Enhancing Photocatalytic Activity Of Two-dimensional ZnSe Nanosheet Via Charge-compensated Codoping

Energy shortage and environmental pollution are the most urgent problems to be solved.Photocatalysis can decompose water to produce hydrogen and decompose part of organic pollutant,which is one of the important ways to solve the above problems.In recent years,the photocatalysts based on two-dimensional(2D)semiconductors have triggered a large number of interest due to the striking physical properties such as extremely large specific surface area and quite high carrier mobility.Among them,a pseudohexagonal ZnSe(h-ZnSe)was synthesized by exfoliating its lamellar hybrid intermediate followed by removing the alkylamine ligand with heating.And under the irradiation of ultraviolet light,its photocurrent density is 4-10 times higher than ligandcoated layers and 192 times higher than its bulk phase.In addition,the existence of tetragonal ZnSe(t-ZnSe)with higher stability is predicted theoretically.However,the band gaps of h-ZnSe and t-ZnSe are 3.50 and 3.17 e V,respectively.They can only absorb ultraviolet light and limit the application in photocatalytic water splitting driven by solar energy due to the wide band gap.Therefore,some strategies have to be adopted to improve the utilization of visible light of h-ZnSe and t-ZnSe.The ion doping method of semiconductor has been widely used to improve the photocatalytic activity due to its advantage such as convenient and simple preparation process,effective regulation of semiconductor band gap and increase of reactive active sites.Besides,density functional theory is an important method to study the electronic structure of semiconductors,and it has been widely used in the field of condensed matter theory calculation.This thesis will investigate the effect of charge compensated codoping on the photocatalytic activity of2 D h-ZnSe and t-ZnSe by employing hybrid density functional theory.The main research conclusions are follows:1.Charge-compensated codoping have been adopted to tune the photocatalytic activity of h-ZnSe.Here charge-compensated codoping is simulated by substituting Sb for Se atom,and Sc or Y for Zn atom,and the effect of the relative position of codopants on photocatalytic activity of h-ZnSe is also considered.The results show that compared with the Sc-,Y-,Sb-monodoping,the Sc-Sb and Y-Sb codoped systems are more beneficial because they do not introduce unoccupied impurity states and effectively decrease the band gap,and the band edges of codoped h-ZnSe straddle water redox potentials at a specific p H value.Besides,when two doped atoms are in adjacent positions,the band gap decrease more obviously.And as the increase of doping concentration,the stability and band gap for codoped systems are reduced.The calculated optical absorption spectra show that,compared with the pure system,the optical absorption curve of Sc-Sb and Y-Sb codoped h-ZnSe show obvious red-shift,and the absorption of visible light has increased.Furthermore,we considered the oxygen evolution reaction(OER)and hydrogen evolution reaction(HER)processes on pure,Sc-Sb and Y-Sb codoped h-ZnSe nanosheets,and consequently,Sc-Sb and Y-Sb codoped h-ZnSe could reduce the overpotential of OER and HER,indicating that codopants can indeed improve the reactive activity of OER and HER.But considering the existence of overpotential,to make sure the oxidation reaction and reduction reaction can occur at the same time,it is necessary to add an approiate amount of co-catalyst.2.The strategies of anion-cation passivation codoping have been adopted to adjust the photocatalytic activities of 2D t-ZnSe.As and Sb are used to simulate anions,and Sc and Y are used to simulate cations for doping.The results show that monodoping can reduce the band gap of t-ZnSe,but it may introduce the unoccupied impurity level and promotes recombination of electron-hole pairs so that suppress the photocatalytic activity.The Sc-As,Y-As,Sc-Sb,Y-Sb codoping can reduce the band gap and avoid the above problems,and their band edge position could straddle the water redox level.And with the increase of doping concentration,the band gap of doping system decrease,but the defect formation energy also increase.The calculated optical absorption spectra show that the Sc-Sb and Y-Sb codoped t-ZnSe absorbs a longer wavelength of visible light spectrum as compared to pure t-ZnSe nanosheet.At last,we compared the oxygen evolution reaction(OER)and hydrogen evolution reaction(HER)of pure,Sc-Sb,Y-Sb codoped t-ZnSe.It is found that Sc-Sb and Y-Sb codoping reduce the free energy change of OER and HER.However,their overpotential is still high,so it is necessary to use co-catalyst to ensure the redox reaction take place.