Mechanistic insights into the photocatalytic properties of metal nanocluster/graphene ensembles. Examining the role of visible light in the reduction of 4-nitrophenol†
Metal nanoclusters (MNCs) based on silver and gold, abbreviated as AgNCs and AuNCs, respectively, were synthesized and combined with functionalized graphene, abbreviated as f-G, forming novel MNC/f-G ensembles. The preparation of MNCs/f-G was achieved by employing attractive electrostatic interactions developed between negatively charged MNCs, attributed to the presence of carboxylates due to α-lipoic acid employed as a stabilizer, and positively charged f-G, attributed to the presence of ammonium units as addends. The realization of MNC/f-G ensembles was established via titration assays as evidenced by electronic absorption and photoluminescence spectroscopy as well as scanning transmission electron microscopy (STEM) and energy-dispersive X-ray (EDX) spectroscopy analyses. Photoinduced charge-transfer phenomena were inferred within MNCs/f-G, attributed to the suppression of MNC photoluminescence by the presence of f-G. Next, the MNC/f-G ensembles were successfully employed as proficient catalysts for the model reduction of 4-nitrophenol to the corresponding 4-aminophenol as proof for the photoinduced hydrogen production. Particularly, the reduction kinetics decelerated by half when bare MNCs were employed vs. the MNC/f-G ensembles, highlighting the beneficial role of MNCs/f-G in catalysing the process. Furthermore, AuNCs/f-G displayed exceptionally higher catalytic activity both in the dark and under visible light illumination conditions, which is ascribed to three synergistic mechanisms, namely, (a) hydride transfer from Au–H, (b) hydride transfer from photogenerated Au–H species, and (c) hydrogen produced by the photoreduction of water. Finally, recycling and re-employing MNCs/f-G in successive catalytic cycles without loss of activity toward the reduction of 4-nitrophenol was achieved, thereby highlighting their wider applicability.