An array of leaf-like Co3Ni microstructures with ferromagnetic properties, superhydrophobic properties and high catalytic performance in the hydrolysis of ammonia borane

Article abstract of DOI:10.1039/c5ra22621j

In this work, a Cu foil supported array of leaf-like Co₃Ni microstructures composed of well aligned nanorods was synthesized by a facile electroless process. It was found that the as-prepared Co₃Ni array exhibited ferromagnetic properties with enhanced coercivity. The static contact angle of water on the surface of the Co₃Ni array could reach 154.7°, indicating that the Co₃Ni array displayed superhydrophobic properties. In addition, the as-prepared Co₃Ni array showed high catalytic activity in the hydrolysis of ammonia borane for hydrogen generation. More importantly, the Co₃Ni array catalyst still retained ca. 91% of its original catalytic activity after 6 cycles, exhibiting significantly improved recyclability and reusability in contrast to many nanocatalysts reported in the literature. The high catalytic performance, together with its ferromagnetic and superhydrophobic properties, makes the Co₃Ni array a multifunctional material with wide applications in different areas.

Full text of DOI:10.1039/c5ra22621j

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An array of leaf-like Co3Ni microstructures with
ferromagnetic properties, superhydrophobic
properties and high catalytic performance in the
hydrolysis of ammonia borane
Cite this: RSC Adv., 2015, 5, 105307
a
b
b
Tao Zeng, Jinyun Liao, Hao Li, Kejun Fengb and Liling Lic
*
In this work, a Cu foil supported array of leaf-like Co3Ni microstructures composed of well aligned nanorods
was synthesized by a facile electroless process. It was found that the as-prepared Co3Ni array exhibited
ferromagnetic properties with enhanced coercivity. The static contact angle of water on the surface of
the Co3Ni array could reach 154.7ꢀ, indicating that the Co3Ni array displayed superhydrophobic
properties. In addition, the as-prepared Co3Ni array showed high catalytic activity in the hydrolysis of
ammonia borane for hydrogen generation. More importantly, the Co3Ni array catalyst still retained ca.
91% of its original catalytic activity after 6 cycles, exhibiting significantly improved recyclability and
reusability in contrast to many nanocatalysts reported in the literature. The high catalytic performance,
together with its ferromagnetic and superhydrophobic properties, makes the Co3Ni array
a multifunctional material with wide applications in different areas.
Received 28th October 2015
Accepted 30th November 2015
DOI: 10.1039/c5ra22621j
www.rsc.org/advances
these catalysts are usually unsupported nanoparticles or nano-
particles supported on powdery substrate, such as activated
carbon, Al2O3, zeolite.11–13 Obviously, those catalysts in the form
Introduction
The worldwide energy consumption is sharply increasing and
the current energy system based on fossil fuels is considered to
be a major cause for global climate changes and environmental
pollution. So, the development of sustainable, environment-
friendly and cost-effective energy sources is undoubtedly the
most important task in the 21st century. Owing to its high
energy density, zero emission and as an abundant source,
hydrogen is regarded as an ideal next-generation fuel.1,2 Many
techniques have been developed to produce and store hydrogen
in the past few decades. In particular, generation of hydrogen
via catalytic hydrolysis of chemical hydrides, such as NaBH4,
N2H4 and NH3BH3 (ammonia borane, AB), has been extensively
investigated in view of its high efficiency, convenience and
safety.3–5 Among these chemical hydrides, AB has the highest
hydrogen storage capacity (19.6 wt%), which renders it more
suitable for the application in the production/storage of
hydrogen. In the catalytic hydrolysis of AB, catalyst is a key
factor which will signicantly affect the efficiency of hydrogen
generation. In addition to noble metal catalysts, some low-cost
catalysts, such as Co,6,7 Co–B,8 Co–Mo–B–P,9 Ni,10 also show
relatively high catalytic activity in the AB hydrolysis. However,
of ne powder are prone to aggregate under the drive of high
surface energy, which will reduce their active surface area and
thus lower their catalytic activity.14 Additionally, the tedious and
costly separation and recovery procedure of the powdery
nanocatalysts is another challenge when these catalysts are in
industry-scale application. So, from a technological and opera-
tional point of view, it is highly expected to design low-cost
catalysts with high stability, good recyclability, as well as high
catalytic activity. On the other hand, hierarchical nano/
microstructures have attracted increasing attention in recent
years because of their excellent performance, enabled by their
unique geometry and efficient charge transfer.15,16 Generally
speaking, hierarchical nano/microstructures are composed of
numerous nanosized subunits, which are arranged in a specic
manner. In the eld of catalysis, it has been well veried in
literature that the hierarchical nanostructures/microstructures
exhibit high catalytic performance in many cases due to their
special architectures.17,18 However, hydrolysis of AB for
hydrogen generation catalyzed by hierarchical nanostructures/
microstructures has been seldom concerned.
