First Principles Study Of The Photocatalytic Properties Of (RuI₂,PtTe₂)/Sb₂S₃,SiSe/(SnSe₂,SnSSe) Heterojunctions And SrTiO₃ Surface Structure

With the acceleration of social industrialization,serious energy crises and environmental pollution have arisen all over the world.In 2020,the Chinese government proposed to achieve the goal of "carbon peak" by 2030 and "carbon neutrality" by 2060.The development of clean energy is an important basis for achieving this goal.Hydrogen energy is a kind of clean energy with good application prospects.Utilizing solar energy to drive the photocatalytic water splitting to produce hydrogen is an important method to develop hydrogen energy.Searching for preferable semiconductors as photocatalysts to produce hydrogen through photocatalytic overall water splitting is a hot research topic at present.Compared with the photocatalysts of three-dimensional（3D）bulk materials,two-dimensional（2D）materials possess superior properties,such as larger specific surface area,abundant active sites,and higher carrier mobility.The probability of electron-hole pair recombination is high because the photogenerated carriers migrate to the photocatalyst surface to participate in the hydrogen evolution reaction（HER）and oxygen revolution reaction（OER）.Moreover,the heterostructures with staggered band alignment for photocatalytic direct Z-scheme have attracted extensive attention,which not only realize the spatial separation of photogenerated electrons and holes,but also retain strong redox ability.However,it is still a great challenge to development 2D photocatalysts with high solarto-hydrogen efficiency（ηSTH）and thermodynamically spontaneous reactions.In addition,understanding the ultrafast dynamics mechanism of photoexcited carriers is also the basis for improving the photocatalytic performance of semiconductors.In this paper,based on the first-principles calculations,the electronic structure properties,optical properties,ηSTH,Gibbs free energy（ΔG）,and non-adiabatic molecular dynamics（NAMD）properties of the multiple monolayers,heterostructures and surface structures were investigated.The manipulation effects of doping,stacking pattern and strain engineering on the electronic structural and photocatalytic properties of these structures were explored,which provided a theoretical basis for the development of photocatalyst materials with good photocatalytic properties.The main research contents are as follows:1.The geometric configurations of the Sb₂S₃ monolayer with P-3ml and P2₁/m space groups were constructed,and phonon dispersions and ab initio molecular dynamics simulation（AIMD）assure the dynamical stability and thermodynamical stability,respectively.The band gap,band edge,density of states,optical properties,and carrier mobility of the two configurations are calculated.The effects of strain engineering on band structure,light absorption,and the ηSTH are also studied.The results show that the band edge of the Sb₂S₃-P2₁/m monolayer can straddle the redox potential for HER to produce hydrogen.The ηSTH of the Sb₂S₃-P2₁/m monolayer is 9.60%when no strain is applied,and the maximum ηSTH of the Sb₂S₃-P2₁/m monolayer can reach 17.51%under+5%biaxial tensile strain.However,the Sb₂S₃-P-3ml monolayer cannot meet the condition of photocatalytic overall water splitting.Therefore,the RuI₂/Sb₂S₃-P-3m1 van der Waals（vdW）heterostructure is constructed to realize the photocatalytic HER with Z-scheme.The results show that the ηSTH of the RuI₂/Sb₂S₃-P-3m1 heterostructure can reach 4.94%without strains and 9.45%under the+9%biaxial tensile strains.The Gibbs free energy in HER（ΔGH*）is used to assess the thermodynamic feasibility of the reaction.All possible adsorption sites are considered.The ΔGH*of the Sb₂S₃-P2₁/m monolayer is 1.037-1.425 eV,and that of the RuI₂/Sb₂S₃-P-3m1 heterostructure is 1.233-2.132 eV.The calculated ΔGH*is smaller than that of the previous reported 2.5 eV for the photocatalytic HER driven by Ni3S2.Therefore,photocatalytic overall water splitting driven by the Sb₂S₃ monolayer and the RuI₂/Sb₂S₃-P-3m1 heterostructure possess thermodynamic feasibility,and can be used as a candidate for developing photocatalytic water splitting photocatalysts for hydrogen production.2.As