Selective synthesis and characterization of metallic cobalt, cobalt/platinum, and platinum microspheres

Article abstract of DOI:10.1016/j.jallcom.2010.09.022

Metallic cobalt microspheres have been successfully synthesized via a facile hydrothermal method in the presence of poly (N-vinyl-2-pyrrolidone) (PVP), and the diameters of these spheres are in the range of 1-2 μm. Bimetallic Co/Pt microspheres and hollow noble metal Pt microspheres have also been selectively synthesized by adjusting the amount of reactants of replacement reaction in which corresponding noble metal compound and metallic cobalt microspheres are used as starting materials. The study on catalytic activities of the as-prepared metallic microspheres on hydrolysis of ammonia-borane for hydrogen generation reveals that Co/Pt (molar ratio 5:1) bimetallic microspheres are far more active than the hollow noble metal Pt microspheres.

Full text of DOI:10.1016/j.jallcom.2010.09.022

Journal of Alloys and Compounds 509 (2011) 338–342
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Selective synthesis and characterization of metallic cobalt,
cobalt/platinum, and platinum microspheres
∗
Wenqing Qin , Congren Yang, Xihong Ma, Shaoshi Lai
School of Minerals Processing and Bioengineering, Central South University, Yueluqu, Changsha, Hunan 410083, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 April 2010
Received in revised form 2 September 2010
Accepted 3 September 2010
Available online 21 September 2010
Metallic cobalt microspheres have been successfully synthesized via a facile hydrothermal method in the
presence of poly (N-vinyl-2-pyrrolidone) (PVP), and the diameters of these spheres are in the range of
1
–2 ␮m. Bimetallic Co/Pt microspheres and hollow noble metal Pt microspheres have also been selectively
synthesized by adjusting the amount of reactants of replacement reaction in which corresponding noble
metal compound and metallic cobalt microspheres are used as starting materials. The study on catalytic
activities of the as-prepared metallic microspheres on hydrolysis of ammonia-borane for hydrogen gen-
eration reveals that Co/Pt (molar ratio 5:1) bimetallic microspheres are far more active than the hollow
noble metal Pt microspheres.
Keywords:
Cobalt
Platinum
Metal catalysts
Hydrogen generation
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
[1], cobalt–molybdenum–boron/nickel foam [23], water-soluble
polymer-stabilized cobalt(0) nanoclusters [24], poly (N-vinyl-
2-pyrrolidone) (PVP) stabilized nickel [25], nanosized Co- and
Ni-based catalysts [26], zeolite confined copper(0) nanoclus-
ters [27], hollow Ni–SiO2 nanosphere [28], Fe–Ni alloy [29]
have also shown high H2 generation kinetics from hydrolysis
of NH BH .
Ammonia-borane (H NBH ) has attracted increasing attention
3
3
as an efficient hydrogen storage material for its high hydrogen con-
tent (19.6 wt% hydrogen) and low molecular weight (30.7 g/mol).
H NBH is highly soluble in water and its solution is stable at room
temperature. Unlike borohydrides, ammonia-borane solutions do
not require any additional stabilization by using bases but only
suitable catalyst (Eq. (1)) [1–14].
3
3
3
3
Various methods have been successfully applied to the synthe-
sis of bimetallic nanoparticles and hollow noble metallic materials
[
15,16,30–39]. An alternative route has been reported to prepare
catalyst
H NBH (aq) + 2H O(l) −→ NH (aq) + BO −(aq) + 3H (g)
+
(1)
hollow noble metal materials with different shapes by a transmet-
alation reaction in which suitable metal ion nanoparticles react
with a sacrificial partner and finally lead to the formation of
hollow spheres inheriting the morphology of the sacrificial part-
ner, and urchinlike hollow metallic and bimetallic nanospheres
have been synthesized [40–42]. Yan et al. [8] reported a ratio-
nal and general strategy for preparing magnetically recyclable
Au@Co core–shell nanoparticles through the one-step seeding-
growth route at room temperature under ambient atmosphere
within a few minutes. The resultant magnetically recyclable Au@Co
nanoparticles exerted excellent catalytic activity and longterm sta-
bility toward the hydrolytic dehydrogenation of aqueous NH BH .
