Study On Electronic And Optical Properties Of Titanate Compounds And Their Doped Systems Using First Principles

Wind energy,solar energy and tidal energy are important green energy sources.Therefore,the opportunity for the development of the energy storage industry has arrived.Lithium-ion batteries,with the advantages of no memory effect,long cycle life and low self-discharge,have attracted growing attention.In terms of environmental protection,industrial wastewater causes pollution of many drinking water sources,and untreated harmful gases cause damage to the atmosphere.Semiconductor photocatalytic technology,which utilizes solar energy,can remove toxic and harmful pollutants through photocatalytic decomposition,and can also be used to produce clean energy,such as decomposing water to produce hydrogen.Thus,semiconductor photocatalytic technology has become an effective way to solve environmental pollution problems and has broad application prospects.At the beginning of this paper,the first principles and density functional theory are explained in detail,including the Born-Oppenheimer approximation,Hartree-Fock approximation,Hohenberg-Kohn equation,Kohn-Sham equation,and exchange-correlation energy.After that,instructions of Materials Studio and CASTEP used in this study has been described,which is rarely mentioned in other similar papers.In the main body of this research,first,we used the first principles based on density functional theory to study LiTi2O4(FD-3M)which is used as superconducting material,Li2Ti3O7(P21/M)which is used as lithium battery material,Li2Ti6013(C2/M)which is used as lithium battery material and photocatalyst,and Li4TiO4(CMCM)which can adsorb carbon dioxide,Their crystallographic properties,electronic properties(band structure and PDOS)and optical properties(dielectric function,refractive index,reflectivity and absorption)have been calculated.The DOS of their conduction band is mainly contributed by the 3d orbital of Ti,and the DOS of the valence band is mainly contributed by the 2p orbital of O.These four lithium titanates have strong absorption and reflectivity in the ultraviolet band.Li4TiO4 has good light transmission performance from visible to infrared,and LiTi2O4 has absorption from visible to infrared.Further,we studied and compared Li2Ti6O13(C2/M),LiNaTi6O13(P2/M),Na2Ti6O13(C2/M),NaKTi6O13(P2/M)and K2Ti6O13(C2/M).Their crystallographic properties,electronic properties(band structure and PDOS)and optical properties(dielectric function,refractive index,reflectivity and absorption)have been calculated.They can be used as both battery materials and photocatalytic materials.The results of calculation show that they have strong absorption and reflectivity in ultraviolet,and limited light absorption in visible light,paving the way for the following study of the photocatalyst K2Ti6O13.Due to the large band gap,K2Ti6O13 can only absorb ultraviolet light which accounts for a fairly small amount of sunlight,which severely limits its application as a photocatalyst.To widen its application,doping is one of the most effective methods.In the following chapters,the research on the improvement of K2Ti6O13's light absorption capacity in the band of visible light has been described.The crystallographic properties,electronic properties(band structure and PDOS)and optical properties(dielectric function,reflectivity and absorption)of pure K2Ti6O13 and of different doping systems have been calculated and compared.First-principles studies were conducted on K2Ti6O13 with different doping concentrations of Zn,Ag,and Cd atoms.Doping concentrations are 8.33%,16.67%,33.33%,and 66.67%.The results of calculation show that in terms of electronic properties,Ag-doped K2Ti6O13 has the best effect-by introducing impurity,the top of the valence band has been significantly elevated,and the bottom of the conduction band has been lowered.Ag-doped K2Ti6O13 with a doping concentration of 33.33%has the smallest band gap.Zn-doped(with 33.33%doping concentration)and Ag-doped(with 33.33%doping concentration)K2Ti6O13 can significantly improve absorption in the visible light range,and the average absorption coefficient in the visible light band has increased by 345.4%and 735.9%compared to that of the pure K2Ti6O13,respectively.Later,studies of C,N,and S doping with different concentrations were carried out.The doping concentrations are 3.85%,7.69%,and 15.38%.The results of calculation show that C-doped K2Ti6O13(with 15.38%doping concentration)has the smallest band gap,which is 0.608 eV.C-doped(with 15.38%doping concentration)and S-doped(with 15.38%doping concentration)K2Ti6O13 can significantly improve absorption in the visible light range,and the average absorption coefficient in the visible light band has increased by 464.6%and 560.3%compared to that of the pure K2Ti6O13,respectively.Finally,the co-doping of Zn-S,Ag-C,Ag-N,and Ag-S with different doping concentration ratios was studied.The results of calculation show that,in terms of crystallographic properties,K2Ti4Ag2O12N and K,Ti4Ag2O25/2N1/2 have better photocatalytic ability due to the minimal lattice distortion.in terms of electronic properties,the Ag-N co-doping reduces the band gap and thus there is red shift in absorption.In terms of optical properties,Ag-N co-doping can significantly improve light absorption.K2Ti4Ag2O12N(with Ag's doping concentration at 33.33%and N's doping concentration at 7.69%)has best absorption capacity of visible light.It is 13.65%higher than that of K2Ti4Ag2O13,43.87%higher than that of K2Ti6O11S2 and 850.05%higher than that of the pure K2Ti6O13.This proves that it can effectively improve the photocatalytic ability of K2Ti6O13 in the visible light range to select the appropriate metal and non-metal atoms and to select the appropriate concentration ratios for doping.This study innovatively used the PBESOL to handle the exchange correlation energy,making results more accurate than previous researches by others.In terms of doped K2Ti6O13,doping with different concentrations,which had not been previously studied,was carried out.Selected doping elements and co-doping combinations had not been previously studied as well.This research fills the research gap of these substances in the field of first principles and provides a basis for the application of these substances as lithium battery materials and photocatalysts.