One-step seeding growth of magnetically recyclable AU@Co core-shell nanoparticles: Highly efficient catalyst for hydrolytic dehydrogenation of ammonia borane

Article abstract of DOI:10.1021/ja910513h

Magnetically recyclable Au@Co core?shell nanoparticles were successfully synthesized in a one-step seeding-growth process within a few minutes. They were thermally stable and exhibited higher catalytic activity toward the dehydrogenation of ammonia borane than Au?Co alloy and the pure metal counterparts. This is a large enhancement in the catalytic activity of core?shell structured nanoparticles and will provide a new design principle for heterogeneous catalysis.

Full text of DOI:10.1021/ja910513h

Published on Web 03/26/2010
One-Step Seeding Growth of Magnetically Recyclable Au@Co Core-Shell
Nanoparticles: Highly Efficient Catalyst for Hydrolytic Dehydrogenation of
Ammonia Borane
Jun-Min Yan,†,§ Xin-Bo Zhang,† Tomoki Akita,†,§ Masatake Haruta,‡,§ and Qiang Xu*,†,§
National Institute of AdVanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan, Graduate
School of Urban EnVironmental Sciences, Tokyo Metropolitan UniVersity (TMU), 1-1 minami-osawa, Hachioji,
Tokyo 192-0397, Japan, and CREST, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan
Received December 14, 2009; E-mail: q.xu@aist.go.jp
2+
Bimetallic NPs with core-shell architecture have attracted growing
research interest in recent years due to their unique and novel optical,
electrical, and catalytic properties compared with their monometallic
counterparts and alloys.1 Containing a magnetic element in NPs is
additionally attractive owing to their applications on biomedical,
information storage, technological, and catalytic applications.2 There-
fore, investigations on the synthesis and application of magnetic core-
shell NPs are of great interest for harvesting the combined advantages of
synergistic structural/electronic effects of core-shell structure and the
intrinsic property of the magnetic element. Up to now, magnetic core-
shell NPs have been successfully synthesized via the two-step seeding
growth (seeds are prepared beforehand), gas condensation, or thermal
decomposition methods, wherein either complicated process or strict
reaction conditions (such as high temperature and high vacuum) are
employed.3 On the other hand, to the best of our knowledge, exploring
and tuning their application performance of the synthesized magnetic
core-shell NPs has rarely been mentioned.3 Compared to these compli-
cated and subeconomic methods, development of a facile and general
strategy for one-step synthesis of magnetic core-shell NPs under mild
conditions in a short time and exploring their potential applications, such
as in catalysis, is of great importance.
reduction potentials of the two soluble metal salts (E°Co /Co ) -0.28 eV
3+/
vs SHE; E°Au Au ) +0.93 eV vs SHE), where a suitable reducing agent
is essential. Alcohol has been used as the reducing agent for the one-step
synthesis of core-shell NPs of dual noble metals,1h,i but this is difficult
for non-noble metal containing core-shell NPs.2f Here, to fulfill the
requirement of the non-noble metal system, AB is employed as a suitable
reducing agent, with which magnetic Au@Co NPs are successfully
synthesized within 2 min. On the contrary, a relative stronger reduction
agent, NaBH4, instead causes the formation of Au-Co alloy NPs.8
The synthesis progress can be visibly monitored by the evolution
of the solution color (Figure 1b). Once the aqueous solution of HAuCl4
and CoCl2 with a molar ratio of 0.06:0.94 in the presence of
polyvinylpyrrolidone K 30 (PVP, 1 wt %) as the stabilizing agent is
mixed with the AB solid,8 the color of the mixture suddenly changes
from its light-pink color to orange-red in a few seconds, indicating
Au3+ cations are preferentially reduced to Au0 NPs prior to Co2+
cations. Then the color became darker and darker and finally changed
to pure black within 2 min. Based on this color evolution and their
intrinsic difference in reduction potentials, we can reasonably assume
that the obtained particles are in the core-shell architecture.
Currently, finding effective hydrogen storage materials is one of
the most difficult challenges toward a hydrogen powered society as a
long-term solution for a secure energy future.4 Ammonia borane
(NH3BH3, AB) has a hydrogen capacity of 19.6 wt %, exceeding that
of gasoline and making itself an attractive candidate for chemical
hydrogen-storage applications.5,6 One of the major obstacles of the
practical application of this system is to develop efficient, economical,
and easily recyclable catalysts for further improving the kinetic and
thermodynamic properties under moderate conditions.6,7
Figure 1. (a) Schematic illustration, (b) color evolution in the formation
process of Au@Co core-shell NPs via a one-step seeding-growth method
at room temperature.
