| Titanium dioxide (TiO2) has been considered as one of the most promising materials for photocatalysis and solar energy conversion due to its high chemical stability, low cost, and long life time of photo-generated carriers. Improving TiO2 photocatalytic property is very significant for the environmental purification and solar energy utilization. In this thesis, we investigate the structural and electronic properties of a series of TiO2 materials by using density functional theory (DFT) calculations. Meanwhile, we designed a variety of TiO2 photocatalysts in visible-light region and the TiO2-based transparent conducting oxide (TCO) and photoanode materials for dye-sensitized solar cells (DSSCs). The main contents of this thesis are as follows.(1) Modulation of the photocatalytic performance of TiO2 through doping. We investigate the electronic structures and the optical absorption properties of a series of heteroatom-doped TiO2 systems by using density functional theory (DFT) calculations. These heteroatom doped TiO2 systems can be divided into three different types:(i) Compensation effect system:An effective non-metal (N) and non-transition metal (Sb) passivated co-doping approach is proposed to improve the photoelectochemical performance of rutile TiO2 for water-splitting by using first-principles calculations. It is found that the band edges of N+Sb co-doped TiO2 match with the redox potentials of water, and a narrow band gap (2.0 eV) is achieved for enhanced visible light absorption. The compensated donor (Sb) and acceptor (N) pairs could prevent the recombination of photo-generated electron-hole pairs. In addition, the N+Sb defect pairs tend to bind with each other, which could enhance the stability and N concentration of the system. (ii) Coupling effect system:The hybrid density functional theory calculations have been used to investigate the electronic structures of (Mg, S), (2A1, S), (Ca, S), and (2Ga, S) codoped anatase TiO2, aiming at improving their photoelectochemical performance for water splitting. It is found that the acceptor metals (Mg, Al, Ca, and Ga), assisting the coupling of the incorporated S with the neighboring O in TiO2, lead to the fully occupied energy levels in the forbidden band of TiO2, which is driven by the antibonding state of the S-O bond. The metal-assisted S-O coupling can prevent the recombination of the photo-generated electron-hole pairs and effectively reduce the band gap of TiO2. Meanwhile, by investigating the electronic structures of B-N codoped TiO2, we found that the band gap narrowing of anatase TiO2 induced by the B-assisted N-O coupling effect is more effective than the compensation effect between the interstitial B donor and the substitutional N acceptors on O site. Results indicate that the (B[sub], N) codoped anatase TiO2 is an intrinsic semiconductor with a band gap of 1.762 eV, exhibiting a figure-of-merit for photoelectrochemical (PEC) catalysis in visible light region. On the basis of the formation energy, we suggest adopting the strong O-rich environment to synthesize the B-N codoped anatase TiO2 with visible-light photocatalytic activity.(iii) Self-compensation effect system:An electron-hole self-compensation effect is proposed and confirmed in nitrogen doped Magneli phase TinO2n-1 (n= 7,8, and 9) by using hybrid density functional theory calculations. We found that the self-compensation effect between the free electrons in Magneli phase TinO2n-1 (n= 7,8, and 9) and the holes induced by p-type nitrogen doping could not only prevent the recombination of photo-generated electron-hole pairs, but also lead to an effective band gap reduction. This novel electron-hole self-compensation effect may provide a new approach for band gap engineering of Magneli phase metal suboxides.(2) Photocatalytic properties of semiconductor/TiO2 composites: The electronic properties of monolayer transition-metal dichalcogenide MX2 (M= Mo and W; X= S and Se) interfaced TiO2(110) composites were investigated by hybrid density functional theory. In the composites, MX2 serves as an efficient photosensitizer, and the electron-hole pair can therefore be easily generated by visible light irradiation and be effectively separated by the electron injection from MX2 to TiO2. The photocatalytic activity of MX2/TiO2(110) composites can be significantly improved due to the separation of photogenerated electron-hole pairs. Moreover, we further find that the monolayer silicane (SiH) and the anatase TiO2(101) composite (i.e. the SiH/TiO2 heterojuction) is a promising TiO2-based photocatalyst under visible light. The band gap of the SiH/TiO2(101) heteroj unction is 2.082 eV, which is ideal for the visible-light photoexcitation of electron-hole pairs. Furthermore, the effective photoexcited electron injection from the conduction band of silicane to that of anatase TiO2 and the carrier separation process in the SiH/TiO2 (101) interface region can prevent the recombination of electron-hole pairs, leading to a more significant photocatalytic enhancement than that induced by doping method.(3) Photocatalytic properties of high-pressure phase TiO2. We use the first-principles calculations to predict a fluorite TiO2(111) surface phase formed on the reconstructed high-energy rutile TiO2(011) surface. The band gap of the fluorite TiO2(111) surface phase is about 2.1 eV. We propose that engineering the high-energy surfaces of common TiO2 to obtain the fluorite TiO2(11l)1surface phase at room conditions is a promising method for the preparation of pure TiO2 materials with visible-light activity.(4) Applications of doped TiO2 in solar energy conversion materials. Applicability of rutile Tii-xInxO2 as a new p-type TiO2-based transparent conducting oxide has been investigated by examining their electronic structure and optical absorption properties, based on density functional theory plus U calculations. It is found that In doped rutile TiO2 displays metal-like characteristics and the edges of the optical absorption spectra are gradually blue-shifted with the increase of In doping concentration. In addition, the proper In doping level,12.5%≤ x≤ 18.75%, is proposed in order to achieve excellent conducting properties and high transparency in the visible light region from the calculations of optical absorption spectra; Moreover, we investigated the electronic structures of N, F, and I doped anatase TiO2 to explore the enhancement mechanism of incident photon-to-current conversion efficiency (IPCE) in dye-sensitized solar cells (DSSCs) based on N, F, and I doped anatase TiO2 photoanodes. The hybrid density functional calculation results indicate that n-type F and I doping is better than p-type N doping. The incorporation of I dopant is very favorable to improve the conductivity, the open-circuit voltage, and the visible-light absorption of anatase TiO2. Moreover, the I doping can facilitate the electron injection from the dye molecule to the TiO:substrate by analyzing the calculated electronic properties of adsorbed dye/TiO2 complexes. As a result, the I doping can significantly enhance the IPCE of DSSCs. In addition, it is found that the metallic n-type doping on the Ti site of the TiO2 photoanode can be an effective approach to improve the performance of DSSCs. |
PDF Full Text Request