Synthesis Of Two-Dimensional Sulfide Heterojunctions And Photocatalytic Properties

In recent years,rapid urbanization and industrialization have led to increasingly significant global energy shortages and environmental pollution problems.Therefore,the development of new environmentally friendly and sustainable energy sources is imminent.Hydrogen energy has the characteristics of high combustion calorific value and non-polluting oxidation products,and is considered to be one of the most ideal environmentally friendly energy sources to replace fossil fuels in the future.Among the various methods for hydrogen production,semiconductor photocatalysis has received extensive attention from researchers due to its ability to split water into hydrogen using renewable solar energy.Compared with other semiconductor materials,transition metal sulfides have suitable band edge positions and wide-band visible light absorption ability,and are ideal photocatalysts for hydrogen production reactions.However,individual transition metal sulfides often exhibit low photocatalytic activity due to excessive charge recombination rates.Studies have shown that cocatalyst modification is an effective way to improve the photocatalytic performance of semiconductors.Cocatalysts can not only form heterojunctions with semiconductors to enhance the charge separation ability,but also provide a large number of catalytic active sites to reduce the activation energy of the reaction.In addition,2D semiconductor materials with unique structural advantages,such as shortened migration distance of photogenerated charges from bulk to surface,larger specific surface area for reactant adsorption and activation,and more surface catalytic active sites,have become One of the most popular materials for photocatalysis research.Therefore,it is expected that the composite materials of transition metal sulfide semiconductors and cocatalysts with two-dimensional structure can obtain excellent performance of photo-splitting water for hydrogen production.Based on the above reasons,in this paper,transition metal sulfides（CdS,Zn In₂S₄）were selected as light absorbers,Mo₂C,Co@C（Co,Co O_x and Co₉S₈ supported by graphitic carbon）and Ni_xCo_1-xS as cocatalysts to design a series of two-dimensional structural composite photocatalytic material taking advantage of its significant advantages such as strong visible light absorption,high charge separation efficiency,and abundant active sites to achieve efficient photolysis of water for hydrogen production.The main research findings are summarized as follows:（1）A unique Mo₂C–CdS–Co@C composite photocatalyst was synthesized by hydrothermally anchoring CdS and Mo₂C nanoparticles to Co@C microflowers.Co@C microflowers consist of Co,Co O_x,and Co₉S₈ nanocrystals partially embedded in graphitic carbon nanoflakes,which are formed by carbonization of metanilic-anion intercalated Co（OH）₂ precursor.Due to excellent visible-light harvesting capacity,effificient charge transmission and isolation caused by the existence of superior electron mediator（graphitic carbon）together with oxidation（Co O_x）and reduction（Co,Co₉S₈,Mo₂C）cocatalysts,as well as the plentiful H₂-evolving active sites originating from the metal（Co）,metal sulfifide（Co₉S₈）,and metal carbide（Mo₂C）,the Mo₂C-CdS-Co@C hybrid micro-flflowers delivered an exceptional HER（λ>400 nm）activity of 76.06mmol·g^-1·h^-1,approximately 32 and 15 folds that of pristine CdS and platinum（3 wt%）-decorated CdS,respectively.Moreover,both the cycling and long-term photocatalytic tests confirm that the Mo₂C–CdS–Co@C possesses an outstanding durability for H₂production.（2）A flower-like structure of Mo₂C-Cd_xZn_1-xIn₂S₄ direct Z-scheme heterojunction was prepared by solvothermal method.It is found that Cd²⁺doping not only broadens the visible light absorption range of Cd_xZn_1-xIn₂S₄ but also enhances its photoelectron reducing ability;meanwhile,the two-dimensional structure significantly increases the coupling interface between Mo₂C and Cd_xZn_1-xIn₂S₄;furthermore,the Z-scheme charge transfer between Mo₂C and Cd_xZn_1-xIn₂S₄ effectively promotes the spatial separation of photogenerated carriers.Therefore,the Mo₂C-Cd_xZn_1-xIn₂S₄ composite exhibits excellent photocatalytic HER activity under visible light（λ>400 nm）irradiation.When x=0.25 and Mo₂C loading is 3 wt%,the hydrogen production rate of the composite(3MC-C_0.25ZIS)reaches the highest value of 28.12 mmol`g<sup>-1</sup>`h^-1,which is 44.2 and 7.2times for Zn In₂S₄ and C_0.25ZIS,respectively.And its apparent quantum yield（AQE）under monochromatic light irradiation atλ=400 nm reached 21.11%.Furthermore,the excellent photocatalytic stability of the Mo₂C-Cd_xZn_1-xIn₂S₄ heterostructure was confirmed by cycling and long-term hydrogen production tests.（3）Using Ni_xCo_1-x-MOF as a self-sacrificial template,Ni_xCo_1-xS amorphous nanoflowers were obtained by solvothermal sulfide reaction,and then CdS-Ni_xCo_1-xS composite nanosheets were synthesized by loading CdS quantum dots under oil bath conditions.The study found that the large specific surface area of CdS quantum dots and Ni_xCo_1-xS amorphous nanoflowers can provide sufficient adsorption and activation sites for the hydrogen production reaction;Ni_xCo_1-xS amorphous nanoflowers as cocatalysts can provide a large number of unsaturated S atoms as active sites for hydrogen production;the type II heterojunction formed between CdS and Ni_xCo_1-xS effectively promotes the separation process of photogenerated charges.Therefore,under visible light（λ>400 nm）irradiation,the CdS-Ni_xCo_1-xS composite(40CdS-N_0.5C_0.5S)exhibits excellent photocatalytic hydrogen production activity,and the highest HER rate can reach 62.75 mmol·g^-1·h^-1,about 20.9 and 11.3 times that of pure CdS QDs and CdS loaded with 3 wt%Pt.Meanwhile,atλ=400 nm,its apparent quantum yield（AQE）for hydrogen production reaches 29.30%.