View Online
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Chromium-assisted synthesis of platinum nanocube electrocatalystsw
Downloaded by STATE UNIVERSITY OF NEW YORK AT BINGHAMTON on 20 September 2010
Published on 24 August 2010 on http://pubs.rsc.org | doi:10.1039/C0CC01379J
Rameshwori Loukrakpam,a Paul Chang,a Jin Luo,a Bin Fang,a Derrick Mott,a In-Tae Bae,b
H. Richard Naslund,c Mark H. Engelhardd and Chuan-Jian Zhong*a
Received 12th May 2010, Accepted 15th July 2010
DOI: 10.1039/c0cc01379j
This report demonstrates a novel strategy of chromium-assisted
synthesis of platinum nanocubes as electrocatalysts for oxygen
reduction reaction with enhanced specific activity.
Platinum and alloy (e.g., PtCo, PtNi, PtCr, etc.) nanoparticles
have been widely exploited as catalysts for different chemical
reactions, including hydrogenation of olefins, oxidation of
carbon monoxide, formic acid and fuel cell reactions such as
oxygen reduction reaction (ORR) and methanol oxidation
reaction (MOR).1–5 In addition to using polyol, borohydride,
hydrazine reducing agents for the synthesis of platinum nanoparticles with different shapes such as spheres, rods, wires,
multipods, cubes, etc.,6–11 the control of the nanocrystal shapes
has been reported using traces of silver compounds,12,13 iron
carbonyl,14 tungsten carbonyl15,16 and cobalt carbonyl.17
Because of the strong correlation of catalytic activity with crystal
face structures, the understanding of the shape–activity
correlation has attracted a great deal of interest. Catalytic
hydrocarbon conversion or hydrogenation reactions by shapecontrolled Pt nanocrystals have been demonstrated with
enhanced activity and selectivity in comparison with regular Pt
catalysts.9,12,18,19 Cube-shaped, multioctahedral-shaped, nanowire or dendritic Pt nanoparticles were shown to exhibit an
enhanced electrocatalytic activity for ORR.14,18,20–23 The
differences in catalytic hydrogenation and electrocatalysis have
been attributed to shape effect in terms of facets, steps, ledges or
kinks, etc.24,25 While different surface adsorption strengths of
capping molecules or foreign metals have been proposed to
account for the shape formation of Pt-based nanocrystals, little
is known about how the different metal precursors or additives
operate mechanistically in the shape formation. Moreover,
assessments of the catalytic activities between cube-shaped and
non-cubic nanoparticles in many of the previous studies were
complicated by differences in the particles size parameter used for
the comparisons, which has a synergistic effect on the activity.
This report describes the findings of an investigation of a novel
strategy for chromium-assisted synthesis of platinum nanocubes
and electrocatalysts. By exploring the energetic differences of
surface adsorption of Cr on different facets of Pt nanocrystals
and propensity of oxidation of Cr (forming Cr2O3),26–29 the shape
control is achieved by manipulating the concentration of
chromium carbonyl precursor in the reaction mixture. In addition
to understanding the shape formation mechanism, the comparison
of the shape effects on catalytic activity was performed under the
condition of similar particle sizes, thus providing meaningful
information for assessing the electrocatalytic activities.
The synthesis started with a modification of the previously
reported methods for the synthesis of PtFe nanoparticles30 and
PtVFe or PtNiFe nanoparticles.31,32 In the reduction reaction of
Pt(II) acetylacetonate with 1,2-hexadecanediol in octyl ether in the
presence of oleic acid and oleylamine as capping agents at a
temperature of 200 1C, Cr hexacarbonyl in the range of 5–15 mM
was introduced under controlled precursor ratios, which underwent a thermal decomposition under the reaction condition (see
ESIw, Table S1 and Fig. S1). Fig. 1(a) shows a representative TEM
image of the nanoparticles synthesized with an Pt : Cr precursor
ratio of 1 : 1. The nanoparticles exhibit predominantly cubic
shape, and are monodispersed (7.0 0.7 nm). The cube-shaped
nanoparticles form ordered domains at an average interparticle
spacing of 2.8 nm, implying an interdigitated structure of the
capping oleylamine and oleic acid molecules (B2.6 nm).
