First-principle Calculation On The Electronic Structures And Charge Carrier Effective Mass Of Photocatalytic Materials

In past several decades,the application of photocatalysis in new energy resources exploitation and environmental remediation has attracted wide attention from human society.After year of research from governments and scientists,great progress has been achieved in this field.However,some of the bottleneck problems of the photocatalysis have not been completely resolved,thereby limiting the wide application of the photocatalysis.Among the problems,the low light utilization and rapid electron-hole recombination have been known as the most critical problems which greatly reduce the photocatalytic activity of the semiconductor.Moreover,several fundamental photocatalytic mechanism remains unclear,restricting the development of photocatalysis.Therefore,extending the light utilization of the photocatalyst,suppressingelectron-holerecombinationandenhancingthe photocatalytic activity are the key scientific issues for achieving the practical application of the photocatalysis.Particularly,the first principles density functional theory can provide the fundamental understanding for the photocatalytic mechanism which can deepen then basic study of the semiconductor in the photocatalysis.Herein,TiO₂,CdS,Zn In₂S₄ with different phases,Non-metal doped ZnO and Ag₂CrO₄photocatalyst were mainly studied and investigated in this work.Using first principle simulation,the key influences of the photocatalytic activity of different phases of TiO₂,CdS and ZnIn₂S₄ were systematically investigated.In addition,the potential mechanism and theoretical study of the photocatalytic activity enhancement of non-metal doped ZnO were analyzed.The details of this work are divided into 8sections and categorized as follows,Chapter 1:The background,basic principles and key influence factors of photocatalytic reaction are discussed.Based on these aspects,the necessities of using first principles simulation are introduced.Finally,the main research contents of this work are presented.Chapter 2:Brief discussion on the first principle simulation and density functional theory are presented.The formation,prerequisite,fundamental principles and commonly used exchange correlation of the density functional theory are also discussed.The importance of the first principle simulation for the understanding of the photocatalytic mechanism is also discussed.Chapter 3:Using density function theory calculation,the band structure,local density state and effective mass of photogenerated charge carriers of anatase,rutile and brookite TiO₂ are systematically studied.According to the calculation results,the lifetime of the photogenerated electron-hole pairs of anatase TiO₂ is longer than that of the rutile and brookite Ti O₂.Moreover,the average photogenerated electron-hole pairs'effective mass of anatase TiO₂ is smaller than that of rutile and brookite TiO₂,leading to rapid migration of photogenerated electron-hole pairs from internal motion of the semiconductor material to the surface.The results indicate that the photogenerated electron-hole pairs of the TiO₂ can be rapidly migrated to its surface for photocatalytic reaction.As a result,the photocatalytic activity of the anatase TiO₂is normally higher than that of the rutile and brookite TiO₂.Chapter 4:The structural characteristics,energy band structure,local density of states,normal distribution,optical properties and the effective mass of the charge carriers of the wurtzite and sphalerite CdS are studied.According to the calculation result,basic structure of wurtzite CdS has internal polarization electric field which is beneficial for the efficient separation and diffusion of photogenerated charge carriers.However,the internal electric field does not exist in the sphalerite CdS.Moreover,the effective mass of wurtzite CdS is smaller than that of sphalerite CdS,implying faster migration of photogenerated charge carriers to perform photocatalytic reactions on wurtzite CdS surfaces.As a result,the electron-hole pairs'separation efficiency of wurtzite CdS is higher than that of the sphalerite CdS,resulting in higher photocatalytic activity.Chapter 5:By analyzing the internal periodic electric field,internal polarization electric field and effective masses of charge carriers of cubic and hexagonal ZnIn₂S₄,the reason of the higher photocatalytic ZnIn₂S₄ in comparison with cubic ZnIn₂S₄ can be found.Chapter 6:The energy band structure,optical properties and effective masses of charge carriers of N,C and S doped ZnO are systematically studied.According to the simulation results,the N,C and S doping can greatly enhance the light absorption ability of the ZnO,especially for C doping.Moreover,ZnO typically possess light electrons and heavy holes,confirming its intrinsic character of n-type semiconductor,while N-,C-and S-doping can generally render electrons lighter and holes heavier,resulting in slower recombination rate of photogenerated electron-hole pairs.Moreover,in comparison with the N-and S-doping,C-doping can discourage such recombination to the greatest extent and separate electron-hole pairs most efficiently on ZnO.Therefore,C-doped ZnO can serve as a one of the most potentially promising pathway to increase the quantum efficiency of ZnO-based photocatalysts.Chapter 7:The band structure,local density of States,charge density and the formation of chemical bonds of Ag₂CrO₄ are systematically studied.The bandgap of Ag₂CrO₄ is calculated to be 1.42 eV.Moreover,it was found that Ag₂CrO₄ has very positive conduction band value and large light absorption range,implying that it has strong oxidation and photocatalytic degradation ability.In addition,according to the calculation result,Ag₂CrO₄ has small effective mass and large relative effective mass which are beneficial for effective photogenerated electron-hole pairs'separation.As a result,Ag₂CrO₄ has high photocatalytic activity.Chapter 8:The main conclusions of all chapters are summarized.The future research direction for photocatalytic reaction are also presented.