Non-adiabatic Molecular Dynamics Study Of Ultrafast Charge Transfer In Photocatalysis

In the field of computational physics,first-principles calculations aim to study the macroscopic properties of condensed-state systems from the properties of atoms and electrons without depending on any parameters.In this field,the first-principles cal-culations have been used to study the physical properties of the ground state of the condensed-state system.However,the description of the properties of the excited state,especially the study of excited-state carrier dynamics,is still very challenging.When dealing with the non-adiabatic processes of the excited states,the traditional Bonn-Oppenheimer approximation is no longer valid.As a result,non-adiabatic molec-ular dynamics methods have been developed,including full quantum dynamics,mixed quantum classical dynamics,quantum dissipation,etc.The most widely used is the Ehrenfest mean field dynamics and the surface hopping.The first chapter briefly in-troduces the Fewest Switch Surface Hopping method,and some developments from Professor Prezhdo's.With the increasingly serious problems of the environment and energy,sustainable new energy has received extensive attention.As early as 1972,people discovered that Ti02 surface photolysis H2O generated clean energy H2.Subsequently,it was also discovered that photo-excited electron-hole pairs can cause molecules adsorbed on the surface of Ti02 dissociated.Even the photosynthesis may be possible.At the same time,the surface of Ti02 that adsorbs dye molecules is also used to make solar cells.This makes TiO2 have important applications in the environment,industry,and clean energy.Chapter 2 will briefly introduce titanium oxide materials and the basic picture of photocatalysis.In the photocatalytic process,the recombination of electron holes will greatly limit the photocatalytic efficiency,and people often delay recombination by introducing elec-trons or hole scavenger.However,the underlying mechanism is not clear.The CH3OH molecule has always been considered as an important hole scavenger on the surface of Ti02.We have used NAMD to investigate the hole transfer dynamics at the interface of Ti02/CH30H.First of all,we found that the adsorption structure of CH3OH plays a crucial role in hole trapping.CH3OH has a weak ability to trap holes.Only dissociated CH3O have found to has a certain ability to capture about 20%of the hole.Secondly,we find that due to the strong electron-phonon coupling,the HOMO level of CH3OH has a strong oscillation over time,and this oscillation will accelerate the decay of photogen-erated holes to VBM.Our research revealed that the CH30H/Ti02 interface determines the photocatalytic efficiency mainly by:1.Energy level alignment;2.Phonons thermal excitation;3.Electron-phonon coupling.This part will be introduced in Chapter 3.NAMD is based on a mixed quantum classical theory framework.The electron's equation of motion follows the time-dependent KS equation,and the movement of the atomic nucleus is described by Newtonian mechanics.In many dynamic problems in-volving light elements,such as the dynamics of H20,the nuclear quantum effect is an important factor that cannot be ignored.We have successfully introduced nuclear quantum effects into NAMD by combining the RPMD with NAMD.We studied the proton-coupled charge transfer process of various adsorbed molecules on the surface of Ti02.It is found that there are quantum tunneling phenomena on various adsorbed molecules on the surface of Ti02,which cannot be simulated by traditional methods.With quantum tunneling,the charge can be transferred very quickly and trapped by the adsorbed molecules.This part will be introduced in Chapter 4.The photoreduction of organic molecules on the surface of Ti02 is another impor-tant direction in the field of photocatalysis.Due to the HOMO level of C02 is high,the theory and experiment of C02 reduction to CO on the surface of Ti02 is scarce.We modified the classical path approximation and used the combination of Constrained DFT and NAMD to study the photodissociation process of CO2.We found that short-time electron injection activates the CO2 bending vibration mode.Under this vibration mode,the energy of CO2 will be gradually stabilized.On the one hand,it is beneficial to directly receive photo-excited electrons.On the other hand,it is beneficial to capture the hot electrons in the conduction band,thereby realizing the reduction of CO2.This section will be introduced in Chapter 5.Perovskite solar cells have become a new favorite in the field of solarcells due to their high conversion efficiency of more than 20%.For a long time,people have been thinking that defects introduce defect energy levels in the gap,that accelerating carrier recombination,which is harmful to photoelectric conversion.We consider the effect of multiple intrinsic defects on carrier recombination in MaPbI3 by considering the deco-herence effect on the basis of NAMD.Surprisingly,these intrinsic defects not only do not become electron-hole recombination channels,but also reduce the coupling between the band gaps,greatly prolonging the carrier lifetime,which is why such materials are so charming.This part will be introduced in Chapter 6.In the last chapter,we will introduce the recent work and future prospects for the development of the suface hopping method.