With octahedral Au nanocrystals as seeds, highly monodisperse Au@Pd and Au@Ag core-shell nanocubes were synthesized by a two-step seed-mediated method in aqueous solution. Accordingly, we have preliminarily proposed a general rule that the atomic radius, bond dissociation energy, and electronegativity of the core and shell metals play key roles in determining the conformal epitaxial layered growth mode.

Tin dioxide (SnO 2 ), an n-type semiconductor with a wide band gap of 3.6 eV, has been widely used in photocatalytic degradation of organic dyes, [1] photovoltaic devices, [2] rechargeable lithium batteries, [3] and so on. In particular, remarkable receptivity to variations in gaseous environments and excellent chemical stability have made SnO 2 the bestknown gas-sensing material. [4][5][6][7][8][9] Over the past decades, considerable efforts have been made to improve the sensitivity and selectivity of SnO 2 -based solid-state gas sensors through modifying the sensing material itself and the fabrication technique, such as doping of catalytic metal particles, [4] hybridization of different sensing materials, [5, 6] and optimization of working temperature. [7] In principle, gas sensing by metal-oxide semiconductors like SnO 2 is based on the oxidation-reduction reaction of the detected gases occurring on the semiconductor surface, which leads to an abrupt change in conductance of the sensor. For this reason, the gas-sensing ability of metal oxide semiconductors is in theory very sensitive to the crystal faces of the sensing materials.[10] From the viewpoint of chemical activity, metaloxide nanocrystals with particular exposed crystal planes, such as high-index facets, may be good sensing materials, because high-index facets having high densities of atom steps, ledges, kinks, and dangling bonds usually exhibit much higher chemical activity. [11][12][13] However, such a strategy to improve sensitivity and selectivity of sensors has not attracted much attention up to now, possibly due to the difficulty of synthesizing metal-oxide nanocrystals with specific exposed crystal planes.Herein we report a simple method for the preparation of octahedron-shaped SnO 2 with exposed high-index {221} facets by exploiting the coordinative-adsorption effect of HCl and poly(vinyl pyrrolidone), PVP, in solution. The {221} facets of SnO 2 have a higher relative surface energy (2.28 J m À2 ) than common low-index facets such as {110} (1.401 J m À2 ), {101} (1.554 J m À2 ), and {100} (1.648 J m À2

National Natural Science Foundation of China [20725310, 20721001, 20673085, 20671079, J0630429]; National Basic Research Program of China [2007CB815303, 2009CB939804]; Fujian Province of China [2005HZ013

Controlled syntheses of multicomponent metal nanocrystals (NCs) and high-index surfaces have attracted increasing attention due to the specific physical and chemical properties of such NCs. Taking advantage of copper underpotential deposition as a bridge, hexoctahedral Au-Pd alloy NCs with {hkl} facets exposed were successfully synthesized, while phase separation occurred in the absence of Cu(2+) ions. The as-prepared hexoctahedral Au-Pd alloy NCs exhibited very excellent performance in terms of both formic acid electro-oxidation and methanol tolerance due to synergism between the high-index facets and the alloy.

The shape of gold nanoparticles has been successfully tuned among penta-twinned decahedrons, truncated tetrahedrons, cubes, octahedrons, hexagonal thin plates by introducing a small amount of salt into a N,N-dimethylformamide (DMF) solution containing poly(vinyl pyrrolidone) (PVP), and changing the temperature or the concentration of the gold precursor.