| Tungsten trioxide materials have been widely studied owing to good sensitivity performances of NOx, NH3, H2S and H2 gas. In order to meet the demands of gas sensors with the feature of high sensitivity, low power consumption and fast response, doping is always used to improve their gas properties. Currently, the research of WO3 gas sensor has been extensive in the experiments, but a little in theory on the mechanism of gas-sensitive. About there has not been any report about gas sensing of doped WO3. In this paper, doped WO3 gas-sensing mechanism also thoroughly studied and analyzed using a first-principles calculation which is based on density functional theory （DFT）.Firstly, Ti-doped WO3 （002） surface electronic properties are explored. When Ti is doped into WO3 surface, the two substitution models are considered: the substitution of Ti for W6c and the substitution of Ti for W5c. The results reveal that substitution of Ti for 5-fold W forms a stable doping structure. Known from the results of band structure and DOS, some new electronic states in the band gap induced by Ti doping may lead to the changes of the surface properties.Secondly, Ti-WO3 （002） surface gas sensitive mechanisms of NO2, NH3 and H2 are studied separately. Four top adsorption models of gas onto Ti doped WO3 （002） surfaces are investigated respectively: adsorptions on bridging oxygen O1c, on plane oxygen O2c, on Ti, and on 6-fold W6c, respectively. The most stable and possible adsorption structures of NO2 and NH3 on Ti-WO3 （002） surface are both N-end oriented to the surface bridge oxygen O1c site, while the favorable adsorption sites for H2 with Ti doped surfaces is not only O2c site but also W6c site. The surface structure, density of states and electronic population of all optimal gas adsorption models are calculated and analyzed, the changes of band structure, Fermi level and electrons transfer are main reasons of resistance changes of WO3 after gas adsorption, which reveals the Ti-WO3 material gas sensitive mechanism.Finally, through the comparison between the gas sensing mechanism of Ti-WO3 （002） and the one of pure WO3 （002） on NO2, NH3 and H2, it is found that changes of band gap and Fermi level by Ti doping results that Ti doped WO3 models transfer more electrons during gas adsorption processes than pure WO3 model and the resistance of Ti-WO3 material changes more after gas adsorption. It can be inferred that the Ti doping is an effective way to improve the gas sensing properties of gas sensor based on WO3. |
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