Ytterbium And Erbium Doping/Heterojunctions Controlling Cadmium-Based Sulfide And Its Photocatalytic Properties

Photocatalytic technology is an essential means to solve the energy crisis and environmental problems by directly utilizing solar energy.At present,the development of high-efficiency photocatalysts is the essential task to advance the photocatalytic reaction in theoretical research and then realize the practical applications of photocatalysts.Metal sulfide semiconductors with generally narrow band gaps can respond to visible light,however their photocorrosion properties greatly limit the application of catalysts.In order to reduce the photocorrosion of sulfides and improve the separation efficiency of photogenerated carriers,cadmium-based sulfides were modified by co-doping with rare earth ion Yb³⁺/Er³⁺and constructing a heterojunction in this paper.The substance are as follows:1.The Yb³⁺/Er³⁺with different molar ratios was doped into Cd_0.5Zn_0.5S through hydrothermal method.Then the characterization presents that Yb³⁺/Er³⁺co-doping induces the generation of sulfur vacancies in Cd_0.5Zn_0.5S,and its activity was analyzed by photocatalytic CO₂ reduction and photolysis of water under visible light.Among them,Cd_0.5Zn_0.5S:15%Yb,Er（15%is molar mass percentage）exhibited supreme photocatalytic performance,and the yield of reducing CO₂ to CO was 55.1μmol·g^-1·h^-1,which was 2.3 times that of Cd_0.5Zn_0.5S.Besides,in the photolysis of water,the hydrogen production rate reached 422.0μmol·g^-1·h^-1,which was3.0 times that of Cd_0.5Zn_0.5S.The primary reason for the improved photocatalytic activity of Cd_0.5Zn_0.5S:Yb,Er was that Yb³⁺/Er³⁺co-doping induces the generation of sulfur vacancies in Cd_0.5Zn_0.5S,then the adsorption and dissociation of substrate and carrier separation efficiency was facilitated by using sulfur vacancies as active centers.2.The electron transfer path of single semiconductor is onefold,so that the photogenerated carriers cannot be separated in time,which limits the photocatalytic application of the samples.Therewith,a heterojunction was constructed by integrating Cd_0.5Zn_0.5S with Co WO₄ through hydrothermal method,and it was proved by characterization that Cd_0.5Zn_0.5S-Co WO₄ was an S-scheme heterojunction,and twin structure existed in Cd_0.5Zn_0.5S.Among them,Cd_0.5Zn_0.5S-10%Co WO₄（10%is mass percentage）exhibited the most excellent photocatalytic performance.The yield and selectivity of reducing CO₂ to CO reached 106.2μmol·g^-1·h^-1 and 95.9%,respectively,which were 4.5 times and 1.6 times higher than those of twin Cd_0.5Zn_0.5S.In addition,Cd_0.5Zn_0.5S-Co WO₄ was more stable than Cd_0.5Zn_0.5S after three cycle experiments.The enhancement of the photocatalytic performance and stability of Cd_0.5Zn_0.5S-Co WO₄ was credited to the charge transfer mechanism of the S-scheme heterojunction,which preserved the optimal reduction and oxidation sites in the system.Additionally,the construction of heterojunction accelerated the charge transport rate of samples and boosted the separation efficiency of photogenerated electrons and holes within the samples.3.The agglomeration of two semiconductors inevitably occurs when a heterojunction is constructed in the aqueous solution,which limits the photocatalytic efficiency of the samples.Therefore,In₂S₃ and Cd S were compounded through the low-temperature molten salt method,which using low-cost molten salt KSCN as the reaction medium and sulfur source,and then Yb³⁺/Er³⁺was doped into Z-scheme In₂S₃/Cd S.The characterization proved that In₂S₃/Cd S:Yb,Er was rich in sulfur vacancies,and its yield of reducing CO₂ to CO under visible light reached62.3μmol·g^-1·h^-1,which was 8.5 times and 7.2 times that of bare In₂S₃ and Cd S,respectively.Meanwhile,the Cr（VI）solution（10 mg/L,30 min）removal rate of In₂S₃/Cd S:Yb,Er was almost 100%under visible light,which was 2.0 times and 7.0 times as high as that of In₂S₃（49.2%）and Cd S（14.0%）,respectively.The raised photocatalytic activity of In₂S₃/Cd S:Yb,Er was mainly due to the charge transfer path of Z-scheme heterojunction,which made the conduction band of samples more negative,and enhanced the reduction ability of samples and the separation efficiency of photogenerated carriers.Simultaneously,Yb³⁺/Er³⁺co-doping regulated the morphology of In₂S₃/Cd S and enlarged its specific surface area,which was beneficial to the adsorption of the reaction substrate by the catalysts.