First Principles Study On The Effect Of The Optical Properties Of TiO₂ After Double Doping And Gas Molecule Adsorption

The worldwide researchers pay attention to energy shortages and environmental pollution problems. In the fields of environmental pollution improvement and photocatalysis applications, as TiO₂ has many excellent characteristics such as chemical stability, neither toxic nor harm, easy to prepare, strong photocatalytic ability, it is extensively employed, which can help to solve environmental problems and energy shortages. However, the photocatalytic activity of TiO₂ is still low. We can reduce the recombination probability of photo-generated holes and photo-generated electrons by incorporating the impurity ions. Thus the optical properties of TiO₂ are also improved. In this thesis, the micromechanism of electronic properties and optical properties of TiO₂ after doping different gas molecules on its surface are investigated. It can provide a theoretical reference for the experimental study of TiO₂ optical gas sensor. The main contents of this thesis are as following:Firstly, by using the first-principles plane-wave ultrasoft pseudopotential method based on density functional theory ?DFT? system, we simulated and calculated codoped TiO₂ such as non-metallic, metal and double metal elements. In this thesis, we studied N and transition metal elements as Sc, V, Cr, Mn, Fe codoped anatase TiO₂ and Cu-Cr codoped rutile TiO₂, respectively. Then we analyzed their band structures, density of states ?DOS?, Mulliken charge distribution and optical properties. The results illustrate that the absorption and reflectivity of the doped system increases in the visible region and a codoping system is more outstanding compared with pure anatase TiO₂. The absorption and reflection capacity of light in the visible region does not increase with the doping atomic number, but with the actual order:N-Fe> N-Cr> N-Mn> N-Sc> N-V> N. Cu/Cr doped rutile TiO₂ can effectively prevent the recombination of photo-generated holes and photo-generated electrons. The optical properties of doped TiO₂ have a red shift in visible region. The visible light absorption ability becomes more strongly. The photocatalytic activity is also improved with the amount of improving in the following order:Cu-Cr codoped>Cu doped>Cr doped.Secondly, gas molecules ?NH₃,H₂O,CH₄? adsorbed on the surface of anatase TiO₂ and rutile TiO₂ are studied respectively. The results reveals that compared with the complete surface, the surface containing oxygen vacancies is much easier to adsorb above gas molecules. The molecular polarization is directly influenced the strength of adsorption and the magnetude of the adsorption energy. In other words, higher polarisation of gas molecule is benefit for its stable adsorption. It is because the negative charge centre is absorbed to the oxygen vacancy on the surface. Electron is transferred from the surface containing oxygen vacancies to the adsorbed molecules ?NH₃,H₂O,CH₄?. The adsorption process is chemical adsorption. The electron on the atoms ?O, N, C? located at negative charge centre is transferred from the adsorbed molecules to the surface. The new peak of density of states ?DOS? is formed at the bottom of conduction band. It helps to improve the resistance characteristics and optical properties after the surface adsorbed gas molecules. New impurity energy level is also appeared at the bottom of conduction band. To some extent, the optical properties of TiO₂ surface are improved. Among the three kinds of gas molecules adsorbed on the anatase TiO₂?101? surface, the improvement effect of adsorption NH₃ is the best. However, among the three kinds of gas molecules absorbed on the rutile TiO₂?110? surface, the improvement effect of adsorption H₂O is the best. These results can provide a theoretical reference for experimental improving the photocatalytic performance of TiO₂.Finally, we studied the redox properties of rutile TiO₂?110? with an oxygen vacancy by calculating the gas molecules ?HF, HCl, HBr? absorbed on the rutile TiO₂?110? surface. The calculated results demonstrate that the dipole moment of molecule is one of the factors that affect the stability and energy of adsorption. In other words, higher dipole moment of molecule is benefit for its stable adsorption. According to the density of states and optical properties, it can be found that a new impurity energy level is formed at the bottom of conduction band after absorbing the gas molecules ?HF, HCl, HBr?. The optical properties of TiO₂?110? surface are improved in different degrees. Among all of three gas molecules ?HF, HCl, HBr?, the improvement effect of adsorption of HF is the best. After adsorption, HF and HCl are with the negative electricity, but HBr is with the positive electricity. By comparative analysis, the oxidation strength of rutile TiO₂ with an oxygen vacancy is between Cl₂ and Br₂. The oxidation strength is in the descending order as follows:F₂, Cl₂, TiO₂ with an oxygen vacancy, Br₂. The reducibility of rutile TiO₂ with an oxygen vacancy is between HBr and HCl. Therefore, the oxidation strength is in the following descending order:HBr, TiO₂ with an oxygen vacancy, HCl, HF.