Study On Two-dimensional MXenes-based Heterostructure In Photocatalytic Water Splitting

Hydrogen production from water splitting is realized from TiO₂ electrode under ultraviolet light irradiation,and semiconductor photocatalysis technology is considered to be one of the most effective ways to solve the energy crisis and reduce the secondary environment.However,due to its large band gap,extremely low solar energy utilization results in very limited hydrogen evolution capacity.Following a series of oxide photocatalysts such as TiO₂,two-dimensional（2D）photocatalysts have gained widespread attention.Among them,2D MXenes have ultra-high carrier mobility and suitable band gaps,showing good prospects in the field of photocatalytic water splitting.However,the photogenerated electron-hole pairs in 2D MXenes monolayers are extremely prone to recombination,which severely inhibits their application in photocatalytic water splitting.The construction of heterostructures based on 2D materials can not only maintain the good properties of the original 2D materials,but also effectively realize the spatial separation of photogenerated electron-hole pairs.Therefore,it is necessary to explore 2D MXenes-based heterostructure composites with good physicochemical properties.At present,exploring the electronic and optical properties of 2D materials based on theoretical design methods has become a powerful tool for the study of semiconductor materials and catalysis.In this thesis,based on first-principles calculations,the electrical,optical,and kinetic properties of several novel MXenes-based 2D heterostructures are explored,and their application prospects in the field of photocatalytic water splitting are predicted.The main research results of this paper are summarized as follows:（1）A new Hf₂CO₂/WS₂ heterostructure is designed and exhibits excellent photocatalytic water splitting ability.The binding energy,and ab initio molecular dynamics（AIMD）simulations exhibit this material’s excellent ambient stability.Furthermore,the band structure confirms that the Hf₂CO₂/WS₂possesses the intrinsic type-Ⅱ heterostructure and are well satisfied with the requirements of redox energy level in water splitting reaction.Meanwhile,the charge transfer occurs from Hf₂CO₂ to WS₂ monolayer,which further induces the separation of photogenerated carriers and prolong the lifetime of carriers.Dramatically high visible-light absorption(～6.3×10⁵ cm^-1)and carrier mobility(～2.2×10³cm² V^-1s^-1)are found,which means that the heterostructure has adequate motive force to make the photogenerated carriers separate into different monolayers quickly before recombination.Moreover,according to Gibbs free energy calculation,HER and OER can be triggered spontaneously under the driving of external potential.The results confirm that the thermodynamic feasibility of water splitting on Hf₂CO₂/WS₂ heterostructure surface.Hence,these distinctive features endow Hf₂CO₂/WS₂ heterostructure with the potential ability of photocatalytic water splitting,which provides a potential avenue for the future experimental project.（2）The application potential of AsP/M₂CO₂（M=Sc,Zr）van der Waals heterostructures in photocatalytic water splitting was systematically explored.The calculated results show that AsP/Zr₂CO₂ heterostructure possesses an unfavorable type-Ⅰband alignment,whereas AsP/Sc₂CO₂ exhibits a desirable type-Ⅱband alignment,which is beneficial for separating the photogenerated electron-hole pairs.Also,the band edge positions of AsP/Sc₂CO₂heterostructure stride the redox potential of water,ensuring favorable reaction kinetics.Besides,the strong optical absorption of AsP/Sc₂CO₂ heterostructure in both visible and ultraviolet regions(especially up to 10^-6 cm^-1 at about 250 nm)makes it possible to utilize solar energy effectively.Meanwhile,AsP/Sc₂CO₂ heterostructure has an exciton binding energy as low as 0.09 e V,which quantitatively illustrates the high separation efficiency of photogenerated charge carrier.Thus,the type-Ⅱband alignment,suitable band edge position,strong light absorption,and low exciton binding energy together indicate that AsP/Sc₂CO₂heterostructure is a potential photocatalytic material.In addition,the obvious redshift phenomenon in the optical spectrum of AsP/Sc₂CO₂ heterostructure shows that biaxial strain can improve its light capture capability.Also,the interconversion between type-Ⅱand type-Ⅰcan be achieved by applying different strains.All these findings suggest that the novel AsP/Sc₂CO₂ heterostructure has significant application prospects in next-generation photovoltaic devices and photocatalysts.（3）Based on the problem that the strong redox capacity of type Ⅱ heterostructures is difficult to be compatible with the light response range,an efficient Sc₂CF₂/Janus MoSSe Z-scheme photocatalyst is designed.The research shows that the traditional type-Ⅱ to direct Z-scheme heterostructure conversion can be realized through different stacking methods of Sc₂CF₂ and Janus MoSSe.Through the built-in electric field and band bending theory,the transition path of photogenerated carriers is analyzed,which reveals the completely different photocatalytic mechanism of type-Ⅱ and direct Z-scheme heterostructure.Surprisingly,the direct Z-scheme heterostructure displays a high overpotential of the hydrogen evolution reaction(χ_H2=1.01 e V)and the oxygen evolution reaction(χ_O2=1.46 e V)compared with the type Ⅱ heterostructure.More importantly,the direct Z-scheme heterostructure has an ultra-high solar-to-hydrogen（STH）efficiency（36.1%）,which breaks through the limitation of traditional theoretical efficiency and reveals a tremendous prospect of commercial application.Furthermore,the calculation of free energy confirms that the water splitting reaction on the direct Z-scheme heterostructure occur spontaneously under the external potential,but not for type-Ⅱ heterostructure.Finally,introducing the additional electronic conductor（N-doped graphene）can accelerate the electron（hole）transfer and interlayer carrier recombination,which will further improve the photocatalytic performance of Z-scheme heterostructure.The research results can lay a theoretical foundation for the preparation of high-performance and stable Z-scheme photocatalysts for water splitting.