Redox-Chemical Electron Storage on Sub-5 nm Gold Nanoparticles Compromised by Coulomb Repulsion

Noble metal nanoparticles are known to act as redox catalysts or cocatalysts in important reactions including water splitting, nitrogen fixation, or CO2 reduction. In case their diameter falls below about 10 nm, the literature is split between, on the one hand, reports of ever-increasing catalytic activity with decreasing diameter and, on the other hand, reports about an optimal diameter of about 3–6 nm, below which the catalytic activity decreases rapidly. In our study, we use the model reduction of ferricyanide to ferrocyanide catalyzed by gold nanoparticles to demonstrate that, determined by the oxidative counter-reaction, different dependencies of the catalytic activity on diminishing nanoparticle diameter exist. If no intermediate charge storage takes place, the catalytic activity increases monotonically roughly with the inverse of the diameter, down to 2 nm. If, however, a strong reductant is present, the nanoparticles act as an intermediate storage of electrons, so-called nanocathodes, which is compromised by Coulomb repulsion. This leads to an optimal diameter of 4–5 nm. It is the strength of this study that exactly the same batches of gold nanoparticles and the same model reduction are used so that the parameter space for the different size dependence of the catalytic activity is limited to the differences in oxidative counter-reactions.