Preparation Of Doped TiO₂ And Theoretical Studies By Density Functional Theory (DFT)

With the explosion of polulation and development of economics, great demands on energy and polution of the environment are the main problems in the face of humanbeings. It is imperative to control evironmental polution and find a renewable energy source. Semiconductor photocatalysis is a promising technology for the two problems.Titania (TiO₂)-based photocatalysts have received intense attention as promising photocatalytic materials due to their high photo-oxidation power, excellent stability, non-toxicity, and low material cost. However, as a wide band gap semiconductor (3.2eV), the limited absorption of ultraviolet irradiation as well as recombination of photogenerated electron-hole pairs restricts somewhat its possible applications in photocatalysts. So the extension of optical absorption in TiO₂-based materials to the visible-light region, in conjunction with a low photogenerated carriers'recombination rate, is of great practical importance.To adress this problem, a combination of experimental and theoretical researches are adopted to find a viable resolution. Firstly, nonmetal (N/C) doped TiO₂ nanoparticals were prepared through solvothermal method and sol-gel method and were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV-visible light photometer (UV-Vis). Secondly, studies on the doping structures, electron structure, populations et al were carried out by first principles calculation. On the base of nonmetal doped TiO₂, transition metal and nonmetal codoped TiO₂ are studied. The results show that a broaden absorption to visible light as well as a reduced electrons and holes recombination is shown in codoped TiO₂ systems. The conclusions are as follows:1. N doped TiO₂ nanoparticals are prepared and analyzed: the N doped TiO₂ samples are anatase with partical size of 4nm; N doping extend the absorption edge redshift to visible light with the N concentration increasing, while the absorption intensity in ultraviolet light decrease.2. Both substitutional N (N_S) and interstitial N (N_I) in the anatase TiO₂ are investigated by density functional theory (DFT) calculations in CASTEP mode of Materials Studio to reveal the microscopic mechanisms of band gap narrowing. For N_S doped TiO₂, N bond with three Ti atoms in the lattice and N2p states are spilted into three states in the bandgap which make the bandgap narrowed to 1.748eV. For N_I doping, N interacts with one crystal O atom and forms N-O bond with twoπbonding states lying below the O₂p states and two antibonding states in the bandgap and narrow the gap to 1.358eV. For the both N species doping systems, the upmost N2p state is partial occupied and act may as recombination centers.3. (N, TM_S) (N=N_S, N_I; TM=V, Nb, Ta)-codoped TiO₂ are designed using density functional theory (DFT) calculations to compensate the defects induced by N species and N/V codoped systems are superior to other codoped systems.Firstly, fully occupied N2p states are formed above the valence band with empty V3d states below the Ti3d states, that reduce the photoelectrons transition energy; Secondly, the fully occupied hybridized states as well as their increased overlapping with O₂p states alleviates greatly the hole trapping ability of N species doped TiO₂; Thirdly, according to the formation energy calculation, the N doping will be enhanced with the incorporation of V ions in two ways: 1) the existence of V ion brings about more distortion of the anatase lattice, which facilitates the incorporation of N species into the lattice. 2) V ion decreases the income resistance of N by the charge neutralityeffects. 4. C doped TiO₂ nanoparticals are prepared and analyzed: the C doped TiO₂ samples are anatase; C doped TiO₂ samples show two optical absorption thresholds in absorption curves; the intensity of the two absorption thresholds are enhanced with the increasing of solvothermal time.5. Both substitutional C (C_S) and interstitial C (C_I) in the anatase TiO₂ are investigated. For C_S doped TiO₂, three C2p states are added into the bandgap with two states under the fermi level (E_F) and another is above E_F and make the bandgap narrowed to 1.114eV. For C_I doping, C interacts with three crystal O atoms and forms CO bond with threeπbonding states lying below the O₂p states, while theπ^* antibonding states are very high so the excess electrons introduced by the C_I are transferred to Ti3d states and make E_F at the bottom of Ti3d states.6.(C, TM_S) (N=N_S, N_I; TM=V,Nb,Cr,Mo)-codoped TiO₂ are designed and investigated. C_S and Cr codoped system is charge neutrality compensated and its bandgap is narrowed greatly. V is the most effective codopant for C_I and make E_F into the bottom of Ti3d states, which enhanced the two absorption thredhold greatly.