Construction Of Photocatalytic Hydrogen Production System And Mechanism Study Of Photo-induced Carries Transfer Based On Metal Sulfide And Metal-organic Frameworks

The development of efficient and cheap visible light catalysts for the photocatalytic splitting of water to produce hydrogen is widely regarded as one of the most potential strategies to solve the energy crisis and environmental pollution.In this paper,metal sulfide and metal-organic framework materials were modified from the point of view of the process of hydrogen production by photocatalytic water splitting.The main methods are to enhance the visible light absorption capacity of photocatalyst,improve the separation and migration rate of photo-generated electron-hole pairs in the semiconductor,and inhibit the recombination of carriers.The main research contents are as follows:（1）The S-scheme Zn_0.5Cd_0.5S/WO₃photocatalyst was prepared by mild solvothermal method and physical mixing method.The potential difference caused by the matched band structure accelerates the photo-induced electrons migration rate between Zn_0.5Cd_0.5S and WO₃,and WO₃consumes the useless holes from the valence band of Zn_0.5Cd_0.5S,so that more photo-induced electrons remain in the conduction band of Zn_0.5Cd_0.5S.The composite catalyst was further modified by monoclinic WP.WP exist in the composite as active sites due to their high electrical conductivity,receiving conduction band electrons from Zn_0.5Cd_0.5S and reducing reaction with hydrogen ions in water.The co-modification of WP and WO₃promoted the full and efficient utilization of photo-induced electrons in Zn_0.5Cd_0.5S,thus significantly improving the hydrogen evolution performance of the catalyst.The hydrogen production rate of the novel composite photocatalyst can reach26262μmol g^-1h^-1,and the apparent quantum efficiency can reach 6.19%.In addition,it shows a good stability in the continuous 25 h hydrogen production experiment.（2）Study on the construction of p-n heterojunction photocatalyst with p-type Co WO₄nanoparticle modified Mn_0.2Cd_0.8S nanorods.The p-type semiconductor Co WO₄nanoparticles were assembled on the surface of rod-shaped Mn_0.2Cd_0.8S by in-situ growth method.The matched morphology increases the specific surface area of the catalyst,which is conducive to sacrificing the contact between the reagent and the active site and the absorption of visible light.More importantly,the directional and rapid transfer of photo-induced electrons from Co WO₄to Mn_0.2Cd_0.8S is facilitated by the presence of inbuilt electric field and the potential difference between the contact interfaces.（3）Modified Mn_0.2Cd_0.8S by metal-organic framework materials.The flake structure of Ni-MOF-74 provides enough space for the dispersion of Mn_0.2Cd_0.8S nanorods,and greatly reduces the aggregation degree of one-dimensional Mn_0.2Cd_0.8S.Meanwhile,the Ni-S bond formed between Ni-MOF-74 and Mn_0.2Cd_0.8S provides a unique transfer channel for photo-induced carriers.Meanwhile,due to the potential energy difference between Mn_0.2Cd_0.8S and Ni-MOF-74,the conduction band electrons of Mn_0.2Cd_0.8S can be rapidly injected into the conduction band of Ni-MOF-74.This indicates that the recombination of photo-induced carriers in Mn_0.2Cd_0.8S is inhibited.Compared with Mn_0.2Cd_0.8S and Ni-MOF-74,the composite catalyst has the longest carrier lifetime,the fastest charge transfer rate and the lowest overpotential.The optimal hydrogen production rate of the composite is 7.104 mmol g^-1h^-1,6.96 times that of the original Mn_0.2Cd_0.8S.（4）Different in-situ treatment methods and the construction of 2D/2D S-scheme heterojunction enhance the hydrogen evolution activity of metal-organic framework materials.First,Ni S₂,Ni O and Ni₂P were introduced into the surface of Ni-MOF by surface sulfonation,oxidation and phosphating.The hyperchromicity effect can effectively improve the absorption capacity of photocatalyst to visible light.Ni S₂,Ni O and Ni₂P on the surface of Ni-MOF can effectively capture electrons in the conduction band of Ni-MOF,thus improving the hydrogen production activity of Ni-MOF.Among them,Ni₂P has strong electronegativity and can adsorb more protons,thus showing the best activity of hydrogen evolution.On this basis,an efficient S-scheme heterojunction was constructed between 2D Ni-MOF/P and 2D Ce O₂interface by one-step high-temperature treatment,which inhibited the recombination of effective photo-induced carriers in Ni-MOF,thus improving the durability of the photocatalyst.The interaction of Ni-MOF with Ni₂P and Ce O₂greatly reduces the resistance of photo-induced electron transfer and hydrogen evolution overpotential.By optimizing a series of photocatalytic hydrogen evolution conditions,the maximum amount of hydrogen evolution of the composite can reach 316.84μmol,which is 14.18 times that of the original Ni-MOF.Finally,by analyzing the characterization results and density functional theory（DFT）calculation,the possible mechanism of photocatalyst hydrogen evolution in eosin system was proposed.This study provides innovative strategies for improving MOFs material derivatives as efficient photocatalysts.