Boosting the Electroreduction of CO2 to CO by Ligand Engineering of Gold Nanoclusters.

The electrochemical CO2 reduction reaction (CO2RR) has been widely studied as a promising means to convert anthropogenic CO2 into valuable chemicals and fuels. In this process, the alkali metal ions present in the electrolyte are known to significantly influence the CO2RR activity and selectivity. In this study, we report a strategy for preparing efficient electrocatalysts by introducing a cation-relaying ligand, namely 6-mercaptohexanoic acid (MHA), into atom-precise Au25 nanoclusters (NCs). The CO2RR activity of the synthesized Au25(MHA)18 NCs was compared with that of Au25(HT)18 NCs (HT = 1-hexanethiolate). While both NCs selectively produced CO over H2, the CO2-to-CO conversion activity of the Au25(MHA)18 NCs was significantly higher than that of the Au25(HT)18 NCs when the catholyte pH was higher than the pKa of MHA, demonstrating the cation-relaying effect of the anionic terminal group. Mechanistic investigations into the CO2RR occurring on the Au25 NCs in the presence of different catholyte cations and concentrations revealed that the CO2-to-CO conversion activities of these Au25 NCs increased in the order Li+ < Na+ < K+ < Cs+, and are gated by the cation-coupled electron transfer step. These results were confirmed by the Nernstian shifts of the polarization curves at different cation concentrations.

Synthesis of RhH-doped Au-Ag alloy nanoclusters and dopant evolution.

Doping atomically precise metal nanoclusters (NCs) with heterometals is a powerful method for tuning the physicochemical properties of the original NCs at the atomic level. While the heterometals incorporated into metal NCs are limited to group 10-12 metals with closed d-shells, the doping of open d-shell metals remains largely unexplored. Herein, we report the synthesis of Rh-doped Au-Ag alloy NCs by a metal-exchange reaction of [RhHAg24(SPhMe2)18]2- NCs with an Au-thiolate complex. Combined experimental and theoretical structural studies revealed that the synthesized product is a dianionic [RhHAuxAg24-x(SPhMe2)18]2- NC (x = 8-12), consisting of RhH dopant, Au-rich kernel, and Ag-thiolate staple motifs, with the superatomic 8-electron configuration (1S21P6). Under aerobic conditions, the synthesized NCs underwent kernel evolution to generate a 6-electron [RhAuxAg24-x(SPhMe2)18]1- NC (1S21P4), which was initiated by the desorption of hydride from the kernel. Structural analysis of the [RhHAuxAg24-x(SPhMe2)18]2- NC suggests that the kernel evolution is induced by the change in chemical bonds surrounding the hydride in the Au-rich kernel.

ClAg14 (C≡Ct Bu)12 Nanoclusters as Efficient and Selective Electrocatalysts Toward Industrially Relevant CO2 Conversion.

Atomically precise metal nanoclusters (NCs) have emerged as a promising frontier in the field of electrochemical CO2 reduction reactions (CO2 RR) because of their distinctive catalytic properties. Although numerous metal NCs are developed for CO2 RR, their use in practical applications has suffered from their low-yield synthesis and insufficient catalytic activity. In this study, the large-scale synthesis and electrocatalytic performance of ClAg14 (C≡Ct Bu)12 + NCs, which exhibit remarkable efficiency in catalyzing CO2 -to-CO electroreduction with a CO selectivity of over 99% are reported. The underlying mechanisms behind this extraordinary CO2 RR activity of ClAg14 (C≡Ct Bu)12 + NCs are investigated by a combination of electrokinetic and theoretical studies. These analyses reveal that different active sites, generated through electrochemical activation, have unique adsorption properties for the reaction intermediates, leading to enhanced CO2 RR and suppressed hydrogen production. Furthermore, industrially relevant CO2 -to-CO electroreduction using ClAg14 (C≡Ct Bu)12 + NCs in a zero-gap CO2 electrolyzer, achieving high energy efficiency of 51% and catalyst activity of over 1400 A g-1 at a current density of 400 mA cm-2 is demonstrated.

Superatom-in-Superatom Nanoclusters: Synthesis, Structure, and Photoluminescence.

We report a new strategy in which a thiolate-protected Ag25 nanocluster can be doped with open d-shell group 8 (Ru, Os) and 9 (Ir) metals by forming metal hydride (RuH2 , OsH2 , IrH) superatoms with a closed d-shell. Structural analyses using various experimental and theoretical methods revealed that the Ag25 nanoclusters were co-doped with the open d-shell metal and hydride species to produce superatom-in-superatom nanoclusters, establishing a novel superatom doping phenomenon for open d-shell metals. The synthesized superatom-in-superatom nanoclusters exhibited dopant-dependent photoluminescence (PL) properties. Comparative PL lifetime studies of the Ag25 nanoclusters doped with 8-10 group metals revealed that both radiative and nonradiative processes were significantly dependent on the dopant. The former is strongly correlated with the electron affinity of the metal dopant, whereas the latter is governed predominantly by the kernel structure changed upon the doping of the metal hydride(s).

