WO 3 bulk and various surfaces are studied by an ab-initio density functional theory technique. The band structures and electronic density states of WO 3 bulk are investigated. The surface energies of different WO 3 surfaces are compared and then the (002) surface with minimum energy is computed for its NH 3 sensing mechanism which explains the results in the experiments. Three adsorption sites are considered. According to the comparisons of the energy and the charge change between before and after adsorption in the optimal adsorption site O 1c , the NH 3 sensing mechanism is obtained.

Density functional theory (DFT) calculations are employed to explore the NO 2 -sensing mechanisms of pure and Ti-doped WO 3 (002) surfaces. When Ti is doped into the WO 3 surface, two substitution models are considered: substitution of Ti for W 6c and substitution of Ti for W 5c . The results reveal that substitution of Ti for 5-fold W forms a stable doping structure, and doping induces some new electronic states in the band gap, which may lead to changes in the surface properties. Four top adsorption models of NO 2 on pure and Ti-doped WO 3 (002) surfaces are investigated: adsorptions on 5-fold W (Ti), on 6-fold W, on bridging oxygen, and on plane oxygen. The most stable and likely NO 2 adsorption structures are both N-end oriented to the surface bridge oxygen O 1c site. By comparing the adsorption energy and the electronic population, it is found that Ti doping can enhance the adsorption of NO 2 , which theoretically proves the experimental observation that Ti doping can greatly increase the WO 3 gas sensor sensitivity to NO 2 gas.