To assess the importance of chromium in the shape control,
attempts were made to synthesize Pt nanoparticles or Cr nanoparticles using only Pt- or Cr-precursors under similar conditions.
It was unsuccessful in obtaining Cr nanoparticles, most likely due
to ineffective reduction and easy re-oxidation of Cr. There are no
prior examples demonstrating strong adsorption of molecules of
similar structures on Cr surfaces. For the as-synthesized Pt
nanoparticles, the resulting particles (Fig. 1(b)) displayed polypod
a
Department of Chemistry, State University of New York at
Binghamton, Binghamton, NY 13902, USA.
E-mail: [email protected]
b 3
S IP Analytical and Diagnostics Laboratories, Binghamton,
NY 13902, USA
c
Department of Geology, State University of New York at
Binghamton, Binghamton, NY 13902, USA
d
Pacific Northwest National Laboratory, Richland, WA 99352, USA
w Electronic supplementary information (ESI) available: Experimental
details, XPS, EDS, TEM and HRTEM. See DOI: 10.1039/c0cc01379j
7184
Chem. Commun., 2010, 46, 7184–7186
Fig. 1 TEM micrographs and size distributions of the as-synthesized
Pt nanocubes (a, 7.0 0.7 nm) and Pt nanoparticles (b, 4.5 0.8 nm).
(c) HR-TEM micrographs for the as-synthesized nanocubes.
This journal is
c
The Royal Society of Chemistry 2010
Downloaded by STATE UNIVERSITY OF NEW YORK AT BINGHAMTON on 20 September 2010
Published on 24 August 2010 on http://pubs.rsc.org | doi:10.1039/C0CC01379J
View Online
structure but relatively monodispersed sizes (4.5 0.8 nm). The
average interparticle spacing, 2.7 nm, is also consistent with the
expected presence of organic capping shells.
The examination of detailed atomic lattices of the Pt-nanocubes
revealed high crystallinity (Fig. 1(c)). The inter-atomic lattice
fringe distance was found to be 0.194 nm, which is very close to
the lattice spacing of (200) planes of face-centered cubic Pt
crystals.14 Composition analyses of the Pt-nanocubes, carried
out using various techniques like EDS, DCP-AES and XPS (ESIw,
Fig. S2 and S3), indicated the presence of traces of Cr in
the Pt nanocubes, which is practically absent considering the
instrumental detection limits (1–2%).
Fig. 2 shows a representative set of XRD patterns for the
as-synthesized Pt nanocubes and the nanoparticles. The peaks
observed experimentally for the as-synthesized nanocubes were
found at 39.5, 46.0 and 67.3 (curve a). In comparison, the peaks
observed for the as-synthesized Pt nanoparticles were found at
39.5, 45.8 and 67.2 (curve b), which were essentially identical to
those found in the as-synthesized nanocubes. The lattice
parameter was found to be 0.3940 nm for the nanocubes, and
0.3947 nm for the nanoparticles. The peaks expected for chromium,
44.4, 64.6, 81.7 for (110), (220), and (211), respectively, were
not detected, implying the absence of significant amount of
crystalline chromium in the nanocubes. Interestingly, the relative
peak intensity ratio of (111) vs. (200) was found to be 1 : 1 for the
nanocubes, which was in sharp contrast to the ratio of 2 : 1
found for the Pt nanoparticles. Considering the flat alignment of
the nanocubes on the substrate as revealed by TEM data, this
finding is indicative of the fact that the shape-induced orientation
is responsible for the enhanced (200) peak intensity.
