The DFT Study Of WO₃ Adsorption Mechanism

With the larger number of exhaust gases in the atmosphere and people's increasing awareness of environment protection, the gas sensor with high performance and low power cost have attracted more and more attentions. In this thesis, gas sensing mechanisms of tungsten oxide crystal with diverse nanostructures to different kinds of gases were studied; the computational results were proved to be reasonable by experimental data.First of all, (001), (100), (101), (010), (020) orientation surfaces were built based on WO₃ crystal. After calculation and comparison of surface energies, it is found that (002) surface has the lowest energy, while both (101) and (020) surfaces have the largest energy. Thus, (002) surface which is the preferred crystallizing surface was selected to investigate NH3 sensing mechanism. On this surface, O1c site adsorption structure with the lowest adsorption energy was considered as the best and the most stable adsorption system. The Fermi energy of the whole system was increased by about 1.08eV after NH3 adsorption. At the same time, NH3 molecular lost 0.35 electrons; that means (002) surface of WO₃ obtained 0.35 electrons. Above both changes are the factors of WO₃ surface resistance decreasing.In addition, WO₃ nanosheets were synthesized by solvothermal method; and the characterization results showed that the nanosheets grow along the [010] and [100] crystallographic direction and is enclosed by±(001) facets. Therefore, the largest exposed (002) surface was considered as the gas adsorption surface. In this work, the NO2 sensing mechanism of WO₃ (002) surface was studied. There are some conclusions as following: O1c site adsorption structure has the lowest adsorption energy and is the most stable adsorption system. NO₂ adsorption in this site makes some changes to partial density of state (PDOS) of surface atoms, and produces extra energy level near the valence band in band gap. Also, NO2 molecular got 0.58 electrons from WO₃ surface after adsorption, which means the surface lost 0.58 electrons. Extra accepter energy level and surface electrons losing contributed to the surface resistance rising after NO2 adsorption. This calculation results are in good agre ement with experimental testing results.