Stimulating Charge Transfer Over Quantum Dots via Ligand-Triggered Layer-by-Layer Assembly toward Multifarious Photoredox Organic Transformation

Semiconductor quantum dots (QDs) have garnered tremendous attention by virtue of their substantial light-harvesting and conversion efficiencies, large number of active sites, unique quantum size confinement, and multiple exciton generation effects. In this regard, recent years have witnessed their widespread applications in photocatalysis. Nonetheless, intrinsic disadvantages of QDs including unfavorable photostability, ultrafast charge recombination rate, and sluggish kinetic of charge carriers retard the construction of high-efficiency QDs-based photocatalysts for solar energy conversion. Thus far, in-depth investigation on the visible-light-driven photoredox organic transformation over QDs has not yet been exhaustively explored, and corresponding photocatalytic mechanisms remain elusive. In this work, selecting cadmium selenide (CdSe) as a quintessential category of semiconductor QDs, we demonstrated a facile, green, easily accessible, and rather efficient electrostatic self-assembly strategy to conspicuously boost the versatile photoredox performances of CdSe QDs toward selective organic transformation under visible light irradiation by intimately integrating with graphene (GR) via judicious surface charge tuning. In this scenario, intrinsically negatively charged CdSe QDs and surface-modified positively charged GRs were utilized as the building blocks for spontaneous electrostatic self-assembly buildup, which gives rise to well-defined CdSe QDs–GR ensembles. More intriguingly, ligands capped on the CdSe QDs surface enable the alternate layer-by-layer (LbL) assembly of CdSe QDs and GR forming three-dimensional spatially multilayered heterostructures. Furthermore, it was significant to unveil that such self-assembled CdSe QDs–GR nanocomposites exhibit remarkably enhanced and multifunctional photoredox performances toward selective oxidation of aromatic alcohols to corresponding aromatic aldehydes and selective reduction of nitroaromatics to amino compounds under visible light irradiation, which far exceeds the pristine CdSe QDs counterpart, which exhibits almost negligible photoactivities. This can be ascribed to the pivotal role of GR for conspicuously capturing and shuttling electrons from band-gap photoexcitation of CdSe QDs, intimate interfacial contact between the building blocks, and enlarged specific surface area stemming from seamless GR encapsulation and intercalation, along with the unique ligand-triggered LbL assembly integration mode between CdSe QDs and GR, hence synergistically reducing the recombination rate and prolonging the lifetime of charge carriers. Furthermore, photoredox mechanisms of the CdSe QDs–GR ensemble were elucidated. It is anticipated that our work would afford an efficacious avenue to finely modulate the charge transport over QDs for solar energy conversion.