In this work, an array of hierarchical leaf-like Co3Ni micro-
structures supported on the Cu substrate was successfully
prepared by a facile electroless approach. Compared with the
traditional electro-deposition method for fabricating dendritic
arrays of metals/alloys of iron group,19,20 the synthetic process
presented in this work is simple and cost-effective. The catalytic
aCollege of Environmental and Chemical Engineering, Shanghai University of Electric
Power, Shanghai 200090, China
bDepartment of Chemical Engineering, Huizhou University, Huizhou, 516007, China.
E-mail: lihao180@126.com; Fax: +86-752-2527229; Tel: +86-752-2527229
cDepartment of Pharmacy, Huizhou Health Sciences Polytechnic, Huizhou, 516007
China
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performance of the as-prepared Co3Ni array in the hydrolysis of determining the accumulative volume of hydrogen generated
AB is tested. As far as we know, an alloy of Co and Ni with during AB hydrolysis reaction in a glass reactor connected with
similar microstructures in the form of an array has not been a gas burette. In a typical procedure, 20 mL of freshly-prepared
previously report. Meanwhile, this is the rst report on the AB AB solution (0.2 M) was added in the reactor, followed by the
hydrolysis for hydrogen generation catalyzed by hierarchical addition of the Co3Ni array supported on of Cu foil (5 cm ꢁ
leaf-like Co3Ni microstructures. Additionally, owing to its 3 cm). The reaction temperature was xed at 308 ꢂ 0.5 K in
special microstructures, the as-prepared Co3Ni array exhibits a thermostated reactor. The reusability of the Co3Ni array was
both ferromagnetic property and superhydrophobic property, evaluated by repeating the hydrolysis reaction 6 times in such
making it a multifunctional material with wide applications in a model: (1) recollecting the catalyst aer reaction; (2) washing
different elds.
the catalyst with water and ethanol alternatively before drying at
50 ꢀC in vacuum oven; (3) starting a new run of AB hydrolysis by
mixing the catalyst with freshly-prepared AB solution.
Experimental section
Materials and synthesis
All reagents were of analytic grade, and double-distilled water
was used throughout the experiments. To prepare the array of
leaf-like Co3Ni microstructures supported on Cu substrate, 1.5
mmol CoCl2$6H2O and 0.5 mmol NiCl2$6H2O was dissolved
into 20 mL water under intense stirring, followed by the addi-
tion of 8 mmol triethylenetetramine and 2 mmol sodium
dodecylsulfate to the mixed solution. Then, 10 mL NaOH
(20 M), 10 mL hydrazine hydrate solution (50%) and 40 mL
ethanol were dropped in sequence to the above solution.
Aerwards, the mixture was quickly transferred into a Teon-
lined steel autoclave, in which a piece of 5 cm ꢁ 12 cm Cu
foil with a thickness of ca. 10 mm and a weight of 0.54 ꢂ 0.02 g
(obtained from Guangzhou Great Power Energy &Technology
CO., Ltd.) closely attached to the inner wall of vessel was used as
support. Then the autoclave was put in the center of a circular
column magnet with the magnetic eld intensity of ca. 0.2 Tesla
around the inner wall of the vessel. Finally, the autoclave, together
with the magnet, was kept in an air oven at 120 ꢀC for 8 h.
