Effect of surface ligands on the photoinduced electron transfer rate and efficiency in ZnO quantum dots and graphene oxide assemblies

Apart from biocompatibility, ZnO quantum dots (QDs) are considered to be an efficient luminescence material due to their low cost and high redox potential. Here, we report the synthesis of ZnO QDs by using five different functionalizing ligands like mercaptoacetic acid (MAA), 3-mercaptopropionic acid (MPA), octadecene (ODE), ethylene glycol (EG), and oleyl amine (OLA) and fabricate their assemblies with graphene oxide (GO). We investigate the role of functionalizing ligands as a surface modifier of ZnO QDs for their attachment to GO. The steady-state photoluminescence (SSPL) and time-resolved photoluminescence (TRPL) analyses demonstrate the photoluminescence (PL) quenching of ZnO QDs in ZnO QDs-GO assembly. The highest reduction in PL intensity is observed with ZnO QDs-GO assembly with EG as a surface functionalizing ligand. Cyclic voltammetry (CV) analysis confirms the feasibility of charge transfer from ZnO QDs to the GO. The maximum (79.43%) charge transfer efficiency (E-CT) is observed in the case of ZnO-MAA-GO as compared to other assemblies. This means the thiol group-containing ligands facilitate charge transfer as compared to hydroxyl and amine group ligands. This leads to the conclusion that charge transfer in ZnO QDs-GO assemblies depends strongly on the nature of surface ligands.