  Electrolysis of water is employed to produce surface nanobubbles on highly
orientated pyrolytic graphite (HOPG) surfaces. Hydrogen (oxygen) nanobubbles
are formed when the HOPG surface acts as negative (positive) electrode.
Coverage and volume of the nanobubbles enhance with increasing voltage. The
yield of hydrogen nanobubbles is much larger than the yield of oxygen
nanobubbles. The growth of the individual nanobubbles during the electrolysis
process is recorded in time with the help of AFM measurements and correlated
with the total current. Both the size of the individual nanobubbles and the
total current saturate after typical 1 minute; then the nanobubbles are in a
dynamic equilibrium, meaning that they do not further grow, in spite of ongoing
gas production and nonzero current. The surface area of nanobubbles shows a
good correlation with the nanobubble volume growth rate, suggesting that either
the electrolytic gas emerges directly at the nanobubbles' surface, or it
emerges at the electrode's surface and then diffuses through the nanobubbles'
surface. Moreover, the experiments reveal that the time constants of the
current and the aspect ratio of nanobubbles are the same under all conditions.
Replacement of pure water by water containing a small amount of sodium chloride
(0.01 M) allows for larger currents, but qualitatively gives the same results.