Earlier studies of the Cr adsorption and reduction on Pt(111)
and polycrystalline Pt substrates showed that there is a kinetic
difference for the onset reduction potentials of Cr(VI) species
on Pt(111) and Pt polycrystalline surfaces.28 The preferential
adsorption and favorable oxidation of Cr species on Pt(111) over
Pt(100) are believed to be operative for the difference in the
adsorption of capping agents on Pt(100) and Pt(111) facets,
leading to an effective blocking of the adsorption of oleylamine
and oleic acid to Pt(111) by the adsorption of Cr (Scheme 1).
While a quantitative explanation for the desorption is not ready at
this time, our qualitative assessment is currently based on the high
kinetic mobility of Cr-oxide species on the surface as shown in an
earlier report,26 which might be responsible for the easy desorption
of Cr-oxide species from the surface. It is also known that the
Fig. 2 XRD patterns for the as-synthesized Pt nanocubes (a, top
curve) and Pt nanoparticles (b, bottom curve).
This journal is
c
The Royal Society of Chemistry 2010
Scheme 1 A schematic illustration of Cr-assisted growth of Pt
nanocube in terms of preferential adsorption and favorable oxidation
of Cr species on Pt(111) over Pt(100), adsorption of capping agents on
Pt(100) and Pt(111) facets, effective blocking of the adsorption of
oleylamine and oleic acid to Pt(111), and the faster growth rate along
h111i than along h100i direction.
capping agent adsorbs more strongly to the (100) face of the
cuboctohedron nanocrystal than (111) based on the surface free
energy consideration.33 The combination of effective oxidative
leaving of the adsorbed Cr species and the weaker adsorption of
capping agents on Pt(111) constitutes thus the kinetic driving force
for a faster growth rate along the h111i direction than along the
h100i direction leading to the formation of a cube-shaped particle.
To examine the shape effect on the ORR electrocatalytic
activity, the Pt-nanocubes were supported on carbon (Pt-nanocubes/C) and subjected to controlled thermal treatment to remove
the organic shell and activate the catalytic sites. The shape was not
affected in a significant way after the thermal treatment, as
demonstrated by the observation of the predominant lattice fringe
(0.194 nm) corresponding to (100) plane of nanocubes (see Fig. S5
and S6, ESIw). The shape feature is in contrast to that for the
largely spherical-shaped Pt nanoparticles supported on carbon
which has similar particle size (B7 nm, see Fig. S4, ESIw).
Cyclic voltammetric (CV) characterization of surface properties
of the nanocube catalysts was performed to determine the
facet-dependent hydrogen adsorption/desorption characteristics
(Fig. 3(A)). For the cathodic sweep, it has been established
that the peak at 0.05–0.15 V corresponds largely to hydrogen
adsorption at surface defects or corners/edges or (110) facets
whereas the peak at 0.20–0.30 V corresponds largely to
hydrogen adsorption at (100) faces. In comparison with the
relatively-larger peak current at 0.05–0.15 V and a relativelysmaller peak current at 0.20–0.30 V for Pt/C, the nanocubes/C
catalyst showed a noticeable change of peak current (ipeak) in
these two potential regions. This change was evidenced by the
increase of ipeak ratio of these two peaks (i.e., ipeak (0.2–0.3 V)/
ipeak (0.05–0.15 V)) from Pt-nanoparticle/C to Pt-nanocube/C
catalysts (Table 1). In addition, the electrochemical active
areas (ECA) measured from the CV curves in the hydrogen
adsorption region, characteristic of the Pt-specific surface sites
on the catalysts, showed the expected decrease as the particle
size increases.
Chem. Commun., 2010, 46, 7184–7186
7185
Downloaded by STATE UNIVERSITY OF NEW YORK AT BINGHAMTON on 20 September 2010
Published on 24 August 2010 on http://pubs.rsc.org | doi:10.1039/C0CC01379J
View Online
the nanocrystal shape, part of our on-going investigation also
involves probing whether the possible presence of traces of
PtCr alloy in the outermost layer of nanocubes is operative in
the enhancement of electrocatalytic activity, and determining
the shape effect on the activity for Pt-based nanocubes with
sizes less than 7 nm.
