First Principle Investigation On G-C₃N₄-base Photocatalytic Hydrogen Production Materials

g-C₃N₄-based photocatalysts have been widely used in many fields such as photocatalytic hydrogen production,photocatalytic CO₂reduction and photocatalytic nitrogen fixation due to their suitable band structure.Combining g-C₃N₄with other semiconductor materials or supporting metal clusters on g-C₃N₄is considered an effective strategy to optimize the photocatalytic activity of g-C₃N₄.In this thesis,the effects of compositing with SnO₂and loading with Ni clusters on the photocatalytic activity of g-C₃N₄are investigated by theoretical calculations.The SnO₂/g-C₃N₄heterojunction has been successfully synthesized through experiments,however,the interaction between SnO₂and g-C₃N₄and the migration process of photogenerated carriers are still unclear.Therefore,we calculated the electronic structure and charge transfer of SnO₂/g-C₃N₄heterojunction by density functional theory method to uncover the interaction between g-C₃N₄and SnO₂.Due to the work functions difference between g-C₃N₄and SnO₂,charge transfer will occur from SnO₂to g-C₃N₄.As a result,a built-in electric field is generated at the interface of g-C₃N₄and SnO₂,and the direction of the electric field is directed from g-C₃N₄to SnO₂.The Valence band maximum（VBM）and conduction band minimum（CBM）of the heterojunction are contributed by g-C₃N₄and SnO₂,respectively.When the heterojunction is excited by light,the electrons on the conduction band of SnO₂are transferred to the valence band of g-C₃N₄under the action of the built-in electric field,forming a Z-scheme heterojunction,promoting the effective separation of electrons and holes in g-C₃N₄and SnO₂.The effect of cluster sizes on the catalytic activity of g-C₃N₄was researched by loading Ni clusters of different sizes in the cavity of g-C₃N₄.The results show that the adsorption of H₂O molecules on the surface of Ni_n/g-C₃N₄is more stable,and Ni_n/g-C₃N₄can significantly reduce the bond energy of O-H,enhance the polarity of water molecules,and facilitate the dissociation process of H₂O.Through TDDFT calculations,it is found that the excitation of g-C₃N₄belongs to local excitation,so photo-generated carriers are easy to recombine;while Ni_n/g-C₃N₄belongs to charge transfer excitation,which can separate electrons and holes,which is conducive to the separation and storage of photo-generated carriers.