The First-Principles Study On Electronic Structures And Photocatalytic Properties Of Zinc Sulfide

Due to the fast generation of photogenerated electron-hole pairs and more nega-tive reduction potential,zinc sulfide(ZnS)is widely investigated in photocatalytic field.The hexagonal wurtzite and cubic zinc-blende structures are two common phases of ZnS.It was reported that the photocatalytic activity of hexagonal ZnS is better than that of cubic ZnS.Experimentally,the difference of photocatalytic activi-ties had been proved,while the theoretical mechanism is not clear yet.It is due to the fact that the chemical activities of semiconductors are determined by the microscopic movements of valence electrons.The first principle calculation based on the density functional theory(DFT)does not rely on any experimental parameters,and solve the electronic waves of Schr?dinger equation after proper approximations to determine the electronic structure and energy of the system containing many electrons and cores in ground state.These data can be used to analyze the photocatalytic activity of the system.Moreover,the band gap of ZnS(about 3.7 eV)is wide,which limits its fur-ther application in visible photocatalysis.Therefore,the dopant Cu was adopted to improve the photocatalytic activity of hexagonal ZnS and the first principle calcula-tion was used to analyze the origin of related properties.The electronic structures of hexagonal and cubic ZnS were calculated by the CASTEP package.First,the internal field is formed along[0001]direction due to the structural deformation in hexagonal ZnS and is beneficial for the separation and migration of photogenerated electrons and holes.Next,the difference of ef-fective masses of electrons and holes in hexagonal ZnS is bigger than that in cubic ZnS,which inhibits the recombination of photogenerated electrons and holes in hexagonal ZnS.Moreover,the bond populations of hexagonal ZnS along[0001]direction are much smaller than the other directions and those of cubic ZnS,which indicates the bonding along[0001]direction is weak and the electrons move freely along[0001]direction.Above all,the hexagonal ZnS is more beneficial than cubic ZnS for the migration and separation of photogenerated electrons and holes,and for the enhancement of photocatalytic activity consequently.The electronic structures of different Cu doping models of ZnS were calculated.The substitutional Cu reduces the band gap of the doping system for visible photo-catalytic response.And the induced Cu 3d state in the top of valence band is much delocalized,which results in the metal conducting character due to the formation of partially filled valence band.The discrete and partially filled levels introduced by in-terstitial Cu is localized seen from corresponding density of states(DOS),which can easily become the recombination centers of photogenerated electrons and holes.Seen from the defect formation energies,the substitutional Cu is stable in hexagonal ZnS,while the interstitial Cu defects are unstable.Moreover,the calculated effective masses indicate that the value of m_h~*/m_e~*is larger in substitutional Cu doping system and the effective masses of photogenerated carriers are more smaller along special[0001]direction,which inhibits the recombination of photoinduced electrons and holes and promotes the movement of photogenerated carriers.