Structure, optical properties, and catalytic applications of alkynyl-protected M4Rh2 (M = Ag/Au) nanoclusters with atomic precision: a comparative study

Abstract

We report two atomically precise alloy nanoclusters of Ag₄Rh₂(CCAr^F)₈(PPh₃)₂ and Au₄Rh₂(CCAr^F)₈(PPh₃)₂ (Ar = 3,5-(CF₃)₂C₆H₃, abbreviated as Ag₄Rh₂ and Au₄Rh₂, respectively) co-protected by alkynyl and phosphine ligands. Both clusters have identical octahedral metal core configurations and can be termed superatoms with two free electrons. However, they possess different optical features, manifested by totally different absorbance peaks, and drastically different emission peaks, and also, Ag₄Rh₂ has a much higher fluorescence quantum yield (18.43%) than Au₄Rh₂ (4.98%). Moreover, Au₄Rh₂ exhibited markedly superior catalytic performance in the electrochemical hydrogen evolution reaction (HER), manifested by a much lower overpotential at 10 mA cm⁻² and better stability. Density functional theory (DFT) calculations revealed that the free energy change of Au₄Rh₂ for the adsorption of two H* (0.64 eV) is lower than that of Ag₄Rh₂ for the adsorption of one H* (−0.90 eV) after stripping a single alkynyl ligand from the cluster. In contrast, Ag₄Rh₂ demonstrated much stronger catalytic capability for catalyzing 4-nitrophenol reduction. The present study provides an exquisite example to understand the structure–property relationship of atomically precise alloy nanoclusters, and emphasizes the importance of fine-tuning of the physicochemical properties and catalytic performance of the metal nanoclusters through modulating the metal core and beyond.