Preparation And Photocatalytic H₂-production Performance Of CdS Composite Photocatalyst

In recent years,energy depletion and environmental degradation have attracted tremendous attention.If the inexhaustible solar energy can be converted into a clean fuel（H₂）with high energy density,human beings may enter a sustainable society.Photocatalytic hydrogen evolution from water splitting with the assistance of semiconducting photocatalysts is one of the most promising methods at present.Among the numerous semiconductor photocatalysts,CdS is extensively used as a promising candidate for hydrogen production due to its narrow band gap（E_g）（about2.4 e V）for visible light absorption and favorable band structure for water splitting.It is a pity that single CdS exhibits poor photocatalytic performance due to the fast recombination rate of photogenerated electron-hole pairs and serious photocorrosion.Therefore,multiple modification approaches,such as modifying the morphology and structure,building heterojunction,and introducing cocatalysts,have been adopted to ameliorate the photocatalytic performance of CdS.Specifically,loading cocatalysts is an outstanding strategy to enhance light harvesting,promote charge separation and provide active sites.Currently,most of the cocatalysts are metallic materials,which are expensive and harmful to environment.Hence,in this study,the strategy of loading two non-metallic cocatalysts（carbon hollow nanosphere（C-HS）and Si O₂nanosphere）and structural regulation was adopted to improve the photocatalytic hydrogen production activity of CdS.This study mainly includes the following two works:The carbon@CdS hollow sphere（C@CdS-HS）composite photocatalyst was fabricated by a facile hydrothermal method with core-shell Si O₂@carbon nanosphere（Si O₂@C）as a semi-sacrificial template and cadmium chloride and thiourea as precursors（thiourea as Si O₂ etchant as well）.For comparison,a pure CdS hollow sphere（CdS-HS）and C-HS were prepared under the same conditions.The C@CdS-HS/Pt exhibits an excellent photocatalytic H₂ generation rate of 20.9 mmol h^-1 g^-1（apparent quantum efficiency of 15.3%at 420 nm）,with 1.0 wt%Pt as a cocatalyst under simulated sunlight irradiation;this rate is 69.7,13.9,and 3.9 times higher than that of CdS-HS,C@CdS-HS,and CdS-HS/Pt,respectively.The charge transfer pathway and process are elucidated and photocatalytic mechanism is proposed on the basis of a series of characterization:1)the C-HS with good electronic conductivity serves as electron reservoir and transporter,promoting the separation and migration efficiency of photogenerated charge carriers;2)the introduction of porous C-HS increases the specific surface area of the composite photocatalyst and provides more active sites;3)the intimate contact interface with strong interaction formed by in situ deposition of CdS nanoparticles（NPs）on the surface of C-HS is advantageous to the interfacial charge transfer;4)the C-HS possesses strong photothermal effect induced by the intense absorption of near infrared（NIR）light,which can increase the surface temperature of CdS,accelerate desorption of H₂ from active sites,and thereby facilitate the photocatalytic H₂production reaction kinetically;and 5)the C-HS,as the interior framework,enhances the mechanical stability of the composite photocatalyst and has no light shielding effect to CdS.The yolk-shell SiO₂@CdS（Y-CdS）composite photocatalyst was prepared by a facile hydrothermal method with SiO₂ nanosphere as a sacrificial template.Subsequently,SiO₂ was completely etched to prepare hollow CdS nanosphere（H-CdS）as a control.Under visible light irradiation,the photocatalytic H₂ production rate of Y-CdS loaded with 1.0 wt%Pt reaches 3884μmol h^-1 g^-1,which is nearly twice as high as H-CdS.Based on characterization results,the mechanism of enhanced photocatalytic activity of Y-CdS is proposed:1)the yolk-shell structure provides good structural stability for the photocatalyst;2)the yolk-shell structure has more mesopores,which is conducive to the transport and diffusion of reactants（sacrificial agent）and products,so that the holes can be consumed quickly to inhibit the recombination of photogenerated electron-hole pairs;3)multiple light reflections and slow-photon effect in the space between the yolk and shell improve the light utilization of CdS;4)the photogenerated electron-hole pairs of Y-CdS has a higher separation efficiency;5)the negatively charged surface of Y-CdS can absorb more H⁺by the Coulomb force to participate in hydrogen reduction reaction,thus increasing the hydrogen production rate kinetically.This study provides a new perspective for improving the photocatalytic hydrogen production activity of CdS by loading non-metallic cocatalysts and regulating the structure.