Forward and Backward Electron Transfer Rates for Cu _<i>x</i> In _<i>y</i> S/ZnS Quantum Dot-Methyl Viologen Redox Couple: Implications for Reductive Photocatalytic Activity

An increase in Cu deficiency leads to the enhancement in photoluminescence quantum yield (PLQY) and lifetime in aqueous L-glutathione-capped CuxInyS/ZnS quantum dots (QDs), due to the increasing preponderance of VCu′ hole trap states and reduction of the surface trap and internal defect states as the extent of diffusion of Zn2+ ions into the crystal lattice increases. Such compositional variation is found to affect the rate of ultrafast photoinduced electron transfer (PET) through the ZnS shell from the QDs to the methyl viologen (MV2+) molecule as well as the slower back electron transfer (BET) from reduced MV2+ (MV+̇) to QDs. The interplay of these two processes plays a crucial role in maximizing charge separation, which is essential for photocatalytic activity. The extent of PL quenching by MV2+ ions gradually decreases with increasing Cu deficiency in the QDs. The PL quenching is due to ultrafast PET from the QDs to MV2+ adsorbed on their surface. Using femtosecond transient absorption spectroscopy (TAS), the rate constant of PET is found to become faster with an increase in Cu deficiency. The rate constant of BET, determined using laser flash photolysis, is the fastest for the most Cu-deficient QDs. The role of different Cux (x = 1+, 2+) oxidation states, hole trapping states, and electron trap states play an important role in PET as well as BET. Hence, it is proposed that QDs with a greater degree of Cu deficiency would be more efficient photosensitizers. Hence, the dynamics of the processes that can trigger reductive photocatalysis like CO2 reduction and hydrogen evolution are worked out.