The research work was supported by the National Science
Foundation (CBET 0709113, CHE 0848701). The XPS was
performed using EMSL, a national scientific user facility
sponsored by DOE’s Office of Biological and Environmental
Research located at Pacific Northwest National Laboratory.
Notes and references
Fig. 3 CV (A) and RDE (B) curves comparing Pt nanoparticles
(B7 nm)/C (a) and Pt nanocubes (B7 nm)/C (b) catalysts in
N2-saturated 0.1 M HClO4 (50 mV s1) (A), and in O2-saturated
0.1 M HClO4 (1600 rpm and 10 mV s1) (B).
Table 1 Comparison of particle size, metal loading, and electrocatalytic properties of Pt/C (Et); Pt/C; and Pt-NC/C
Particle
Catalysts size/nm
ML
ECA/
(wt%) m2 gPt1 ipeak
Pt/C (Et) 2.5 0.5 20
Pt/C
7.1 1.6 23
Pt-NC/C 7.9 1.1 24
91
55
26
0.3
0.4
0.6
ratio
MA/
SA/
A mgPt1 mA cm2
0.22
0.11
0.13
0.24
0.20
0.49
Note: ipeak ratio is defined as ipeak (B0.2 V)/ipeak (B0.1 V). ML: Metal
loading; Et: E-tek; NC: nanocubes; ECA: electrochemical active area;
MA: mass activity; SA: specific activity.
The electrocatalytic activity of the Pt-nanocube/C catalysts
for ORR was compared with Pt/C catalysts based on rotating
disk electrode (RDE) measurements (Fig. 3(B) and Table 1).
In comparison with Pt/C of similar particle sizes, the
Pt-nanocube/C catalyst showed a small increase in the kinetic
current region (e.g., at 0.900 V). The mass activity of the Pt
nanocube/C catalyst was found to be basically comparable
with that for Pt/C (B7 nm), and about half of that for the
commercial Pt/C catalysts with a much smaller size (B3 nm).
Clearly, the size effect is more significant than the shape effect
for the mass activity because of the difference in active surface
areas. However, by taking the particle size and the ECA value
into consideration, the specific activity of the nanocube
catalyst was found to display a 2–3 fold increase in comparison
with those of Pt/C (3 nm and 7 nm).
This finding demonstrates that the presence of significant
(100) faces in the Pt-nanocubes/C is at least partially responsible
for the increase in the specific activity. In addition to delinating the
detailed correlation between the electrocatalytic activity and
7186
Chem. Commun., 2010, 46, 7184–7186
1 Y. D. Qian, W. Wen, P. A. Adcock, Z. Jiang, N. Hakim,
M. S. Saha and S. Mukherjee, J. Phys. Chem. C, 2009, 112, 1146.
2 V. A. Grinberg, T. L. Kulova, N. A. Maiorova, Zh.
V. Dobrokhotova, A. A. Pasynskii, A. M. Skundin and
O. A. Khazova, Russ. J. Electrochem., 2007, 43, 77.
3 E. Antolini, J. R. C. Salgado, L. G. R. A. Santos, G.
Garcia, E. A. Ticianelli, E. Pastor and E. R. Gonzalez, J. Appl.
Electrochem., 2006, 36, 355.
4 R. C. Koffi, C. Coutanceau, E. Garnier, J. M. Leger and C. Lamy,
Electrochim. Acta, 2005, 50, 4117.
5 P. P. Wells, Y. Qian, C. R. King, R. J. K. Wiltshire, E. M. Crabb,
L. E. Smart, D. Thompsett and A. E. Russell, Faraday Discuss.,
2008, 138, 273.