Results and discussion
The crystal structures of the Cu substrate, as well as the Cu foil
supported Co3Ni array, are investigated by XRD and the results
are shown in Fig. 1. It is observed that bare Cu substrate shows
three characteristic peaks at 2q ¼ 43.4ꢀ, 50.6ꢀ and 74.2ꢀ, cor-
responding to the (111), (200) and (220) planes of face-centered
cubic (fcc) phase of Cu (PDF#89-2838). Apart from these peaks,
other three peaks at 2q ¼ 44.4ꢀ, 51.6ꢀ and 76.1ꢀ are observed in
the XRD pattern of the Cu foil supported Co3Ni array, which can
be indexed to the diffractions from the (111), (200) and (220)
planes of the fcc phase of CoNi alloy.21
Fig. 2a and b display the digital camera photos of the bare Cu
foil and the Cu foil supported Co3Ni array, respectively. As can
be seen, the Cu foil is originally purple yellow in color and
changes into gray black aer the deposition of Co3Ni. The
morphology of the Cu supported Co3Ni array, as well as the Cu
foil, is analyzed by FE-SEM and the results are shown in Fig. 2c–
f. Clearly, the bare Cu foil is relatively smooth on its surface
(Fig. 2c). Aer the deposition of Co3Ni, the surface becomes very
coarse (Fig. 2d). Fig. 2e indicates that the Co3Ni layer is
composed of numerous leaf-like Co3Ni microstructures, which
are perpendicular to the Cu substrate. The high magnication
SEM image in Fig. 2f displays the architecture of a piece of
Characterization
X-Ray diffraction (XRD) patterns were recorded using a PAN-
alytical B. V. Empyean X-ray diffractometer with Cu Ka radiation
˚
(l ¼ 1.5406 A). The morphology of the array was investigated
using a Hitachi S-4800 eld emission scanning electron
microscope (FE-SEM) equipped with an X-ray energy-dispersive
spectrometer (EDS). Transmission electron microscopy (TEM)
images were obtained on a JEOL JEM-2100 transmission elec-
tron microscope operated at 200 kV. The mass of Co and Ni
deposited on the Cu foil was measured by a Varian 720 Induc-
tively Coupled Plasma-Optical Emission Spectrometer (ICP-
OES). The magnetic property of the sample was measured by
a Quantum Design MPMS XL-7 superconducting quantum
interference device magnetometer (SQUID). Contact angle was
measured by a contact angle meter (OCA20, DataPhysics
Instruments GmbH, Germany) at ambient temperature. The
specic surface area was measured on a Quantachrome
Autosorb-1 volumetric analyzer, using nitrogen adsorption and
the Brunauer–Emmett–Teller method.
Catalytic performance testing
The catalytic performance of the array of leaf-like Co3Ni
Fig. 1 XRD patterns of the Cu foil (a) and the Cu foil supported Co3Ni
microstructures supported on Cu foil was assessed by array (b).
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growth. As the reduction reaction proceeds, more and more
Co3Ni clusters will be formed and move to the Cu substrate,
then add themselves to existing nuclei. In such model, nano-
structures on the Cu substrate are formed.
To determine the magnetic properties of the Cu supported
array of leaf-like Co3Ni microstructures, magnetic measure-
ments at 298 K were carried out on the array, with the applied
eld parallel and vertical to the Cu substrate. The magnetic
hysteresis loops are displayed in Fig. 3, which demonstrate the
array has typical ferromagnetic properties. When the applied
external magnetic was parallel to the Cu substrate, the satura-
tion magnetization, the remnant magnetization and the coer-
civity of the array is 114.0 emu gꢃ1, 58.8 emu gꢃ1 and 79.5 Oe,
respectively. While when the applied external magnetic was
vertical to the Cu substrate, the saturation magnetization, the
remnant magnetization and the coercivity of the array is
96.7 emu gꢃ1, 11.1 emu gꢃ1 and 286.8 Oe, respectively.
Compared with the coercivity of the nanosized Co75Ni25 alloy
(31.69 Oe),22 our Co3Ni array exhibits much higher coercivity. It
has been demonstrated that
a nonspherical nanosized
magnetic materials can possess shape anisotropy and a small
departure from sphericity in shape will cause a signicant
increase of coercivity.23 In the present study, the shape anisot-
ropy of the leaf-like Co3Ni microstructures may be responsible
for the improved coercivity.
