Highly efficient and selective photocatalytic CO₂ reduction based on water-soluble CdS QDs modified by the mixed ligands in one pot

To date, the facile preparation of photocatalysts with high efficiency, selectivity and photostability to promote CO₂ conversion remains a big challenge. Here, we fabricated a novel photocatalyst, namely, MPA&MUA QDs prepared from CdS quantum dots (QDs) anchored by mixed MPA (3-mercaptopronic acid) and MUA (11-mercaptoalkanoic acid) ligands in one pot. Assisted by the sacrificial agent, the MPA&MUA QDs exhibited excellent performance and selectivity for photocatalytic CO₂ reduction to carbonaceous products HCOOH, CO and CH₄ with the corresponding rates of 2.8, 5.4 × 10^-2 and 6.3 × 10^-3 μmol mg(cat)^-1 h^-1, respectively, under irradiation for 32 h. The apparent quantum efficiency (AQE) for the production of HCOOH achieved a high value up to 23.2%, while for CO and CH₄, it was 0.4% and 0.2%, respectively. The attractive duration for photocatalysis under irradiation implied that the MPA&MUA QDs possessed enhanced photostability. Compared with the QDs solely decorated by MPA or MUA, the MPA&MUA QDs could enhance the energy gap E_g and tune the conduction band to smoothly generate CH₄, CO and HCOOH. From the combination of steady-state photoluminescence and time-resolved lifetime, we inferred that the MPA&MUA coatings had a decreased number of available hole trapping sites and improved passivation on the surface, which allowed for efficient hole and electron migration. With this work, we introduce a facial and feasible strategy to create an inexpensive and efficient visible-light photocatalyst for the multi-electron reduction of CO₂.