Constructing The Visible-light Photocatalytic H₂ Generation System Based On CdS Nanosheets Modified By Noble-metal-free Cocatalysts

In recent years,the global energy crisis and environmental pollution caused by the burning of fossil fuels have become more and more serious.Hydrogen energy?H₂?is an ideal candidate for the replacement of fossil fuels in the future,capturing freely available solar energy and converting it into hydrogen energy has received much attention.Since the groundbreaking reported by Fujishima and Honda about the photoelectrochemical splitting of water on TiO₂ electrodes using UV light in 1972,photocatalytic H₂ generation over the semiconductor materials has attracted the attention of scientists.Cadmium sulfide?CdS?has been considered to be an attractive semiconductor for visible light H₂ evolution because of its narrow band gap?2.4 eV?and suitable conduction band position.However,several persistent issues still markedly hinder the scale-up application of CdS catalysts,including the rapid charge-carrier recombination,the weak redox ability of photogenerated electrons/holes and the strong photocorrosion,thus leading to the low photocatalytic activity and durability of CdS.In order to overcome these challenging problems,many modification strategies have been widely proposed to improve the photocatalytic H₂-evolution activity over CdS photocatalysts.Among them,fabricating nanostructured CdS and loading noble-metal-free cocatalysts were considered as the most promising methods to significantly improve the photocatalytic activity.In this paper,CdS nanosheet was synthesized by simple methods.Then loading MoS₂,carbon black?CB?,NiS₂,Ni₃C as noble-metal-free cocatalysts for efficient photocatalytic performance,which can reduce band gap,enhance visible light absorption,promote photongenerated electron-hole pair separation,accelerate the surface H₂-evolution kinetics and increase the reactive sites.The crystal structure,morphology,elemental composition and optical property of prepared samples were characterized by XRD,TEM,HRTEM,EDX,XPS,BET and DRS.Their photocatalytic activities of hydrogen were also measured under the visible light irradiation.The possible photocatalytic H₂ evolution and charge transfer mechanisms were investigated by PL and electrochemical test.The results are as follows:?1?The CdS nanosheets were successfully synthesized by a simple solvothermal method,Then,2D-2D CdS/MoS₂ composite photocatalyst was prepared by ultrasonic loading MoS₂on the surface of CdS nanosheets.The photocatalytic H₂-evolution rates of CdS-1%MoS₂sample in lactic acid,formic acid and Na₂S/Na₂SO₃ aqueous solution were 8.72,5.32,1.75 mmolg^-1h^-1,respectively.?2?The ternary CdS/CB/NiS₂ composites were synthesized by ultrasonic dispersion treatment and subsequent precipitation of NiS₂ cocatalyst.CdS-0.5%CB-1%NiS₂ sample exhibited the hightest H₂-evolution rate of 166.7?molg^-1h^-1,which was approximately 5.16and 1.87 times higher than those of pure CdS NSs and CdS-1.0%NiS₂,respectively..?3?The Ni₃C/CdS NSs composite photocatalysts were successful synthesized by a simple grind method.The CdS-1%Ni₃C hybrid sample exhibited the highest photocatalytic H₂-evolution rates of 357?molh^-11 in 0.25M Na₂S-Na₂SO₃ and 450.5 umolh^-1 in lactic acid,which were approximately 7.76 and 4.79 times higher than that of pure CdS NSs,respectively.