Preparation And Properties Of Polyoxometalate-based Composite Photocatalysts

At present,energy shortages and environmental pollution are becoming more and more serious.Especially with the development of metal smelting,leather,and other industries,it will produce sewages containing organic dyes and heavy metal ions that are difficult to degrade.Therefore,it is urgent to deal with these problems.Solar photocatalysis has received comprehensive attention in solving energy and environmental concerns due to its mild reaction conditions,sustainability,and non-toxicity.Up to now,various photocatalysts have been developed.Among them,polyoxometalates have received extensive attention due to their excellent photochemical stability,oxidation activity and photoelectric properties,and discrete molecular structure.Therefore,this topic focuses on the polyoxometalate-based(K₃PW₁₂O₄₀)composite photocatalyst,which is a special kind of early transition metal oxygen cluster with unique structural characteristics and adjustable energy band structure.In addition,K₃PW₁₂O₄₀ is a compound with characteristics like semiconductors and is composed of inorganic monovalent cations K⁺and Keggin type[PW₁₂O₄₀]^3-anions.Although it has the above advantages and is easy to prepare,when K₃PW₁₂O₄₀ is used as homogeneous catalyst,the photoelectric charge recombination is faster,which leads to the low solar energy conversion ability.Therefore,to suppress photo-induced electronic recombination,this paper combines K₃PW₁₂O₄₀ with semiconductor photocatalysts to form a heterojunction to increase the charge separation rate,thereby further improving the photocatalytic efficiency.In this paper,Cd S/K₃PW₁₂O₄₀ and Cd S/K₃PW₁₂O₄₀/meso-g-C₃N₄ photocatalysts are synthesized by simple hydrothermal,in-situ chemical deposition and template calcination methods.Under visible light irradiation,the photocatalytic performance of the prepared composite photocatalytic material was tested,including the photocatalytic cracking of aquatic hydrogen and the photocatalytic degradation of target pollutants.The main research contents of this paper are as follows:（1）The K₃PW₁₂O₄₀ dodecahedron is prepared by a simple hydrothermal method,and then Cd S quantum dots are loaded on the K₃PW₁₂O₄₀ dodecahedron by in-situ chemical deposition to prepare the Cd S/K₃PW₁₂O₄₀ heterojunction photocatalyst.The reduction rate of Cr⁶⁺by Cd S/K₃PW₁₂O₄₀ reaches 73.7%,and the removal rate of BPA reaches 49.4%in 180 min.In addition,the photocatalytic hydrogen production rate of Cd S/K₃PW₁₂O₄₀ reaches 267μmol h^-1 g^-1,which is 17.8 times that of pure K₃PW₁₂O₄₀.The formation of a Z-scheme heterojunction between Cd S and K₃PW₁₂O₄₀ inhibits the recombination of photoinduced electron-hole pairs and increases the photoresponse range of the photocatalyst.（2）The Cd S/K₃PW₁₂O₄₀/meso-g-C₃N₄ tandem heterojunction is prepared by template calcination method and in-situ chemical deposition method.Various characterization methods prove that Cd S and K₃PW₁₂O₄₀ are successfully loaded on meso-g-C₃N₄ nanosheets.The reduction rate of Cd S/K₃PW₁₂O₄₀/meso-g-C₃N₄ to Cr⁶⁺is as high as 97%in 180 min.In addition,the maximum hydrogen production of the ternary composite material is as high as 568μmol h^-1 g^-1,which is 37.9 times that of the original K₃PW₁₂O₄₀.The improvement of photocatalytic activity may be since the oxygen-doped mesoporous structure shortens the band gap of the catalyst and improves the separation of photogenerated charge.In addition,the synergistic effect between the type II heterojunction and the Z-scheme heterojunction is also beneficial to the spatial separation and transfer of photogenerated charge.