Potential of zero charge of platinum stepped surfaces:: a combined approach of CO charge displacement and N2O reduction

The relationship between the potential of zero total charge (pztc) and step density for a series of n (111) x (111) stepped platinum single-crystals in perchloric acid using CO displacement is presented. Since CO displacement gives rise to a 'global average' of the local the pztc of step and terrace surface contributions to the total charge, we have tried to use N2O reduction as a local probe of the pztc in order to generate analogous data to that found with CO displacement, but derived from local values of the pztc. This procedure is outlined in detail. Agreement between CO and N2O determinations of the pztc is found to be good for highly stepped surfaces (average terrace width < 7) but increasingly poor for surfaces containing wide (111) terraces. This behaviour has been rationalised within a framework emphasising the role played by electrosorbed water in blocking N2O adsorption sites, which would not necessarily be detected via CO displacement. It is proposed that for surfaces that exhibit overlap between the hydrogen and anion adsorption potential ranges, N2O reduction is a local probe of the pztc. For surfaces in which hydrogen and anion adsorption are well separated (Pt(111) in HClO4), the N2O reduction maximum is displaced positively with respect to the pztc determined using CO charge displacement. As the step density is increased it is found that the local pztc of step sites shifts to more positive potentials whereas terrace sites exhibit a shift to more negative potentials. It is suggested that this phenomenon is a reflection of charge redistribution between the step and terrace sites as the step density increases. (C) 2002 Elsevier Science B.V. All rights reserved.