Energy Material Design Based On Electronic-Structure Engineering

The band structure engineering of semiconductors can not only expand their application scope,but also improve key performances.To date,a variety of physical and chemical methods have been developed to achieve this goal,yet the underlying mechanism is not thoroughly understood,which prevents people from conducting rational design of semiconductor materials for energy related applications.First-principles studies enable us to simulate the electronic structures and properties of materials at the microscale towards comprehensive understanding of those mechanisms in the physical and chemical processes of interest.In this thesis,we presented first-principles investigations on electronic structures of a series of semiconductors,based on which we developed novel designs for potential applications as energy materials.This thesis contains four chapters,which are organized as follows:The first chapter introduces the background of the thesis works,providing an overview of electronic structure engineering of semiconductors materials.We focus on several important fields including metal oxide materials and graphene like two-dimensional materials.For photocatalysis application which is regarded as a promising approach to address environment and energy issues,achieving effective charge separation is a prominent challenge for photocatalytic materials design.Among various potential solutions,Z-scheme heterojunctions are appealing because they can achieve simultaneous broad spectrum sunlight adsorption and charge separation.Then we introduce the hydrogen doping of metal oxides and chemical modification of graphene like two-dimensional materials,which cause electronic band structure engineering toward high-performance utilization in photocatalysis.The second chapter provides a brief introduction of first-principles theory and the development of computational programs built on it.First-principles calculation enables us to acquire the electronic structures of systems of interest by solving Kohn-Sham equation,in which the complex multi-body interactions can be approximate as one-electron interacting with an effective mean field.Particularly,the exchange and correlation interactions can be approximately described with exchange correlation functional.The third chapter discusses the rational design of electronic structures of metal oxides materials.(1)We have developed a sophisticated method for hydrogenating metal oxides under mild conditions to tune the optical,electric and magnetic properties of materials.Hydrogenation is an effective approach to tailor physical properties of metal oxides for their applications,yet conventional processes using hydrogen molecules(H2)as the H source are energy demanding and expensive.This is because it requires harsh reaction conditions(high temperature and high pressure)and noble metal catalysts to break H-H bond and inject H into lattice phase of metal oxides.Meanwhile,there are abundant cheap hydrogen atoms(protons)in the acid solution.Unfortunately,most metal oxide semiconductors cannot be directly hydrogenated via acid treatment,because they either are chemically inert(such as TiO2)or easily undergo corrosion(like VO2)in acids.In order to achieve direct hydrogenation of metal oxides in an acid solution,we propose a metal-acid treatment to realize the electron-proton co-doping hydrogenation of metal oxide,which has been successfully applied to several metal oxides(TiO2/WO3/MoO3/Nb2O5).The proposed mechanism can be divided into three steps:(?)work-function difference between metal and metal oxide drives electrons to transfer from the metal to the oxide through their interface;(?)the negative charges in the oxide attract the surrounding protons and drive them to diffuse into the lattice;(?)a stable H-doping is formed after neutralization of positive and negative charges in the lattice.Further,this methods enable a smooth control of hydrogenation of metal oxides,which facilitates fine tuning of these materials.(2)We investigated the cause of metal-like behavior of vanadium dioxide attached by ascorbic acid molecules at room temperature.Pristine vanadium dioxide is an insulator at room temperature,which will transform into a metal phase at 68?.We simulate the adsorption of ascorbic acid on the surface of vanadium dioxide and find that deprotonated ascorbic acid hold larger adsorption energy and electron transfer tendency.Both the resulting anions and protons are adsorbed on the surface of vanadium dioxide,maintaining the neutrality.From the simulation and analysis of electronic density of state,it is found that the Fermi level of vanadium dioxide shifts up after hydrogenation as the electrons fill the bottom of conduction band,which effectively reduces the band gap of vanadium dioxide and eventually transforms it into a metal phase at room temperature.The hypothetical mechanism from theoretical simulations are then supported by experiments.The fourth chapter is mainly about the band structure engineering of two-dimensional carbon-nitrogen materials.(1)We investigate the systematic regulation of energy band gaps of two-dimensional C3N materials.The original band gap of C3N material is 1.03 eV.Using the first principle calculation,we investigated how the band gap is tuned with different surface modification,including defect engineering,surface decoration,and substitutional doping.The simulation results show that the band structure can be tuned effectively by introducing impurity states,orbital rehybridization,and n-or p-type doping simultaneously.Besides,we find the linear relationship between the band gap width and doping concentration in a certain concentration range,which provides important information for the precise regulation of band gap width.(2)We investigate the band structure of graphene like two-dimensional carbon nitrogen material(g-CN)to realize photocatalytic water splitting.The calculation results show that the valence band and conduction band of g-CN satisfy the requirements of photocatalytic water splitting.Although reactions can occur in pure two-dimensional carbon nitrogen material,only ultraviolet light can be utilized because of its wide band gap(3.18 eV).We then load different metal groups on g-CN to adjust the band gap.It is found that g-CN with doping of Pd(OH)2 and Co(OH)2 groups can not only adsorb visible and infrared light,but also be used for the photocatalytic generation of O2 and H2.The simulated density of state results show that each part retains the respective electronic structure in the composite system,and can generate photo generated electrons and holes for photocatalytic generation of O2/H2 reaction,respectively.Further non-adiabatic molecular dynamic simulations demonstrate that the charge can be effectively delivered to two parts.Based on our theoretical simulations,a new design of Z-scheme photocatalytic system is proposed:a g-CN combined with Pd(OH)2 and Co(OH)2 fragments is used to harvest full-solar spectrum for catalyzing O2 and H2 production.While lower energetic electrons and holes will be annihilated on g-CN,higher energetic charge carriers are kept around Pd(OH)2 and Co(OH)2 parts and then used for redox and oxidation reactions.This design provides a useful model system for the rational design of Z-schemes.