Investigation On The Mechanisms Of HfS₂/GaS And PtSe₂/SnS₂ Heterostructures As Photocatalysts For Water Decomposition

Exploring excellent performance of photocatalysts for hydrogen/oxygen evolution reactions is an effective way for solar energy applications.Two-dimensional semiconductor photocatalysts have large specific surface area and unique physical characteristics,but the rapid recombination of photogenerated electrons and holes in the material limits the photocatalytic efficiency.When the two semiconductor materials are vertically stacked to form type-Ⅱheterostructure,the photogenerated carriers can be separated effectively,so as to prolong the life of photogenerated charge and improve the photocatalytic performance obviously.Therefore,HfS₂/GaS and PtSe₂/SnS₂ heterostructures have been constructed in this paper.And their photoelectric properties and photocatalytic performance have been studied in depth.Firstly,a HfS₂/GaS van der Waals heterostructure has been designed to achieve efficient and spontaneous water decomposition.Through first-principles calculation,HfS₂/GaS heterostructure has been proved to be a stable and promising photocatalyst with the following obvious advantages:both water reduction and oxidation potentials are located in the band gap（2.04 e V）of HfS₂/GaS heterostructure,ensuring the water splitting can occur.The type-Ⅱband structure and built-in electric field ensure the effective separation of photoelectron-hole,and the high mobility of electrons and holes can also predict high photocatalytic activity.By calculating the free energy of the oxygen evolution reaction on the surface of GaS and the hydrogen evolution reaction on the surface of HfS₂ of the heterostructure,it’s found that the water decomposition reaction can occur spontaneously under light.It is worth noting that the hydrogen evolution efficiency after the construction of heterostructure is significantly improved,reaching 14.91%,which is higher than the standard for commercial use of water decomposition photocatalysts.Furthermore,after 1-6%in-plane biaxial tensile strain are applied,the redox reaction of photocatalytic decomposition water can still occur on the HfS₂/GaS heterostructure.And the application of strain can effectively regulate the optical and electronic properties.Therefore,HfS₂/GaS heterostructure could be a promising candidate material for the photocatalytic decomposition of water.In addition,the direct Z-scheme heterostructure has attracted much attention due to its strong photocatalytic and visible light absorption capacity.Using first principles calculations,we explore the electronic and optical properties and photocatalytic mechanism of PtSe₂/SnS₂heterostructure.Compared with PtSe₂,SnS₂ monolayer and traditional type-Ⅱphotocatalyst（HfS₂/GaS heterostructure）,PtSe₂/SnS₂ heterostructure has smaller band gap,more significant light absorption capacity and better hydrogen evolution efficiency.The results of band and state density show that the heterostructure form the type-Ⅱband arrangement.Further work function calculations indicate that the direction of the built-in electric field is PtSe₂ to SnS₂,confirming the charge transfer mechanism is Z-scheme.In the water decomposition reaction,the free energy calculation results show that hydrogen evolution reaction on PtSe₂ side and oxygen evolution reaction on SnS₂ side can occur spontaneously under light.Besides,PtSe₂/SnS₂heterostructure has obvious light absorption in the range of ultraviolet and visible light,indicating it’s sensitive to sunlight.Interestingly,the band edge position still meets the requirement of water decomposition and retains considerable visible light absorption capacity after biaxial strain is applied.Therefore,PtSe₂/SnS₂ heterostructure can be used as a potential direct Z-scheme photocatalyst for hydrogen and oxygen production.