Synthesis of tin dioxide octahedral nanoparticles with exposed high-energy {221} facets and enhanced gas-sensing properties.

Tin dioxide (SnO2), an n-type semiconductor with a wide band gap of 3.6 eV, has been widely used in photocatalytic degradation of organic dyes, photovoltaic devices, rechargeable lithium batteries, and so on. In particular, remarkable receptivity to variations in gaseous environments and excellent chemical stability have made SnO2 the bestknown gas-sensing material. Over the past decades, considerable efforts have been made to improve the sensitivity and selectivity of SnO2-based solid-state gas sensors through modifying the sensing material itself and the fabrication technique, such as doping of catalytic metal particles, hybridization of different sensing materials, and optimization of working temperature. In principle, gas sensing by metal-oxide semiconductors like SnO2 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. 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. 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 SnO2 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 SnO2 have a higher relative surface energy (2.28 Jm ) than common low-index facets such as {110} (1.401 Jm ), {101} (1.554 Jm ), and {100} (1.648 Jm ). As we expected, the as-prepared SnO2 octahedra exhibit far better gas-sensing performance over ethanol than those mainly having exposed {110} facets. Usually, a crystal prefers to expose crystal planes with low surface energy during growth. For example, both natural and synthetic forms of SnO2 crystals are usually enclosed by {110}, {101}, or {100} facets with low surface energy. In recent years, special attention has been paid to developing methods to acquire nanocrystals with exposed high-energy surfaces, which usually show good chemical performance, for example, high catalytic activity. Recently, Lu et al. and we prepared TiO2 nanostructures with a large percentage of highenergy (001) facets, by taking advantage of the selective adsorption of fluoride ions. These results inspired us to synthesize other metal-oxide nanoparticles with high-energy crystal facets with the assistance of halide ions and other inorganic species. The octahedral SnO2 particles with high-index {221} facets were obtained through a hydrothermal route at 200 8C for 12 h, whereby SnCl4·5H2O was gradually hydrolyzed to form SnO2 in an appropriate acidic environment adjusted with HCl in the presence of PVP. Figure 1a shows a typical powder Xray diffraction (XRD) pattern of the as-prepared product, which can be indexed to the rutile phase of bulk SnO2 with cell constants of a= 4.738 and c= 3.187 (JCPDS No. 411445). Scanning electron microscopy (SEM) showed that the product consists of high-purity particles with smooth surfaces (Figure 1b). These particles are of well-defined octahedral shape with an edge-to-edge width W of about 200 nm, and an apex-to-apex length L around 300 nm (see the inset of Figure 1b). More-detailed structural information on the octahedral SnO2 particles was provided by transmission electron micro-