Electrochemical Generation of Individual Nanobubbles Comprising H₂, D₂, and HD

The electrochemical reduction of deuterons (2D⁺ + 2e^- → D₂) at Pt nanodisk electrodes (radius = 15-100 nm) in D₂O solutions containing deuterium chloride (DCl) results in the formation of a single gas nanobubble at the electrode surface. Analogous to that previously observed for the electrochemical generation of H₂ nanobubbles, the nucleation and growth of a stable D₂ nanobubble is characterized in voltammetric experiments by a highly reproducible and well-resolved sudden drop in the faradaic current, a consequence of restricted mass transport of D⁺ to the electrode surface following the liquid-to-gas phase transition. D₂ nanobubbles are stable under potential control due to a dynamic equilibrium existing between D₂ gas dissolution and electrochemical generation of D₂ at the circumference of the Pt nanodisk electrode. Remarkably, within the error of the experimental measurement (<6%), the electrochemical current required to nucleate a D₂ gas phase in a D₂O solution is identical to that for the H₂ gas phase in a H₂O solutions, indicating that the concentration required for nucleating a D₂ nanobubble in D₂O (0.29 M) is ∼1.25 times larger than that for a H₂ nanobubble (0.23 M), while the supersaturation is ∼300 in each case. We further demonstrate that individual nanobubbles can be electrogenerated in mixed D₂O/H₂O solutions containing both D⁺ and H⁺ at respective individual concentrations well below those required to nucleate a gas phase containing either pure D₂ or H₂. This latter finding indicates that the resulting nanobubbles comprise a mixture of D₂, H₂, and HD molecules with the chemical composition of a nanobubble determined by the concentrations and diffusivities of D⁺ and H⁺ in the mixed D₂O/H₂O solutions.