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.
Density functional theory (DFT) calculations are conducted to explore the interaction of H_2 with pure and Tidoped WO_3 (002) surfaces.Four top adsorption models of H_2 on pure and Ti-doped WO_3 (002) surfaces are investigated respectively,they are adsorption on bridging oxygen O_(1c),absorption on plane oxygen O_(2c),absorption on 5-fold W_(5c) (Ti),and absorption on 6-fold W_(6c).The most stable and H_2 possible adsorption structure in the pure surface is H-end oriented to the surface plane oxygen O_(2c) site,while the favourable adsorption sites for H_2 in a Ti-doped surface is not only an O_(2c) site but also a W_(6c) site.The adsorption energy,the Fermi energy level E_F,and the electronic population are investigated and the H_2-sensing mechanism of a pure-doped WO_3 (002) surface is revealed theoretically:the theoretical results are in good accordance with our existing experimental results.By comparing the above three terms,it is found that Ti doping can obviously enhance the adsorption of H_2.It can be predicted that the method of Ti-doped into a WO_3 thin film is an effective way to improve WO_3 sensor sensitivity to H_2 gas.
