Electrode Surface Engineering Using the Langmuir-Schaefer Method: Benefits of Controlled Distribution of Catalytic...

Investigating and improving highly efficient nanoelectrocatalysts requires precise control over the arrangement and organization of active sites on the electrode surface and at the reactant interface. Conventional methods for modifying electrodes with gold nanoparticles or nanoclusters typically involve drop-casting and drying a nanoparticle dispersion to form a thin film. In our study, the approach is different: the Langmuir-Schaefer (LS) technique is used to spread [Au₂₅(SC₄)₁₈]⁰ clusters (AuNCs) at the air-water interface and transfer the assembly onto highly oriented pyrolytic graphite (HOPG) surfaces under different surface pressures. We investigated how changes in surface pressure affect both the amount of AuNCs on the electrode and their electrochemical CO₂RR activity. We present the previously unrecognized interfacial mechanism that governs the behavior of atomically precise Au nanoclusters: the transition from a condensed 2D monolayer to an irreversible 3D aggregated phase when transfer is carried out under surface pressures above 30 mN/m. This directly determines the number of catalytically accessible gold atoms and thus the catalytic activity per nanocluster. The AuNCs distribution under different surface pressures was monitored using atomic force microscopy and field-emission scanning electron microscopy. We demonstrate, with rigorous triangulation of Langmuir isotherm-derived densities and oxidation-based quantification of accessible Au atoms, that only the surface-exposed fraction contributes to catalysis and that high loading can reduce the per-cluster activity. Application of the Langmuir-Schaefer method revealed that the catalytic efficiency per AuNC is maximized when AuNCs are well dispersed and do not exhibit substantial aggregation or multilayer formation, resulting in improved electrode performance and higher catalytic activity at the electrode surface.