Nonadiabatic Molecular Dynamics Study Of Charge Carriers Recombination On The Hematite Surface

Photoelectrochemical（PEC）water splitting technique can realize the direct conversion of solar energy to hydrogen fuel,which provides a powerful strategy to meet the growing energy demand and mitigate the current environmental pollution.The key to the realization of the technique is to find the stable and highly active semiconductor photoelectrode materials,especially the photoanode materials for water oxidation.Hematite（α-Fe₂O₃,hereafter referred to as Fe₂O₃）,as a typical semiconductor photoanodic oxide,exhibits extremely attractive properties in PEC water spiltting and has a potential application prospect.However,the rapid electron-hole recombination in Fe₂O₃ results in serious energy loss,especially the highly complex reaction interface,which hinders the further improvement of water splitting efficiency of PEC.Therefore,in-depth understanding of the electron-hole recombination mechanism of surface-interface for Fe₂O₃has a great significance for the development of new types of highly active Fe₂O₃ photoanodes.Taking the Fe₂O₃（0001）surface as the research model in the tehesis,the charge recombination dynamics of bulk Fe₂O₃ and Fe₂O₃（0001）surface charge are simulated by applying Fewest Switches Surface Hopping（FSSH）and Decoherence Induced surface hopping（DISH）methods,using ab initio nonadiabatic molecular dynamics based on time-dependent density functional theory,combined with the newly proposed phase correction of electronic state wave function and all-electron correction methods.The effects of semiconductor oxide（α-Ga₂O₃）passivation and surface water molecular adsorption on the charge recombination dynamics of Fe₂O₃（0001）surface were investigated.The detailed research contents are as follows:（1）Charge recombination dynamics in bulk Fe₂O₃The development of the NAMD method and the phase correction and all-electron correction of the NAC term enable us to simulate the charge recombination process of the system more accurately at the ab initio level.In this chapter,the recombination process of conduction band electron-valence band hole in bulk Fe₂O₃ is simulated by FSSH and DISH methods,and the effects of phase correction and all-electron correction to NAC on charge recombination rate are emphasized analyzed.The research results first verify the reliability of the DISH method,indicating that it is necessary to use the phase and all-electron correction NAC to simulate the DISH for the electron transition with large energy gap and transition metal subshell.Further simulation results reveal that the charge recombination time scale of Fe₂O₃ is on the order of microseconds,which is obviously slower than the defect-induced charge recombination process.This is consistent with the long-lived charge carriers observed by experimens,which proves the advantages of Fe₂O₃ photoanode in the water oxidation charge dynamics.The work of this chapter promotes our understanding of the dynamics of charge recombination in bulk Fe₂O₃ and provides a basis for the subsequent study of surface charge recombination.（2）Charge recombination dynamics for Fe₂O₃（0001）surfaceThe surface charge recombination dynamics is extremely important for the water oxidation reaction on the photoanode surface.Due to the existence of unsaturated chemical bonds on the surface,the surface state will have a direct effect on the surface charge recombination.In this chapter,the Fe₂O₃（0001）surface wuth Fe termination is taken as the research object,and the charge recombination process on the Fe₂O₃（0001）surface is simulated by FSSH and DISH using phase correction and all-electron correction to NAC.The simulation results show that there are two surface states in the band gap of Fe₂O₃（0001）surface.These surface states directly participate in the surface charge recombination process and greatly accelerate the surface charge recombination.The main reason for the decrease of the charge carriers lifetime of Fe₂O₃（0001）is that the surface states involved in the charge recombination process appear in the band gap of Fe₂O₃,which reduces the energy gap between the surface states,thus accelerating the surface charge recombination process.The charge recombination on the Fe₂O₃（0001）surface is much faster than that of bulk Fe₂O₃,so it is necessary to prolong the lifetime of charge carriers by modifying its surface.In the work,ab initio NAMD simulation is used for the first time at the atomic level to study the role of surface states in surface charge recombination dynamics,which lays a foundation and provides a reference basis for subsequent surface passivation and water adsorption.（3）Passivation effect of charge recombination dynamics for Fe₂O₃（0001）surfaceThe surface modification of Fe₂O₃ photoanode,especially the introduction of passivation layer,can effectively improve the efficiency of water oxidation reaction.The research model of this chapter is established by placingα-Ga₂O₃ with monatomic layer thickness on both sides of the Fe₂O₃（0001）surface model.The effect of semiconductor overlayer on the charge recombination rate of Fe₂O₃（0001）surface is simulated by FSSH and DISH methods.The simulation results show that after theα-Ga₂O₃ atomic overlayer is deposited on the Fe₂O₃（0001）surface,the surface states that originally exist in the band gap are eliminated,resulting in increase of energy gap between the electronic states related with charge recombination on the Fe₂O₃（0001）surface,which slows down the charge recombination rate on the Fe₂O₃（0001）surface and prolongs the lifetime of photogenerated carriers by about 4 times.It is found that the pure-dephasing time of the two surfaces is almost equal,but the contribution of the energy gap is stronger than that of NAC,which leads to a slower charge recombination process on the passivated surface.This result is consistent with the phenomenon observed in transient absorption experiments,which reasonably explains the decrease of anode onset potential observed in photoelectrochemical experiments.This work reveals the details of the surface charge recombination process of Fe₂O₃passivation byα-Ga₂O₃ on the atomic scale,and points out that the elimination of surface states by surface passivation is of great significance to prolong the lifetime of photogenerated charge carriers.（4）Adsorption effect of charge recombination dynamics for Fe₂O₃（0001）surfaceIn the photoelectrochemical experimental study,the Fe₂O₃ photoanode is in direct contact with the aqueous electrolyte solution,and the surface is easy to react with water molecules.Based on the Fe₂O₃（0001）surface model,the research model of this chapter is established by adsorbing three water molecules at each end of the surface model.The effect of water adsorption on the charge recombination rate of Fe₂O₃（0001）surface is simulated by NAC with phase correction and all-electron correction and DISH methods.The simulation results show that the adsorption of water molecules can significantly inhibit the recombination of surface charge and prolong the carriers lifetime by about 6 times.This is mainly due to the effect of water adsorption on the electronic structure of Fe₂O₃（0001）surface,which increases the width of energy gap while eliminating the surface state.This result can be explained by the energy gap between electronic states related with the charge recombination,the pure-dephasing time and the NAC intensity.The adsorption of water molecules leads to the increase of NAC and the pure-dephasinge time on the Fe₂O₃（0001）surface,but the results show that the charge recombination time is still prolonged.This is due to the fact that the contribution of the two surface energy gaps is stronger than that of NAC and pure-dephasinge time,resulting in a slower charge recombination rate on the surface of water adsorption.Compared with the Ga₂O₃ passivation layer,although both of them affect the charge recombination on the Fe₂O₃（0001）surface by eliminating surface states,the composition and spatial distribution of electronic states are different,resulting in different charge recombination dynamics.Although the inhibition of charge recombination dynamics by water adsorption is slightly stronger than that of Ga₂O₃ overlayer,the structure and composition of surface water adsorption may be destroyed during photoanodic water oxidation.Therefore,it is of great significance to study the surface-interface charge recombination in the photoanodic water oxidation process.In the thesis,the charge recombination processes of bulk Fe₂O₃,surface,semiconductor oxide（α-Ga₂O₃）passivation and adsorption surface of water molecule are investigated in turn,revealing that the chemical environment control can have a great influence on the charge recombination dynamics,deepening people’s understanding of the excited state dynamics of Fe₂O₃,and providing theoretical guidance for designing novelα-Fe₂O₃ photoanode materials.