Electrocatalytic hydrogen evolution reaction (HER) holds promise in the renewable clean energy scheme. Crystalline Au and Ag are, however, poor in catalyzing HER, and the ligands on colloidal nanoparticles are generally another disadvantage. Herein, we report a thiolate (SR)-protected Au 36 Ag 2 (SR) 18 nanocluster with low coverage of ligands and a core composed of three icosahedral (I h ) units for catalyzing HER efficiently. This trimeric structure, together with the monomeric I h Au 25 (SR) 18− and dimeric I h Au 38 (SR) 24 , constitutes a unique series, providing an opportunity for revealing the correlation between the catalytic properties and the catalyst's structure. The Au 36 Ag 2 (SR) 18 surprisingly exhibits high catalytic activity at lower overpotentials for HER due to its low ligand-to-metal ratio, low-coordinated Au atoms and unfilled superatomic orbitals. The current density of Au 36 Ag 2 (SR) 18 at −0.3 V vs RHE is 3.8 and 5.1 times that of Au 25 (SR) 18 − and Au 38 (SR) 24 , respectively. Density functional theory (DFT) calculations reveal lower hydrogen binding energy and higher electron affinity of Au 36 Ag 2 (SR) 18 for an energetically feasible HER pathway. Our findings provide a new strategy for constructing highly active catalysts from inert metals by pursuing atomically precise nanoclusters and controlling their geometrical and electronic structures.

Temperature-dependent optical properties are of paramount importance for fundamentally understanding the electron−phonon interactions and phonon modes in atomically precise nanocluster materials. In this work, low-temperature optical absorption spectra of the icosahedral [Au 25 (SR) 18 ] − nanocluster are measured from room temperature down to liquid helium temperature by adopting a thin-film-based technique. The thin-film measurement is further compared with results from the previous solution-based method. Interestingly, the previously missing core phonon is revealed by a quantitative analysis of the film data through peak deconvolution and fitting of the temperature trend with a theoretical model. The two lowest-energy absorption peaks (at 1.6 and 1.8 eV) of Au 25 are determined to couple with the staple-shell phonon (average energy ∼350 cm −1 ) in the solution state, but in the solid state these electronic transitions couple with the core phonon (average energy ∼180 cm −1 ). The suppression of the staple-shell phonon in the solid state is attributed to the intracluster and cluster−matrix interactions.

This work presents the synthesis and intriguing photoluminescence of the Au 42 (PET) 32 (PET = 2-phenylethanethiolate) nanocluster (NC). The Au 42 (PET) 32 NC exhibits dual emission at 875 and 1040 nm, which are revealed to be fluorescence and phosphorescence, respectively. The emission quantum yield (QY) of Au 42 (PET) 32 in dichloromethane is 11.9% at room temperature in air, which is quite rare for thiolate-protected Au NCs. When Au 42 (PET) 32 NCs are embedded in polystyrene films (solid state), the fluorescence was dramatically suppressed while the phosphorescence was significantly enhanced. This divergent behavior is explained by dipolar interaction-induced enhancement of intersystem crossing from singlet to triplet excited state.

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Understanding the electron–phonon interaction in Au nanoclusters (NCs) is essential for enhancing and tuning their photoluminescence (PL) properties. Among all the methods, ligand engineering is the most straightforward and facile one to design Au NCs with the desired PL properties. However, a systematic understanding of the ligand effects toward electron–phonon interactions in Au NCs is still missing. Herein, we synthesized four Au25(SR)18 – NCs protected by different −SR ligands and carefully examined their temperature-dependent band-gap renormalization behavior. Data analysis by a Bose–Einstein two-oscillator model revealed a suppression of high-frequency optical phonons in aromatic-ligand-protected Au25 NCs. Meanwhile, a low-frequency breathing mode and a quadrupolar mode are attributed as the main contributors to the phonon-assisted nonradiative relaxation pathway in aromatic-ligand-protected Au25 NCs, which is in contrast with non-aromatic-ligand-protected Au25 NCs, in which tangential and radial modes play the key roles. The PL measurements of the four Au25 NCs showed that the suppression of optical phonons led to higher quantum yields in aromatic-ligand-protected Au25 NCs. Cryogenic PL measurements provide insights into the nonradiative energy relaxation, which should be further investigated for a full understanding of the PL mechanism in Au25 NCs.