Electronic Structure And Photocatalytic Properties Of Several Bi-based Semiconductors From Theoretical Studies

Because photocatalysts have increasing utilizations in the degradation of organic pollutants, inorganic pollutants and water-splitting into hydrogen, photocatalysis is catching more and more attention in the environmental decontamination and renewable clean energy production field. TiO2is the earliest photocatalyst used in water splitting and degradation of organic pollutants. However, the large band gap of TiO2(～3.2eV) makes it can only absorb a small portion of the solar spectrum in the ultraviolet (UV) light region, a small portion (<5%) of sunlight.Searching for sufficiently stable and efficient photocatalysts under visible light is a strongly appealing challenge. The efficiency of a photocatalytic process involving visible light can be improved by extending the energy range of photoexcitation and then increasing the separation efficiency of photogenerated carriers for the system. Besides TiO2, metal oxides and their composite oxides become popular materials for photocatalysts due to their inherent stability and relatively low cost of synthesis. Among the large number of metal oxides and their composite oxides, Bi-based photocatalysts have attracted much attention because of their layered structure and the up-shifted valence band (VB) and a smaller band gap resulted from hybridization between Bi6s and O2p states. At the same time, the hybridization makes the VB more dispersive, which is benefit to the transition of photogenerated holes in the valence band. Therefore they have good optical-absorption properties and then photo-split water and photo-degrade organic pollutants.In this dissertation, we studied the stability, optical absorption and photocatalytic properties of several typical Bi-based oxides, such as Bi2MoO6, Bi2WO6, BiVO4, and BiNbO4, including their pure and doped systems. We gave some reasonable explanations for some important experimental phenomena and proposed some novel ideas to improve the photocatalytic efficiency. The dissertation is divided into eight chapters. In the first chapter, we briefly present the mechanism of photocatalysis and development of Bi-based photocatalytic oxides and the main content of this dissertation. In the second chapter, we introduced the density functional theory and gave a brief description for the first-principles software packages. In the third chapter, we comparatively studied the electronic structures and related properties of Bi2MOg (M=Cr,Mo,W) systems, and in the fourth chapter, the geometric and electronic structures of Bi2MoO6and Bi2WO6with and without oxygen vacancy and the effects of N-doping have been examined by means of DFT. In the fifth chapter, we further study the photocatalytic water splitting mechanisms of Bi2WO6and effects of the monodoping and codoping of N and Mo on the photocatalytic water splitting activity. In the sixth chapter, we elucidated the doping synergistic effect on the photocatalytic O2evolution in BiVO4. In the seventh chapter, formation energies, transition energies, and electron properties of various intrinsic defects in BiNbO4systems were studied based on the density functional theory. In the eighth chapter, we summarized the research contents and innovations in this dissertation and pointed out some main problems that need to be solved urgently as well as the further research directions. The main research work and contents are listed as follows:(1) We performed electronic structure calculations based on DFT method within GGA scheme for photocatalysts Bi2MO6(M=Cr, Mo and W). Our results show that the calculated band-gap nature of Bi2MO6is direct. The rules of Bi5d electrons effect on Bi2CrO6, Bi2MoO6, and Bi2WO6electronic structures were studied, such as band gap, bonding. Besides the influence on band gap,. It shows that Bi5d states widen the valence-bands and conduction-bands of Bi2MoO6and Bi2WO6Mulliken population analyses reveal that the atomic populations of Bi increase while the atomic numbers of M decrease, showing the reduction in oxidative capability of Bi2MO6.(2) The electronic properties of Bi2MO6(M=Mo and W) are studied by using the first-principles calculations. It is attributed to its smaller electron effective mass that has higher photocatalytic activity than Bi2MoO6. The oxygen vacancy in serves as a trapping center of photogenerated electrons and thus is in favor of the photocatalytic efficiency. Nitrogen-doping induces localized structure distortion and thus improves the separation of photogenerated electron-hole pairs. Moreover, band gaps decrease obviously with doping concentration increasing, therefore the photoabsorption edges will give rise to a redshift in Bi2MO6.(3) To understand the photocatalytic activity of Bi2WO6for water splitting, the electronic structures of N-and Mo-monodoped and N/Mo-codoped Bi2WO6are investigated based on first-principles calculations. Our results indicate that the band gap of N/Mo-codoped Bi2WO6reduction by0.19eV and the smaller driving force are required for the water-splitting oxidation process. The calculated binding energies of the defect pairs show that N/Mo-codoping in Bi2WO6is more feasible than mono-doping, indicating N/Mo-codoped Bi2WO6is a potential candidate for the photocatalytic water splitting.(4) The electronic structures and related properties of N-and M-(M=Cr, Mo) doped BiVO4are investigated by means of first-principles calculations. The synthetic codoping effect of N/Cr and N/Mo in BiVO4are also examined. Our results demonstrate that the electronic band edge positions are aligned with respect to the water oxidation/reduction potential to evaluate the photocatalytic O2evolution activity of doped BiVO4. The band gap of N/Cr-codoped BiVO4is reduced by about0.34eV compared with undoped system, with the redox potential still lying at a level suitable for water splitting, thus making it a potential candidate for the photocatalysis of O2evolution. The calculated formation energies indicate that the introduction of M in BiVO4favors incorporation of N. The binding energies of defect pair show the codoped material is more stable.(5) Formation energies, transition energy levels and electronic properties of various intrinsic defects in BiNbO4systems are studied using the density-functional theory. Our results indicate that the acceptor defects form easier than donor defects under O-rich condition, while it is opposite under Bi-rich condition. Under O-rich condition, Bi vacancy (Bivac) leading to p-type conductivity is the dominant intrinsic defect whereas, O vacancies (Ovac) inducing moderate n-type conductivity is the dominant intrinsic defect under Bi-rich condition. Among these Intrinsic defects, Ovac is a deep donor, on the contrary, Bivac is found to be a shallow acceptor which is benefit to the separation and migration of the photogenerated carriers. Consequently, the BiNbO4with Bivac under O-rich growth condition should be of better photocatalytic performance.