, Appl. Phys. Lett. 92 (2008) 254101.
[33] A. Alberti, C. Bongiorno, P. Alippi, A.L. Magna, C. Spinella, E. Rimini, Acta Crystal-
logr. B 62 (2006) 729–736.
[12] A.Q. Zhao, X.F. Zhang, X. Chen, J.C. Guan, C.H. Liang, J. Phys. Chem. C 114 (2010)
3962–3967.
[13] Y.P. Song, A.L. Schmitt, S. Jin, Nano Lett. 7 (2007) 965–969.
[14] Y. Wu, J. Xiang, C. Yang, W. Lu, C.M. Lieber, Nature 430 (2004) 61–65.
[15] G.X. Wang, L. Sun, D.H. Bradhurst, S. Zhong, S.X. Dou, H.K. Liu, J. Alloys Compd. 306
(2000) 249–252.
[16] C.Y. Lee, M.P. Lu, K.F. Liao, W.F. Lee, C.T. Huang, S.Y. Chen, L.J. Chen, J. Phys. Chem. C
113 (2009) 2286–2289.
[17] R.G. Nuzzo, L.H. Dubois, N.E. Bowles, M.A. Trecoske, J. Catal. 85 (1984) 267–271.
[18] X. Chen, A.Q. Zhao, Z.F. Shao, Z.Q. Ma, C.H. Liang, Stud. Surf. Sci. Catal. 175 (2010)
77–84.
[19] X.W. Xie, Y. Li, Z.Q. Liu, M. Haruta, W.J. Shen, Nature 458 (2009) 746–749.
[20] Y. Li, B. Zhang, X.W. Xie, J. Liu, Y. Xu, W.J. Shen, J. Catal. 238 (2006) 412–424.
[21] Y. Li, X.W. Xie, J.L. Liu, M. Cai, J. Rogers, W.J. Shen, Chem. Eng. J. 136 (2008)
398–408.
[34] C.J. Kim, K. Kang, Y.S. Woo, K.G. Ryu, H. Moon, J.M. Kim, D.S. Zang, M.H. Jo, Adv.
Mater. 19 (2007) 3637–3642.
[35] N.J. Ikarashi, Appl. Phys. 107 (2010) 033505.
[36] X.Q. Yan, H.J. Yuan, J.X. Wang, D.F. Liu, Z.P. Zhou, Y. Gao, L. Song, L.F. Liu, W.Y. Zhou,
G. Wang, S.S. Xie, Appl. Phys. A 79 (2004) 1853–1856.
[37] I. Jarrige, N. Capron, P. Jonnard, Phys. Rev. B: Condens. Matter 79 (2009) 035117.
[38] R.G. Nuzzo, L.H. Dubois, Appl. Surf. Sci. 19 (1984) 407–413.
[39] T.R. Hogness, T.L. Wilson, W.C. Johnson, J. Am. Chem. Soc. 58 (1936) 108–112.
[40] A. Franciosi, J.H. Weaver, Phys. Rev. B: Condens. Matter 26 (1982) 546–553.
[41] H. Jiang, T. Zhao, C.Y. Yan, J. Ma, C.Z. Li, Nanoscale 2 (2010) 2195–2198.
[42] Z.X. Liang, T.S. Zhao, J.B. Xu, L.D. Zhu, Electrochim. Acta 54 (2009) 2203–2208.
´
´
[43] M. Grden, A. Czerwinski, J. Solid State Electrochem. 12 (2008) 375–385.
[44] J.W. Lee, S.I. Pyun, S. Filipek, Electrochim. Acta 48 (2003) 1603–1611.
[45] Y.L. Lo, B.J. Hwang, Langmuir 14 (1998) 944–950.
[22] A.A. El-Gendy, V.O. Khavrus, S. Hampel, A. Leonhardt, B. Bchner, R. Klingeler, J.
Phys. Chem. C 114 (2010) 10745–10749.
[46] E.G. Colgan, Thin Solid Films 279 (1996) 193–198.
[23] G. Glaspell, V. Abdelsayed, K.M. Saoud, M.S. El-Shall, Pure Appl. Chem. 78 (2006)
1667–1689.
[47] , D.R. Lide (Ed.), 81st edition, CRC Handbook of Chemistry and Physics, CRC Press,
Boca Raton, 2000, pp. 12–46.
[24] D.W. Wang, F. Li, M. Liu, G.Q. Lu, H.M. Cheng, Angew. Chem. Int. Ed. 47 (2008)
373–376.
[48] Q.W. Jiang, G.R. Li, S. Liu, X.P. Gao, J. Phys. Chem. C 114 (2010) 13397–13401.
[49] C.Z. Li, Y.L. Liu, J.H.T. Luong, Anal. Chem. 77 (2005) 478–485.