First Principles Study On Photocatalytic Decomposition Of Water By Heterojunction In Two-dimensional Materials

The development of human society is closely related to materials,information and energy.In order to meet the demand for energy and reduce environmental pollution,the development of new energy materials has become an urgent task.Photocatalysis technology,which converts inexhaustible solar energy into chemical energy,is one of the important means to solve the above problems.In this process,the selection of efficient photocatalysts is very important.Two-dimensional semiconductor materials are potential candidates due to their large specific surface area and low carrier migration distance.Many one-component two-dimensional semiconductors have more or less certain performance defects and high photoelectron-hole pair recombination rate,which affects the photocatalytic efficiency and hinders their application range.The stacking of two two-dimensional materials into type II heterojunction materials can effectively reduce the carrier recombination rate and significantly improve the photocatalytic performance.Based on this,this paper designed several efficient photocatalytic systems for the decomposition of water to produce hydrogen,and systematically studied the electronic structure and photocatalytic properties of several heterojunction using the first-principles method.Specific research contents are as follows:（1）Design of Ga Se/Mo S₂heterojunction and study on its electronic structure and photocatalytic performance.The two two-dimensional materials are stacked so that the mismatch is less than 5%,and the first principles method is used to study.The results show that the Ga Se/Mo S₂heterojunction has an indirect band gap of 1.835 e V.The valence band top of the heterojunction is contributed by Ga Se,while the conduction band bottom is contributed by Mo S₂.Therefore,the Ga Se/Mo S₂heterojunction is a Type II heterojunction,which is favorable for the separation of photogenerated electron-hole pairs.Differential charges are studied,and the results show that electrons are transferred from the Ga Se layer to the Mo S₂layer,thus forming a built-in electric field.In addition,the edging position of the Ga Se/Mo S₂heterojunction at different p H values indicates that it has a strong redox potential under acidic and neutral conditions.The in-plane biaxial strain shows that the heterojunction is a good photocatalyst at 2%tensile stress.Finally,the absorption coefficient of the heterojunction is studied.It is shown that the absorption coefficient of the heterojunction increases greatly in the visible range under 2%tensile stress.In conclusion,Ga Se/Mo S₂heterojunction is a potential catalyst for photocatalytic water decomposition.（2）Design of Ga Se/Mo SSe heterojunction and study on its electronic structure and photocatalytic performance.Janus Mo SSe can be obtained by destroying the structural symmetry of Mo S₂.Due to its asymmetric structure,it has different characteristics when Ga Se and Mo SSe contact with different surfaces to form van der Waals heterojunction.Ga Se is named Model A when it comes into contact with the S atomic plane,and model B when it comes into contact with the Se atomic plane.It is found that model A has an indirect band gap of 1.939 e V and model B has a direct band gap of 2.125 e V when using the HSE06 functional.The top of the valence band of model A is contributed by Ga Se,while the bottom of the conduction band is contributed by Mo SSe,so it is a Type II heterojunction.For model B,both the valence band top and conduction band bottom are contributed by Mo SSe,so it is a Type-I heterojunction.The work function shows that model A heterojunction forms an internal electric field from Mo SSe layer to Ga Se layer.In addition,the edge positions of model A at different p H values indicate that the heterojunction has a strong redox potential under acidic and neutral conditions.The in-plane biaxial strain shows that the heterojunction is a good photocatalyst in the small stress range.Finally,the absorption coefficient of heterojunction model A is studied,and the results show that the absorption coefficient is greatly improved in the visible range.In conclusion,Ga Se/Mo SSe heterojunction model A is a potential catalyst for photocatalytic water decomposition.（3）Design of Ga₂SSe/Mo S₂heterojunction and study on its electronic structure and photocatalytic performance.When the S atom of Ga₂SSe contacts Mo S₂,it is named as model A,whereas when the Se atom contacts Mo S₂,it is named as model B.It is found that model A and Model B have indirect bandgaps when the HSE06functional is used,and their bandgaps are 2.116 e V and 2.003 e V respectively.The top of the valence band of model A is contributed by Ga₂SSe layer,while the bottom of the conduction band is contributed by Mo S₂layer,so it is a Type II heterojunction.For model B,both the valence band top and conduction band bottom are contributed by Ga₂SSe,so it is a Type-I heterojunction.The work function shows that the electrostatic potential difference of model A heterojunction is zero.In addition,the edge positions of model A at different p H values indicate that the heterojunction has a strong redox potential under acidic and neutral conditions.The in-plane biaxial strain indicates that the heterojunction is a good photocatalyst at equilibrium.Finally,the absorption coefficient of heterojunction model A is studied,and the results show that the absorption coefficient is greatly improved in the visible range.In conclusion,Ga₂SSe/Mo S₂heterojunction model A is a potential catalyst for photocatalytic water decomposition.