Dual-functional Photocatalysis Of ZnIn₂S₄/MOFs For Simultaneous Photocatalytic Hydrogen Generation And Organic Wastewater Degradation

Photocatalytic hydrogen production has great potential to solve problems such as the energy crisis and environmental pollution.However,in current photocatalytic hydrogen production systems,a sacrificial agent,such as methanol,ethanol,and triethanolamine,must be introduced.These materials are expensive,toxic,and harmful to the environment,and they are originally energy carriers.Coupling the degradation of organic pollutants or the production of value-added organic products with photocatalytic hydrogen production is an economical and sustainable solution.It utilizes the holes left by the consumption of electrons during hydrogen evolution and also replaces the role of sacrificial agents in promoting hydrogen production.The dual-function photocatalytic system has great potential for development in the areas of new energy development and environmental pollution treatment.To couple these two systems,it is important to develop highly efficient dual-functional photocatalysts that have more active reaction sites for oxidation and for reduction.Zinc indium sulfide（Zn In₂S₄,ZIS）is considered to be an excellent photocatalyst because of its strong absorption ability in the visible light region,excellent chemical stability and suitable bandgap position.Metal-organic frameworks（MOFs）are a new type of porous materials with semiconductor-like properties,with rich pore structure and high specific surface area.In addition,MOFs provide sufficient active sites for oxidation and reduction reactions,which also provide the possibility for efficient hydrogen production and pollutant degradation at the same time.This study opens up new avenues for environmental restoration and non-fossil fuel production.The main research contents and results are presented as follows:1.The controlled synthesis of ultrasmall metal carbides effectively enhances the photocatalytic efficiency.Here,we report a new type of cocatalyst in which a three-dimensional（3-D）nitrogen-doped carbon cage（NGC）of metal–organic framework derivatives encapsulate ultrasmall MoC nanoparticles（MoC@NGC）,promoting simultaneous degradation of organic pollutants and hydrogen production by Zn In₂S₄（ZIS）.Characterization analyses showed that MoC accelerated the separation of the photogenerated carrier and effectively reduced the overpotential of hydrogen evolution,while NGC promoted the good dispersion of MoC particles and provided sufficient sites.The MoC@NGC/ZIS composite exhibited a high hydrogen（H₂）evolution rate of 1012μmol g^–1 h^–1,which exceed that of ZIS loaded with platinum.In the coupled system,where the electron donor has been substituted by Rhodamine B（Rh B）,the mechanism analysis showed that Rh B and the as-generated intermediates consumed holes and facilitated hydrogen evolution.In addition,we designed a combined photocatalytic anoxic and oxic sequence process to achieve the recovery of hydrogen energy during the treatment of dye wastewater.2.The competition for oxidation and reduction sites and the low carrier utilization efficiency remain important factors limiting the development of the couple system.We report a three-layered,dual-functional photocatalysts of Cu atomics@Ui O-66-NH₂（U6N）/Zn In₂S₄nanosheets（ZIS）,for the simultaneous production of photocatalytic hydrogen and degradation of organic pollutants.When common and highly toxic organic pollutants（phenolic compounds and antibiotics）were substituted for the sacrificial agents,Cu@U6N/ZIS exhibited superior sustained hydrogen production,which was about 28 times greater than that of purely ZIS.The experimental results suggested that the window size of U6N（<organic pollutants molecular size）separates the reaction sites of organic pollutants oxidation and reduction of hydrogen ions（H⁺）,thereby achieving spatial separation of redox sites.Besides,the structure of Cu@U6N/ZIS,which was anchored Cu within U6N and encapsulated by ZIS nanosheets,forming Z-type heterojunctions,achieved the control of the directional transfer of photogenerated electrons.And the coupled system can effectively enhance water quality by reducing the biological toxicity of the wastewater.3.The weak oxidation capacity of ZIS has some disadvantages in the complete mineralization of organic pollutants,but it has advantages in the selective oxidation process.ZIF-67 derivative was successfully decorated on ultrathin ZIS nanosheets to form CoNC/ZIS composite for simultaneous photocatalytic H₂ evolution and selective benzyl alcohol oxidation to benzaldehyde under visible light irradiation.ZIF-67 was pyrolyzed at different temperatures to obtain CoNC with different Co metal exposure surface by reducing Co²⁺,promoting enrichment and directional migration of electron and providing sufficient reaction sites for hydrogen evolution.In addition,the photogenerated holes on ZIS nanosheets could rapidly consume by benzyl alcohol,which promoted the utilization rate of photogenerated carrier and improved the catalytic efficiency of the whole system.And two high value-added products were obtained,which could be separated spontaneously because the products were in the gas phase（H₂） and in the solution（benzaldehyde）,respectively.