Construction Of The ZnS Based Heterojunction From Metal-Organic Frameworks For Photocatalytic Hydrogen Evolution

The solar-driven photocatalytic decomposition of water to produce hydrogen is considered to be the most promising method for producing hydrogen fuel.There are many photocatalysts for hydrogen evolution reaction.Among them,transition metal sulfides or oxides have become a class of ideal photocatalytic material due to theirs suitable energy band and electron mobility.However,single-component transition metal sulfides or oxides have problems of easy recombination of photo-generated electrons and holes.Therefore,heterojunction structures are often constructed to improve the stability of photocatalysts.Metal-organic framework materials(MOFs)are a class of porous and ordered materials with high specific surface area,controllable pore structure,and adjustable chemical and physical properties.Due to the fact that ligands can orderly space the metal nodes,the MOFs precursor method can be used to prepare well-dispersed metal-based photocatalysts and avoid the problem of metal aggregation.In addition,since the composition and structure of MOFs are easily controlled at the molecular level and combined with the characteristics of semiconductors,the MOFs derived porous materials are liable to be rational designed and possess high porosity and uniform load state,which promote the contact between photogenerated carriers and reactants,reduce the recombination rate of electrons and holes and control the morphological characteristics.Therefore,in this paper,heterojunction photocatalysts were designed and synthesized by the MOFs precursor method for photocatalytic hydrogen production.In the first part:A new type of /CdS composite photocatalytic heterojunction was constructed through the one-step simple sulfidation method of bimetallic Zn/Cd-MOFs.Due to the inherent porosity of the MOFs derivative,the reactant and the active sites were more easily contacted and the carrier migration distance was shortened,thereby improving the carrier separation efficiency.By adjusting the ratio of Zn/Cd in the materials,the hydrogen production rate could reach 2348.5?mol g^-1h^-1 under the excitation of visible light in the Na₂S/Na₂SO₃ sacrificial agent system.The relationship between the efficiency of electron-hole separation in the heterojunction and the performance of hydrogen production explained by the photoelectric characterization.And through Mott Schottky,a possible electron-hole transfer method proposed.In the second part:By oxidizing core-shell ZnCo-ZIF@ZIF-8 precursor,the Co₃O₄-Zn O@Zn O composite was obtained.Then,thioacetamide was used as the sulfur source,Co₃O₄-Zn O@ZnS was obtained by hydrothermal anion exchange method.Finally,the Co₃O₄-Zn O@ZnS@Pt heterojunction was successfully synthesized by the photodeposition method.The hydrogen production rate of Co₃O₄-Zn O@ZnS@Pt heterojunction can reach 3269.3?mol g^-1 h^-1 under the condition of sacrificial agent system.Without sacrificial agent,the heterojunction still showed photocatalytic hydrogen production activity and the hydrogen production rate could reach 523.1?mol g^-1 h^-1.Through transient fluorescence spectroscopy,impedance spectroscopy and photocurrent,the reason for the improved performance of Co₃O₄-Zn O@ZnS@Pt heterojunction was discussed and the reason for no oxygen generation without sacrificial agent was explained.