Heterostructures of Ni(II)-doped CdS quantum dots and β-Pb0.33V2O5 nanowires: Enhanced charge separation and redox photocatalysis via doping of QDs

We synthesized heterostructures by tethering Ni(II)-doped CdS (Ni:CdS) quantum dots (QDs) to β-Pb0.33V2O5 nanowires (NWs) using L-cysteine as a molecular linker, and we evaluated the influence of doping on their redox photocatalytic reactivity. We initially hypothesized that incorporating Ni:CdS QDs into heterostructures could alter excited-state dynamics and mechanisms, and that the localization of excited electrons on Ni 3d states could promote redox photocatalytic mechanisms including reduction of CO2. Isolated Ni:CdS QDs were ferromagnetic, and they exhibited enhanced photocatalytic hydrogen evolution and photostability relative to undoped CdS QDs. Both Pb0.33V2O5/CdS heterostructures (with undoped QDs) and Pb0.33V2O5/Ni:CdS heterostructures (with Ni(II)-doped QDs) exhibited substantial energetic overlap between valence-band states of QDs and intercalative mid-gap states of β-Pb0.33V2O5 NWs. Within Pb0.33V2O5/CdS heterostructures, photoexcitation of CdS QDs was followed by rapid (50–100 ps) transfer of both holes and electrons to β-Pb0.33V2O5 NWs. In contrast, within Pb0.33V2O5/Ni:CdS heterostructures, holes were transferred from Ni:CdS QDs to β-Pb0.33V2O5 NWs within 100 ps, but electrons were transferred approximately 20-fold more slowly. This difference in electron- and hole-transfer kinetics promoted charge separation across the Pb0.33V2O5/Ni:CdS interface and enabled the photocatalytic reduction of CO2 to CO, CH4, and HCO2H with > 99.9% selectivity relative to the reduction of H+ to H2. These results highlight the opportunity to fine-tune dynamics and mechanisms of excited-state charge-transfer, and mechanisms of subsequent redox half-reactions, by doping QDs within heterostructures. Moreover, they reveal the promise of heterostructures comprising QDs and MxVyO5 materials as CO2-reduction photocatalysts.