the ηSTH of the RuI₂/Sb₂S₃-P-3m1 heterostructure is not too high,then we further constructed the PtTe₂/Sb₂S₃ heterostructure.The geometric and electronic structures of the PtTe₂ and Sb₂S₃ monolayers are studied firstly.The results show that the conduction band minimum（CBM）of the PtTe₂ monolayer is suitable for HER,while the valence band minimum（VBM）of Sb₂S₃ is suitable for OER.The maximum lattice mismatch ratio between the two monolayers is 2.30%.Hence,the construction of the PtTe₂/Sb₂S₃ heterostructure with the two monolayers is feasible in terms of the geometries and band alignment.By rotating the two monolayers 180° along the x and y axes respectively,nine configurations for the PtTe₂/Sb₂S₃ heterostructure are obtained.According to the formation energy and structural symmetry,two configurations with higher stabilities in energy are selected among the nine configurations.Both configurations can drive the direct Z-schemes for overall water splitting based on the band alignment and the direction of the built-in electric field.The results show that the maximum η’STH of the PtTe₂/Sb₂S₃ heterostructure is 28.82%,and the biaxial compression strains can significantly reduce η’STH,while the tensile strains have insignificant influence.Therefore,in practice,the compressive strains should be avoided to maintain high η’STH.After optimizing the intermediates,the preferable adsorption sites are identified and ΔGH*S are calculated.The maximum ΔGH*for HER is 1.396 eV,and the Gibbs free energy of the rate-determining steps in OER（ΔGmax）by the dual-site process is 2.089 eV.By comparing the ΔGH*and ΔGmax of the HER and OER that have been experimentally achieved,it is found that both reactions are thermodynamically feasible.Therefore,the 2D PtTe₂/Sb₂S₃ heterostructure is more superior for the development of overall water splitting photocatalyst with high ηSTH than the RuI₂/Sb₂S₃-P-3m1 heterostructure.3.The effects of element substitution and stacking patterns on photocatalytic performance and carrier ultrafast dynamics of SiSe/SnSe₂,SiSe/SnSSe-I and SiSe/SnSSe-Ⅱ heterostructures were studied.Based on the first-principles calculations,the direct Z-schemes for overall photocatalytic water splitting of the three heterostructures were identified.The times of the carrier transfer and recombination were calculated by the NAMD simulation.The maximum η’STH of the considered configurations is 19.18%and can be enhanced up to 28.71%under tensile biaxial strains.The NAMD results indicated that the transfer of electrons for HER and holes for OER for the SiSe/SnSe₂ heterostructure were slower than those for the other two heterostructures,indicating that the reduction and oxidation activities of the heterostructure were well-protected.The shortest electron-hole recombination time is attributed to SiSe/SnSSe-I,indicating it holds superior photocatalytic performance.The obtained ΔGH*and ΔGmax indicate that both HER and OER with SiSe/SnSSe-Ⅰ can spontaneously proceed,while SiSe/SnSe₂ and SiSe/SnSSe-Ⅱ can perform OER but cannot drive HER spontaneously.Therefore,the newfound SiSe/SnSe₂ and SiSe/SnSSe heterostructures,especially SiSe/SnSSe-Ⅰ,are promising candidates in photocatalytic overall water splitting.4.The electronic properties and photocatalytic performance of the SrTiO₃（100）surface with different atom termination and their doped structures were investigated.The SrTiO₃ bulk is inefficient for solar photocatalytic performance because its wide bandgap restricts their response to visible light.Different surface terminals and the doped atoms affect the photocatalytic behavior of the surface.The band edges for the Ti/O-terminated（100）surfaces of SrTiO₃（SS-Ⅰ）can straddle the redox potentials for the HER and OER,while this is not true for the Sr/O-terminated one（SS-Ⅱ）.However,we successfully make the band edges of SS-Ⅱ meet the HER and OER conditions by doped Te and Po atoms（Te-SS-Ⅱ and Po-SS-Ⅱ）,after trying the doped elements of theⅢA-ⅥA group.The ηSTHs are 14.50%,9.89%,and 12.75%for SS-Ⅰ,Te-SS-Ⅱ and PoSS-Ⅱ,respectively.The result of-0.34 eV for the HER on SS-Ⅰ indicates that this reaction can spontaneously proceed.Therefore,these surfaces are promisingly used for photocatalytic overall water splitting.