Design And Theoretical Study Of Two-Dimensional Layered CdS/C₂N Heterostructure Photocatalyst

Semiconductor photocatalytic technology has gradually formed two major application directions of solar energy conversion and environmental purification,and is one of the important ways to solve energy shortage and environmental pollution issues.In recent years,the use of various semiconductor photocatalysts to decompose water for hydrogen production under sunlight irradiation has triggered intensive interest of researchers around the world,and has become a hot topic in the field of materials science.However,the traditional semiconductor photocatalysts has a wide band gap and only absorbs ultraviolet light with a wavelength of less than 400 nm,which has caused poor response to visible light.In addition,due to the high rate of electron-hole recombination in the conventional photocatalysts,the light quantum efficiency is low.Based on the above reasons,the utilization of solar energy and photocatalytic activity have been greatly restricted.Therefore,it is necessary for researchers to search for and design some highly efficient,environmentally friendly semiconductor photocatalysts.Two-dimensional semiconductors,including transition metal dichalcogenides,hexagonal boron nitride,and graphitic carbon nitrides,have become the focus of attention because of the large surface areas available and short migration distance for photogenerated carriers.Among them,two-dimensional nanosheets of cadmium chalcogenides?CdX,X=S,Se,Te?have been successfully synthesized by solvothermal and colloidal techniques.Subsequently,some experimental reports have indicated that CdS nanosheet exhibits excellent photocatalytic activity and good stability for hydrogen evolution.However,CdS nanosheet is a semiconductor with a direct bandgap of 2.94 eV,which restricts the visible light absorption.More importantly,CdS nanosheet has self-oxidation effect and a high rate of photogenerated carriers recombination,which has caused low photocatalytic efficiency and poor structural stability.The construction of two-dimensional heterostructures has been adopted to restrain the recombination of photogenerated carriers,expand the visible light absorption range,and improve the stability of photocatalysts.In this thesis,the photocatalytic activity and efficiency of the CdS/C₂N heterostructure have been systematically investigated by using density functional theory,so as to reveal the photocatalytic mechanism and design an efficient photocatalyst.The main research contents and conclusions are listed as follows:1.The geometric structure and electronic property of CdS?C₂N?monolayer have been investigated.The results show that both CdS and C₂N monolayers are direct band-gap semiconductors,and the calculated band gaps for CdS and C₂N monolayers are 2.81 eV and 2.46 eV,respectively.2.Due to the existence of symmetry,three heterostructures consisting of C₂N monolayer and CdS monolayer with various rotation angles??=0°,60°,and 120?have been constructed,which are denoted as CdS/C₂N?i?,CdS/C₂N?ii?,and CdS/C₂N?iii?,respectively.The calculatedinterface binding energies have indicated that all the systems have the van der Waals interactions.In addition,CdS/C2N?iii?heterostructure has the lowest energy and the smallest interlayer distance,which indicates that this system is the most stable.3.The density of states,Bader charge analysis,charge density difference,and band edge positions of CdS/C₂N?iii?heterostructure have been explored.The results of analysis show that the CdS/C₂N composite system has the characteristics of type-II heterostructures.The photogenerated electrons migrate from the CdS monolayer to the C₂N monolayer,which can effectively suppress the recombination of electron-hole pairs and improve the photocatalytic efficiency.4.The calculated optical absorption spectrum of the CdS/C₂N composite shows that this system has excellent visible light absorption behavior and the absorption edge has a slightly red shift,indicating that the C₂N monolayer acting as a cocatalyst can expand the visible light response range of the CdS monolayer.5.The effects of biaxial strain on the electronic structures and optical properties of CdS/C₂N heterojunction have been investigated.The results have shown that the compositions of band edge,band edge positions,and optical properties of CdS/C₂N heterojunction can be effectively modulated via applying a certain range of strain,implying that the CdS/C₂N heterostructure has a wide potential application in solar cells and photodetectors.In addition,uniaxial strain can also tune electronic structures.These findings could provide some valuable information for experimental researchers to design highly active and efficient photocatalysts for visible light photocatalytic water splitting to produce hydrogen.