Structural Regulation And Hydrogen Evolution Performance Of ZnIn₂S₄-Based Photocatalysts

The conversion of solar energy into low-density and zero-pollution hydrogen energy through photocatalytic water splitting technology is an effective way to solve the two major problems of energy crisis and environment pollution.The development of green and efficient photocatalysts is the key to realize the conversion of solar energy to hydrogen energy.ZnIn₂S₄（ZIS）with layered structure is one of the most promising photocatalysts for hydrogen evolution due to its suitable band gap（2.06-2.85 e V）,stable physical and chemical properties,controllable morphology and adjustable electronic structure.However,the monomer ZIS has the shortcomings of easy aggregation,narrow visible light absorption range,low separation efficiency of photogenerated carriers,and weak redox ability,which seriously limits its development and wide application in the field of photocatalysis.In view of the above problems,we modified and optimized ZIS through strategies such as element doping,construction of heterojunctions,and micro-morphology control,thereby achieved an effective improvement in photocatalytic performance.The mechanism of hydrogen evolution performance improvement of different photocatalytic systems was studied by combining experimental technical analysis with theoretical calculation.The following are the specific research contents:（1）Moin-situ doped ZIS hollow hierarchical nanotube photocatalyst（ZNT-Mox）was successfully synthesized by solvothermal method using one-dimensional（1D）MoO₃ nanobelts as template and Mosource,and the synchronous regulation of micro-morphology and element doping on ZIS was realized.The prepared tubular Mo-doped ZIS photocatalyst exhibits excellent photocatalytic hydrogen evolution performance.The photocatalytic hydrogen evolution rate of the sample with the best Modoping content（ZNT-Mo5）is 12.10 mmol h^-1 g^-1,which is about 4.7 times that of the monomer ZIS(2.58 mmol h^-1 g^-1).Its excellent photocatalytic hydrogen evolution performance is attributed to the following three aspects:（1）High specific surface area and abundant pore structure provide more adsorption and reaction sites for photocatalytic reactions;（2）Modoping regulates the electronic structure and band structure of ZIS,effectively shortens the band gap,and broadens the visible light absorption range;（3）High conduction band potential provides a strong driving force for photocatalytic hydrogen evolution reaction.（2）Using a co-regulation strategy of micro-morphology and composition,the in-situ Ce-doped ZIS hierarchical nanocage photocatalyst（ZTNs-Cex）was synthesized by one-step solvothermal method using tetrakaidecahedron mesoporous Ce-MOF as dopant and template.The unique three-dimensional（3D）hollow structure effectively increases the specific surface area of the photocatalyst,and shortens the vertical migration distance of photogenerated carriers from bulk to surface.Through experimental characterization techniques and density functional theory（DFT）calculations,it is revealed that Ce doping increases the density of electronic states near the valence band of ZIS,thus providing more photogenerated carriers to participate in photocatalytic reactions.Moreover,Ce doping narrows the band gap of ZIS,thereby expanding its visible light absorption range.After component optimization and micro-morphology control,the hydrogen evolution rate of the nanocage photocatalyst（ZTNs-Ce20）with the optimal Ce doping content reached7.46 mmol h^-1 g^-1,which was much higher than that of the monomer ZIS(2.61 mmol h^-1 g^-1),and showed good cycle stability.（3）The p-type semiconductor CuS has a narrow band gap and strong absorption capacity for visible light,which can be used as a cocatalyst for ZIS to improve its photocatalytic hydrogen evolution performance.Therefore,in this work,cubic hierarchical nanocages assembled by two-dimensional（2D）CuS nanosheets were synthesized by template method.Then,n-type 2D ZIS nanosheets were grown on the surface of cubic nanocages,and hierarchical 2D/2D CuS@ZIS nanocages were successfully synthesized.The introduction of CuS nanocages not only effectively improves the visible light absorption capacity of the composite photocatalyst but also increases the specific surface area.Combined with experimental characterization techniques and DFT calculations,it is revealed that the built-in electric field formed by strong electron interaction at the interface of CuS and ZIS p-n heterojunctions is a powerful driving force for the separation and migration of photogenerated carriers.After micro-morphology control and interface optimization,the 3%-CuS@ZIS composite photocatalyst exhibits excellent photocatalytic hydrogen evolution performance,and its photocatalytic hydrogen evolution rate is 7.91 mmol h^-1 g^-1,which is about 3 times that of monomer ZIS(2.63 mmol h^-1 g^-1).（4）The 0D/2D Co₃O₄@ZIS hierarchical nanoboxes（NBs）heterojunction composite photocatalyst（CZ-x NBs）was successfully prepared by growing ZIS nanosheets on the surface of hierarchical nanoboxes assembled by ultra-small Co₃O₄nanoparticles（NPs）.The introduction of 3D nanoboxes assembled by Co₃O₄ NPs not only significantly increases the specific surface area of the catalyst,but also effectively broadens the visible light absorption range of the catalyst.The S-scheme charge transfer mechanism of Co₃O₄@ZIS 0D/2D heterojunction was revealed by combining experimental technical analysis and DFT calculation.Under the action of built-in electric field and Coulomb force,the separation and migration efficiency of photogenerated carriers has been significantly improved,while retaining the strong reducibility of photogenerated electrons.The optimized composite photocatalyst（CZ-5 NBs）exhibits an efficient photocatalytic hydrogen evolution performance under visible light irradiation,and its hydrogen evolution rate is 39.38 mmol h^-1 g^-1,which is about 12.9 times that of monomer ZIS.After four cycles of experiments,the photocatalytic hydrogen evolution activity can still maintain 82.8%,with good photocatalytic stability.Moreover,this work provides an effective synthesis strategy for constructing novel hollow 0D/2D heterojunction nanocomposites.