Tuning the size of quantum dots to enhance charge transfer and photocatalytic CO2 reduction

Coupling graphene oxide (GO) with functionalized CdS quantum dots (QDs) can form a promising assembly for the photocatalytic reduction of CO2. The functionalizing ligand mercaptoacetic acid (MAA) controls the size of the QDs as well as assists in linking to the surface of GO through the polar groups present on both the QDs and GO. Steady-state photoluminescence (SSPL) and time-resolved photoluminescence (TRPL) analyses reveal photoluminescence (PL) quenching of QDs and suggest the photoexcited charge transfer from QDs to GO. Ultrafast femtosecond transient absorption (TA) spectroscopy corroborates the process of charge transfer in the QDs and GO assembly. Cyclic voltammetry (CV) analysis reveals the difference in the energy levels of QDs and GO, which favors the photoexcited electron transfer from the QDs to GO in the assembly. Electrochemical impedance spectroscopy (EIS) also provides evidence for electron transfer and suppression of electron–hole pair recombination in the assembly. It is found that, among the various QDs-GO assemblies, those with the smallest QDs exhibit the highest charge-transfer efficiency (∼36%) and charge-transfer rate (1.83 × 107 s−1). It is further found that the photocatalytic conversion of CO2 into formaldehyde strongly depends on the size of the QDs in the QDs-GO assembly. The small QDs assembly exhibits higher photocatalytic performance towards CO2 reduction than the large QDs assembly. These findings suggest that QDs-GO assemblies with small QDs facilitate the charge-carrier separation and enable carriers to be available for CO2 reduction.