Herein, we report a rational and general strategy for preparing
magnetically recyclable Au@Co core-shell NPs through the one-
step seeding-growth route at room temperature under ambient
atmosphere within a few minutes. Unexpectedly, in contrast to its
monometallic and alloy counterparts, the resultant magnetically
recyclable Au@Co NPs exert excellent catalytic activity and long-
term stability toward the hydrolytic dehydrogenation of aqueous
AB under ambient atmosphere at room temperature.
This assumption has immediately been confirmed by monitoring
the UV-vis spectra.8 To further confirm the core-shell architecture,
X-ray photoelectron spectroscopy (XPS) with Ar etching was also
applied to the prepared NPs.8 Well-defined peaks of metallic Au specie
can be detected only after Ar etching, while those of metallic Co can
be found both before and after Ar etching and the intensities are slightly
decreased, indicating all the Au ions are reduced prior to the Co ions.
This means that the atomic ratio of Au/Co increases from the surface
to the inner part of the prepared NPs, which, combined with the
UV-vis spectra results, gives convincing evidence for the homoge-
neous core-shell structures, with the cores mainly composed of Au
and the shells mainly composed of Co, of the whole specimen particles.
The transmission electron microscopy (TEM) images with different
magnifications of the Au@Co NPs are shown in Figure 2. The NPs
are roughly spherical in shape and ∼7 nm in size. A distinct contrast
This one-step synthetic method was achieved by exposing a mixture
of Au3+ and Co2+ precursors to the reducing agent at the same time,8
where the core Au NPs can be formed first and served as the in situ
seeds for the successive catalytic reduction leading to the growth of
outer Co NPs (Figure 1a). The basic concept of this one-step synthesis
of the Au@Co core-shell NPs is to take advantage of the difference in
† AIST.
§ JST.
‡ TMU.
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5326 J. AM. CHEM. SOC. 2010, 132, 5326–5327
10.1021/ja910513h  2010 American Chemical Society

C O M M U N I C A T I O N S
of core and shell metals can be observed clearly, and the dark cores
are Au (ca. 2.5 nm) and the gray shells are Co. The high resolution
image (Figure 2c) indicates this sample is basically amorphous, which
agrees well with its XRD result.8 It can be well crystallized into metallic
Au and Co while keeping its core-shell architecture after heat
treatment at 873 K for 3 h,8 suggesting the core-shell architecture
has good thermal stability. The EDS analysis8 reveals the Au/Co atomic
ratio to be 7/93, which agrees well with the target atomic ratio. In a
word, robust Au@Co core-shell NPs have been successfully synthe-
sized with the appointed atomic ratio through a one-step seeding-
growth pathway under mild conditions within a few minutes.
The lifetime/stability is very important for the practical applica-
tion of catalysts. We tested the catalytic activity of the Au@Co
NPs every 24 h in solution under ambient atmosphere. There was
no significant decrease (17% decrease) in catalytic activity even
after 120 h.8 Moreover, this high performance catalyst is magnetic
and thus can be easily recovered by an external magnet.8
In summary, we have demonstrated a novel methodology for the
synthesis of Au@Co magnetic core-shell structured NPs through a one-
step seeding-growth method at room temperature under ambient atmo-
sphere within a few minutes. In addition to possessing thermal stability
and easy recovery functions, the resultant Au@Co NPs exhibit high
catalytic activity and long-term stability for the hydrolytic dehydrogenation
of aqueous AB. Moreover, this rational and general method can be easily
extended to the other metallic systems, which are used as optical, magnetic,
and electrical materials as well as heterogeneous catalysts.
Acknowledgment. The authors thank the reviewers for valuable
suggestions and thank AIST and JST for financial support.
Supporting Information Available: Experimental procedures,
XRD, TEM, XPS, UV-vis, stability, and lifetime/stability results of
prepared samples. This material is available free of charge via the
Internet at http://pubs.acs.org.
Figure 2. TEM images of Au@Co NPs with (a) low, (b) middle, and (c)
high magnifications.
