Atomically precise thiolate-protected noble metal nanoclusters (NCs) have emerged as promising electrocatalysts due to their well-defined structures and compositions, offering a unique platform to study structure-performance relationships at the molecular and atomic levels. Their ultrasmall size (<3 nm) leads to strong quantum confinement effects, resulting in molecular-like properties including quantized charging and discrete energy levels. The electrocatalytic activity and stability of these NCs are strongly dependent on their structure, which can be tuned by modifying the metal core, the metal-thiolate interface, and the protecting ligands. While the influence of the metal core and ligands on electronic structure and catalytic performance has been studied, a detailed understanding of how ligands affect the rate-determining step (RDS) in electrocatalytic reactions at a molecular level remains limited. This gap in knowledge is largely due to the difficulty in identifying key reaction intermediates during electrocatalytic processes. This research aims to address this knowledge gap by exploring the effect of ligands with similar size but different electron-withdrawing capabilities on the OER kinetics of Au25 nanoclusters. The researchers hypothesize that varying the electron-withdrawing ability of the ligands will influence the electronegativity of Au(I) active sites, ultimately affecting the adsorption of reactants and the overall OER pathway. The successful demonstration of ligand-controlled RDS switching would significantly advance the understanding and application of atomically precise metal nanoclusters as effective electrocatalysts.

Extensive research has explored the tunability of metal NCs' catalytic properties through structural engineering. Modifying the metal core's composition and packing structure has yielded significant improvements in electrocatalytic activity, primarily due to changes in superatomic electronic configuration and resulting shifts in reduction potentials. The role of protecting ligands in influencing electrocatalytic performance has also been investigated. Ligands, as the outermost layer, directly interact with the reaction environment and determine the electronic structure of the NCs through orbital coupling between the anchoring atoms (e.g., sulfur) and metal atoms. Studies have focused on the ligand effects on HOMO-LUMO gaps and reactant adsorption, but the impact on the electrocatalysis RDS remains poorly understood. This knowledge gap stems from the challenges in identifying and characterizing key reaction intermediates in electrocatalytic reactions involving metal NCs. This research builds on previous work demonstrating the catalytic activity of Au25 nanoclusters and the influence of ligands on their electronic properties. However, this study goes further by directly addressing the role of ligands in altering the RDS of the OER, providing a more detailed understanding of the underlying mechanism.

The researchers synthesized Au25 nanoclusters capped with three different thiolate ligands: para-mercaptobenzoic acid (pMBA), 6-mercaptohexanoic acid (MHA), and homocysteine (HCys). These ligands have similar molecular weights but varying electron-withdrawing abilities (pMBA > HCys > MHA). The Au25 nanoclusters were synthesized using established methods, including a mild-reduction strategy and NaBH4 reduction, depending on the ligand. The synthesized nanoclusters were characterized using various techniques including UV-Vis spectroscopy, electrospray ionization mass spectrometry (ESI-MS), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) to confirm their structure, composition, and size. XPS was used to investigate the effect of ligands on the electronic structure of Au(I) atoms, which are considered the active sites for OER. DFT calculations were performed to analyze the electronic structures and differential charge density maps to visualize charge accumulation/deficiency around the sulfur and gold atoms. The electrocatalytic OER activities of the synthesized Au25 nanoclusters were evaluated using a standard three-electrode system in O2-saturated 1.0 M KOH electrolyte. Linear sweep voltammetry (LSV) was used to obtain OER polarization curves, and Tafel plots were constructed to determine the Tafel slopes. Electrochemical impedance spectroscopy (EIS) was employed to analyze charge-transfer resistance, and the turnover frequency (TOF) was calculated to assess the intrinsic activity. In situ Raman spectroscopy was used to identify key reaction intermediates during OER at various applied potentials. Finally, DFT calculations were performed to determine adsorption energies of OH and O on Au25 NCs with pMBA and MHA ligands to further understand the ligand effect on OER performance. The synthesis of Au nanoparticles (NPs, ~5 nm) capped by PMBA was carried out using NaBH4 reduction for comparison with the Au25 NCs.

