A Quantum Alloy: The Ligand-Protected Au_{25–<i>x</i>}Ag_<i>x</i>(SR)₁₈ Cluster

Recent synthetic advances have produced very small (sub-2 nm), ligand-protected mixed-metal clusters. Realization of such clusters allows the investigation of fundamental questions: (1) Will heteroatoms occupy specific sites within the cluster? (2) How will the inclusion of heteroatoms affect the electronic structure and chemical properties of the cluster? (3) How will these very small mixed-metal systems differ from larger, more traditional alloy materials? In this report we provide experimental and computational characterization of the ligand-protected mixed-metal Au_{25–<i>x</i>}Ag_<i>x</i>(SC₂H₄Ph)₁₈ cluster (abbreviated as Au_{25–<i>x</i>}Ag_<i>x</i>, where <i>x</i> = 0–5 Ag atoms) compared with the unsubstituted Au₂₅(SC₂H₄Ph)₁₈ cluster (abbreviated as Au₂₅). Density functional theory analysis has predicted that Ag heteroatoms will preferentially occupy sites on the surface of the cluster core. X-ray photoelectron spectroscopy revealed Au–Ag state mixing and charge redistribution within the Au_{25–<i>x</i>}Ag_<i>x</i> cluster. Optical spectroscopy and nonaqueous electrochemistry indicate that Ag heteroatoms increased the cluster lowest unoccupied molecular orbital (LUMO) energy, introduced new features in the Au_{25–<i>x</i>}Ag_<i>x</i> absorbance spectrum, and rendered some optical transitions forbidden. In situ spectroelectrochemical experiments revealed charge-dependent Au_{25–<i>x</i>}Ag_<i>x</i> optical properties and oxidative photoluminescence quenching. Finally, O₂ adsorption studies have shown Au_{25–<i>x</i>}Ag_<i>x</i> clusters can participate in photomediated charge-transfer events. These results illustrate that traditional alloy concepts like metal-centered state mixing and internal charge redistribution also occur in very small mixed-metal clusters. However, resolution of specific heteroatom locations and their impact on the cluster’s quantized electronic structure will require a combination of computational modeling, optical spectroscopy, and nonaqueous electrochemistry

Experimental and Computational Investigation of Au₂₅ Clusters and CO₂: A Unique Interaction and Enhanced Electrocatalytic Activity

Atomically precise, inherently charged Au₂₅ clusters are an exciting prospect for promoting catalytically challenging reactions, and we have studied the interaction between CO₂ and Au₂₅. Experimental results indicate a reversible Au₂₅–CO₂ interaction that produced spectroscopic and electrochemical changes similar to those seen with cluster oxidation. Density functional theory (DFT) modeling indicates these changes stem from a CO₂-induced redistribution of charge within the cluster. Identification of this spontaneous coupling led to the application of Au₂₅ as a catalyst for the electrochemical reduction of CO₂ in aqueous media. Au₂₅ promoted the CO₂ → CO reaction within 90 mV of the formal potential (thermodynamic limit), representing an approximate 200–300 mV improvement over larger Au nanoparticles and bulk Au. Peak CO₂ conversion occurred at −1 V (vs RHE) with approximately 100% efficiency and a rate 7–700 times higher than that for larger Au catalysts and 10–100 times higher than those for current state-of-the-art processes

Photomediated Oxidation of Atomically Precise Au₂₅(SC₂H₄Ph)₁₈^– Nanoclusters

The anionic charge of atomically precise Au₂₅(SC₂H₄Ph)₁₈^– nanoclusters (abbreviated as Au₂₅^–) is thought to facilitate the adsorption and activation of molecular species. We used optical spectroscopy, nonaqueous electrochemistry, and density functional theory to study the interaction between Au₂₅^– and O₂. Surprisingly, the oxidation of Au₂₅^– by O₂ was not a spontaneous process. Rather, Au₂₅^––O₂ charge transfer was found to be a photomediated process dependent on the relative energies of the Au₂₅^– LUMO and the O₂ electron-accepting level. Photomediated charge transfer was not restricted to one particular electron accepting molecule or solvent system, and this phenomenon likely extends to other Au₂₅^––adsorbate systems with appropriate electron donor–acceptor energy levels. These findings underscore the significant and sometimes overlooked way that photophysical processes can influence the chemistry of ligand-protected clusters. In a broader sense, the identification of photochemical pathways may help develop new cluster-adsorbate models and expand the range of catalytic reactions available to these materials

Electrocatalytic Oxygen Evolution with an Atomically Precise Nickel Catalyst

The electrochemical oxygen evolution reaction (OER) is an important anodic process in water splitting and CO₂ reduction applications. Precious metals including Ir, Ru. and Pt are traditional OER catalysts, but recent emphasis has been placed on finding less expensive, earth-abundant materials with high OER activity. Ni-based materials are promising next-generation OER catalysts because they show high reaction rates and good long-term stability. Unfortunately, most catalyst samples contain heterogeneous particle sizes and surface structures that produce a range of reaction rates and rate-determining steps. Here we use a combination of experimental and computational techniques to study the OER at a supported organometallic nickel complex with a precisely known crystal structure. The Ni₆(PET)₁₂ (PET = phenylethyl thiol) complex out performed bulk NiO and Pt and showed OER activity comparable to Ir. Density functional theory (DFT) analysis of electrochemical OER at a realistic Ni₆(SCH₃)₁₂ model determined the Gibbs free energy change (Δ<i>G</i>) associated with each mechanistic step. This allowed computational prediction of potential determining steps and OER onset potentials that were in excellent agreement with experimentally determined values. Moreover, DFT found that small changes in adsorbate binding configuration can shift the potential determining step within the OER mechanism and drastically change onset potentials. Our work shows that atomically precise nanocatalysts like Ni₆(PET)₁₂ facilitate joint experimental and computational studies because experimentalists and theorists can study nearly identical systems. These types of efforts can identify atomic-level structure–property relationships that would be difficult to obtain with traditional heterogeneous catalyst samples