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Science 4 May 2007:
Vol. 316. no. 5825, pp. 732 - 735
DOI: 10.1126/science.1140484
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Reports
Synthesis of Tetrahexahedral Platinum Nanocrystals with High-Index Facets and High Electro-Oxidation Activity
Na Tian,1 Zhi-You Zhou,1 Shi-Gang Sun,1* Yong Ding,2 Zhong Lin Wang2*
The shapes of noble metal nanocrystals (NCs) are usually defined by polyhedra that are enclosed by {111} and {100} facets, such as cubes, tetrahedra, and octahedra. Platinum NCs of unusual tetrahexahedral (THH) shape were prepared at high yield by an electrochemical treatment of Pt nanospheres supported on glassy carbon by a square-wave potential. The single-crystal THH NC is enclosed by 24 high-index facets such as {730}, {210}, and/or {520} surfaces that have a large density of atomic steps and dangling bonds. These high-energy surfaces are stable thermally (to 800°C) and chemically and exhibit much enhanced (up to 400%) catalytic activity for equivalent Pt surface areas for electro-oxidation of small organic fuels such as formic acid and ethanol.
1 State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China.
2 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332–0245, USA.
* To whom correspondence should be addressed. E-mail:
sgsun@xmu.edu.cn (S.G.S.);
zhong.wang@mse.gatech.edu (Z.L.W.)
Generally, catalytic performance of nanocrystals (NCs) can be finely tuned either by their composition, which mediates electronic structure (1, 2), or by their shape, which determines surface atomic arrangement and coordination (3, 4). Fundamental studies of single-crystal surfaces of bulk Pt have shown that high-index planes generally exhibit much higher catalytic activity than that of the most common stable planes, such as {111}, {100}, and even {110}, because the high-index planes have a high density of atomic steps, ledges, and kinks, which usually serve as active sites for breaking chemical bonds (5–7). For example, a bulk Pt(210) surface possesses extremely high catalytic reactivity for electroreduction of CO2 (8) and electro-oxidation of formic acid (9). The bulk Pt(410) surface exhibits unusual activity for catalytic decomposition of NO, a major pollutant of automobile exhaust (10). Thus, the shape-controlled synthesis of metal NCs bounded by high-index facets is a potential route for enhancing their catalytic activities.
It is, however, rather challenging to synthesize shape-controlled NCs that are enclosed by high-index facets because of their high surface energy. Crystal growth rates in the direction perpendicular to a high-index plane are usually much faster than those along the normal direction of a low-index plane, so high-index planes are rapidly eliminated during particle formation (11). During the past decade, a variety of face-centered cubic (fcc) structured metal NCs with well-defined shapes have been synthesized, but nearly all of them are bounded by the low-index planes, such as tetrahedron, octahedron, decahedron, and icosahedron, enclosed by {111} facets (12–14), cube by {100} (12, 15), cuboctahedron by {111} and {100} (16), and rhombic dodecahedron by {111} (17). Here we describe an electrochemical method for the synthesis of tetrahexahedral (THH) Pt NCs at high purity. The THH shape is bounded by 24 facets of high-index planes {730} and vicinal planes such as {210} and {310}. The synthesized THH Pt NCs show enhanced catalytic activity in electro-oxidation of small organic fuels of formic acid and ethanol, demonstrating their potential for use in the traditional applications of Pt group metal nanoparticles, including catalysts (18), automotive catalytic converters (19), fuel cells (20), and sensors (21).
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