Yi et al. [42] demonstrated that hollow metallic microspheres (e.g.,
Ni, Ni/Au, Ni/Pt, Ni/Pd, Pt, Pd, etc.) could be successfully synthesized
based on the reduction and replacement reaction and revealed that
the catalytic activity of hollow bimetallic microspheres was much
higher than those of pure noble metal microspheres. Those stud-
ies have successfully expanded the catalyst materials for hydrogen
generation from noble metal such as Pt and Rh to more abundant
and economical first-row metals.
3
3
2
4
2
2
Recently, various catalysts for promoting the hydrolysis of
NH BH have been reported. In particular, noble metal-based cata-
lysts such as Fe@Pt core–shell nanoparticles [9], Au@Co core–shell
3
3
nanoparticles [8], PtxNi1 nanoparticles [15,16], ruthenium sup-
−x
ported on carbon [17], water-soluble poly (4-styrenesulfonic
acid-co-maleic acid) stabilized ruthenium(0) and palladium(0)
nanoclusters [18], zeolite confined palladium(0) nanoclusters [5],
zeolite confined rhodium(0) nanoclusters [6], zeolite framework
stabilized rhodium(0) nanoclusters [19], water soluble laurate-
stabilized ruthenium(0) nanoclusters [20] have exhibited very
high catalytic activity for hydrolysis of NH BH . Non-noble metal
3
3
3
3
catalysts such as Co nanoparticles [21], Ni nanoparticles [7],
intrazeolite cobalt(0) nanoclusters [2], cobalt chloride CoCl2 [3],
nanoparticle-assembled Co-B thin film [22], electroplated Co–P
∗ Corresponding author. Tel.: +86 731 88830884; fax: +86 731 88710804.
E-mail address: qinwenqing369@126.com (W. Qin).
0
925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.09.022

W. Qin et al. / Journal of Alloys and Compounds 509 (2011) 338–342
339
Fig. 3. XRD patterns of the as-prepared spheres: (A) Co/Pt (molar ratio 10:1), (B)
Co/Pt (molar ratio 5:1) and (C) Pt.
Fig. 1. XRD patterns of the as-prepared metallic Co spheres.
2
.2. Synthesis
Herein, we demonstrate a facile synthesis strategy for fab-
ricating metallic cobalt microspheres via a direct reduction of
cobalt(II) chloride by hydrazine hydrate in the presence of poly
1
mmol of CoCl2·6H2O and 0.5 g of poly (N-vinyl-2-pyrrolidone) (PVP) were
put into a beaker of 100 mL and dissolved in 30 mL of ethylene glycol before the
beaker was sealed and stirred vigorously for 1 h at room temperature. Then 2 mL of
hydrazine hydrate was added into the beaker and stirred for 1 h in confined condi-
tions at room temperature. After that, the solution was sealed in a 50 mL Teflon-lined
autoclave, and was maintained at 200 C for 4 h without shaking or stirring, and
then the system was cooled to room temperature naturally. The resulting products
were collected by filtration and washed with absolute ethanol and distilled water
(
N-vinyl-2-pyrrolidone) (PVP) stabilizer in ethylene glycol solu-
tion. PVP-stabilized cobalt(0) nanoclusters are found to be stable
in solution, highly active and long-lived in hydrolyses of ammonia-
borane. The direct reduction route provides a new method for
large-scale synthesis of metallic cobalt spheres and other metallic
spheres. On the basis of replacement reaction, bimetallic (Co/Pt)
and noble metal (Pt) spheres have also been obtained as corre-
sponding noble metal compounds interacting with as-prepared
metallic cobalt spheres. The investigation on catalytic activities of
hollow noble metal and bimetallic spheres reveals the superior cat-
alytic activities of Co/Pt bimetallic spheres for hydrogen generation,
respectively.
◦
◦
in sequence several times. The final product was dried in a vacuum box at 60 C.
