The nucleation rate of single O₂ nanobubbles on Pt nanoelectrodes

Nanobubble nucleation is a problem that affects efficiency in electrocatalytic reactions, since those bubbles can block the surface of the catalytic sites. We focus on the nucleation rate of O2 nanobubbles resulting from electrooxidation of H2O2 at Pt disk nanoelectrodes. By applying a critical peak current, inbp, bubbles form almost instantaneously. However, for lower currents, bubble nucleation is a stochastic process in which the nucleation time, tind, dramatically decreases as the applied current approaches inbp, a consequence of the local supersaturation level, ζ, increasing at high currents. This fact provides a means to measure the stochastic tind. We study in detail the different conditions in which nanobubbles appear, concluding that the electrode surface needs to be pre-conditioned for achieving reproducible results. We also measure the activation energy for bubble nucleation, Ea and we determine the footprint diameter L=8-15 nm, the contact angle to the electrode surface θ=135-155o and the number of O2 molecules contained in the nucleus (50 to 900 molecules), assuming that the nucleus has a spherical cap shape.