Electronic Properties, Photocatalytic Water Splitting Mechanism, And Strain And Electric Field Regulation Of ZnS/SnS₂ And ZnSe/SnSe₂ Heterojunction

A fast-growing industry and the rising global population are the key factors contributing to the energy shortage and environmental pollution.Photocatalytic water splitting to produce hydrogen,which converts solar energy into chemical energy,is one technology that promises to solve the both problems.However,the single semiconductor photocatalyst has some defects,such as photogenerated electron-hole recombination and small photoresponse range,which leads to its low efficiency of hydrogen production and can not meet the practical needs.The engineering heterojunctions can promote the separation of electron-hole pairs and enlarge the photoresponse range,thereby improving the photocatalytic performance.In this paper,the electronic properties of ZnS/SnS₂ and ZnSe/SnSe₂heterojunctions and their mechanism of photocatalytic water splitting were studied by the first-principle method.In addition,ZnS/SnS₂ and ZnSe/SnSe₂heterojunctions were regulated by biaxial strain and applied electric field to explore the changes of their electronic properties and photocatalytic water splitting mechanism.The main findings are as follows:The ZnS/SnS₂ heterojunction was constructed,and its structure,electronic properties,optical properties and details of photocatalytic water splitting were studied.Monolayer ZnS and SnS₂ can form a stable heterojunction with a binding energy of-18.63 me V/?².And ZnS/SnS₂ heterojunction is a direct bandgap semiconductor with a bandgap value of 1.79 e V.The electron mobility of ZnS/SnS₂ heterojunction is 6362.56 cm² V^-1 S^-1,which is significantly higher than that of monolayer ZnS(915.42 cm² V^-1 S^-1)and SnS₂(1043.35 cm² V^-1 S^-1).The initial absorption position of ZnS/SnS₂ heterojunction was 2.00 e V,which was much lower than the initial absorption energy of ZnS（3.30 e V）and SnS₂（2.50e V）,indicating that the ZnS/SnS₂ heterojunction has stronger visible light absorption.ZnS/SnS₂ heterojunction is a Z-type photocatalyst.ZnS and SnS₂ form a staggered band structure.The valence band maximum and conduction band minimum of SnS₂ are lower than those of ZnS,respectively.Under the effect of the built-in electric field,the photogenerated electrons and holes accumulate in conduction band of ZnS and valence band of SnS₂,respectively.Electrons in the ZnS conduction band reduce H⁺to H₂,and holes in the SnS₂ valence band oxidize O^2-to O₂,finally realizing the goal of photocatalytic water splitting.The ZnSe/SnSe₂ heterojunction was constructed,and its structure,electronic properties,optical properties and details of photocatalytic water splitting were studied.Monolayer ZnSe and SnSe₂ can form a stable heterojunction with a binding energy of-19.47 me V/?².And ZnSe/SnSe₂ heterojunction is a direct bandgap semiconductor with a bandgap value of 1.21 e V.The electron mobility of ZnSe/SnSe₂ is 5572.04 cm² V^-1 S^-1,which is significantly higher than that of monolayer ZnSe(661.71 cm² V^-1 S^-1)and SnSe₂(889.28 cm² V^-1 S^-1).The initial absorption position of ZnSe/SnSe₂ heterojunction was 1.72 e V,which is much lower than the initial absorption energy of ZnSe（2.83 e V）and SnSe₂（1.84 e V）,indicating that the heterojunction has stronger light absorption.ZnSe/SnSe₂heterojunction is a Z-type photocatalyst.ZnSe and SnSe₂ form a staggered band structure.The valence band maximum and conduction band minimum of SnSe₂are lower than those of ZnSe,respectively.Under the effect of the built-in electric field,the photogenerated electrons and holes accumulate in conduction band of ZnSe and valence band of SnSe₂,respectively.Electrons in the ZnSe conduction band reduce H⁺to H₂,and holes in the SnSe₂ valence band oxidize O^2-to O₂,finally realizing the goal of photocatalytic water splitting.The effects of biaxial strain on the electronic properties and