As-prepared metallic Co spheres (1 mmol) were suspended in deionized water
by ultrasonic treatment. Afterward, a freshly prepared HCl solution (5 wt%) was
added slowly to remove the oxidation layer on metallic Co spheres. The upper solu-
tion was decanted immediately when continuous bubbles were observed, indicating
metallic Co started to react with HCl to release H2 and the removal process of oxida-
tion layer was complete. The Co powders were washed with distilled water several
times by centrifugation and then freshly processed Co powders were added to a
certain amount of the chloroplatinic acid (H2PtCl6·6H2O) solution according to the
designed molar ratio of Co: Pt (10:1, 5:1 and 2:1, M). The replacement reaction was
allowed to proceed for 6 h without stirring and shaking at room temperature. Finally,
the obtained products were collected and washed with absolute ethanol and dis-
◦
2.
Experimental
tilled water in sequence several times and then dried in vacuum box at 60 C for
30 min.
2.1. Chemicals
2
.3. Catalyst study
Ammonia-borane (NH3BH3) was of 90% grade from the Aldrich, cobalt(II)
chloride hexahydrate (CoCl2·6H2O) was of analytical reagent grade and obtained
from the Beijing Chemical Reagents Factory, chloroplatinic acid (H2PtCl6·6H2O),
hydrazine hydrate, ethylene glycol and poly (N-vinyl-2-pyrrolidone) (PVP) were of
analytical grade and purchased from the Sinopharm Chemical Reagent Co., Ltd. All
chemicals were used as starting materials without further purification. Deionized
water was used throughout the experiment.
The hydrolysis of ammonia borane (NH BH ) was carried out at 35 ◦C. A 0.0012 g
3
3
sample of catalyst (metallic Co, Pt spheres and Co/Pt bimetallic spheres) was kept
in a round-bottom flask with the opening connected to an inverted, water-filled
graduated burette. The reaction was started and proceeded under constant stirring
with the addition of aqueous NH BH solution (50 mL, 0.25 wt%). The volume of
3
3
generated H2 was recorded per minute.
Fig. 2. SEM images of the as-prepared metallic Co spheres.

3
40
W. Qin et al. / Journal of Alloys and Compounds 509 (2011) 338–342
Fig. 4. SEM images of the as-prepared the Co/Pt bimetallic spheres: (A and B) Co/Pt (molar ratio 10:1), (C and D) Co/Pt (molar ratio 5:1) and (E and F) Pt. EDS spectrum of the
as-prepared the Co/Pt bimetallic spheres: (insert of A) Co/Pt (molar ratio 10:1) and (insert of C) Co/Pt (molar ratio 5:1).
2.4. Characterization
diffraction peaks can be indexed as (1 1 1) and (2 2 0) crystal planes
of face-centered cubic Co, in good accordance with the reported
The obtained samples were characterized on a Rigaku Dmax-2000 X-ray powder
diffractometer (XRD) with Cu K␣ radiation (ꢀ = 1.5418 A˚ ). The operation voltage and
data (space group Fm3m (2 2 5), PDF# 15-0806, a = 3.5477 A˚ ), and
two characteristic diffraction peaks can be indexed as (1 0 0) and
(1 0 1) crystal planes of hexagonal Co, in good accordance with
the reported data (space group P63/mmc (1 9 4), PDF# 05-0727,
a = 2.0531, c = 4.0605 A˚ ). Moreover, the sharp and narrow diffrac-
tion peaks indicated the well-crystallinity of the Co spheres. The
peaks of any impurities were not detected, which indicated that
the metallic Co was successfully obtained with high purity.
current were kept at 40 kV and 40 mA, respectively. The size and morphology of the
as-synthesized products were determined at 20 kV by a XL30 S-FEG scanning elec-
tron microscope (SEM). Energy-dispersive X-ray spectroscopy (EDS) was taken on
the SEM. The specific surface area of the cobalt/platinum and platinum microspheres
was detected by Brunauer–Emmett–Teller (BET) nitrogen adsorption–desorption
measurements (NOVA-1000). Before the analysis, the samples were outgassed at
◦
0 C under vacuum.
9
Scanning electron microscopy (SEM) analysis (Fig. 2A) shows
metallic cobalt microspheres synthesized via a direct reduction of
cobalt(II) chloride by hydrazine hydrate in the presence of poly (N-
vinyl-2-pyrrolidone) (PVP) stabilizer in ethylene glycol solution.
