Regulation Of Surface Structure Cooperating With Interfacial Photo-generated Carrier Transport Based On Hollow Spherical NiCo₂S₄ For Photocatalytic Hydrogen Evolution

In this paper,the surface of double transition metal sulfide NiCo₂S₄ was adjusted to prepare a hollow spherical NiCo₂S₄.On this basis,different materials were used to rationally regulate the surface structure and electron transfer path of hollow spherical NiCo₂S₄.Graphene quantum dots（GQDs）sensitization type-Ⅰ,type-Ⅱand Z-scheme heterojunction were utilized to construct composite materials,respectively,and these composite photocatalytic materials were used to achieve efficient photocatalytic hydrogen evolution.In addition,the transition metal sulfide was extended to non-metallic photocatalytic materials.The principle of co-regulation of morphology and electron transport mechanism were used to further construct 2D Co MOF/p-g-C₃N₄ with directional coupling of dye sensitizer and sacrificial agent.This hydrogen evolution system effectively improves the utilization efficiency of the sensitizer and sacrificial agent in the dye sensitized system.（1）Environmentally friendly materials and green synthesis methods were used to prepare GQDs with a small particle size（4～5 nm）.The GQDs were coupled to the surface of the hollow spherical bimetallic sulfide NiCo₂S₄ by hydrothermal deposition.Thus,GQDs/NiCo₂S₄ organic-inorganic hybrid materials were prepared.The hollow spherical structure effectively improves the light absorption of the photocatalytic material.Further research shows that the LUMO energy level of GQDs is higher than the conduction band potential of NiCo₂S₄.Therefore,GQDs and Eosin Y（EY）molecules play the co-sensitization effect in this system.They give great photocatalytic hydrogen evolution performance for GQDs/NiCo₂S₄ composite material relying on NiCo2S4 hollow spheres with the high light absorption capacity.The hydrogen production rate of the sample with the optimal activity reaches 118.14μmol.h^-1.（2）On the basis of the hollow spherical NiCo₂S₄,the photocatalytic hydrogen evolution material NiCo₂S₄@Ni₂P with a unique hollow core-shell structure was successfully prepared.The hollow sphere structure of the composite material gives it great light absorption performance.Simultaneously,this structure forms a type-Ⅰheterojunction with uniform surface contact,which makes the composite photocatalytic material exhibit higher photocatalytic hydrogen evolution activity and stability than pure NiCo₂S₄ in the dye-sensitized system.The highest photocatalytic hydrogen evolution rate of NiCo₂S₄@Ni₂P 10%reached 77.97μmol.h^-1.（3）On the basis of hollow spherical NiCo₂S₄,a photocatalytic hydrogen evolution composite material was constructed using Zn_xCd_1-xS with adjustable energy band structure.The hollow spherical structure of NiCo₂S₄ makes Zn_xCd_1-xS have more contact positions to form the heterojunction.By adjusting the energy band structure of Zn_xCd_1-xS the induced electron transfer mode is changed to Z-scheme,which effectively improves the photo-generated carrier separation efficiency of the composite material.At the same time,the high conductivity of the hollow spherical NiCo₂S₄ enables rapid electron transfer.Compared with Zn_0.5Cd_0.5S（3.9 ns）,this allows the composite to obtain a shorter average lifetime（0.15 ns）.The composite NiCo₂S₄@Zn_0.5Cd_0.5S with Z-scheme heterojunction has excellent photocatalytic activity and stability due to its unique spatial structure and effective Z-scheme electron transfer path.The optimized composite NiCo₂S₄@Zn_0.5Cd_0.5S1:1 photocatalytic hydrogen evolution rate reached 233.68μmol.h^-1.（4）In the dye-sensitized hydrogen evolution system,in order to solve the problem of the reduction of the hydrogen production rate caused by the rapid degradation of the dye,combined with the structural characteristics of the NiCo₂S₄ hollow spheres,a NiCo₂S₄@Mo S₂ material with a type-Ⅱheterojunction was constructed.The type-Ⅱheterojunction realizes the directional transport control of photogenerated carriers between the interfaces.The electrons move to the outer Mo S₂ edge for hydrogen evolution reaction,and the holes move to the inner layer of NiCo₂S₄ to reduce the contact with the EY molecule,thereby slowing down degradation of EY molecules.The kinetics of photocatalytic hydrogen evolution in the sensitized system showed relatively good first-order kinetic characteristics.On this basis,in-situ hydrothermal synthesis of Mo S₂ in the outer layer supports Ni₂P material with electron enrichment effect to further improve the photocatalytic hydrogen evolution activity of the system.（5）On the basis of the idea of charge-oriented regulation in the previous chapter to reduce the sensitizer decay rate,in this chapter,a dye-sensitized photocatalytic hydrogen production system with carrier-oriented conduction combined with sensitizer and sacrificial agent directionally coupling is constructed.Protonated graphite phase carbon nitride（g-C₃N₄）with opposite surface charge and sheet-like Co-based metal organic framework material（MOF）form 2D/2D heterojunction by electrostatic self-assembly.The protonated g-C₃N₄ and 2D Co MOF respectively adsorb the EY molecule and the triethanolamine（TEOA）molecule through chemical bonds to form a complete photocatalytic system.In photocatalytic hydrogen evolution,this structure effectively improves the utilization of dye sensitizers and hole sacrificial agents.Thus,a good photocatalytic hydrogen evolution effect is achieved.On the basis of 2D/2D heterojunction,Co MOF is doped with a small amount of rare earth element Sm.In the transient fluorescence decay experiment,the fluorescence lifetime of the Sm-doped composite system is extended to 4.547 ns compared to 2.975 ns for binary composites.This further improves the efficiency of photo-generated carrier separation to achieve high-efficiency photocatalytic hydrogen evolution.The directional coupling of sensitizer and electron sacrificial agent combined with rare earth element doping makes the photocatalytic hydrogen evolution rate of the composite material reach 73.42μmol.h^-1 within 5 hours.