6 T. S. Ahmadi, Z. L. Wang, T. C. Green, A. Henglein and M. A.
El-Sayed, Science, 1996, 272, 1924.
7 X. Teng and H. Yang, Nano Lett., 2005, 5, 885.
8 J. Chen, T. Herricks and Y. Xia, Angew. Chem., Int. Ed., 2005, 44, 2589.
9 H. Lee, S. E. Habas, S. Kweskin, D. Butcher, G. A. Somorjai and
P. Yang, Angew. Chem., Int. Ed., 2006, 45, 7824.
10 T. Herricks, J. Chen and Y. Xia, Nano Lett., 2004, 4, 2367.
11 J. Chen, T. Herricks, M. Geissler and Y. Xia, J. Am. Chem. Soc.,
2004, 126, 10854.
12 M. E. Grass, Y. Yue, S. E. Habas, R. M. Rioux, C. I. Teall,
P. Yang and G. A. Somorjai, J. Phys. Chem. C, 2008, 112, 4797.
13 H. Song, F. Kim, S. Connor, G. A. Somorjai and P. Yang, J. Phys.
Chem. B, 2005, 109, 188.
14 C. Wang, H. Daimon, Y. Lee, J. Kim and S. Sun, J. Am. Chem.
Soc., 2007, 129, 6974.
15 J. Zhang and J. Fang, J. Am. Chem. Soc., 2009, 131, 18543.
16 J. Zhang, H. Yang, J. Fang and S. Zou, Nano Lett., 2010, 10, 638.
17 S. I. Lim, I. Ojea-Jimenez, M. Varon, E. Casals, J. Arbiol and
V. Puntes, Nano Lett., 2010, 10, 964.
18 J. Chen, B. Lim, E. P. Lee and Y. Xia, Nano Today, 2009, 4, 81.
19 C. K. Tsung, J. N. Kuhn, W. Huang, C. Aliaga, L. I. Hung,
G. A. Somorjai and P. Yang, J. Am. Chem. Soc., 2009, 131, 5816.
20 B. Lim, M. Jiang, P. H. C. Camargo, E. C. Cho, J. Tao, X. Lu,
Y. Zhu and Y. Xia, Science, 2009, 324, 1302.
21 Q. Yuan, Z. Zhou, J. Zhuang and X. Wang, Chem. Commun.,
2010, 46, 1491.
22 B. Y. Xia, J. N. Wang and X. X. Wang, J. Phys. Chem. C, 2009,
113, 18115.
23 H. Zhou, W. Zhou, R. R. Adzic and S. S. Wong, J. Phys. Chem. C,
2009, 113, 5460.
24 R. Narayanan and M. A. El-Sayed, Top. Catal., 2008, 47, 15.
25 G. A. Somorjai and D. W. Blakely, Nature, 1975, 258, 580.
26 L. Zhang, M. Kuhn and U. Diebold, Surf. Sci., 1997, 375, 1.
27 L. Zhang, M. Kuhn, U. Diebold and J. A. Rodriguez, J. Phys.
Chem. B, 1997, 101, 4588.
28 R. Bujak and K. Varga, Electrochim. Acta, 2006, 52, 332.
29 A. B. Anderson and P. Shiller, Surf. Sci., 1996, 345, 274.
30 S. Sun, C. B. Murray, D. Weller, L. Folks and A. Moser, Science,
2000, 287, 1989.
31 J. Luo, L. Wang, D. Mott, P. N. Njoki, N. N. Kariuki, C. J. Zhong
and T. He, J. Mater. Chem., 2006, 16, 1665.
32 J. Luo, L. Han, N. N. Kariuki, L. Wang, D. Mott, C. J. Zhong and
T. He, Chem. Mater., 2005, 17, 5282.
33 D. Mott, J. Galkowski, L. Wang, J. Luo and C. J. Zhong,
Langmuir, 2007, 23, 5740.
This journal is
c
The Royal Society of Chemistry 2010