Scanning tunneling microscopy of electrochemically activated platinum surfaces. A direct ex-situ determination of the electrode nanotopography

Abstract

A direct scanning tunneling microscopy ex-situ determination on the nanometer scale of the topography of electrochemically highly activated platinum electrodes is presented. A correlation between catalytic activity and surface microtopography becomes evident. This result gives support to a structural model for the activated electrode surface. In the model, a volume with a pebble-like structure allows electrocatalytic processes to occur practically free of diffusion relaxation contributions under usual voltammetric conditions. Catalytic activity and surface roughness are of the outmost importance in heterogeneous catalysis, including electrocatalysis. The term roughness usually implies the existence of both macropores (macroroughness), which to a great extent are responsible for additional diffusional relaxation,' and micropores (microroughness), which concern the effective catalytic area.2 Despite the close relationship between microroughness and catalytic activity, many real systems involve complex macro-and micropore structures which make the direct determination of microroughness a difficult task. A new approach to overcome this drawback is forseen by using metal surfaces which offer large catalytic activity, negligible micropore diffusional relaxation, and distribution of active sites very close to that of the starting materiaL3 This is the case, among others, with platinum electrodes in acid solutions, which have been subjected to a relatively fast square potential cycling, over a potential range such that a hydrous metal oxide multilayer is formed and immediately afterwards is electroreduced to yield a substantially increased active area. The new surface

Changes in real surface area, crystallographic orientation and morphology of platinum electrodes caused by periodic potential treatments: phenomenological approach

“…13c,d) . Surprisingly, this platinum structure is similar to that described earlier for the surfaces resulting from the electroreduction of a platinum oxide layer formed after applying to pc platinum a symmetric RSWPS at 1 kHz but at E" > 2.1 V [5,12,28] .…”

“…This situation also prevails for asymmetric RSWPS involving a short T, value which contributes to maintain continuously a fresh platinum surface and to avoid local acidification and water depletion at the metal/solution interface . The structure of the platinum electroreduced layer, as revealed through SEM micrographs, consists of sphere-like sticking clusters like that found recently for electroreduced platinum and gold on the basis of scanning electron tunneling microscopy imaging [12,41] .…”

“…It is well known that the surface of fee metal electrodes immersed in aqueous solutions can be modified substantially in different ways through the application of periodic potentials of various waveforms, that is, by increasing the real surface area [1][2][3][4][5][6][7], by changing the relative proportion of crystallographic faces through a process denoted as electrochemical faceting [7][8][9][10][11] and by producing some particular surface morphologies [4,10,12] . The prevailing type of change depends principally on the characteristics of the periodic potential, and secondarily on the waveform and the electrolyte composition [7,11] .…”

Scanning tunneling microscopy characterization of electrode materials in electrochemistry

“…It has long been recognized that the electrochemical oxidation reduction cycle can modify the structure of the Pt electrode surface [51]. Vazquez et al [52] first used ex-situ STM to investigate polycrystalline Pt electrode surface roughening in air by electrochemical activation. The nonhomogeneous topography of treated platinum is observed by the following three main morphologies: (1) A relatively high-density structure, which appears either as high or low domes, localized in certain regions which are randomly distributed over the surface.…”

The Electrochemical Development of Pt(111) Stepped Surfaces and Its Influence on Methanol Electrooxidation

“…It is well known that the application of fast potential periodic programs on platinum, rhodium, iridium, gold, or palladium produces permanent morphological changes with defined orientations, which depend on the potential limits and frequency of the routine by a microfacetting process [16][17][18]. The electrochemical responses of these stepped surfaces are similar to those of stepped crystalline surfaces [19,20].…”