Synthesis Of 2D Metal Phosphorous Trichalcogenides And Applications For Photocatalytic Hydrogen Evolution

Solar-driven water splitting hydrogen production technology is one of the most promising strategies to address the ever-rising energy demand and related environmental problems.The prerequisite for realizing an efficient photocatalytic water-splitting process relies on developing a semiconductor photocatalyst with excellent performance,which should have a suitable band alignment and electronic structure,ensuring effective light-harvesting and photochemical energy conversion efficiency while withstanding photocorrosion in solution.The newly emerging two-dimensional（2D）layered metal phosphorous trichalcogenides（MPX₃）semiconductors are acknowledged theoretically and experimentally to have various intriguing properties including wide spectral response range,high charge carrier mobility,thermodynamically appropriate band structure,and abundant surface catalytic active sites.In this thesis,2D MPX₃ semiconductors are presented as research subject to study their photocatalytic hydrogen production activities.The chemical vapor conversion method was utilized as efficient and robust technique to rationally synthesize the different MPX₃ nanocrystals,and their photogenerated charge carriers separation and transfer efficiency are studied.Meanwhile,the influence of their energy band structure,surface state,crystal thickness,and free radicals in the solution on the photocatalytic hydrogen evolution reaction（HER）was systematically investigated;thereby a significant improvement in photocatalytic performance was finally achieved.Further,the catalytic reaction mechanism was deeply studied through systematic experiments and theoretical calculations.The main research contents of this thesis are as follows:1.The unique physical and chemical properties of 2D layered MPX₃semiconductors have aroused widespread research interest,and the synthesis of MPX₃nanomaterials has always been a bottleneck hindering their development and application.The high-quality In_4/3P₂Se₆ nanosheets were firstly controllably synthesized on flexible carbon fibers via the two-step chemical vapor conversion method,and their potential applications in the field of photocatalytic water splitting and photodetector were explored.Spectral characterizations and theoretical calculations revealed the direct bandgap properties and an appropriate band alignment structure of In_4/3P₂Se₆ nanosheets（1.9 e V）.Therefore,these characteristics endow In_4/3P₂Se₆ nanosheets with efficient photocatalytic activity towards water splitting,achieving an efficient hydrogen production rate of 4110μmol g^-1 h^-1 under xenon light irradiation.Moreover,the photodetector fabricated based on two-terminal In_4/3P₂Se₆ nanosheet exhibits ultralow dark current（～80 f A）,ultrahigh photoswitching ratio（6.8×10⁵）,external quantum efficiency（1485%）,specific detectivity(6.3×10¹²Jones),and ultrafast photoresponse speed(τ_rising=470μs,τ_decay=440μs).Based on the excellent photocatalytic water-splitting activity and photoresponse characteristics of In_4/3P₂Se₆ nanosheets,this will provide guidance for the application of 2D In_4/3P₂Se₆ semiconductors in photoelectric energy conversion and photoelectronic integrated systems.2.The poor efficiency of charge separation and migration has always been a key factor that limits the application of photocatalytic water splitting.In this regard,designing vacancies on the surface of 2D photocatalyst is a feasible strategy to improve the efficiency of separation of photoexcited charge carriers.The introduced vacancies can serve as charge separation centers to capture photoexcited electrons,thereby inhibiting recombination with photoexcited holes.Herein,the ultrathin Cu In P₂S₆ nanosheets with sulfur vacancies were synthesized on carbon fibers via the chemical vapor conversion method.The effects of thickness and sulfur vacancies on the energy band structure,charge carrier separation,and photocatalytic H₂ evolution activity of Cu In P₂S₆ are investigated.Spectral characterization and theoretical calculation results unveil that the thickness change of Cu In P₂S₆ can affect the band-edge positions,even though there is imperceptible change in bandgap value.Meanwhile,the introduced sulfur vacancies reduce the bandgap and then enhance photoabsorption ability,as well as regulate the charge distribution on the surface and optimize the charge carrier separation for effective photocatalytic reaction.Benefiting from the ultrathin structural feature and abundant surface sulfur vacancies,the Cu In P₂S₆ nanosheets demonstrated superior photocatalytic activity towards hydrogen production.This study provides a new idea for the designing and optimizing of high-efficiency water splitting photocatalysts in the future.3.The exploitation of photocatalysts with high catalytic activity and stability remains a major challenge in realizing large-scale photocatalytic water splitting applications.However,different from design nanocatalysts with complex structures,the hydrogen evolution activity and stability of semiconductor photocatalysts can be significantly improved by introducing CO₃^2-anions in the photocatalytic system.Inspired by our seminal works on photocatalytic H₂ evolution over 2D MPX₃semiconductors,the high-quality In_4/3P₂S₆ nanosheets were synthesized through the two-step chemical vapor conversion strategy.Band structure characterization and theoretical calculation revealed the appropriate bandgap value（2.7 e V）and band alignment structure of In_4/3P₂S₆ nanosheets,indicating the thermodynamic appropriateness for photocatalytic water splitting under light illumination.Taking into account the specific reaction selectivity and charge storage capacity of CO₃^●-radicals,we utilized CO₃^●-/CO₃^2-redox pairs as a robust hole reservoir to accelerate hole transfer kinetics in the In_4/3P₂S₆ nanosheets photocatalytic water splitting process,thereby improving the separation efficiency and lifetime of the photogenerated charge carriers.Therefore,carbonate-redirected hole transfer results in In_4/3P₂S₆ nanosheets exhibit excellent photocatalytic hydrogen evolution activity and photostability,thus breaking the activity/stability trade-off.This work utilizes CO₃^●-radicals that are widely present in natural water bodies to redirect the hole transfer pathway in the photocatalytic water-splitting process,providing a new strategy for exploring efficient and stable photocatalytic water splitting systems,which could open a new avenue for further research on CO₃^●-radical to regulate the hole transfer process.