To evaluate the wettability of the Co3Ni array, the static
contact angle was determined. Fig. 4 is the typical photograph
of a water droplet on the Cu supported Co3Ni array. The contact
angle of the Co3Ni array is 154.7ꢀ, indicating that the Co3Ni
array exhibit superhydrophobic property. It is known that the
superhydrophobic materials can be obtained by the construc-
tion of appropriate surface hierarchical micro/nanostructures
and chemical composition.24 As far as our sample concerned,
the leaf-like Co3Ni microstructures in the array consist of well
aligned nanorods. Such hierarchical micro/nanostructures on
the surface of the Cu substrate account for the super-
hydrophobic properties of the sample.
Fig. 2 Digital camera photos of the Cu foil (a) and the Cu foil sup-
ported Co3Ni array (b); SEM images of the Cu foil (c) and the Cu foil
supported Co3Ni array (d–f); TEM image (g) and EDS pattern (h) of the
Co3Ni array.
Co3Ni leaf, which is consisting of a main stem with a diameter
of 80–200 nm and many parallel rod-like branches with
a diameter of ca. 150 nm on both sides of the stem. The TEM
image in Fig. 2g further conrms the leaf-like structure of Co3Ni
in the array. It should be mentioned that even a 30 min ultra-
sonic treatment can not detach these Co3Ni microstructures
from the Cu substrate, implying that the as-prepared Co3Ni
array is very stable. The typical EDS pattern of a leaf-like Co3Ni
microstructure in Fig. 2h indicates the atomic ratio of Co to Ni
is ca. 72 : 28. The ICP-OES analysis demonstrates the atomic
ratio of Co to Ni is ca. 74 : 26. Both ratios are very close to the
targeted composition of 3 : 1. According to the ICP-OES result,
the mass of the Co3Ni array deposited on Cu foil (5 ꢁ 12 cm) is
ca. 36.4 mg, and the corresponding catalyst loading on Cu foil is
0.61 mg cmꢃ2
.
In this study, Co3Ni alloy was produced by the co-reduction
of Co2+ and Ni2+. At the early stage of the reduction process,
plenteous primary Co3Ni alloy nuclei are formed in solution,
which will be magnetized and migrate to the Cu substrate under
the magnetic force provided by the external magnet. Aer they
are deposited on the Cu foil, these nuclei will act as crystal
nuclei and provide heterogeneous nucleation sites for crystal
Fig. 3 Magnetic hysteresis loops of the Cu foil supported Co3Ni array
with the applied external magnetic field parallel and vertical to the Cu
substrate.
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Table 1 Comparison of rate constants and BET surface normalized
rate constants of AB hydrolysis when Co3Ni array and Co3Ni nano-
particles act as catalysts
BET surface area
normalized rate
Rate constant (mL minꢃ1
)
constant (mL minꢃ1 m2)
Catalyst
dosage
Co3Ni
nanoparticles
Co3Ni
array
Co3Ni
nanoparticles
Co3Ni array
4.6 mg
9.1 mg
13.7 mg
18.4 mg
4.5
7.4
10.5
11.7
3.3
4.8
6.6
7.2
72.5
60.2
56.8
47.1
32.8
24.1
22.0
17.9
Fig. 4 Typical photograph of a water droplet on the surface of Cu foil
supported Co3Ni array.