More details of those spheres can be obtained from the high-
magnification SEM image (Fig. 2B). The diameters of these spheres
3
. Results and discussion
X-ray diffraction (XRD) was carried out to determine the chemi-
cal composition and crystallinity of the as-prepared products. Fig. 1
shows the typical XRD pattern of Co spheres. Two characteristic

W. Qin et al. / Journal of Alloys and Compounds 509 (2011) 338–342
341
Fig. 5. EDS spectrum of the as-prepared the Co/Pt bimetallic spheres: (A) Co/Pt (molar ratio 10:1, insert of Fig. 4A) and (B) Co/Pt (molar ratio 5:1, insert of Fig. 4C).
are in the range of 1–2 ␮m, and the surface of the Co spheres
is very tight. The diameter of metallic cobalt microspheres (sec-
ondary particles) is far larger than the particle size based on X-ray
diffraction (XRD) analysis, which indicates that the microspheres
are composed of individual nanoparticles (primary particles) with
an average size of about 28 nm.
of EDS analysis (Fig. 5) reveals the sample mainly consists of Co
and Pt, therefore, it indicates that the Co/Pt bimetallic spheres have
been successfully obtained, which is in good accordance with XRD
results.
The SEM images of the as-prepared Pt samples are shown in
Fig. 4E and F, in which some broken spheres can be clearly seen,
exhibiting the hollow structure of the obtained Pt spheres. The
hollow Pt spheres are obtained by replacement reaction between
The Co/Pt bimetallic spheres and hollow Pt spheres are obtained
by replacement reaction between metallic Co spheres and the
chloroplatinic acid. The driving force of those reactions comes from
the large standard reduction potential gap between the Co2 /Co
redox pair (−0.280 V vs standard hydrogen electrode (SHE)) and
metallic Co spheres and the chloroplatinic acid because the Pt lat-
+
tice constant (a = 3.878 A˚ ) is larger than the Co lattice constant
(a = 3.5477 A˚ ), and two Co atoms are replaced by one platinum
2
−
/Pt redox pair (0.74 V vs standard hydrogen electrode
the PtCl
atom. Furthermore, the surface of the hollow Pt spheres with a
diameter range of 1–2 ␮m is very tight, resembling those of metal-
lic Co spheres. The diameter of the hollow Pt spheres (secondary
particles) is far larger than the particle size based on X-ray diffrac-
tion (XRD) analysis, which indicates that the microspheres are
composed of individual nanoparticles (primary particles) with an
average size of about 11 nm.
6
(
SHE)). Fig. 3A and B displays XRD patterns of the as-prepared
Co/Pt bimetallic spheres. The characteristic diffraction peaks of Co
(1 1 1) and (2 2 0)) and Pt ((1 1 1), (2 0 0), (2 2 0) and (3 1 1)) are
(
also found in the patterns of the Co/Pt bimetallic spheres sample,
indicating the successful replacement of non-noble metal. Fig. 3C
displays XRD patterns of the as-prepared hollow Pt spheres. Promi-
nent peaks ((1 1 1), (2 0 0), (2 2 0) and (3 1 1)) in the XRD patterns
correspond to the reflections of the face-centered cubic structure of
Pt, in good accordance with the reported data (space group Fm3m
The catalytic activities of the as-prepared spheres for hydrogen
generation from ammonia borane (NH BH ) were investigated and
3
3
the results are shown in Fig. 6. In comparison, the catalytic activ-
ities of products containing Pt are favorable, among which Co/Pt
(
2 2 5), PDF# 87-0647, a = 3.878 A˚ ). The sharp and narrow diffrac-
tion peaks indicate the well-crystallinity of the as-prepared Co/Pt
bimetallic spheres and hollow Pt spheres (Fig. 3).
