First-principles Study On Photocatalytic Properties Of Two-dimensional C₂N/WS₂ Van Der Waals Heterojunctions

The semiconductor photolysis water hydrogen production technology can directly use solar energy to decompose water to obtain hydrogen energy,which has the characteristics of high efficiency,environmental protection and renewable,and is of great significance for the realization of"carbon peak"and"carbon neutral"strategy.Two-dimensional semiconductor photocatalytic materials have a larger specific surface area and a shorter carrier migration path,thus providing more efficient surface activity to accelerate redox reactions.However,the rapid recombination of carriers in single-component two-dimensional materials and the limitation of light absorption range further hinder the improvement of photocatalytic efficiency.The construction of two-dimensional van der Waals heterojunction can effectively separate photogenerated electrons and holes,and further enhance the overall physical properties of the heterojunction by using the interface characteristics and the synergistic effect between two-dimensional materials,thus greatly improving the photocatalytic efficiency and stability.Therefore,it is of great value to optimize the design of two-dimensional semiconductor photocatalytic complex heterojunction and study the regulation and enhancement mechanism of its photocatalytic performance.In this paper,based on density functional theory,the electronic structure,interface effect and photocatalytic performance of double and multilayer C₂N/WS₂ heterojunction were comprehensively and deeply studied by using first-principles calculation method,and efficient C₂N/WS₂ heterojunction photocatalyst was constructed,revealing the enhancement mechanism of photocatalytic performance.The photocatalytic performance was further enhanced by applying biaxial strain and adjusting interface characteristics.The main research contents are as follows:（1）The effects of biaxial strain on the electronic properties,interfacial properties and photocatalytic performance of C₂N/WS₂ heterojunction were studied.The results show that the tensile strain can reduce the band gap and enhance the visible light absorption,and the compressive strain can promote the interfacial charge transfer and separation and enhance the redox capacity.For example,the transition from indirect band gap to direct band gap can be realized at 2%strain.Interestingly,spontaneous H^*adsorption and H₂ release can be achieved at 2%and-2%strains respectively,and the Gibbs free energy is in the ideal range,so the dynamic strain between±2%can achieve ideal hydrogen evolution reactivity.（2）The electronic properties,interfacial properties and photocatalytic properties of WS₂/C₂N/WS₂ and C₂N/WS₂/C₂N sandwich heterojunctions were investigated.The calculation results show that both WS₂/C₂N/WS₂ and C₂N/WS₂/C₂N are type II heterojunction with indirect band gap.Compared with the C₂N/WS₂ bilayer,the two sandwich structures have stronger interfacial effect,charge transfer,visible light absorption and excellent hydrogen evolution activity.WS₂/C₂N/WS₂ structure has the strongest charge transfer and interfacial effect,while C₂N/WS₂/C₂N has the strongest visible light absorption.In addition,applying biaxial strain to the C₂N/WS₂sandwich structure can further enhance the photocatalytic performance,achieve a wide range of adjustable band gap size and the transition from indirect band gap to direct band gap.At 1%strain,both kinds of sandwich heterojunctions can achieve direct band gap and have excellent photocatalytic hydrogen production performance.The above results show that the C₂N/WS₂sandwich structure has better photocatalytic performance than the double-layer structure.Changing the stacking order,increasing the number of heterojunction layers and applying biaxial stress are all effective methods to regulate and enhance the photocatalytic performance of C₂N/WS₂heterojunction.