Fig. 2.

(A) SEM image of the glassy carbon surface with well-dispersed 50-nm-radius SiO₂ nanoparticles. Circles represent locations of droplets left from SECCM measurement, while colors stand for SiO₂ nanoparticle number within the droplet, with yellow for one, green for two, and red for three. (B) SECCM mapping of bubble nucleation current, $i_{\text{b}}^{\text{p}}$, across the same surface as in A. (C) Zoomed-in SEM images of partial locations with nanoparticles and (D) corresponding voltammograms for single–H₂ bubble formation. The Insets in C are the corresponding magnified SEM images of individual SiO₂ nanoparticles.The peak currents at locations I, II, III, and V are –124.4, –103.3, –107.1, and –105.1 nA, respectively. In the experiment, a pipette with a 1.5-μm-radius tip opening and 2.0 M H₂SO₄ solution was used. Voltammograms were recorded at 0.5 V/s with a step size of 7 μm.