The First Principles Study Of Doped TiO₂Surfaces

Nowadays, energy crisis and environmental pollution problem is becoming more and more serious, and this restricts the further development of social and economic. Photocatalytic technology is an ideal technology which can use solar power to drive some reactions. It is the key to solve environmental pollution and produce clean energy so that people pay more extensive attention to it now. TiO2photocatalytic material is reseached and used the most widely because it has the advantages of non-toxic, stability, low cost and easy to synthesis. But as a result of the big band gap width, only ultraviolet light can be absorbed, and the recombination of the electronic-hole is easy which cause low quantum efficiency. As a simple and effective method, doping is widely used for modification of TiO2to improve the photocatalysis activity in the visible light.The researches of doping TiO2concentrate on the bulk, but as is well-known, most physical and chemical phenomena occur in the surfaces and the structures and properties of the surfaces have a huge impact on its application. The study about surface structures is lack of depth and it is very difficulty to obtain more information in the atom scale from the existing experiment methods. In this paper, the surface energy, band structure, DOS of0vacancy defects, I-doped, C/N-codoped rutile TiO2(110) and anatase TiO2(101) surfaces are calculated to explore the impact on photocatalytic performance. The adsorption and dissociation of H2O, CH3OH, HCHO on undoped and N-doped rutile TiO2(110) and anatase TiO2(101) surfaces are also discussed. The obtained results are as follows.For undoped rutile TiO2(l10) and anatase TiO2(101) surfaces, the band gap of surface structures is larger than that of corresponding bulk structures. This is because eletrons of surface layer transfer to bulk. O vacancy defects prefer to be formed on the reducing conditions than on the oxidition conditions. The electrons transfer to adjacent Ti atoms reducing the charges of Ti due to the generation of O vacancy. The band structure and DOS indicate that the band gap decreases by0.35eV and0.27eV for rutile TiO2(110) and anatase TiO2(101) surfaces after surface O vacancy forms. As a result, the light absorption could be extended to455nm and419nm respectively, and the photocatalysis could be improved.For I substitutional doped rutile TiO2(110) and anatase TiO2(101) surfaces, O atom is easier to be replaced by I on the reducing conditions and I prefer to replace Ti atom on the oxiding conditions. When I replace O or Ti of rutile TiO2(110) surface, the band gap reduces by0.41eV and0.34eV and the light absorption range is extended to465nm and454nm respectively. For1doped anatase TiO2(101) surface, when O is replaced the band gap has a reduction of0.28eV and the light absorption can be extend to420nm. When Ti is replaced, there is a band in the forbidden gap which has the advantage of improving light absorption.There are two kinds of C/N codoped anatase TiO2(101) surfaces, the first is C, N replaces Ti, O respectively and the second is C, N both replace O. For the first structure, the band gap reduce to1.703eV which is smaller than that of C, N single doped TiO2(101) surface. The reason is that C could shift down the conduction band and N2p state is located the top of valence band to reduce the band gap. There exists synergistic effect between C and N. For the second structure, there is a band which has a1.493eV distance from the top of valence band which can extend the light absorption to the visible light. Both of the two structures are beneficial to improve photocatalysis of the visible light.For H2O, CH3OH, HCHO adsorption on undoped rutile TiO2(110) and anatase TiO2(101) surfaces, CH3OH prefers to dissociation adsorption on the undoped anatase TiO2(101) surface, The rest are adsorbed molecularly mainly. After N is doped on these surfaces, the dissociation adsorption is more stable for three molecule, the dissociation reaction is exothermic. Except the reaction activation of CH3OH on N doped rutile TiO2(110) surface increases by0.1eV, All of the other reaction activation reduces to varying degrees which can make the dissociation reaction easier to occur.