Single-crystal electrodes, preparation

Underpotential deposition is described as less than monolayer metal deposition on a foreign metal substrate, which occurs at more positive potentials than the equilibrium potential of a metal ion deposed on its own metal, expressed by the Nemst equation. Kolb reviewed state-of-the-art Underpotential deposition up to 1978. As Underpotential deposition is a process indicative of less than a monolayer metal on a substrate, it is expected to be quite sensitive to the surface stmcture of the substrate crystal a well-defined single-crystal electrode preparation is a prerequisite to the study of Underpotential deposition. In the case of Au and Ag single-crystal electrodes, Hamelin and co-workers extensively studied the necessary crystal surface structure, as reviewed in Ref. 2. 

Preparation of the Pt(lll) Single-Crystal Electrode. The Pt single-crystal electrode was prepared according to the procedure developed by Clavilier (7) and oriented using the methodology of Hamelin (8). It was subsequently polished with alumina to a mirror-like finish. The quahty of the Pt(l 11) surface was verified by recording CV... 

Kibler, L.A. Preparation and Characterization of Noble Metal Single-Crystal Electrodes, http //www.uni-ulm.de/ echem/index.html id=1023001. 

The chemisorption of species occurs at specific sites on the electrode, for example on top of certain atoms, or in the bridge position between two atoms. Therefore, most adsorption studies are performed on well-defined surfaces, which means either on the surface of a liquid electrode or on a particular surface plane of a single crystal. Only fairly recently have electrochemists learned to prepare clean single crystal electrode surfaces, and much of the older work was done on mercury or on amalgams. 

The use of single crystal electrodes. While the polycrystalline electrodes that were used in older works gave reproducible results in studies of reactions, they did not possess a definite structure. Only recently have electrochemists learned to prepare and characterize single crystal electrodes. This has greatly extended the study of the structure of the electrochemical interface. 

Adsorption of acetic acid on Pt(lll) surface was studied the surface concentration data were correlated with voltammetric profiles of the Pt(lll) electrode in perchloric acid electrolyte containing 0.5 mM of CHoCOOH. It is concluded that acetic acid adsorption is associative and occurs without a significant charge transfer across the interface. Instead, the recorded currents are due to adsorption/desorption processes of hydrogen, processes which are much better resolved on Pt(lll) than on polycrystalline platinum. A classification of adsorption processes on catalytic electrodes and atmospheric methods of preparation of single crystal electrodes are discussed. 

Figure J. Preparation of a welJ-defined single-crystal electrode by four steps, (a) Mirror-rinished crystal is annealed under gas flame, and (b) quenched in air and then pure water, (c) The crystal is covered with a droplet of pure water to protect it from pollution by impurities in air and (d) is introduced into the cell, where the crystal contacts the meniscus of the electrolyte solution. (From Ref. 25.)...

Reduction of N2O was used as model electrocatalytic reaction on well-defined surfaces [102, 103]. Pt(lll) and Pt(lOO) electrodes covered with Rh adlayers were prepared for these studies. It was shown that the adsorptive and catalytic activity of the adlayers differs from those of the bulk single-crystal electrodes. 

An important consideration, from the practical point of view, is that thin film polycrystalline photoelectrodes can be prepared, by various methods, with conversion efficiencies of more than half of those obtained with single crystal electrodes and with better stability characteristics than those obtained with single crystal based PEC s (1,4.,5). ... 

For comparison, figure 3B shows results not only for the relatively most stable face of a single crystal (2) and a polycrystalline thin layer prepared by pasting 5), but also for a pressed pellet of CdSe (2). This last electrode shows stability behaviour intermediate between that of the two other electrodes. This behaviour is evident also in the decrease in output stability, which is less steep than that of the single crystal electrode. 

Surface Electrochemistry is an important field in Surface Science, which is undergoing a very important development. In spite of the complexity of the systems, the experimental measurements have acquired a very high degree of sophistication and atomic resolution has almost been reached. After the development of special techniques that allowed the preparation of well defined single crystal electrodes [1, 2], attempts have been made to use surface analysis techniques in an electrochemical environment. However, one must... 

The authors are grateful to Mr. Andrea Pozzi and Mr. Francesco Gualchieri for their contribution to the set up of the automated deposition system, and Mr. Ferdinando Capolupo lor the preparation of the silver single crystal electrodes. The financial support of the Italian CNR and of the Murst is gratefully acknowledged. 

---