Effect of optical gas sensing properties rutile phase oxide TiO<sub>2</sub>, SnO<sub>2</sub> and GeO<sub>2</sub> surface oxidation with HCl molecules

Abstract

Recently, optical gas sensing materials become a research hotspot, which can detect the concentration of environment gas sensitively and quickly. Environment gas molecule adsorbed on gas sensing materials surface will cause changes in the optical properties of materials, so as to achieve the parameters of gas composition and concentration. Rutile phase TiO2, SnO2 and GeO2 are widely used in optical gas sensing semiconductor materials. The three semiconductor have the same structure type, and the (110) surface has the lowest energy. The band gap of the three are 3.0, 3.6, 5.04 eV, and their photo-chemical properties have been used in many fields. Hydrogen chloride is a dangerous gas, which has a very strong odor, colorless, non-flammable, easily soluble in water and strong corrosive. So detection the concentration of HCl gas in environment is very important. In this paper, the microscopic mechanism of HCl gas adsorption on TiO2, SnO2 and GeO2(110) surface is studied, which adopted first-principles plane-wave ultrasoft pseudo-potential method based on density functional theory (DFT-D). The calculation used the castep module in Material Studio software. The DFT-D method deal with electron-related energy of HCl gas, TiO2, SnO2 and GeO2 surface by Perdew-Burke-Ernzerh (PBE) exchange correlation functional under generalized gradient approximation (GGA) respectively. After HCl molecule adsorbed on three kinds oxide surface, micro-change and optical gas sensing properties of the three surface were researched including adsorption energy, adsorption distance, density of states, charge population distribution, differential charge density, dielectric function, absorption coefficient and reflectivity, et al. The results showed that: after HCl gas adsorbed on rutile TiO2, SnO2 and GeO2(110) surface, the distance of molecule and surface is reduced respect to the initial distance, and the order of distance is: d (TiO2) d (SnO2) d (GeO2). The corresponding adsorption energy order is: E (TiO2)> E (SnO2)> E (GeO2). This results indicates that HCl gas molecule on oxygen vacancies are easily adsorbed by rutile TiO2, SnO2 and GeO2(110) surface. Adsorption stability is as follows: TiO2>SnO2>GeO2. Oxidation-reduction reaction of molecule and surface is an important factor influencing adsorption energy and optical gas sensing properties. By charge population analysis, it found on TiO2(110) surface, HCl molecules charge number increased, and on SnO2 and GeO2 surface, HCl molecules charge number decreased. In addition, the HCl molecule decrease charge on GeO2 surface was larger than on SnO2 surface. This conclusion illustrated that the oxidation ability when HCl gas molecules adsorbed on surface are as follows: TiO2>SnO2>GeO2. From state density analysis, the 3p electrons peak value of Cl atom near the Fermi level order is: TiO2(110) surface<SnO2(110) surface<GeO2(110) surface, which corresponds to the number of electron transfer charges in the 3p orbital of the Cl atom (0.03, 0.04, 0.06). This result is consistent with the Cl atom charge population distribution. On the analysis of optical properties, it found that HCl molecules when adsorption on TiO2(110) surface oxygen vacancies, optical properties including dielectric function, absorption coefficient and reflectivity changed the most obvious. Especially for light of 500–700 nm, TiO2 having excellently optical sensing effect, it can be used as an ideal optical gas sensing material.