The study revealed that the ligand significantly influences the OER activity and kinetics of Au25 nanoclusters. Au25(pMBA)18 exhibited significantly higher OER activity than Au25(MHA)18 and Au25(HCys)18, achieving a current density of 10 mA/cm² at an overpotential of 360 mV, compared to 470 mV and 540 mV, respectively. The Tafel slopes for Au25(pMBA)18 (62 mV/dec), Au25(MHA)18 (304 mV/dec), and Au25(HCys)18 (196 mV/dec) suggest different RDS for each ligand-capped cluster. The lower Tafel slope for Au25(pMBA)18 suggests that the decomposition of Au–O–OH is the slower step, while for Au25(MHA)18 and Au25(HCys)18, the RDS is deprotonation of Au–OH. EIS results indicated faster electrode kinetics for Au25(pMBA)18. The TOF of Au25(pMBA)18 was about four times higher than that of the other two nanoclusters. In situ Raman spectroscopy provided evidence for the presence of different intermediates on the surface of the nanoclusters during OER, supporting the different RDS observed for different ligands. Specifically, Au-O-OH was observed for Au25(PMBA)18 while Au-OH was observed for Au25(MHA)18 and Au25(HCys)18. DFT calculations showed that the adsorption energies of OH and O on Au(I) are lower with pMBA than with MHA, indicating easier activation of these species on Au25(pMBA)18. Long-term stability testing showed Au25(pMBA)18 to be the most stable under OER conditions. Overall, the results demonstrate a strong correlation between the electron-withdrawing ability of the ligand, the partial positive charge on Au(I) active sites, and the OER activity and kinetics.

The findings of this study clearly demonstrate that the ligand plays a crucial role in determining the OER kinetics and RDS of atomically precise Au25 nanoclusters. The stronger electron-withdrawing ability of pMBA leads to a higher partial positive charge on the Au(I) active sites, making OH− adsorption more favorable and altering the RDS. This highlights the importance of ligand engineering in optimizing the electrocatalytic performance of metal nanoclusters. The observed differences in Tafel slopes and the detection of distinct intermediates by in situ Raman spectroscopy provide strong evidence for the ligand-induced switching of the RDS. The superior activity and stability of Au25(pMBA)18 underscore the potential of rational ligand design for developing high-performance electrocatalysts. This work provides a molecular-level understanding of the interplay between ligand electronic effects, active site characteristics, and electrocatalytic behavior. The insights gained are valuable for the broader field of electrocatalysis, particularly for designing and developing advanced catalysts based on atomically precise metal nanoclusters.

This research successfully demonstrated the significant impact of ligand engineering on the oxygen evolution reaction kinetics of atomically precise Au25 nanoclusters. The use of para-mercaptobenzoic acid (pMBA) as a ligand led to a significant enhancement in OER activity compared to other ligands. The findings illustrate the ability to tune the rate-determining step of the OER through careful ligand selection and highlight the importance of considering ligand effects in the design of highly active electrocatalysts based on atomically precise metal nanoclusters. Future research could explore a wider range of ligands with different electronic and steric properties to further optimize OER performance. Investigating the scalability of the synthesis and exploring the application of these ligand-modified nanoclusters in practical water-splitting devices would also be beneficial.

While this study provides valuable insights into the ligand effects on OER kinetics, some limitations should be acknowledged. The study focused on three specific ligands, and the findings might not be generalizable to all types of ligands. The DFT calculations used simplified models of the Au25 nanoclusters to reduce computational cost. The exact nature of the interactions between the ligand and the Au25 core in the OER process was not fully elucidated. Furthermore, the study was conducted under specific experimental conditions (1.0 M KOH electrolyte), and the results might vary under different conditions.