Mechanistic insights into the photocatalytic properties of metal nanocluster/graphene ensembles. Examining the role of visible light in the reduction of 4-nitrophenol

Abstract

Metal nanoclusters (M_NCs) based on silver and gold, abbreviated as Ag_NCs and Au_NCs, respectively, were synthesized and combined with functionalized graphene, abbreviated as f-G, forming novel M_NC/f-G ensembles. The preparation of M_NCs/f-G was achieved by employing attractive electrostatic interactions developed between negatively charged M_NCs, 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 M_NC/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 M_NCs/f-G, attributed to the suppression of M_NC photoluminescence by the presence of f-G. Next, the M_NC/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 M_NCs were employed vs. the M_NC/f-G ensembles, highlighting the beneficial role of M_NCs/f-G in catalysing the process. Furthermore, Au_NCs/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 M_NCs/f-G in successive catalytic cycles without loss of activity toward the reduction of 4-nitrophenol was achieved, thereby highlighting their wider applicability.