Surface And Interface Regulation Of CdSe Quantum Dots And The Utilization For Photocatalytic CO₂ Reduction

Photocatalytic carbon dioxide（CO₂）reduction is the important reaction in the field of artificial photosynthesis.The conversion of CO₂ to value-added solar fuels（such as carbon monoxide,methane,methanol,etc.）driven by solar energy,is the ideal way to realize the resource utilization of CO₂.Currently,the main task is to develop photocatalysts/systems with high activity and selectivity for photocatalytic CO₂ reduction.Cadmium selenide quantum dots（CdSe QDs）have been proven to be active for the conversion of CO₂-to-CO under visible light irradiation.However,it suffered from unsatisfactory efficiency and selectivity,especially in aqueous solution.In response to the above problems,we regulated surface and/or interface chemical environments of CdSe QDs photocatalyst and thus developed three systems of photocatalytic CO₂ reduction.These systems realized photocatalytic conversion of CO₂-to-CO with high efficiency and selectivity in an organic solution or with improved selectivity in aqueous solutions.The main contents and results are as follows.（1）By regulating the feeding ratio of Cd/Se and stripping the surface organic ligands further,the surface ligand-removed and Cd-rich CdSe quantum dots（LRCR-CdSe QDs）were prepared.The system of photocatalytic CO₂ reduction was constructed in DMF by using LRCR-CdSe QDs as the photocatalyst,triethylamine as an electron donor.The system photo-catalyzed CO₂ to CO with high efficiency of 789 mmol g^-1 h^-1 and selectivity up to 95%.Photocatalytic mechanism revealed that the improvement of the feeding ratio of Cd/Se and surface organic ligands stripped of CdSe QDs can expose more active cadmium atoms on the surface of CdSe QDs.The photoinduced electrons transfer from CdSe QDs to surface Cd-C₂O₄^-species during photocatalysis.Surface oxidation and decrease of surface Cd during long-time photocatalysis lead to inactivation of CdSe QDs.（2）MPA-CdSe@Chitosan assemblies were formed by enwrapping MPA-CdSe QDs into Chitosan.MPA-CdSe@Chitosan assemblies enable to improve activity and selectivity of CO₂-to-CO conversion compared to MPA-CdSe QDs in aqueous solution.The aqueous system with MPA-CdSe@Chitosan assemblies as the photocatalyst,ascorbic acid as an electron donor,photo-catalyzed the conversion of CO₂ to CO with production efficiency of73.6 mmol g^-1 and selectivity of 51.0%after irradiation of 60 hours.The production efficiency and selectivity of the MPA-CdSe@Chitosan system were dramatically improved compared to MPA-CdSe QDs（without Chitosan）.Mechanism of photocatalytic CO₂ reduction revealed that amino groups in Chitosan can coordinate to CdSe QDs and thus form MPA-CdSe@Chitosan assemblies.The hydrophobicity and CO₂ solubilization effect originated from Chitosan can provide the hydrophobic and CO₂-rich micro-environment for CdSe QDs.These advantages contributed to the enhanced activity and selectivity of CO₂reduction in aqueous solution.（3）MUA-CdSe@SDS micelles were constructed by co-assembly of MUA-CdSe QDs（MUA=11-mercaptoundecanoic acid）and SDS（sodium dodecyl sulfonate）in TEOA/H₂O solution（TEOA=triethanolamine）.The system of photocatalytic CO₂ reduction based on MUA-CdSe@SDS micelles（TEOA as an electron donor）exhibited excellent stability and selectivity,and CO was the main product of CO₂ redution.The production efficiency and selectivity of CO were 2.47 mmol g^-1 and 91.5%,respectively,after irradiation of 80 hours under visible light.Experimental results revealed that CdSe QDs were located in the hydrophobic and CO₂-rich micro-environment of micelles,these were beneficial to highly selective conversion of CO₂-to-CO.Cadmium oxide formed on the surface of QDs during photocatalysis,the shedding of ligands on the surface of MUA-CdSe QDs can destroy the structure of micelles and lead to the aggregation of QDs,these resulted in the inactivation of MUA-CdSe@SDS micelles.