First-principles Study Of Janus Indium Chalcogenide Monolayers For Photocatalytic Water Splitting

For a long time,photocatalytic water splitting reaction using the solar energy over semiconductor photocatalysts is considered as an effective strategy to solve the future energy crisis and environmental pollution.Semiconductor photocatalysts play an important role in this process.In general,the three basic conditions at least for an ideal photocatalyst are satisfied and they are the following:（1）the range of the band gap is1.6³.3 eV,which is used to absorb the visible light;（2）the band edge positions including the conduction band minimum（CBM）and valence band maximum（VBM）should straddle the oxidation and reduction potentials of water and be able to perform hydrogen evolution and oxygen evolution reactions（HER and OER）simultaneously;（3）the stability including insoluble in water and corrosion resistance should be enough high.Moreover,a large specific surface area,high carrier mobility and small exciton binding energy also are extremely important conditions.Two-dimensional（2D）Janus indium chalcogenide In₂XY（X,Y=S,Se and Te）monolayers was proposed to be potential photocatalysts for water splitting by using first-principles calculations.The research content mainly includes the following three parts:（1）The atomic structure of Janus In₂XY monolayers was proposed and designed,and their dynamical stability was confirmed by the phonon spectrum calculation.According to the band structures calculated by using HSE06 hybrid functional,the band gaps of In₂SSe,In₂STe and In₂SeTe monolayers are 2.30,1.54 and 1.82 eV,respectively,which meet the requirements of photocatalytic water splitting reaction.In addition,different chalcogen atoms on the upper and lower surfaces of In₂XY monolayers obtain unequal amounts of electrons due to the out-of-plane asymmetry,thereby inducing a built-in electric field that promotes the separation of photogenerated electrons and holes,which is conducive to improving the efficiency of photocatalytic water splitting reaction.（2）By calculating the band edge position of the In₂XY monolayers with regard with the redox potential of water,it was found that the CBM is slightly higher or lower than the reduction potential of water,making HER difficult to occur.It is possible to move the band edge positions by applying strain because in the elastic-plastic deformation region,In₂XY monolayers have more atoms in the unit cell and every atom has three directions of movement（x,y and z axis directions）to complete the translation and more axis of rotation to complete the lattice twist under stress compared with MoS₂monolayer.Thus,in theory In₂XY monolayers should have more degrees of freedom for the stress and be more sensitive to the lattice strain/stress.The results show that the CBM and VBM shift upward and they straddle the oxidation and reduction potentials of water under the compressive strain,which improving the performance of photocatalytic water splitting reaction.（4）Since the compressive strain is favorable for the photocatalytic water splitting reaction of In₂XY monolayers,we further calculated the carrier mobility,exciton binding energy and light absorption coefficient under compressive strain.Compared with MoS₂ monolayer,In₂XY monolayers was found to have a high and anisotropic carrier mobility and a small exciton binding energy,so that the photogenerated electrons and holes generated in them can be effectively separated and transferred to the surface to participate in the next photocatalytic water splitting reaction.In addition,the light absorption coefficients in the visible light region have one or two absorption peaks,indicating that In₂XY monolayers can effectively absorb visible light and those absorption peaks have undergone a red shift under the compressive strain,thus further improving the efficiency of photocatalytic water splitting reaction.This general strain-driven energy band engineering is also applicable to other 2D material photocatalysts with good flexibility,and also promotes further research in related experiments.