New advances in ohmic microscopy

Abstract

A series of electrochemical measurements involving metal disks, ring-disks and facetted single crystals in aqueous 0.1 M H₂SO₄ have been performed to monitor differences in the electrostatic potential in the electrolyte, Δϕ_sol, induced by the passage of current, and thus assess the prospects of ohmic microscopy as an in situ imaging tool of electrodes in solution. Excellent quantitative agreement was found between the experimental values of Δϕ_sol and those predicted by theory for current pulses several milliamperes in magnitude and tens of microseconds in duration, applied to a Pt disk electrode embedded in a coplanar insulating surface, assuming a strict primary current distribution. Cyclic voltammetry measurements involving a gapless Pt–Ir ring|Au disk electrode yielded Δϕ_sol versus potential, E, curves, consistent with contributions derived from each of the two electrodes assuming a uniform current distribution. Also explored were extensions of ohmic microscopy to the study of facetted Pt single crystals using microreference electrodes housed in a double barrel capillary. Data collected in voltammetric experiments in which the tip of the capillary was placed directly above and at very close distance from one of the (111) facets recorded with the entire single crystal immersed in the electrolyte, yielded Δϕ_sol vs. E curves displaying pronounced features believed to be characteristic of that surface. Possible strategies toward improving the spatial resolution of this emerging technique are also discussed.