Jane A. Schimpf

Electrochemistry of the I-on-Pd single-crystal interface : studies by UHV-EC and in situ STM

Abstract A single chemisorbed layer of zerovalent I atoms has been found to enhance, at ambient temperatures, the reactivity of Pd electrode surfaces. Three unique reactions (anodic dissolution in non-corrosive electrolyte, regeneration of well-ordered single-crystal surfaces, and “electrochemical digital etching”) have been investigated at Pd(111) and Pd(100) single-crystal electrodes and are described in this paper. Experimental measurements were based upon a combination of electrochemistry (EC), low-energy electron diffraction (LEED), and in-situ scanning tunneling microscopy (STM).

Electrochemical regeneration of clean and ordered Pd(100) surfaces by iodine adsorption-desorption: evidence from low-energy electron diffraction

In atomic-level investigations of electrocatalytic processes, the preparation and regeneration of clean and well ordered electrode surfaces under electrochemical conditions constitute major concerns [1,2]. Standard single-crystal orientation procedures performed outside an inert environment do not guarantee the existence of clean and ordered surfaces [1,3]; even for well defined surfaces prepared in ultrahigh vacuum (UHV), interfacial disorder occurs at potentials where surface oxide layers are formed 141. Towards the solution of this problem, two general schemes have been adopted [l]. One employs high temperatures (thermal annealing), and the other applies electrode potentials (electrochemical annealing). Thermal annealing has been done in UHV (following which surface analysis is performed to ascertain the interfacial structure and composition) [1,51 and at ambient-pressure conditions (provided precautions are taken to prevent surface contamination from atmospheric impurities) [6-91. Electrochemical annealing is based on the likelihood that, at an appropriate potential, disordered interfacial atoms can either be activated to diffuse to ordered sites or be dissolved to expose ordered layers. Electrochemical annealing may be electrolyte unassisted (such as the ordering of Au(ll1) electrodes by sequential voltammetric scans between the oxygen and hydrogen evolution regions [lo]) or electrolyte assisted (such as the etching of

Electrochemical digital etching in inert electrolyte: Reordering of ion-bombarded Pd(100) by chemisorbed-iodine-catalyzed dissolution

The anodic dissolution of an extensively disordered (ion bombarded) Pd(100) single-crystal surface in inert (halide-free) H2SO4 solution, catalyzed by a single adsorbed layer of iodine, has been found to generate a well-ordered Pd(100)-c(2 × 2)-I adlattice; reductive desorption of the adsorbed iodine yields the clean and ordered Pd(100) surface. Experimental measurements were based upon a combination of linear-sweep voltammetry, potential-step coulometry, low-energy electron diffraction, and Auger electron spectroscopy. This dissolution-reordering phenomenon is unique since the process occurs in the absence of bulk corrosive reagents, and only if a monolayer of chemisorbed iodine is present. This process may be viewed analogously to electrochemical digital etching except that bulk material is not needed to replenish the adsorbed iodine that activates the surface, and the dissolution is not immediately terminated upon regeneration of the ordered interface.

Adsorbate-catalyzed layer-by-layer metal dissolution in inert electrolyte: Pd(100)-c(2 × 2)-I

Abstract Studies on the corrosion of Pd in inert ( halide-free ) H 2 SO 4 solution, catalyzed by a single adsorbed layer of iodine, have been extended to a Pd(100) single-crystal electrode that contained an ordered c(2 × 2) adlattice of iodine. Experimental measurements were based upon a combination of linear-sweep voltammetry, potential-step coulometry, low-energy electron diffraction, and Auger electron spectroscopy. As was earlier noted with polycrystalline electrodes, Pd dissolution occurred only when iodine was present on the surface. More significantly, neither the coverage nor the ordered structure of the iodine adlattice was affected by the dissolution process. These observations strongly suggest that the iodine-catalyzed corrosion occurs one layer at a time.

In situ reordering by iodine adsorption-desorption of extensively disordered (ion-bombarded) Pd(100) electrode surfaces

Abstract Previous studies have shown that the single-crystallinity of a Pd(100) electrode surface mildly disordered by electrochemical oxidation can be reestablished if the spent surface is immeresed at ambient temperatures in aqueous iodide followed by reductive desorption of the interfacial iodine. In this short note, we show that an extensively disordered (Ar+-ion-bombarded) Pd(100) surface can also be reordered by the iodine-chemisorption method. Multiple surface oxidation—reduction cycles on the ion-bombarded electrode did not regenerate an ordered surface, but the nature and/or degree of disorder was altered to resemble an anodically oxidized surface. Reordering was attained only when multiple sequences of iodine oxidative chemisorption (deposition) and reductive desorption (stripping), at potentials close to the hydrogen evolution region, were performed. Experiments were carried out in alkaline solutions to ensure that the reordering process is driven predominantly by the strong chemisorption of iodine and not by the dissolution of Pd. Electrode-surface characterization consisted of cyclic voltammetry, low-energy electron diffraction, and Auger electron spectroscopy.