photocatalytic water splitting mechanism of ZnS/SnS₂ and ZnSe/SnSe₂ heterojunctions were investigated.In the strain range from-10%to 10%,the ZnS/SnS₂ heterojunction transitions from direct band gap to indirect band and then to metal as the strain increase,while the ZnSe/SnSe₂ heterojunction transitions from metal to direct band gap and then to indirect band gap.Especially when the strain value is 4%,both heterojunctions change from direct band gap to indirect band gap.Under strain,electrons transfer from ZnS（ZnSe）to SnS₂（SnSe₂）.In the strain range of-10%to 10%,the charge transfer between the layers of ZnS/SnS₂ heterojunction increases gradually with the increase of strain.In the strain range of-6%to 10%,the interlayer charge transfer of ZnSe/SnSe₂ heterojunction gradually increases with the increase of strain.In addition,the tensile strain can improve the light absorption of the two heterojunctions in the visible light range,while compressive strain is the opposite.In the strain range of-4%to 2%,ZnS/SnS₂ and ZnSe/SnSe₂heterojunctions remain Z-type photocatalysts and meet the potential requirements of photocatalytic water splitting.In this strain range,with the increase of strain,the redox ability of ZnS/SnS₂ heterojunction is weakened,while the reduction ability of ZnSe/SnSe₂ heterojunction is weakened and the oxidation ability is enhanced.When the strain is too small（-6%）,the reduction ability of ZnS/SnS₂heterojunction is enhanced and the oxidation ability is weakened,but it still meets the potential requirement of photolysis of water;However,ZnSe/SnSe₂heterostructures do not meet the potential requirements for photodegradation of water.When the strain is too large（4%）,the two heterojunctions will change from Z-type photocatalysts to Type-I photocatalysts,so the excessive tensile strain is not conducive to the photocatalytic decomposition of water.The effects of external electric field on the electronic properties and photocatalytic water splitting mechanism of ZnS/SnS₂ and ZnSe/SnSe₂heterojunctions were investigated.In the electric field range from-0.9 V/?to 0.8V/?,the band gap of ZnS/SnS₂ heterojunction gradually increase with the increase of electric field strength.In the electric field range from-0.8 V/?to 0.5V/?,the band gap of ZnSe/SnSe₂ heterojunction gradually increases with the increase of electric field strength.Excessive positive or negative electric fields will cause the breakdown of the heterojunction.The breakdown voltages of the positive and negative electric fields of ZnS/SnS₂ heterojunction are-0.9 V/?and1.0 V/?respectively,and the breakdown voltages of the positive and negative electric fields of ZnSe/SnSe₂ heterojunction are-0.8 V/?and 0.9 V/?respectively.When a positive electric field is applied,electrons transfer from SnS₂（SnSe₂）to ZnS（ZnSe）,and the amount of charge transfer increases gradually with the increase of electric field strength.When a negative electric field is applied,electrons transfer from ZnS（ZnSe）to SnS₂（SnSe₂）,and the amount of charge transfer gradually increases with the increase of electric field strength.In the electric field range of-0.8 V/?to 0.8 V/?,the light absorption of ZnS/SnS₂ and ZnSe/SnSe₂ gradually weakens with the increase of electric field intensity.The ZnS/SnS₂ and ZnSe/SnSe₂ heterojunctions remain as Z-type photocatalysts when a negative electric field is applied.In the range of-0.6 V/?to 0 V/?,the redox property of the ZnS/SnS₂ heterojunction gradually increases with the decrease of the electric field strength.In the range of-0.4 V/?to 0 V/?,the redox property of ZnSe/SnSe₂ heterojunctions gradually increases with the decrease of the electric field strength.When a positive electric field is applied to ZnS/SnS₂ and ZnSe/SnSe₂ heterojunctions,both heterojunctions change from Z-type to Type-Ⅱor type-Ⅰphotocatalysts,and their redox ability becomes weaker,which is not favorable to photocatalytic water splitting.