Fig. 4A–D shows the SEM images of Co/Pt bimetallic spheres. The
diameters of these spheres are in the range of 1–2 ␮m. The diameter
of the Co/Pt bimetallic spheres microspheres (secondary particles)
is far larger than the particle size based on X-ray diffraction (XRD)
analysis, which indicates that the microspheres are composed of
individual nanoparticles (primary particles) with an average size
of about 28 nm Co nanoparticles and 11 nm Pt nanoparticles. It is
known that the bimetallic nanoparticles prepared by using solid
templates could be alloy nanoparticles in which the two constituent
metals are mixed at the atomic level [43–45] or core/shell nanopar-
ticles in which the two components are separated by distinct phase
boundaries [46,47] or even core/shell nanoparticles with an alloy
shell and a pure core [48]. However, all the peaks of the Pt and Co
can be observed without shift compared with those of pure Pt and
Co in our work (Figs. 1 and 3), distinct from the XRD pattern of a
Co–Pt alloy [42]. Meanwhile, many flakes are in the surface of Co/Pt
bimetallic spheres (Fig. 4A–D), which indicate that the PtCl62 may
react with Co atoms on the surface, in the case of the formation of
the core/shell structure. As the molar ratio of cobalt to platinum
decreases, the surface of spheres becomes smoother. The result
−
Fig. 6. Hydrogen release from the hydrolysis of aqueous NH3BH3 solution (50 mL,
0.25 wt%) in the presence of various as-prepared catalysts (12 mg) at 35 C.
◦

3
42
W. Qin et al. / Journal of Alloys and Compounds 509 (2011) 338–342
(
molar ratio 5:1) bimetallic spheres show the highest activity. It
[3] U.B. Demirci, P. Miele, J. Power Sources 195 (2010) 4030–4035.
[
[
4] N. Mohajeri, A. T-Raissi, O. Adebiyi, J. Power Sources 167 (2007) 482–485.
5] M. Rakap, S. Özkar, Int. J. Hydrogen Energy 35 (2010) 1305–1312.
is noteworthy that Co/Pt (molar ratio 5:1) bimetallic spheres can
catalyze the hydrolysis of NH BH3 at 35 C, and about more than
◦
3
[6] S. C¸ alı s¸ kan, M. Zahmakıran, S. Özkar, Appl. Catal. B 93 (2010) 387–394.
2
4
.5 out of 3.0 (H /NH BH , molar ration) can be generated over
h. BET nitrogen-adsorption measurements show that the spe-
[7] J.M. Yan, X.B. Zhang, S. Han, H. Shioyama, Q. Xu, Inorg. Chem. 48 (2009)
2
3
3
7
389–7393.
8] J.M. Yan, X.B. Zhang, T. Akita, M. Haruta, Q. Xu, J. Am. Chem. Soc. 132 (2010)
326–5327.
[
cific surface area of the Co/Pt (molar ratio 10:1) bimetallic spheres,
Co/Pt (molar ratio 5:1) bimetallic spheres and hollow Pt spheres is
5
[9] X.B. Zhang, J.M. Yan, S. Han, H. Shioyama, Q. Xu, J. Am. Chem. Soc. 131 (2009)
2778–2779.
10] M. Chandra, Q. Xu, J. Power Sources 156 (2006) 190–194.
11] Q. Xu, M. Chandra, J. Alloys Compd. 446–447 (2007) 729–732.
[12] T. Umegaki, J.M. Yan, X.B. Zhang, H. Shioyama, N. Kuriyama, Q. Xu, Int. J. Hydro-
gen Energy 34 (2009) 2303–2311.
