The Loading-Dependent Evolution of Ru Cocatalysts on CdS Nanorods for Photocatalytic Hydrogen Evolution: Unveiling the Critical Role of Neighboring Single-Atom Site Engineering

Abstract Modulating cocatalyst loading and its concomitant structural evolution represents a key strategy for optimizing the photocatalytic hydrogen evolution reaction (HER). However, a quantitative investigation into the loading-dependent evolution of active sites, its kinetic impacts on the H2 evolution rate (rH2), and the inherent trade-off between atomic utilization and cocatalyst stability remains elusive. Herein, a library of over twenty Ru/CdS nanorod samples with gradient loadings (0.009–2.88%, wt %) was constructed via a one-pot solvothermal method. By introducing the relative enhancement coefficient (ke) and marginal promotion rate (ηm), the influence of Ru site evolution on HER performance was investigated from both macroscopic cumulative and microscopic instantaneous perspectives. A distinct three-stage nonlinear evolutionary profile was revealed for the HER with increasing Ru content. Specifically, according to the ηm results, at the initial low-loading stage (<0.03%), positive cooperativity occurs among neighboring Ru single atoms, resulting in a quadratic increase in HER activity. Despite maximizing Ru utilization, these single-atom sites undergo progressive structural reconstruction during continuous operation, leading to a decay in activity. As the loading amount enters the coalescence transition regime [0.03–0.20%], reduced Ru atom-utilization efficiency and the parasitic light-shielding effect drag ηm into a sharp exponential decay, thereby attenuating the acceleration of rH2. Further increasing the loading amount [0.25, 2.88%] drives Ru site transition to nanoparticles, and a saturation plateau is subsequently approached due to an accentuated light-blocking effect and the conversion of active sites into carrier recombination centers. Although nanoparticle sites compromise Ru atom efficiency, they deliver enhanced durability, revealing a trade-off between atomic efficiency and catalytic lifespan. These findings demonstrate that, rather than merely maximizing rH2 by tuning the cocatalyst amount, more effort should be devoted to enhancing the critical loading amount of neighboring robust single-atom Ru cocatalysts. Overall, this work develops an alternative paradigm for cocatalyst evaluation and underscores the urgent imperative to develop robust neighboring single-atom catalysts for highly efficient solar-to-hydrogen conversion.