Controlled Chemical Coversion Synthesis Of ZnS@CdS Double-layered Porous Nanotubes For Photocatalytic Hydrogen Evolution

Solar energy conversion of water into H₂ through photocatalysis is envisaged to be an attractive approach for H₂ production.Fabricating heterojunction and loading cocatalysts represent two promising approach to improve photocatalytic activity.However,some issues still exist in most heterojunctions.For example,the charge carriers get encountered and recombined before they arrive the surface;and randomly distributed dual cocatalysts resulted in unordered flow of photogenerated charge carriers,leading to a high recombination probability.Recently,an advanced strategy have adopted to fabricate hollow-nanostructured photocatalysts with a thin layer of heterojunction and spatially separated reduction and oxidation cocatalysts on different?inner/outer?surface of shell.Nevertheless,these photocatalysts are always restricted to hollow spheres with close-ended structure which increased the mass diffusion resistance,and the high-cost of Pt nanoparticles used as the electron collector limits their practical application.In addition,the random chemical-deposition of cocatalysts on the inner or outer surface of shell could not be taken full advantage because some of them may be loaded on the unreactive sites.Therefore,the designed synthesis of noble-metal-free heterojunction endowed with open-ended channels and spatially separated active dual cocatalysts is highly desirable.The main works in this dissertation contain following parts:1.The ZnS@CdS PNTs are realized by using ZnO nanorods as templates through sequentially interfacial anion and cation exchange reactions and template etching.This strategy can endow ZnS@CdS PNTs with mesoporous wall,macropore cavities and open-ended channels,which are in favor of the photocatalysis.Besides,benefiting from the efficient spatially charge separation,the as-prepared ZnS@CdS PNTs exhibit significantly enhanced photocatalytic HER under visible light irradiation compared to pure CdS and ZnS PNTs.The mesoporous wall and macroporous cavity of the ZnS@CdS nanotubes allow the penetration and scattering of light for an efficient utilization of solar energy.The thin mesoporous heterojunctions can significantly provide driving force for the order transfer of photogenerated electrons and holes toward opposite direction and promote the surface catalytic reaction,leading to greatly enhanced photocatalytic performance.2.The results of in-situ photo-deposition of metal/metal oxide demonstrate that the photo-reduction deposition of Ni NPs and photo-oxidation deposition of CoOx NPs are distributed on the outer surface and inner surface of ZnS@CdS shell,respectively,suggesting the spatially separated photoredox reaction sites in ZnS@CdS double-layered shell.Furthermore,the thin mesoporousheterojunctions and spatially separated dual cocatalysts of CoO_x/ZnS@CdS/Ni PNTs can significantly provide driving force for the order transfer of photogenerated electrons and holes toward opposite direction and promote the surface catalytic reaction,leading to greatly enhanced photocatalytic performance?This activity of the as-prepared samples is 1.8 and 13.5 times higher than those of ZnS@CdS PNTs and CdS PNTs,respectively?.Besides,our proposed self-template strategy can be broadened to the synthesis of other hollow nanostructure with spatially separated photoredox reaction sites.For instance,when NaHSe was used to replace Na₂S and other condition remained unchanged,ZnSe@CdSe PNTs can be also obtained.The CoO_x/ZnSe@CdSe/Ni PNTs could be also obtained and showed the significantly increased photocatalytic activity.This suggests that our facile and robust self-template strategy for the designed preparation of porous tubular heterojunctionphotocatalysts is practicable and general.Furthermore,the tubular photocatalysts with thin mesoporous heterojunctions and spatially separated dual cocatalysts may be used in other photocatlytic and photoelectrochemical fields besides photocatalytic HER.