First-principles Studies On Structural And Physical Properties Of Bi-based Photocatalysts Containing Alkali Or Alkaline-earth Metals

Photocatalytic materials have attracted much attention from scientific and industrialfields, since they are able to utilize the sunlight effectively and achieve energytransformation and environmental management. Currently, the wide applications of thecommercialized TiO2photocatalyst are still limited, since it has a broad band gap and onlyresponses to the ultraviolet light which is less than5%of total solar energy. In thisdissertation, first-principles method based on density functional theory (DFT) was used toinvestigate the structural and physical properties of BaBiO3, CaBiO2Cl and KBiO3potential photocatalytic materials via structure optimization and electronic structurecalcualtions on band structures, density of states, doping modification, mechanical property,and lattice dynamic. The achieved results are as follow:The origin of difference in electronic and mechanical properties between BaBiO3andBa2BiTaO6has been understood from the atomic level by performing electronic structurecalculations like band structure and density of states. Our results show that the top ofvalence band (TVB) is mostly composed of the mixing states of O-2p and Bi(II)-6s orbitalelectrons and the bottom of conduction band (BCB) is mainly ascribed to O-2p andBi(I)-6s orbital electons. However, the contribution from Bi with+5valence in Ba2BiTaO6can be ignored. The BiO6octahedra centered in Bi(I) with+3valence and in Bi(II) with+5valence connect with the corner-shared O atoms from each other and form the passway fortransporting electrons, leading to charge disproportionate and underestimation of band gaps.Band gaps can be modulated by changing the Ta/Bi atomic ratio during Ta doping inBaBiO3, since charge transport between Bi ions with different valence is blocked.Calculated results on elastic property indicate that the elastic constants Cijof Ba2BiTaO6isincreased about20~50%in comparison with BaBiO3, but the mechanical anisotropy ofBa2BiTaO6becomes more notable because of Ta incorporation.Pressure-enthalpy calcualtions show that the P21/m monoclinic CaBiO2Cl becomesunstable and transforms into an orthorhombic structure with space group Cmcm whenpressure goes up to4.46GPa. With increasing pressure the band gap of the P21/m phase isdereased continuously, which means the range responsing to visible light in CaBiO2Cl canbe broadened drastically and provide an effective pathway to improving its photocatalyticperformance. Further phonon calculations indicate that at ambient conditions the P21/mand Cmmm phases are dynamically unstable, but the Cmcm phase would becomedynamically stable with increasing pressure to20GPa. In addition, Ag doping leads to the formation of intermediate energy band in the forbidden band of CaBiO2Cl, which canadjust the band gaps of doped sysem freely by controlling the amount of Ag substitutionsand enhance the photocatalytic activity of CaBiO2Cl.The controllable effect on band gaps of KBiO3has been investigated by consideringthe single doping or co-doping effect from Sb and Ag. Electronic structure calcuationssuggest that the TVB composed of O-2p states and BCB mainly contributed by Bi-6s andO-2p states play a key role in determining the photocatalytic performance of KBiO3.Single Sb doping and Ag doping can increase and decrease the band gap of KBiO3,respectively. However, the co-doping of Sb and Ag can not only tune the band papbilaterally in order to achieve the optimal design of band structure, but also lead to theefficient absorption of visible light and effective segregation of photon-inducedelectrion-hole pairs. This will be helpful for improving the performance of KBiO3catalystunder visible light greatly.