Chem. Sci. http://doi.org/vvb (2014)

When a particle hits a catalytic surface, diffusion can no longer be described using Brownian motion because attractive surface–particle interactions begin to play a role. This type of behaviour, which is known as near-wall hindered diffusion, can significantly increase the time the particle spends near the surface and is particularly important for electron-transfer reactions, but studying the effect remains challenging. Enno Kätelhön and Richard Compton at the University of Oxford have now developed a model that could help experimentalists account for this behaviour in their measurements.

The researchers divide the space above a surface into three zones. The layer nearest to the surface is where particles are permanently adsorbed; above that, there is a tunnelling region that defines the distance in which electron transfer can occur; and above that, there is a non-tunnelling region in which no electrochemical reactions can occur, but the particle is still considered to be in the near-wall hindered diffusion regime and is likely to return to the tunnelling region within the sampling time of a single measurement.

Each time a particle enters the tunnelling region, a tiny current is generated at the electrode. The duration of this current, however, can change significantly depending on the size of the particle and the height of the non-tunnelling region. As a consequence, most impacts are too short to be detected by experiments and only contribute to the noise. In order not to lose this valuable information, Kätelhön and Compton recommend that physicochemical parameters are tuned so that the average time spent by a nanoparticle in the tunnelling and non-tunnelling region before it returns to the bulk is on the order of the measurement sampling rate.