2
2
2
3
.50 m /g, 3.21 m /g and 2.43 m /g, respectively, which is in good
[
[
accordance with the spheres surface features in Fig. 4. According
to the results presented above, Co/Pt (molar ratio 5:1) bimetallic
spheres exhibit higher catalytic activity than that of Co/Pt (molar
ratio 10:1) bimetallic spheres and hollow Pt spheres for the hydrol-
[
13] H.L. Jiang, S.K. Singh, J.M. Yan, X.B. Zhang, Q. Xu, ChemSusChem 3 (2010)
41–549.
5
ysis of NH BH and the interpretation of the reason follows. First,
3
3
[14] I.P. Jain, P. Jain, A. Jain, J. Alloys Compd. 503 (2010) 303–339.
[15] F. Cheng, H. Ma, Y. Li, J. Chen, Inorg. Chem. 46 (2007) 788–794.
the BET surface area of the Co/Pt (molar ratio 5:1) bimetallic spheres
is much larger than that of the hollow Pt spheres, exposing more
active sites for the catalytic reaction to happen. Second, the hollow
Pt spheres have inevitably collapsed after complete replacement
reaction, making the surface area decrease to some extent and fur-
ther weakening the catalytic activities. Third, the BET surface area of
the Co/Pt (molar ratio 5:1) bimetallic spheres is smaller than that
of the Co/Pt (molar ratio 10:1) bimetallic spheres, but the Co/Pt
[
16] X. Yang, F. Cheng, J. Liang, Z. Tao, J. Chen, Int. J. Hydrogen Energy 34 (2009)
785–8791.
8
[
17] S. Basu, A. Brockman, P. Gagare, Y. Zheng, P.V. Ramachandran, W.N. Delgass, J.P.
Gore, J. Power Sources 188 (2009) 238–243.
18] Ö. Metin, S. Sahin, S. Özkar, Int. J. Hydrogen Energy 34 (2009) 6304–6313.
19] M. ZahmakIran, S. Özkar, Appl. Catal. B 89 (2009) 104–110.
20] F. Durap, M. ZahmakIran, S. Özkar, Int. J. Hydrogen Energy 34 (2009)
7223–7230.
[
[
[
[
[
21] J.M. Yan, X.B. Zhang, H. Shioyama, Q. Xu, J. Power Sources 195 (2010)
1091–1094.
(
molar ratio 5:1) bimetallic spheres have much more Pt than that
22] N. Patel, R. Fernandes, G. Guella, A. Miotello, Appl. Catal.
137–143.
B 95 (2010)
of the Co/Pt (molar ratio 10:1) bimetallic spheres, making more
active sites for the catalytic reaction. Due to their relatively higher
catalytic activities and much lower cost compared to hollow Pt
spheres, Co/Pt (molar ratio 5:1) hollow bimetallic spheres may have
potential applications in portable hydrogen generation systems.
[23] H.B. Dai, L.L. Gao, Y. Liang, X.D. Kang, P. Wang, J. Power Sources 195 (2010)
07–312.
24] O. Metin, S. Özkar, Energy Fuels 23 (2009) 3517–3526.
3
[
[25] T. Umegaki, J.M. Yan, X.B. Zhang, H. Shioyama, N. Kuriyama, Q. Xu, Int. J. Hydro-
gen Energy 34 (2009) 3816–3822.
[
26] T. He, Z. Xiong, G. Wu, H. Chu, C. Wu, T. Zhang, P. Chen, Chem. Mater. 21 (2009)
315–2318.
[27] M. Zahmakıran, F. Durap, S. Özkar, Int. J. Hydrogen Energy 35 (2010).
2
4
. Conclusions
[
28] T. Umegaki, J.M. Yan, X.B. Zhang, H. Shioyama, N. Kuriyama, Q. Xu, J. Power
Sources 191 (2009) 209–216.
