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.
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.
The NO2 gas sensing behavior of porous silicon(PS) is studied at room temperature with and without ultraviolet(UV) light radiation.The PS layer is fabricated by electrochemical etching in an HF-based solution on a p +-type silicon substrate.Then,Pt electrodes are deposited on the surface of the PS to obtain the PS gas sensor.The NO2 sensing properties of the PS with different porosities are investigated under UV light radiation at room temperature.The measurement results show that the PS gas sensor has a much higher response sensitivity and faster response-recovery characteristics than NO2 under the illumination.The sensitivity of the PS sample with the largest porosity to 1 ppm NO2 is 9.9 with UV light radiation,while it is 2.4 without UV light radiation.We find that the ability to absorb UV light is enhanced with the increase in porosity.The PS sample with the highest porosity has a larger change than the other samples.Therefore,the effect of UV radiation on the NO2 sensing properties of PS is closely related to the porosity.
WO3 thin films were sputtered onto alumina substrates by DC facing-target magnetron sputtering.One sample was rapid-thermal-annealed(RTA) at 600℃ in a gas mixture of N2:O2=4:1,and as a comparison,another was conventionally thermal-annealed at 600℃ in air.The morphology of both was investigated by scanning electron microscopy(SEM) and atomic force microscopy(AFM),and the crystallization structure and phase identification were characterized by X-ray diffraction(XRD).The NO2-sensing measurements were taken under LED light at room temperature.The sensitivity of the RTA-treated sample was found to be high,up to nearly 100,whereas the sensitivity of the conventionally thermal-annealed sample was about five under the same conditions.From the much better selectivity and response-recovery characteristics,it can be concluded that compared to conventional thermal annealing,RTA has a greater effect on the NO2-sensing properties of WO3 thin films.
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.
