Adsorbate effects on the surface stress-charge response of platinum electrodes

The length change in response to changes in the surface stress during scans of the electrode potential was measured for nanoporous platinum samples immersed in aqueous NaF, an electrolyte with weak ion adsorption. The surface stress-charge response may be characterized separately for four different processes, selected by the potential range and by the surface pretreatment: hydrogen adsorption/desorption, oxygen adsorption/desorption (and/or surface oxidation/reduction), and nominally capacitive charging of the Pt surface in two different states, clean and oxide-covered. While each process exhibits a roughly linear response, the magnitude and even the sign of the slope, which determines the surface stress-charge coefficient, ζ, differ. We suggest that the sign of ζ depends on the penetration depth of the excess charge: for strong screening the electronic charge is located outside of the surface, and ζ is negative as found previously for clean metal surfaces. For weaker screening, the wider space charge layer implies a trend for the excess charge to fill bulk-like unoccupied states. These states are here antibonding, giving positive-valued ζ.