Study On The Structure Control Of Sulfide Solid Solution Composites For Photocatalytic Hydrogen Production

Solar-powered photocatalytic water splitting for hydrogen?H₂?evolution via semiconductor photocatalysis has been regarded as an effective solution to address the increasing environmental and energy issues.Since Fujishima and Honda first reported in 1972 that the photoelectrocatalytic water splitting could be achieved on TiO₂ electrodes,a large amount of semiconductor based photocatalysts have been applied for H₂ production.Ideal photocatalysts should have the capability of wide band light absorption and effectively separation and utilization of photogenerated charge.At present,oxide semiconductor photocatalysts are widely used for H₂ evolution from water splitting,but they just absorb the ultraviolet?UV?light which account for about 4% of the sunlight because of their wide band gaps,while the visible light,the vast majority?about 45%?of solar energy has not been effectively utilized.Therefore,the development of visible light responsive photocatalysts has attracted much attention.Among them,sulfide materials such as CdS,ZnS,etc.have become a research hotspot due to their adjustable band gaps and excellent photoelectric properties.For example,the conduction band position of Cd_1-xZn_xS can be regulated through adjusting the ratio between Zn and Cd.However,the application of pure CdS or ZnS is still limited due to their serious photocorrosion or the rapid recombination of photogenerated carriers.In order to overcome the above shortcomings,metal sulfides CdS,ZnS and Cd_1-xZn_xS were used as the main catalysts to design and synthesize a series of nanocomposites by adjusting the morphology,constructing heterojunction and loading cocatalysts for improving the efficiency of H₂ production.The detailed studies are as follows:?1?The well-defined 1D Cd_1-xZn_xS@O-MoS₂/NiO_x hybrid nanostructures with enhanced activity and stability for photocatalytic HER was prepared for the first time by adjusting the Zn-doping content of Cd_1-xZn_xS,growing defect rich MoS₂ layer and loading NiO_x nanoparticles.The results show that the maximum H₂ evolution rate?HER?of CZ_0.15S@0.2M-Ni O_x in lactic solution is 66.08 mmol?h^-1?g^-1(about 25 times that of Pt loaded CZ_0.15S).In addition,when Na₂S/Na₂SO₃ was used as a sacrificial agent,the rate further increased significantly to 223.17 mmol?h^-1?g^-1.The reason for the enhancement of photocatalytic activity is the band alignments,exposure of active sites,and interfacial charge separation of Cd_1-xZn_xS@O-MoS₂/NiO_x are optimized by tuning the Zn-doping content as well as the growth of defect-rich O-MoS₂ layer and NiO_x nanoparticles.The heterojunction formed between MoS₂ and Cd_1-xZn_xS significantly inhibited the recombination of photochargic carriers.Meanwhile,in Na₂S/Na₂SO₃ solution,the Cd_1-xZn_xS@O-MoS₂/NiO_x sample demonstrates an apparent quantum yield of 64.1% at 420 nm and a good stability for HER under long-time illumination.?2?T-Cd_xZn_1-xS/MoS₂ with twinned Cd_xZn_1-xS solid solution and multilayer defective MoS₂ nanosheets was prepared.The alternating zinc blende/wurtzite?ZB/WZ?homojunctions in T-Cd_xZn_1-xS and intimate heterojunctions formed between T-Cd_xZn_1-xS and MoS₂ facilitated the efficient separation and transfer of both bulk and surface charge carriers.Moreover,the good visible-light responses of T-Cd_xZn_1-xS and MoS₂ as well as the existence of abundant S₂^2-and Mo⁵⁺active species in defective MoS₂ nanosheets also contributed significantly to the activity improvement of T-Cd_xZn_1-xS/MoS₂.Benefiting from the above advantages,T-Cd_xZn_1-x S/MoS₂ exhibited superior HER rates of 37.22 mmol·h^-1·g^-1 in lactic acid solution and 69.25mmol·h^-1·g^-1 in Na₂S/Na₂SO₃ solution,corresponding to the apparent quantum yield?AQY?of 36.3%and 55.2%at 420 nm,respectively.?3?A template-assisted ion-exchange strategy combined with subsequent electrostatic assembly process were employed to prepare uniform ZnS@CdS@Cd_0.5Zn_0.5S-MoS₂ hollow nano-spheres for the first time.Under visible-light irradiation??>420 nm?,the ZnS@CdS@Cd_0.5Zn_0.5S-MoS₂ hybrid exhibits the optimal HER activity of 50.65 mmol·h^-1·g^-1,which is 20 times more than that of the Pt-decorated ZnS@CdS@Cd_0.5Zn_0.5S counterpart and superior to that of most CdS-based photocatalysts ever reported.Moreover,the ZnS@CdS@Cd_0.5Zn_0.5S-MoS₂ hybrid is also in possession of a good stability for long-term HER.The outstanding HER capability of ZnS@CdS@Cd_0.5Zn_0.5S-MoS₂could be attributed to the dramatically facilitated separation of charge carriers,abundant active sites from the defect-rich 1T/2H MoS₂ few-layered nanosheets,and enhanced light-harvesting of the hollow nanostructure.?4?P-doped Cd_1-xZn_xS solid solution was prepared by thermal phosphating reaction,then a one-step solvothermal reaction was used to simultaneously grow Ni₂P nanoparticles onto Cd_1-xZn_xS-P nanospheres and assemble Cd_1-xZn_xS-P@Ni₂P to rGO nanosheets to form a Cd_1-xZn_xS-P@Ni₂P/rGO composite photocatalyst.The results show that the Cd_1-xZn_xS-P@Ni₂P/rGO exhibits an impressive HER rate of 616.80mmol?h^-1?g^-1 in Na₂S/Na₂SO₃ solution under visible-light irradiation??>420 nm?,corresponding to an AQE of 63.14%at 420 nm.