The Modulation Of Surface Microstructures And Enhanced Photocatalytic Mechanism Of CdS For Hydrogen Production

CdS is considered to be one of the most potential photocatalysts because of its suitable band structure,which can make effective use of visible light.In addition,the band edge position of CdS meets the thermodynamic requirements of many different photocatalytic reactions.However,the traditional pure-phase CdS photocatalysts still have problems such as low separation efficiency of photogenerated charges and easy photocorrosion in the photocatalytic process,greatly restricting their practical applications.In order to further synthesize efficient CdS photocatalysts,this paper focuses on the modulation of surface microstructure of CdS photocatalysts to enhance its photocatalytic hydrogen-production activity.The main results are shown as follows:First,a small amount of traditional TiO₂ was modified on the surface of the CdS photocatalyst by a simple calcination method,and then Pt was deposited on TiO₂/CdS under visible light irradiation by a photodeposition method to obtain the Pt-TiO₂/CdS photocatalyst.The results of photocatalytic hydrogen production show that with the introduction of anatase phase TiO₂ into the Pt/CdS system,the resultant Pt-TiO₂/CdS photocatalysts exhibit an obviously improved hydrogen-generation rate.Especially,when the TiO₂ content reaches 8%,the Pt-TiO₂/CdS（8%）achieves the best photocatalytic rate(5.88 mmol h^-1 g^-1),which is distinctly superior than that of CdS and Pt/CdS about 5.8 and 3.2 times,respectively.Based on the experimental results,a dual-function TiO₂-mediated mechanism was proposed for the superior H₂-production activity:the dual-function TiO₂layer in Pt-TiO₂/CdS photocatalyst not only works as electron-transport layers to effectively transmit photogenerated electrons to promote the H₂-production reaction,but also acts as hole-block layer to prevent the rapid recombination of photogenerated charges on the Pt active sites,leading to an extraordinary improvement of final hydrogen-generation rate.Second,the colloidal CdS nanocrystals（CdS-NCs）with massive S^2--adsorption were synthesized by a one-step synthetic method through the direct introduction of limited Cd²⁺ions in a sulfur-rich Na₂S/Na₂SO₃ solution.It is found that the massive S^2-ions were adsorbed on colloidal CdS-NC surface,resulting in an excellent solubility of colloidal CdS-NCs in the sulfur-rich solution.The prepared colloidal CdS-NCs reach a superb photocatalytic H₂-production rate of 6.86 mmol h^-1 g^-1 in the absence of a cocatalyst,which was 6.0 and 3.7 times higher than that of traditional c-CdS and h-CdS,respectively,as well as achieve an excellent photocorrosion resistance.Based on the experimental results,a possible S^2-active site-mediated mechanism is proposed for the high H₂-production activity of the colloidal CdS nanocrystals,namely,the massive adsorbed S^2-ions not only act as efficient hole scavengers to rapidly consume the photogenerated holes,but also function as effective H⁺-capturing active sites to boost the interfacial H₂-generation rate.Moreover,the above synthetic strategy can be extended to synthesize colloidal Cd_xZn_1-xS-NCs with a controllable bandgap structure（from 2.26 to 3.51 e V）,indicating that the present synthetic route realizes the universality for preparing highly efficient CdS-based photocatalysts.