Mn(ii)-doped CdS/ZnS core/shell quantum dot films photocatalyze reductive organic transformations with a boost in efficiency from enhanced Auger processes

Abstract

We investigated the reduction of organic compounds photocatalyzed by Mn²⁺-doped CdS/ZnS quantum dot (Mn:CdS/ZnS QD) films. The incorporation of Mn²⁺ ions into CdS nanocrystals as a dopant promotes Auger cross-relaxation, a phenomenon that yields hot electrons with high reducing power. Using QD-coated vessels or substrates, we examined several model organic reactions to establish the versatility and superior performance of Mn:CdS/ZnS QDs over undoped CdS QDs. First, Mn:CdS/ZnS QDs exhibited a 3.4-fold increase in the photoreduction of methyl viologen compared with CdS QDs. Second, the viologen can subsequently be utilized as an electron shuttle in biphasic reactions, such as the photoreduction of meso-stilbene dibromide, allowing for flexible design where reactants were separate from the photocatalysts. Third, Mn:CdS/ZnS QDs photocatalyzed the 6-electron reduction of nitrobenzene to aniline with an overall internal quantum efficiency of ∼3%. Impressively, the photoreduction of nitrobenzene was complete in 15 min by doped QDs vs. >48 h by undoped QDs – an enhancement of >190-fold. Moreover, we investigated the recyclability of the Mn:CdS/ZnS film and detected changes in the surface species, indicating some oxidation of the film upon its use as a photocatalyst. Different from previous studies, the QD films and coatings facilitate the rapid separation of catalysts from solution and significantly reduce post-reaction workups. Our work demonstrates a drastic improvement in the efficiency of CdS QD-based photocatalysis that may have broad implications in photoredox reactions.