Photocatalytic Mechanism Of G-C₃N₄-based Oxygen-deficient Heterojunctions Based On Density Functional Theory

g-C₃N₄has been investigated generally on account of its eminent photocatalysis effect.But it still has been shown some features such as the high electron hole recombin action rate and large band gap,which results in its extensive use under the retraint.It is an effective way to improve the photocatalytic effect to find suitable materials to construct heterojunction.Oxygen defect is an easy defect in the preparation of materials.Previous experimental studies have shown that the presence of oxygen defect will affect the photocatalytic performance of materials.However,it is not clear why the presence of oxygen defects will change the photocatalytic performance of materials,and how oxygen defects affect the photogenerated carrier transport mechanism of heterojunction materials.Therefore,In this paper,the charge transfer,density of states,electron band arrangement and electron orbital wave function of g-C₃N₄/（101）-Ti O₂,g-C₃N₄/V_O-（101）-Ti O₂,g-C₃N₄/（001）-Ti O₂and g-C₃N₄/V_O-（001）-Ti O₂heterojunctions are systematically studied based on the hybrid density functional method.The results show that the pure g-C₃N₄/（101）-Ti O₂,g-C₃N₄/V_O-（101）-Ti O₂,and g-C₃N₄/（001）-Ti O₂heterojunction for Z-scheme,and g-C₃N₄/V_O-（001）-Ti O₂for type-II heterojunction.In addition,the electron cloud of impurity level in the oxygen defect g-C₃N₄/（101）-Ti O₂heterojunction is distributed in the whole heterojunction in a delocalized way,which makes the interface between Ti O₂and g-C₃N₄form a better contact,and improves the photocatalytic effect.For the oxygen defect g-C₃N₄/（001）-Ti O₂heterojunction,there are localized deep defect levels in the energy gap,and the wave function of the deep impurity level is distributed on one side of the heterojunction.They can only act as electron-hole pairs complex center,making the photocatalytic effect worse.In addition,in order to further explore the role of oxygen defects in different materials,the g-C₃N₄/Bi₂WO₆heterojunction and g-C₃N₄/V_O-Bi₂WO₆heterojunction were also studied in this paper.The results show that both pure g-C₃N₄/Bi₂WO₆and g-C₃N₄/V_O-Bi₂WO₆are Z-scheme heterojunctions.Due to the existence of oxygen defect,the Fermi level is raised,so that the two heterojunctions have the opposite direction of the internal electric field.Moreover,g-C₃N₄/Bi₂WO₆with oxygen defect can effectively improve the defects of bismuth tungstate material,such as weak reduction ability and high electron hole recombination rate.The electron cloud of impurity level in g-C₃N₄/V_O-Bi₂WO₆heterojunction is also distributed in the whole heterojunction,which improves the transport effect of material carrier.In addition,due to the improvement of reducing ability,hydroxyl radicals can be generated,which makes more oxidizing active substances participate in the degradation reaction of pollutants,so that the g-C₃N₄/V_O-Bi₂WO₆heterojunction has a better photocatalytic effect.In the two kinds of heterojunctions with oxygen defects calculated in this paper,some shallow local impurity levels and delocalized levels are generated due to the existence of oxygen defects.The delocalized defect state is an extended state,which promotes the photogenerated electrons to pass through the intermediate interface region of the heterojunction to the interface of another material,accordingly accelerating the combination of photogenerated electrons and holes in the process of Z-scheme mechanism in the two materials.There is no doubt that the number of charge carriers may be more than before involved in the photocatalytic reaction.Finally,it will increase the reaction.This paper provides a method to improve the photocatalytic effect of materials by introducing oxygen defects.