Photocorrosion Mechanism And Enhanced Photocatalytic Hydrogen-Evolution Activity Of Cadmium Sulfide

Among many photocatalysts,cadmium sulfide（CdS）photocatalysts have been widely investigated for their excellent light-trapping ability（E_g=2.4 e V）and suitable conduction band position（-0.51 V vs NHE）for hydrogen production.However,CdS usually suffers from severe photocorrosion and the lack of hydrogen-producing active sites in photocatalytic reactions,resulting in its low photocatalytic hydrogen-producing activity and poor stability.Meanwhile,the photocorrosion mechanism of CdS photocatalytic materials is still unclear.In order to solve the above problems,in this thesis,the structural evolution and photocorrosion progress of CdS photocatalysts during photocatalytic process were firstly investigated under different sacrificial agents to reveal the structure-activity relationship,and an efficient MoOS_xcocatalyst-modified CdS photocatalytic material was synthesized by a one-step low-temperature method.The main results are shown as follows:First,the photocorrosion mechanism of CdS photocatalysts during the photocatalytic hydrogen-production process was carefully investigated in both of conventional Na₂S-Na₂SO₃ and lactic acid sacrificial agents.It is found that the photocorrosion process of CdS not only causes the oxidation of surface lattice S^2-to S,but also induce the reduction of lattice Cd²⁺to metallic Cdby photogenerated electrons.In addition,different sacrificial agents have a great influence on the structure and morphology of metal Cd.In the Na₂S-Na₂SO₃ system,the photocorrosion process of CdS causes the lattice Cd²⁺to be reduced to metallic Cdwith a particle size of about 5-50 nm,while in the lactic acid system,the reduced metallic Cdis tightly encapsulated on the surface of CdS with a thin layer of 2-3 nm thickness.Based on the above experimental results,a photoinduced self-stabilization mechanism of the CdS photocatalyst is proposed,which means that the photocorrosion process of CdS will spontaneously generate metal Cdon the surface of the CdS material.Once a certain amount of metal Cdis generated on the surface of CdS,the obtained CdS-Cdcomposite has excellent stability,resulting in the good stability of hydrogen-production performance of the formed CdS-Cdphotocatalyst.Second,with thioacetamide as the S source of both CdS photocatalytic material and MoOS_x cocatalyst,the one-step thioacetamide-induced method was developed to synthesize MoOS_x/CdS hollow sphere photocatalyst with high hydrogen-production activity.It was found that the formation of MoOS_x/CdS hollow spheres followed the Ostwald ripening mechanism,namely,the formation of hollow spheres is a ripening process from solid to hollow and from amorphous to crystallization.The photocatalytic hydrogen-production tests showed that the MoOS_x cocatalyst could significantly increase the hydrogen-production activity of CdS photocatalyst,and the MoOS_x/CdS（1 wt%）sample had the best hydrogen-production activity with a hydrogen production rate of 46.47μmol h^-1 and an apparent quantum efficiency of3.51%,which was 3.73 times higher than that of the pure CdS sample.The improved hydrogen-production activity could be attributed to the loading of MoOS_x cocatalyst,which not only provides a large number of hydrogen-production active sites,but also accelerates the separation efficiency of photogenerated carrier in CdS photocatalysts.