In the present study, the as-prepared Cu foil supported array
of leaf-like Co3Ni microstructures as a catalyst for the hydrolysis
of AB is investigated. To clarify the role of Cu substrate,
hydrolysis of AB in the presence of bare Cu foil is carried out. It
is found that nearly no hydrogen is generated, suggesting that
the Cu foil in our work has no catalytic activity in the hydrolysis
reaction. In contrast, when the array of leaf-like Co3Ni micro-
structures acts as a catalyst, hydrogen is continuously gener-
ated. Fig. 5 shows the relationship of the accumulated volume
of hydrogen vs. reaction time at different catalyst dosages in
terms of geometrical surface areas of lm catalyst, which are the
product of the length and the width of the lm. As can be
observed, the accumulated volume of hydrogen is directly
proportional to the reaction time, hinting that the hydrolysis of
AB catalyzed by the Co3Ni array is zero-order with respect to the
concentration of AB. This implies that the adsorption of AB on
the array surface may be a rate-limiting step.25 Noting that the
volume of hydrogen at the late stage of the hydrolysis (aer
20 min) is smaller than the normal value at catalyst dosages of
22.5 and 30.0 cm2. Similar results have also been reported by
other groups.8,10 The corresponding rate constants can be calcu-
lated from the slopes of tting lines in Fig. 5 and the results are
listed in Table 1. The total turnover frequency (TOF) of catalyst
can also be calculated based on these data. At catalyst dosage of
9.1 mg, the value of TOF is ca. 2.03 mol H2 minꢃ1 (mol cat.)ꢃ1
.
It is worth noting that production of hydrogen via AB
hydrolysis catalyzed by various catalysts has been widely re-
ported and a huge number of data on the rate constants of AB
hydrolysis have been published in literature. However, the
detailed reaction conditions of AB hydrolysis, such as reaction
model, reaction temperature, catalyst dosage, concentration of
AB and even the support, are not the same in different reports.
So, it may be unsound to assess these catalysts by comparing
their rate constants or TOF without normalizing these param-
eters. In this study, for comparison, Co3Ni nanoparticles with
a diameter of 40–50 nm synthesized by a modied polyol
method26 instead of the leaf-like Co3Ni array were used as
a catalyst while other reaction conditions were kept the same.
The corresponding results are also shown in Table 1. As can be
seen, our Co3Ni array exhibit higher catalytic activity than Co3Ni
nanoparticles in the hydrolysis of the AB. Two possible reasons
account for the fact that the Co3Ni array shows much higher
catalytic activity than Co3Ni nanoparticles. Firstly, Co3Ni layers
supported on substrate are composed of shape-anisotropic leaf-
like microstructures, which have more corners and edges than
the spherical nanoparticles. Generally, the atoms on the corners
or edges have unsaturated valency with less number of bonds
around them than those in the interiors or on the faces.27 Such
atoms have much higher intrinsic catalytic activity. In this
study, the BET surface areas of Co3Ni array and the Co3Ni
nanoparticles were measured, which were 13.5 and 21.9 m2 gꢃ1
,
respectively. Correspondingly, the BET surface area normalized
rate constants can be gured out. As shown in Table 1, BET
surface area normalized rate constants of Co3Ni array is
signicantly larger than those of Co3Ni nanoparticles, hinting
that Co3Ni array has higher intrinsic catalytic activity than
Co3Ni nanoparticles. Secondly, these Co3Ni nanoparticles sus-
pended in AB solution during hydrolysis reaction tend to
agglomerate due to their high surface energy, as well as the
magnetic attraction among the Co3Ni nanoparticles resulted
from their ferromagnetic property.14 In this case, the surface
area of particulate Co3Ni catalyst will decrease, resulting in the
decline of catalytic performance. In contrast, the leaf-like Co3Ni
microstructures in the array are immobilized on the surface of
Fig.
5 Accumulative volume of hydrogen generated during AB
hydrolysis vs. reaction times at different catalyst dosages in terms of
geometrical surface (T ¼ 308 K).
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substrate and therefore the decrease of catalytic activity related
to the aggregation is evitable.
The apparent activated energy (Ea) is a very important
parameter of a catalyst, which can be applied to evaluate its
catalytic performance. In general, a catalyst with lower activation
energy in the catalytic reaction always shows a higher catalytic
activity. In this study, the hydrolysis of AB catalyzed by the Co3Ni
array was carried out at various temperatures to obtain the Ea
value. As displayed in Fig. 6a, the hydrolysis of AB is enhanced as
the reaction temperature increases. The hydrolysis rate is 5.0,
7.4, 10.5 and 19.4 mL minꢃ1 at reaction temperature of 298 K,
308 K, 318 K and 328 K, respectively. On the basis of these data,
Ea of the AB hydrolysis catalyzed by the leaf-like Co3Ni array can
be calculated according to the classical Arrhenius equation,
which is 35.6 kJ molꢃ1 (Fig. 6b). This value is higher than that of
Fig. 7 Normalized rate constant at different cycles.
