Preparation Of CdS-Based Composites For Photocatalytic Hydrogen Production

Using solar energy to decompose aquatic hydrogen is one of the effective ways to relieve the pressures on energy shortage and environmental conservation.Much attention has been paid to the development of photocatalyst for this application.In order to maximize the conversion of solar energy,the photocatalyst needs to meet a wide range of solar absorption and a high efficiency of photogenic charge separation and transmission.Since it is difficult for a single group of spectroscopic catalysts to satisfy the above two conditions at the same time,the construction of multi-component composite photocatalysts has aroused extensive research.In this paper,starting from the design of photocatalyst,three kinds of CdS-based composite photocatalysts were constructed,their structures and physical properties were characterized by XRD,SEM,HRTEM,BET,UV-vis and EIS,and their visible light photocatalytic hydrogen production activities and recycling stabilities were studied,and also the separation and transfer mechanism of photogenerated electron-hole pairs were analyzed.1?CdS/SiO₂ composite photocatalytic materials:The CdS/SiO₂ composite photocatalytic materials were prepared by sol-gel-anion exchange method.The prepared CdS is cubic crystal phase,3⁷ nm in size,68.99 m²/g in specific surface area,and 605 nm in the absorption band edge.Compared with hexagonal phase CdS prepared by hydrothermal synthesis method and cubic phase CdS prepared by polyvinylpyrrolidone?PVP?-assisted precipitation method,the CdS prepared by silica gel confined has the characteristics of small particle size,high surface area and good dispersion.With the increase of CdS content in the composite,the photocatalytic activity of hydrogen production first increased and then decreased.In the absence of co-catalyst,the photocatalytic hydrogen production rate of CdS/SiO₂ reached 1925.29?mol·h^-1·g^-1,which was47 and 2.3 times than CdS nanoparticles prepared by hydrothermal method and PVP-assisted precipitation method,respectively.Besides,CdS/SiO₂ shows better cycle stability during 21 h photocatalytic cycle activity test.2?g-C₃N₄/CdS heterojunction composite photocatalytic materials:With melamine as the precursor,the g-C₃N₄ nanosheets were synthesized by the continuous thermal polymerization,and then in-situ growth CdS nanoparticles on g-C₃N₄ nanosheets were achieved by the hydrothermal method.g-C₃N₄/CdS heterojunction composite photocatalysts were prepared by different quality percentage content of g-C₃N₄ nanosheets.The results show that the introduction of the g-C₃N₄ nanosheet significantly improves the dispersion of the CdS nanoparticles.For the g-C₃N₄?10%?/CdS heterojunction composite photocatalyst,the hexagonal CdS nanoparticles with an average particle size of about 100 nm are uniformly dispersed on the surface of the g-C₃N₄ nanosheets or embedded between the g-C₃N₄ nanosheets.Compared with CdS and g-C₃N₄,g-C₃N₄?10%?/CdS has a higher specific surface area?65.32 m²/g?,a red shift of absorption band edge?610 nm?,a narrower band gap?2.03 eV?and a lower charge transfer resistance.The construction of g-C₃N₄/CdS heterojunction can effectively separate the photogenerated electron hole pairs in space.In the absence of co-catalyst,the photocatalytic decomposition rate of hydrogen production is 1846.47?mol·h^-1·g^-11 for g-C₃N₄?10%?/CdS,which is better than CdS,g-C₃N₄ and g-C₃N₄?X=1%,5%,15%,20%?/CdS obviously.During 21 h photocatalytic cycle activity test,the catalytic activity keeps stable.3?CuS/Zn_0.8Cd_0.2S composite photocatalytic materials:The solid solution of Zn_0.8Cd_0.2S with rectangular nano sheet morphology was prepared by solvothermal method with ethylenediamine as structure guide agent.Then,the composite photocatalyst of CuS/Zn_0.8Cd_0.2S was prepared by cation exchange method.Compared with Zn_0.8Cd_0.2S,CuS/Zn_0.8Cd_0.2S has narrower band gap?2.16 eV?,lower charge transfer resistance,and its absorption band edge shifts to 572 nm.With the increase of mass percent of CuS,the photocatalytic activity of CuS/Zn_0.8Cd_0.2S increased first and then decreased.In the absence of co-catalyst,CuS?3%?/Zn_0.8Cd_0.2S shows more excellent photocatalytic hydrogen production activity at a rate of 1870.32?mol·h^-1·g^-1,which was 2.9 times higher than Zn_0.8Cd_0.2S and 31 times higher than CdS.During 21 h photocatalytic cycle activity test,the catalytic activity keeps stable.The formation of CuS/Zn_0.8Cd_0.2S solid solution accelerates the separation rate of photogenerated electron-hole pairs,reduces the bulk phase recombination of photogenerated charges,and thus improves the photocatalytic activity.