Electrogeneration of Single Nanobubbles at Sub-50-nm-Radius Platinum Nanodisk Electrodes

2013-09-03T00:00:00Z (GMT) by Long Luo Henry S. White
The electrochemical generation of individual H<sub>2</sub> nanobubbles at Pt nanodisk electrodes immersed in a 0.5 M H<sub>2</sub>SO<sub>4</sub> solution is reported. A sudden drop in current associated with the transport-limited reduction of protons is observed in the <i>i</i>–<i>V</i> response at Pt nanodisk electrodes with radii of less than 50 nm. This decrease in current (∼95% blockage) corresponds to the formation of a single H<sub>2</sub> nanobubble attached to the nanoelectrode that blocks proton transport to the surface. The current at which nanobubble formation occurs, <i>i</i><sub>nb</sub><sup>p</sup>, is independent of scan rate and H<sub>2</sub>SO<sub>4</sub> concentration (for [H<sub>2</sub>SO<sub>4</sub>] > 0.1 M), indicating a critical concentration profile of electrogenerated H<sub>2</sub> required to nucleate a nanobubble. Finite element simulation based on Fick’s first law, combined with the Young–Laplace equation and Henry’s law, indicates that the concentration of H<sub>2</sub> near the nanoelectrode surface at <i>i</i><sub>nb</sub><sup>p</sup> exceeds the saturation concentration necessary to generate a nanobubble with a size comparable to the electrode size. The rapid dissolution of the nanobubble due to the high inner Laplace pressure is precisely balanced by the electrogeneration of H<sub>2</sub> at the partially exposed Pt surface, resulting in a dynamically stabilized nanobubble. Preliminary measurements of the <i>i</i>–<i>t</i> response during nanobubble formation indicate a two-step nucleation and growth mechanism with time scales on the order of 100 μs (or less) and ∼1 ms, respectively.