Co35Pd65/C catalyst (27.5 kJ molꢃ1
)
and Co–P catalyst (22 kJ
25
molꢃ1),28 but is signicantly lower than those of recently re-
this work, the normalized rate constant, i.e. kn/k1, where k1 and
kn represents respectively the rate constant of AB hydrolysis in
cycle 1 and cycle n, is applied to assess the durability and
reusability of the as-prepared array of leaf-like Co3Ni micro-
structures supported on the Cu foil. As can be observed in Fig. 7,
the normalized rate constant is as high as 0.91 aer 6 cycles,
indicating that the used catalyst still retains ca. 91% catalytic
activity of its original activity. In contrast, the Co3Ni nano-
particles obtained via the modied polyol method lose ca. 40%
activity aer 6 cycles. Moreover, the as-prepared Cu supported
array of leaf-like Co3Ni microstructures also exhibits much
better durability and reusability than many Co or Co-containing
nanocatalysts in literature, such as intrazeolite Co nanoclusters
(31% activity loss in the h run),13 CoNi/RGO (31% activity loss
in the h run).32 For those Co or Co-containing nanocatalysts
in the form of powder, the material loss during the process of
separation, drying and re-dispersed is a common reason that
causes the decrease in the activity of the catalysts.13 Also, under
the drive of high surface energy and the attraction of magnetic
force among each other, the ne powdery particles are very
likely to aggregate during the process of usage, isolation and
desiccation, which will result in a remarkable loss of the cata-
lytic activity in the long-term run. In contrast, the leaf-like Co3Ni
microstructures in the array are immobilized on the Cu foil, so
ported Co-based and Ni-based catalysts, such as Co/g-Al2O3 (62
kJ molꢃ1),11 Co/zeolite (56 kJ molꢃ1),13 PVP–Co (46 kJ molꢃ1
)
29
and Co0.75B0.25 (40.85 kJ molꢃ1),30 bulk Ni (70 kJ molꢃ1),31 CoNi/
RGO (39.89 kJ molꢃ1 32 and Ni0.97Pt0.03 (57 kJ molꢃ1).33
)
The reusability and durability of a catalyst are important
concerns to be considered before its practical applications. In
Fig. 6 (a) Accumulative volume of hydrogen generated during AB
hydrolysis vs. reaction times at different reaction temperatures; (b) the
logarithm of rate constants vs. reciprocal of reaction temperature.
Fig. 8 SEM image of the Co3Ni array after 6 cycles.
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the activity drop caused by the migration and aggregation of
Co3Ni microstructures is evitable. Besides, the alteration of
shapes and morphology of micro- or nanocatalysts during
catalytic process can also inuence their catalytic perfor-
mance.34 To clarify this, the SEM image of the Co3Ni array
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In summary, an array of leaf-like Co3Ni microstructures sup-
ported on the Cu foil was prepared, which showed high catalytic
activity in the hydrolysis of AB. The apparent activated energy of
the AB catalyzed by the as-prepared Co3Ni array was calculated
to be ca. 35.6 kJ molꢃ1, which is much lower than those of many
Co-based and Ni-based nanocatalyst in literature. Also, it dis-
played better durability and reusability in contrast to many
powdery Co-based nanocatalysts. Owing to its high catalytic
activity, good durability and reusability, as well as the low cost,
the array of leaf-like Co3Ni microstructures showed great
potential in the applications in the hydrolysis of ammonia
borane for hydrogen generation. In addition, the as-prepared
Co3Ni array exhibits both ferromagnetic property and super-
hydrophobic property due to its special architecture, which
make it a multifunctional material with wide applications in
different elds.
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Acknowledgements
This work was nancially supported by the Science and Tech-
nology Commission of Shanghai Municipality (No.
14DZ2261000), the National Natural Science Foundation of
China (No. 51001052), the High-level Talent Project of the
University in Guangdong Province (184), the Distinguished
Young teachers in Higher Education of Guangdong Province
(No. Yq2013154), and the Characteristic and Innovative Project
of the Education Department of Guangdong Province (No.
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