First Principles Study Of The Structure And Photocatalytic Hydrogen Splitting Of Two-dimensional A₂B₃ (A=As, Sb, Bi, Ga; B=S, Se, Te) Monolayer

To achieve the goal of "double carbon",clean energy has become an important research direction in many disciplines.Currently,solar energy and hydrogen energy are the most concerned new energy sources.The research of converting solar energy into hydrogen energy with semiconductor photocatalysts for overall water splitting to manufacture hydrogen has important academic significance,social benefits and economic prospects.However,due to the insufficient exploration of the relationship between photocatalyst structure and its photocatalytic mechanism and performance,very limited photocatalysts with high solar-to-hydrogen(STH)conversion efficiency have been discovered.Two-dimensional materials with large specific surface area,many catalytic sites,and high carrier mobility have the potential with high STH conversion efficiency,and have received the attention of experimental and theoretical researchers in photocatalysis for overall water splitting.Although new photocatalytic materials with monolayer thickness are continuously reported,there are still not many photocatalysts with high STH conversion efficiency.Therefore,the discovery and design of novel monolayer photocatalysts with high STH conversion efficiency is still an area that needs to be explored in depth.In this paper,the possible structures of A2B3(A=As,Sb,Bi,Ga;B=S,Se,Te)monolayers are discovered based on first-principles calculations combined with the method for structure searches:Universal Structure Predictor:Evolutionary Xtallography(USPEX).First,we confirm the stability of A2B3 monolayer and its doped structure.Then we investigate the electronic structural and optical properties,construct a photocatalytic scheme for overall water splitting to produce hydrogen(hydrogen evolution HER/oxygen evolution OER),and explore the effect of different configurations on STH efficiency and thermodynamic feasibility.This work will provide candidates for the development of high performance photocatalysts and a theoretical basis for constructing new schemes of photocatalytic water splitting for hydrogen evolution.1.The Bi2Se3 monolayer with P-3ml space group is determined and optimized,and its stability is confirmed by phonon dispersions and ab initio molecular dynamics(AIMD)simulation.Bandgap and band edges,density of states,optical properties,and carrier mobilities are calculated and analyzed,and effects of strain engineering on band structures and optical absorption are explored.The results show that the Bi2Se3 monolayer has an indirect bandgap of 1.42 eV(HSE06)/1.96 eV(GW),and strain engineering can significantly modulate band edges and bandgaps.The electron and hole mobilities show anisotropy,with large mobilities in both zigzag and armchair directions,the largest mobility can reach 1.96 × 105 cm2V-1s-1,and the spatial charges separation is obvious.The high optical absorption in the UV-visible region can be observed and can be further enhanced by strain.Despite the excellent photoelectronic properties,its band edges cann’t meet the requirements of overall water splitting for hydrogen generation.Therefore,we consider to find its isomers to improve band edges.Using the USPEX method,20 Bi2Se3 monolayer isomers are found.According to HER and OER requirements for bandgap and band edges,3 isomers satisfying the conditions are selected for further study.Their stabilities are confirmed by phonon dispersions and AIMD simulation The band structures,density of states,electrostatic potential,mobilities and strain engineering effects are calculated.The results show that the direct bandgap ofⅠ-Bi2Se3 monolayer is about 1.44/1.77 eV(HSE06/GW).As a comparison,the indirect bandgaps of Ⅱ-Bi2Se3 monolayer are 1.91/2.15 eV,and the indirect bandgaps ofⅢ-Bi2Se3 monolayer are 1.99/2.84 eV.We find that the potentials of conduction band minimum and valence band maximum match the HER and OER conditions,although one of them needs to be applied with a biaxial compression strain greater than 3%.Enhanced optical absorption in the UV-visible region,as well as significant mobility differences between electrons and holes,are also observed.Therefore,the new discovered Bi2Se3 monolayer can be used as candidates for overall water splitting to produce hydrogen,indicating that monolayer isomers do improve photocatalytic performance.2.This chapter