Density functional theory study of NO 2 -sensing mechanisms of pure and Ti-doped WO 3 (002) surfaces

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.