29] J.M. Yan, X.B. Zhang, S. Han, H. Shioyama, Q. Xu, J. Power Sources 194 (2009)
In summary, metallic cobalt microspheres have been success-
[
fully synthesized via a facile hydrothermal method in the presence
of poly (N-vinyl-2-pyrrolidone) (PVP), and the diameters of these
spheres are in the range of 1–2 ␮m. Bimetallic Co/Pt microspheres
and hollow noble metal Pt microspheres have also been obtained
via a replacement reaction route using metallic Co microspheres
as sacrificial templates. The catalytic activities of the as-prepared
metallic microspheres for hydrogen generation by hydrolysis of
ammonia borane have been investigated; the results show much
enhanced favorable catalytic performance of Co/Pt (molar ratio 5:1)
bimetallic microspheres with the larger BET surface area compar-
ing with the hollow Pt spheres, providing a new possible alternative
in the making of portable fuel cells. This strategy provides an effec-
tive method for the fabrication of other pure and composite hollow
spheres.
4
78–481.
[30] K.D. Beard, D. Borrelli, A.M. Cramer, D. Blom, J.W. Van Zee, J.R. Monnier, ACS
Nano 3 (2009) 2841–2853.
[
31] W.R. Lee, M.G. Kim, J.R. Choi, J.I. Park, S.J. Ko, S.J. Oh, J. Cheon, J. Am. Chem. Soc.
27 (2005) 16090–16097.
1
[32] Q. Liu, Z. Yan, N.L. Henderson, J.C. Bauer, D.W. Goodman, J.D. Batteas, R.E. Schaak,
J. Am. Chem. Soc. 131 (2009) 5720–5721.
33] Y. Vasquez, A.K. Sra, R.E. Schaak, J. Am. Chem. Soc. 127 (2005) 12504–12505.
34] J. Li, Y. Liang, Q.C. Xu, X.Z. Fu, J.Q. Xu, J.D. Lin, D.W. Liao, J. Nanosci. Nanotechnol.
6 (2006) 1107–1113.
35] J.M. Sieben, M.M.E. Duarte, C.E. Mayer, J. Alloys Compd. 491 (2010) 722–728.
36] G. Singh, I.P.S. Kapoor, S. Dubey, J. Alloys Compd. 480 (2009) 270–274.
37] G. Wang, H. Wu, D. Wexler, H. Liu, O. Savadogo, J. Alloys Compd. 503 (2010)
L1–L4.
[
[
[
[
[
[38] H. Wu, D. Wexler, G. Wang, J. Alloys Compd. 488 (2009) 195–198.
[
[
39] T. Yang, L. Zhang, X. Li, D. Xia, J. Alloys Compd. 492 (2010) 83–87.
40] H.P. Liang, H.M. Zhang, J.S. Hu, Y.G. Guo, L.J. Wan, C.L. Bai, Angew. Chem. Int.
Ed. 43 (2004) 1540–1543.
41] Y. Sun, B. Mayers, Y. Xia, Adv. Mater. 15 (2003) 641–646.
42] R. Yi, R.R. Shi, G.H. Gao, N. Zhang, X.M. Cui, Y.H. He, X.H. Liu, J. Phys. Chem. C
[
[
Acknowledgment
113 (2009) 1222–1226.
[
43] M.P. Mallin, C.J. Murphy, Nano Lett. 2 (2002) 1235–1237.
This work was supported by the National Natural Science Foun-
dation of China (Project No. 50774094).
[44] I. Lee, S.W. Han, K. Kim, Chem. Commun. (2001) 1782–1783.
[
45] R.G. Sanedrin, D.G. Georganopoulou, S. Park, C.A. Mirkin, Adv. Mater. 17 (2005)
027–1031.
46] Y.W. Cao, R. Jin, C.A. Mirkin, J. Am. Chem. Soc. 123 (2001) 7961–7962.
1
[
References
[47] M. Tsuji, N. Miyamae, K. Matsumoto, S. Hikino, T. Tsuji, Chem. Lett. 34 (2005)
518–1519.
48] Q.B. Zhang, J.P. Xie, J.Y. Lee, J.X. Zhang, C. Boothroyd, Small
067–1071.
1
[
4 (2008)
[
[
1] K. Eoma, K. Choa, H. Kwon, Int. J. Hydrogen Energy 35 (2010) 181–186.
2] M. Rakap, S. Özkar, Int. J. Hydrogen Energy 35 (2010) 3341–3346.
1