Theoretical Study On Electronic Structure Regulation And Photocatalytic Water Splitting For Hydrogen Production In Two-dimensional Materials

Under the dual pressure of energy crisis and environmental pollution,hydrogen energy has become one of the most promising clean energy sources to replace traditional fossil fuels because of its advantages of no pollution and high combustion calorific value.Among various hydrogen production methods,photocatalytic water splitting can directly convert inexhaustible solar energy into hydrogen energy only by the aid of the semiconductor materials,which is considered as the most ideal route to obtain hydrogen energy.In this process,the properties of the semiconductor materials play an important role in realizing the above energy conversion.Two dimensional materials have become potential candidates in the field of photocatalysis because of their ultra-high specific surface area and excellent photoelectric properties.However,the practical application of two-dimensional materials in photocatalytic water splitting is still limited due to the rapid recombination of photogenerated carriers for a single system and the limitations of the band gaps and band edge positions of the materials.Therefore,the key to improve the efficiency of photocatalytic water splitting is to explore and design novel two-dimensional photocatalytic systems and effectively regulate the properties of the avaliable materials.In this dissertation,we systematically studied the geometric structures,electronic and photoelectric related properties of several two-dimensional materials based on the first-principles calculations.We also explored the regulation of the electronic structure and photocatalytic properties of the above materials by heterostructure,strain and dipole built-in electric field,and revealed its underlying physical mechanism.These studies will provide theoretical guidance for the application of two-dimensional materials in the field of photocatalytic water splitting.The main research contents and conclusions of this paper are as follows:(1)We constructed the blue P/?-AsP vd W heterostructure with a type-II band alignment by stacking vertically blue P and?-AsP monolayers,and explored its application potential in photocatalytic water splitting.It is found that blue P/?-AsP vd W heterostructure is an indirect band gap semiconductor with band gap value of1.46e V.Its band edge position crosses the redox potential of water and meets the requirements of photocatalytic water splitting.The efficient charge transfer between blue P and?-AsP monolayers in the interface region ensures that blue P/?-AsP heterostructure has high utilization rate of photogenerated carriers,improving its photocatalytic performance.In addition,blue P/?-AsP heterostructure has a strong absorption capacity in the visible region and even ultraviolet light.At the same time,it also maintains a high carrier mobility.We explored the adsorption and dissociation process of water molecules on the surface of this heterostructure,and demonstrated the reaction path of water molecules decomposition,which provides theoretical reference for the applications of blue P/?-AsP heterostructure in photocatalysis.(2)We proposed a novel Sn₂S₂P₄ monolayer structure based on the Sn P₃monolayer,and demonstated that the structure is a visible light driven high-efficiency photocatalytic material,which improves the situation that its parent Sn P₃ monolayer cannot be used for photocatalytic water splitting.The stability of this structure is confirmed by phonon dispersion curves and molecular dynamics simulation.It is found that Sn₂S₂P₄ monolayer has a larger 1.77e V band gap and suitable band edge positions than Sn P₃ monolayer,and it possesses strong light absorption capacity in the visible region.The free energy calculations demonstrate the superiority of Sn₂S₂P₄monolayer in the ready occurrence of both OER and HER under visible light irradiation.The kinetic overpotential and STH efficiency are tunable with the p H value.The high STH efficiency of 17.51%at p H=4 is close to the conventional theoretical limit of 18%.This study provides theoretical guidance for realizing the optimal photocatalytic water splitting activity and designing ideal photocatalytic materials.(3)We explored the effect of biaxial strain regulation on the photocatalytic activity of MoSSe/g-GeC heterostructure,and further revealed the physical mechanism of the enhancement of catalytic activity caused by compressive strain.Our results demonstrate that the MoSSe/g-GeC heterostructure is a direct band gap semiconductor with 1.83e V band gap value,and possess a noticeable visible-light adsorption intensity.Its type II band alignment is conducive to promoting the separation of photogenerated electrons and holes.Moreover,the optical absorption,band edges,bandgap,and charge transfer of the MoSSe/g-GeC heterostructure can be effectively tuned by external strain.We find an appropriate compressive strain will lead to a charge transfer variation between the MoSSe and g-GeC layer and further improve the catalytic activities.Meanwhile,the enhanced optical absorption in the visible light range and the promotion of energy conversion efficiency are observed at2%tensile strain,which supports the heterostructure to serve as an excellent candidate for photovoltaic devices.Our results provide an effective regulation strategy to realize the multifunctionality of MoSSe/g-GeC heterostructure for phtotcatalyric water splitting.(4)We predicted a series of polar MSi₂N₂XY(M=Mo,W;X/Y=N,P,As,S,Se,Te;X?Y)monolayers with asymmetric configurations,and explored their application potentials in photocatalytic water splitting.Among 30 candidates,MoSi₂N₃P,MoSi₂N₃As,WSi₂N₃P and WSi₂N₃As are identified as excellent photocatalytic materials for overall water spliiting.It is found that the built-in electric field caused by intrinsic polarization in MSi₂N₃Y monolayer not only promotes the separation of photogenerated electrons and holes,but also endows the photogenerated carriers adequate driving forces for spontaneous OER and HER under illumination.In addition,MSi₂N₃Y monolayer has a wide light absorption range and strong visible-light absorption capacity.Benefiting from built-in electric field,the solar hydrogen conversion efficiency of MSi₂N₃Y monolayer can reach 32.93%,which is much higher than that of previously reported traditional photocatalysts.Our findings provide theoretical guidances for the design of new photocatalytic materials with high energy conversion efficiency.