Theoretical Study On Band Alignment Of Semiconductor/Water Interfaces

Semiconductors have been widely used in photocatalysis and photoelectrocatalysis,where the performance is related to the band alignment of semiconductors.Take photocatalytic water splitting as an example,the catalytic activity of water splitting photocatalysts are largely depended on their band gap and band edge positions.Firstly,the optimal band gap of the solid should be larger enough to split water and smaller than 3.1 eV in order to fall within the visible range of the solar spectrum;Secondly,band edge positions must straddle the water oxidation and reduction potentials.As the band structure(band gap,band edge positions)is one of the most important properties of semiconductors which decides the usage and performance of materials,it is very important to study the band alignment of semiconductors.At present,there are some difficulties in measuring of the band alignment of semiconductors in experiment,such as the synthesis of single crystal.As for theoretical studies,up to now,lots of the theoretical studies of band alignment are confined to the clean surface calculations and 1ML water adsorbed slabs calculations.Though there are some researches of interfaces,most of them built the interfaces using implicit water model without taking into account electronic structure of water molecule and the statistical mechanical nature of solvent fluctuation.But from some former studies,we know that the band alignment of real solid-water interface is different,which means the research to band alignment of surfaces or interfaces using implicit water model is not enough to calculate band alignment of real solid-water interfaces where most of the reactions take place.In this thesis,we used the method which combines density functional theory based molecular dynamics(DFTMD)and free energy perturbation theory(FEP)to simulate several semiconductor water interfaces,study the band alignment and the effects of water to band edge positions.1.As the water introduced will shift the band edge positions of semiconductors,the band alignment of clean surface,1ML water adsorbed and fully solvated semiconductors are different.And the band edge positions shift varies for different semiconductors.It is not enough to estimate the band alignment of interfaces by using only clean surface or 1 ML water adsorbed slabs for most of semiconductors.2.The shift of band edge positions is caused by the interaction between the surface water and the non-surface water(including part of the interaction between surface and the non-surface water),which will change the interfacial dipole.For most of semiconductors,the band edge positions of fully solvated slabs will be shifted up compared with that of 1 ML water adsorbed slabs.And the band edge positions shift is related to the surface structures,radius of ions and degree of dissociation for surface water,and so on.We are trying to find a simple relationship to combine these properties and the effects of the surface water and non-surface water to the movement of band edge positions.