A novel three-dimensional(3D) hierarchical structure and a roughly oriented one-dimensional(1D) nanowire of WO_3are selectively prepared on an alumina substrate by an induced hydrothermal growth method.Each hierarchical structure is constructed hydrothermally through bilateral inductive growth of WO_3 nanowire arrays from a nanosheet preformed on the substrate.Only roughly oriented 1D WO_3 nanowire can be obtained from a spherical induction layer.The analyses show that as-prepared 1D nanowire and 3D hierarchical structures exhibit monoclinic and hexagonal phases of WO_3,respectively.The gas-sensing properties of the nanowires and the hierarchical structure of WO_3,which include the variations of their resistances and response times when exposed to NO_2,are investigated at temperatures ranging from room temperature(20 ℃) to 250 ℃ over 0.015 ppm-5 ppm NO_2.The hierarchical WO_3 behaves as a p-type semiconductor at room temperature,and shows p-to-n response characteristic reversal with the increase of temperature.Meanwhile,unlike the1 D nanowire,the hierarchical WO3 exhibits an excellent response characteristic and very good reversibility and selectivity to NO_2 gas at room temperature due to its unique microstructure.Especially,it is found that the hierarchical VO_3-based sensor is capable of detecting NO_2 at a ppb level with ultrashort response time shorter than 5 s,indicating the potential of this material in developing a highly sensitive gas sensor with a low power consumption.
The effects of the surface and orientation of a WO3 nanowire on the electronic structure are investigated by using first principles calculation based on density functional theory（DFT）.The surface of the WO3 nanowire was terminated by a bare or hydrogenated oxygen monolayer or bare WO2 plane,and the[010]- and[001]-oriented nanowires with different sizes were introduced into the theoretical calculation to further study the dependence of electronic band structure on the wire size and orientation.The calculated results reveal that the surface structure, wire size and orientation have significant effects on the electronic band structure,bandgap,and density of states （DOS） of the WO3 nanowire.The optimized WO3 nanowire with different surface structures showed a markedly dissimilar band structure due to the different electronic states near the Fermi level,and the O-terminated[001] WO3 nanowire with hydrogenation can exhibit a reasonable indirect bandgap of 2.340 eV due to the quantum confinement effect,which is 0.257 eV wider than bulk WO3.Besides,the bandgap change is also related to the orientation-resulted surface reconstructed structure as well as wire size.
