CdS-based Heterojunction Catalysts For Synergistic Photocatalytic CO₂ Reduction

Photocatalytic conversion of CO2to fuels and chemicals driven by solar energy is one of the sustainable and effective ways to address the global environmental and energy crisis.Although photocatalytic reduction of CO2 has been extensively studied,energy conversion efficiency of this process is still very low.Design and development of catalysts with high conversion efficiency for photocatalytic CO2 reduction is still a great challenge.There are two major problems encountered in photocatalytic water splitting and photocatalytic reduction of CO2,:rapid recombination of photogenerated electron-hole pairs(e--h+)and wide band gap of some photocatalysts.Apart from these two issues,photocatalytic CO2 reduction not only suffers from the inertness of CO2,but also competes with water for surface adsorption on photocatalysts.Photocatalytic CO2 reduction involves very complex reactions,including photo adsorption to generate carriers,separation and migration of photogenerated carriers,redox reactions by photogenerated e-or h+ on the surface of catalyst.Although a large number of studies have been reported to address these problems,lack of systematic studies on all steps involved in the concerted photocatalytic CO2 reduction has been limiting the improvement of the efficiency of photocatalytic reduction of CO2.Based on the idea of "Multi-pronged Approach",the following three parts of work about CdS-ZnO(or CeO2)heterojunction for photocatalytic reduction of CO2 were carried out.It is expected that photons absorption by CdS,photo-generated electrons and holes separation and migration,CO2 adsorption and activation on ZnO(or CeO2)may be harmoniously compatible with each other for highly efficient photocatalytic CO2 reduction owing to the well-matched structural and intrinsic properties of CdS/ZnO(or CeO2)catalysts.This work highlights the potential of improving the performance of CO2 photocatalytic reduction based on a multi-pronged approach.(1)CdS/ZnO photocatalysts with heterojunction structure were successfully designed and prepared.Compared with CdS or ZnO,the activity of photocatalytic CO2 reduction over CdS/ZnO was significantly improved.The activity of photocatalytic reduction of CO2 over CdS/ZnO is～11 times higher than that over CdS under visible light irradiation at 383 K.This improvement of photocatalytic performance can be ascribed to the well-matched structural and intrinsic properties of CdS/ZnO heterojunction.The synergistic effects of CdS-ZnO such as the visible light harvested by CdS,enhanced charge separation through migration of electrons from CdS to ZnO,along with the improved adsorption and activation of CO2 on ZnO,are harmoniously compatible with each other to enhance photocatalytic activity of inert CO2 reduction.The study of multi-pronged catalyst design strategy provides a feasible strategy to develop new catalyst platforms for harvesting solar-energy in the conversion of CO2 to valuable fulels and chemicals.At the same time,it is found that ZnO is unstable under the photocatalytic CO2 reduction conditions.(2)To improve the stability of the catalyst,we successfully prepared a CeO2/CdS heterojunction photocatalyst by a facile two-step solvothermal method which exhibited good performance in photocatalytic reduction of CO2.By simulating solar irradiation without adding any external sacrificial agent,photocatalytic reduction of CO2 not only yielded the main products of CO and CH4,but also produced a small amount of a liquid product CH3OH.The properties of CeO2 are similar to that of ZnO for combination with CdS,including the formation of heterojunction structure between CeO2 and CdS,which favors the separation of electrons and holes.In addition,CeO2 can effectively adsorb and activate CO2.Development of CeO2/CdS for photocatalytic CO2 reduction is another successful application inspired by the"Multi-pronged Approach" strategy.More importantly,CeO2 is more stable than ZnO under the condition for photocatalytic CO2 reduction,which is desired for photocatalytic CO2 reduction(3)In order to investigate the effects of oxygen vacancies from CeO2 on the performance of photocatalytic reduction of CO2,uniform and single-crystalline nanocubes,nanopolyhedra,and nanorods CeO2 with different amount of oxygen vacancies were prepared by a hydrothermal method at different temperatures under different NaOH concentrations with Ce(NO3)3 as cerium source.CeO2 with different morphologies exposes different facets with different amount of oxygen vacancies concentrations,leading to distrinct performance of photocatalytic reduction of CO2.Our experimental results show that CeO2 nanorods with the highest concentration of oxygen vacancies can enhance the adsorption and activation of CO2,leading to the best activity of photocatalytic reduction of CO2.We further show that the effective separation of photogenerated electrons and holes by heterojunction and the adsorption and activation of CO2 by CeO2 with abundant oxygen vacancies can be achieved over a CeO2/CdS heterojunction catalyst.The surface oxygen vacancy concentration can be regulated by the morphology of CeO2,which can further increase its adsorption and activation of CO2 and thus obtain improved performance of photocatalytic reduction of CO2.