Characterization of Platinum Electrode Surfaces by Electrochemical Surface Forces Measurement

The surface forces between platinum, Pt, electrodes and those between the Pt electrode and mica in aqueous HClO₄ were measured at various potentials (E) applied to the electrodes using an electrochemical surface forces apparatus (EC-SFA). This apparatus uses the twin-path surface forces apparatus, recently developed for opaque samples. The influence of the proton adsorption on the surface interactions was studied. The Pt electrodes were prepared by the template-stripping procedure using glass templates. The electrode surfaces were smooth (RMS roughness: 0.26 nm for a 5 μm × 5 μm area) and polycrystalline based on the atomic force microscopy and cyclic voltammetry results, respectively. When the applied potential E was decreased from 0.5 to 0.2 V (vs Ag/AgCl), the electric double layer (EDL) repulsion between the Pt electrodes decreased. The absolute values of the surface potentials, |₀|, calculated using the EDL theory were 58 and 43 mV at E = 0.5 and 0.2 V, respectively. The EDL force at E = 0.2 V was the local minimum, suggesting that the potential of the zero charge (PZC) of the Pt electrode was around 0.2 V in the 1 mM HClO₄ solution. With the further decreasing potential E from 0.2 to -0.2 V, the EDL repulsion remained similar in amplitude, took another minimum, |₀| = 40 mV, at E = -0.1 V, and started to increase again at E = -0.1 V. These behaviors could be caused by proton adsorption on the Pt surface (Pt^δ-···H⁺), the electrochemical hydrogen adsorption (Pt-H), and the subsequent hydrogen evolution (H₂). The possibility for characterizing the hydrogen evolution processes on the Pt electrodes based on the surface forces measurement is discussed for the first time.