Electrochemical Nucleation of Stable N2 Nanobubbles at Pt Nanoelectrodes

Exploring the nucleation of gas bubbles at interfaces is of fundamental interest. Herein, we report the nucleation of individual N-2 nanobubbles at Pt nanodisk electrodes (6-90 nm) via the irreversible electrooxidation of hydrazine (N2H4 -> N-2 + 4H(+) + 4e(-)). The nucleation and growth of a stable N, nanobubble at the Pt electrode is indicated by a sudden drop in voltammetric current, a consequence of restricted mass transport of N2H4 to the electrode surface following the liquid-to-gas phase transition. The critical surface concentration of dissolved N-2 required for nanobubble nucleation, C-N2,critical(5), obtained from the faradaic current at the moment just prior to bubble formation, is measured to be similar to 0.11 M and is independent of the electrode radius and the bulk N2H4 concentration. Our results suggest that the size of stable gas bubble nuclei depends only on the local concentration of N-2 near the electrode surface, consistent with previously reported studies of the electrogeneration of H-2 nanobubbles. C-N2,critical(5) is similar to 160 times larger than the N-2 saturation concentration at room temperature and atmospheric pressure. The residual current for N2H4 oxidation after formation of a stable N-2 nanobubble at the electrode surface is proportional to the N2H4 concentration as well as the nanoelectrode radius, indicating that the dynamic equilibrium required for the existence of a stable N-2 nanobubble is determined by N2H4 electrooxidation at the three phase contact line.