investigates the modulation of Bi2Se3 monolayers doped with a single atom.First,the monolayer structures doped with As and Sb elements homologous to Bi,XBiSe3(X=As,Sb)monolayer,are constructed.Their stabilities are confirmed by phonon dispersions and AIMD simulation.The electronic structural properties are calculated.The results show that the indirect band gaps of AsBiSe3 and SbBiSe3 monolayers are 1.2 eV(HSE06)/1.43 eV(GW)and 1.1 eV(HSE06)/1.32 eV(GW),respectively.Both monolayers have high electron(～105)and hole(～104 cm2V-1s-1)mobilities with significant anisotropy and optical absorption in the UV-visible region.Although the band edges of AsBiSe3 monolayer can cross the HER and OER potentials at 4%compressive biaxial strain,the band edges of both monolayers in the natural state cannot meet the requirements of HER and OER.Considering the effect on the first ionization potential and affinity potential with different atomic numbers,we further modulated the system with group ⅢA elements to improve band edges.The configurations of the XBiSe3(X=Ga,In,Tl)monolayer are constructed and optimized.Their stabilities are confirmed by phonon dispersions,AIMD simulation and binding energy,respectively.The calculations of electronic structural properties show that the indirect bandgaps of XBiSe3 monolayers are 1.14～1.69(HSE06)or(1.20～1.84)eV(GW).Optical absorption,carrier mobilities and strain engineering effects are favorable for photoelectron behavior.Therefore,these monolayers have potential applications in optoelectronic devices.Besides,the band edges of InBiSe3 monolayer can meet the redox potential conditions for HER under compressive strains greater than 3%.In general,the structure doped wiht a single atom can modulate the photoelectronic behavior of Bi2Se3 monolayer,but the photocatalytic performance is improved but relatively limited.3.In this chapter,the Bi2Se3 monolayers are doubly modulated by using two different kinds of elements.The AIBiX3(X=S,Se,Te)monolayers are constructed in the same way similar to single element doping and their stabilities are confirmed.The electronic structural properties show that the indirect bandgaps of AlBiS3,AlBiSe3,and AlBiTe3 monolayers calculated by HSE06/GW are 2.44/3.20,1.87/2.41 and 0.97/1.35 eV,respectively,and the carrier mobilities and optical absorption are also favorable for photoelectron behavior.Meanwhile,the bande edges of AlBiS3 and AlBiSe3 monolayers can satisfy the HER and OER conditions without strain.The Gibbs free energies for HER are moderate,and the STH efficiency maximum can reach 17.51%.Therefore,AlBiS3 and AlBiSe3 monolayers are efficient photocatalyst candidates for overall water splitting for hydrogen production.4.This chapter extends to the study of Ga2S3 monolayer.The isomers and their photocatalytic properties are investigated.Using the USPEX method,20 isomers are found from 1038 Ga2S3 monolayers.The phonon dispersions confirm that 10 monolayers are stable structures,and there are still 7 stable monolayers after eliminating the duplicate structures.The calculated electronic properties show that the vacuum levels of two surfaces are different due to the existence of polarized electric fields in some Ga2S3 monolayers.For 7 monolayers,12 schemes of sunlight photocatalytic overall water splitting to manufacture hydrogen are constructed,but only 10 schemes can meet the band edge requirements of HER and OER.The Gibbs free energies for HER and OER are within the range of the corresponding reactions that have been achieved experimentally,although the overpotentials of some monolayers are not sufficient to drive the reactions spontaneously.Therefore,HER and OER of the 7 monolayers are thermodynamically feasible.In particular,the STH efficiency of GS-7 monolayer with PI space group is up to 17.71%,which can be increased to 19.58%with 2%compression strain.The Gibbs free energies of HER and OER are only 0.16 eV and 1.80 eV(limiting step),which are smaller than the corresponding overpotentials of 0.91 eV and 2.11 eV,respectively,indicating that only photogenerated electrons(without other energy)can drive HER and OER.Therefore,GS-7 monolayer is a promising candidate material for overall water splitting to manufacture hydrogen.In conclusion,the 7 obtained Ga2S3 monolayers can be used as photocatalyst candidates for overall water splitting,especially GS-7 monolayer has the potential to be used for the development of overall water splitting to manufacture hydrogen.