Preparation Of MoS₂/ZnCoS Composites For Photocatalytic Hydrogen Evolution By Water Splitting

Photocatalytic water decomposition using solar energy for hydrogen production has attracted much attention because it could alleviate the dependence on fossil fuels and eliminate environmental pollution.Although great progress has been made in the development of high-efficiency photocatalysts,the utilization of light source,stability,and hydrogen production rate of the photocatalysts are still far from the levels of practical applications.Increasing solar hydrogen production efficiency basically relies on the improvement of photocatalysts with low cost while high stability,thereby enhancing their light-harvesting ability and promoting charge transfer.Among the developed photocatalysts,transitionmetal chalcogenides have attracted a lot of attention due to their inherent semiconductor properties and good optoelectronic properties.ZnS is widely considered to be an effective water-splitting photocatalyst due to its low cost,high electrochemical stability,fast electron mobility,and high reduction potential of photoexcited electrons.However,in the visible range,ZnS generally exhibits poor photocatalytic activity because of its large optical band gap and rapid recombination of photogenerated electron-hole pairs.Metal-Organic Frameworks?MOFs?are a class of porous materials formed by coordination and self-assembly of inorganic metal ions with nitrogen or oxygen-containing organic ligands.Compared with traditional porous materials,MOFs derivatives have unique character such as large specific surface area,structural designability,ordered pore size and pore tailorability,which show outstanding advantages in catalysis.In this thesis,a simple,low-cost,structure-controllable and facile synthetic route to improve the performance of a low-cost metal sulfide semiconductor,consisting of the bimetallic MOFs-templating and the simultaneous sulfidation of the photocatalyst and loading of MoS₂ co-catalyst is developed.Since the mutual sulfur atom shared by all the transition metal sulfides allowed the formation of ZnCoS solid-solution structure and stabilization of the metallic 1T-MoS₂ phase.This novel approach to prepare advanced materials could be further extended to the phase-controllable preparation of MoS₂ and the discovery of other transition-metal chalcogenides with high activity and stability for various applications.Based on the structural advantages of bi-metallic organic frameworks,we used zinc and cobalt-based bimetallic MOF?ZnCo-ZIF?as templatesto prepare stable derivatives?ZnO-Co₃O₄?by calcining ZnCo-ZIF under air atmosphere.Subsequently,the derivative ZnO-Co₃O₄was simultaneously vulcanized with molybdate and sulfur source to obtain 1T-MoS₂ modified ZnCoS solid-solution hollow dodecahedral nanocomposites?MoS₂/ZnCoS?.The prepared composite materials were applied to photolysis of water to produce hydrogen.The main research contents and findings are as follows:?1?Mixing the salts of different molar amounts of zinc and cobalt and 2-methylimidazole at room temperatureyielded bimetallic organic framework materials Zn_xCo_1-x-ZIF?x=0.1,0.3,0.5,0.7,0.9?with different molar ratios of zinc and cobalt substances.Then,the Zn_xCo_1-x-ZIF materials were calcined in air to obtain stable oxides ZnO-Co₃O₄.It was found that when x was0.5,its oxide ZnO-Co₃O₄ exhigited a hollow structure.On the other hand,the oxides obtained by pyrolyzing ZnxCo₁-x-ZIF?x=0.1,0.3,0.7,0.9?with other zinc and cobalt ratios did not show the hollow structure.The hollow structure could enable multiple reflections of light in the cavity,thereby increasing the light utilization.?2?Simultaneous hydrothermal sulfuration of oxide ZnO-Co₃O₄,sodium molybdate,and thioacetamide obtained a hollow dodecahedral composite material(MoS₂/Zn_0.5Co_0.5S)composed of metal 1T phase MoS₂ modified ZnCoS solid solution.Since all the transition metal sulfides shared a sulfur atom,the formation of the ZnCoS solid solution and the metal 1T phase MoS₂ was promoted.And thus,abundant catalytically active sites and high electron conductivity can be obtained at the close contact of the ZnCoS solid solution with 1T-MoS₂.The obtained composite material MoS₂/Zn_xCo_1-xS maintained an intact dodecahedral structure,indicating that the derivative of the MOF material has excellent thermal stability.?3?A series of photoelectrochemical characterization methods such as UV absorption spectroscopy,fluorescence spectroscopy,Mott-Schottky,photocurrent response,and electrochemical impedance spectroscopy test have confirmed that the band gap of ZnS semiconductor can be adjusted by doping other metal sulfides.Moreover,the recombination velocity of the photogenerated electrons-hole pairs was reduced,which accelerated the separation and transfer ability of the photogenerated electrons,thereby facilitating the photocatalytic reaction.In the presence of dyes,the hydrogen production rate of the dodecahedral MoS₂/Zn_0.5Co_0.5S material with hollow structure can be optimized to 15.47 mmol h^-1 g^-1 under Xe lamp radiation.