Transplanting Gold Active Sites into Non-Precious-Metal Nanoclusters for Efficient CO2-to-CO Electroreduction.

Electrocatalytic CO2 reduction reaction (CO2RR) is greatly facilitated by Au surfaces. However, large fractions of underlying Au atoms are generally unused during the catalytic reaction, which limits mass activity. Herein, we report a strategy for preparing efficient electrocatalysts with high mass activities by the atomic-level transplantation of Au active sites into a Ni4 nanocluster (NC). While the Ni4 NC exclusively produces H2, the Au-transplanted NC selectively produces CO over H2. The origin of the contrasting selectivity observed for this NC is investigated by combining operando and theoretical studies, which reveal that while the Ni sites are almost completely blocked by the CO intermediate in both NCs, the Au sites act as active sites for CO2-to-CO electroreduction. The Au-transplanted NC exhibits a remarkable turnover frequency and mass activity for CO production (206 molCO/molNC/s and 25,228 A/gAu, respectively, at an overpotential of 0.32 V) and high durability toward the CO2RR over 25 h.

Superatom-in-Superatom [RhH@Ag24 (SPhMe2 )18 ]2- Nanocluster.

Heterometal doping is a powerful method for tuning the physicochemical properties of metal nanoclusters. While the heterometals doped into such nanoclusters predominantly include transition metals with closed d-shells, the doping of open d-shell metals remains largely unexplored. Herein, we report the first synthesis of a [RhHAg24 (SPhMe2 )18 ]2- nanocluster, in which a Rh atom with open d-shells ([Kr]4d8 5s1 ) is incorporated into the Ag24 framework by forming a RhH superatom with closed d-shells ([Kr]4d10 ). Combined experimental and theoretical investigations showed that the Ag24 framework was co-doped with Rh and hydride and that the RhH dopant was a superatomic construct of a Pd atom. Additional studies demonstrated that the [RhHAg24 (SPhMe2 )18 ]2- nanocluster was isoelectronic to the [PdAg24 (SPhMe2 )18 ]2- nanocluster with the superatomic 8-electron configuration (1S2 1P6 ). This study demonstrated for the first time that a superatom could be incorporated into a cluster superatom to generate a stable superatom-in-superatom nanocluster.

Synthesis and Photophysical Properties of Light-Harvesting Gold Nanoclusters Fully Functionalized with Antenna Chromophores.

The development of efficient light-harvesting systems is important to understand the key aspects of solar-energy conversion processes and to utilize them in various photonic applications. Here, atomically well-defined gold nanoclusters are reported as a new platform to fabricate artificial light-harvesting systems. An efficient amide coupling method is developed to synthesize water-soluble Au22 clusters fully protected with pyrene chromophores by taking advantage of their facile phase-transfer reaction. The synthesized Au22 clusters with densely packed 18 pyrene chromophores (Au22 -PyB18 ) exhibit triple-emission in blue, green, and red wavelength regions arising respectively from pyrene monomer, pyrene excimer, and Au22 emission, producing bright white light emission together. The photoluminescence of Au22 is enhanced by more than tenfold, demonstrating that pyrenes at the periphery efficiently channel the absorbed energy to the luminescent Au22 at the center. A combination of femtosecond transient absorption and anisotropy measurements of Au22 -PyB18 explicitly reveals three main decay components of 220 fs, 3.5 ps, and 160 ps that can be assigned to energy migration between pyrenes and energy transfer processes from pyrene monomer and excimer to the central Au22 , respectively.

Unique Energy Transfer in Fluorescein-Conjugated Au22 Nanoclusters Leading to 160-Fold pH-Contrasting Photoluminescence.

Accurate measurements of intracellular pH are of crucial importance in understanding the cellular activities and in the development of intracellular drug delivery systems. Here we report a highly sensitive pH probe based on a fluorescein-conjugated Au22 nanocluster. Steady-state photoluminescence (PL) measurements have shown that, when conjugated to Au22, fluorescein exhibits more than 160-fold pH-contrasting PL in the pH range of 4.3-7.8. Transient absorption measurements show that there are two competing ultrafast processes in the fluorescein-conjugated Au22 nanocluster: the intracore-state relaxation and the energy transfer from the nonthermalized states of Au22 to fluorescein. The latter becomes predominant at a higher pH, leading to dramatic PL enhancement of fluorescein. In addition to the intrinsically low toxicity, fluorescein-conjugated Au22 nanoclusters exhibit high pH sensitivity, wide dynamic range, and excellent photostability, providing a powerful tool for the study of intracellular processes.