References
It should be noted that this strategy employed here for preparing
Au@Co core-shell NPs can be easily expanded to the other system. For
example, changing the core metal from Au to Ag or shell metal from Co
to Ni can also lead to core-shell architecture.8 However, changing the
reduction agent of AB to conventional NaBH4 only leads to the formation
of Au-Co alloy8 with inferior catalytic activity (vide infra), indicating
both the relative reduction potentials of the metal ions and the reduction
ability of the reduction agent are the key factors for this synthesis strategy.
To compare the catalytic properties of Au@Co NPs with those of
pure Au, Co, and AuCo alloy NPs, we evaluated their catalytic
activities for the dehydrogenation of AB, which is a promising chemical
hydrogen storage material. Figure 3 shows the amount of H2 generated
as a function of reaction time. The core-shell Au@Co NPs exhibit
markedly high catalytic activity (Figure 3a) to complete the dehydro-
genation reaction of AB within 11 min. This excellent catalytic activity
is much higher than that of the AuCo alloy (Figure 3b, 25 min) and
Co (Figure 3c, 169 min) with similar morphologies and sizes as those
of Au@Co.8 Au NPs, which are in interlaced branch shapes with
diameters less than 10 nm, have the worst activity (Figure 3d; only
70% of H2 is released even after more than 800 min).8
(1) (a) Alayoglu, S.; Zavalij, P.; Eichhorn, B.; Wang, Q.; Frenkel, A. I.; Chupas, P.
ACS Nano 2009, 3, 3127. (b) Lee, Y. W.; Kim, M.; Kim, Z. H.; Han, S. W.
J. Am. Chem. Soc. 2009, 131, 17036. (c) Tao, F.; Grass, M. E.; Zhang, Y. W.;
Butcher, D. R.; Renzas, J. R.; Liu, Z.; Chung, J. Y.; Mun, B. S.; Salmeron, M.;
Somorjai, G. A. Science 2008, 322, 932. (d) Alayoglu, S.; Nilekar, A. U.;
Mavrikakis, M.; Eichhorn, B. Nat. Mater. 2008, 7, 333. (e) Cui, Y.; Ren, B.;
Yao, J. L.; Gu, R. A.; Tian, Z. Q. J. Phys. Chem. B 2006, 110, 4002. (f) Scott,
R. W. J.; Wilson, O. M.; Oh, S. K.; Kenik, E. A.; Crooks, R. M. J. Am. Chem.
Soc. 2004, 126, 15583. (g) Mallin, M. P.; Murphy, C. J. Nano Lett. 2002, 2,
1235. (h) Toshima, N.; Harada, M.; Yamazaki, Y.; Asakura, K. J. Phys. Chem.
1992, 96, 9927. (i) Toshima, N.; Harada, M.; Yonezawa, T.; Kushihashi, K.;
Asakura, K. J. Phys. Chem. 1991, 95, 7448.
(2) (a) Gao, J. H.; Gu, H. W.; Xu, B. Acc. Chem. Res. 2009, 42, 1097. (b) Frey,
N. A.; Peng, S.; Cheng, K.; Sun, S. H. Chem. Soc. ReV. 2009, 38, 2532. (c)
Lu, A. H.; Salabas, E. L.; Schuth, F. Angew. Chem., Int. Ed. 2007, 46, 1222.
(d) King, N. C.; Blackley, R. A.; Wears, M. L.; Newman, D. M.; Zhou,
W. Z.; Bruce, D. W. Chem. Commun. 2006, 32, 3414. (e) Himpsel, F. J.;
Ortega, J. E.; Mankey, G. J.; Willis, R. F. AdV. Phys. 1998, 47, 511. (f)
Toshima, N.; Lu, P. Chem. Lett. 1996, 9, 729.
(3) (a) Xu, Y. H.; Wang, J. P. AdV. Mater. 2008, 20, 994. (b) Choi, W. S.; Koo,
H. Y.; Kim, D. Y. AdV. Mater. 2007, 19, 451. (c) Lee, W.-r.; Kim, M. G.;
Choi, J.-r.; Park, J.-I.; Ko, S. J.; Oh, S. J.; Cheon, J. J. Am. Chem. Soc.
2005, 127, 16090. (d) Sobal, N. S.; Ebels, U.; Mohwald, H.; Giersig, M. J.
Phys. Chem. B 2003, 107, 7351. (e) Park, J.-I.; Cheon, J. J. Am. Chem. Soc.
2001, 123, 5743.
(4) (a) Grochala, W.; Edwards, P. P. Chem. ReV. 2004, 104, 1283. (b) Chen,
P.; Xiong, Z.; Luo, J.; Lin, J.; Tan, K. L. Nature 2002, 420, 302. (c)
Schlapbach, L.; Zuttel, A. Nature 2001, 414, 353. (d) Ogden, J. M. Annu.
ReV. Energy EnViron. 1999, 24, 227.
The electronic structure in the bimetallic alloy or core-shell architecture
can be modified through the interaction of the two kinds of atoms in
relation to the so-called ligand and strain effects.1c,9 Here, the better
catalytic activity of Au@Co core-shell than AuCo alloy NPs might result
from the modification of the electronic structure in a core-shell NP which
is superior to that in an alloy NP for AB dehydrogenation. Further
experimental and theoretical work is needed for elucidating this.
(5) (a) Gutowska, A.; Li, L. Y.; Shin, Y. S.; Wang, C. M. M.; Li, X. H. S.; Linehan,
J. C.; Smith, R. S.; Kay, B. D.; Schmid, B.; Shaw, W.; Gutowski, M.; Autrey,
T. Angew. Chem., Int. Ed. 2005, 44, 3578. (b) Chen, Y. S.; Fulton, J. L.;
Linehan, J. C.; Autrey, T. J. Am. Chem. Soc. 2005, 127, 3254. (c) Sit, V.;
Geanangel, R. A.; Wendlandt, W. W. Thermochim. Acta 1987, 113, 379.
(6) (a) Himmelberger, D. W.; Yoon, C. W.; Bluhm, M. E.; Carroll, P. J.; Sneddon,
L. G. J. Am. Chem. Soc. 2009, 131, 14101. (b) Davis, B. L.; Dixon, D. A.;
Garner, E. B.; Gordon, J. C.; Matus, M. H.; Scott, B.; Stephens, F. H. Angew.
Chem., Int. Ed. 2009, 48, 6812. (c) Stephens, F. H.; Pons, V.; Baker, R. T.
Dalton Trans. 2007, 2613. (d) Stephens, F. H.; Baker, R. T.; Matus, M. H.;
Grant, D. J.; Dixon, D. A. Angew. Chem., Int. Ed. 2007, 46, 746. (e) Keaton,
R. J.; Blacquiere, J. M.; Baker, R. T. J. Am. Chem. Soc. 2007, 129, 1844. (f)
Denney, M. C.; Pons, V.; Hebden, T. J.; Heinekey, D. M.; Goldberg, K. I. J. Am.
Chem. Soc. 2006, 128, 12048. (g) Bluhm, M. E.; Bradley, M. G.; Butterick Iii,
R.; Kusari, U.; Sneddon, L. G. J. Am. Chem. Soc. 2006, 128, 7748.
(7) (a) Yan, J. M.; Zhang, X. B.; Han, S.; Shioyama, H.; Xu, Q. Inorg. Chem.
2009, 48, 7389. (b) Yan, J.-M.; Zhang, X.-B.; Han, S.; Shioyama, H.; Xu, Q.
Angew. Chem., Int. Ed. 2008, 47, 2287. (c) Clark, T. J.; Whittell, G. R.; Manners,
I. Inorg. Chem. 2007, 46, 7522. (d) Chandra, M.; Xu, Q. J. Power Sources
2007, 168, 135. (e) Paul, A.; Musgrave, C. B. Angew. Chem., Int. Ed. 2007,
46, 8153. (f) Yoon, C. W.; Sneddon, L. G. J. Am. Chem. Soc. 2006, 128,
13992. (g) Xu, Q.; Chandra, M. J. Power Sources 2006, 163, 364.
(8) See Supporting Information.
(9) (a) Kitchin, J. R.; Norskov, J. K.; Barteau, M. A.; Chen, J. G. Phys. ReV.
Lett. 2004, 93, 156801. (b) Zhai, H. J.; Li, J.; Wang, L. S. J. Chem. Phys.
2004, 121, 8369. (c) Wang, L. M.; Bai, J.; Lechtken, A.; Huang, W.; Schooss,
D.; Kappes, M. M.; Zeng, X. C.; Wang, L. S. Phys. ReV. B 2009, 79, 033413.
Figure 3. Hydrogen generation from AB aqueous solution (0.26 M, 10 mL)
catalyzed by (a) Au@Co, (b) Au-Co, (c) Co, and (d) Au NPs under ambient
atmosphere at room temperature. Catalyst/AB ) 0.02 (molar ratio).
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