Clavilier bead

Figure 2. (A) The arrangement of single-crystal basal planes in a flame-annealed Clavilier bead. (B) Single-crystal facets on a Clavilier bead of Pt. (C). Hanging-meniscus configuration for voltammetric experiments with a single-crystal facet of a Clavilier bead.

Figure 6 shows the LEED pattern and AES spectram of a typical UHV-prepared clean and well-ordered Pd(lll) electrode surface. For a representative Pd(lll) faeet on a flame-aimealed Clavilier bead, STM images obtained under an Ar atmosphere and in sulfuric acid under (double-layer) potential control are shown in Fig. 7. It will be mentioned that no experiments are carried out unless the surfaees are verified to be as untainted and as highly organized as those exemplified in Figs. 6 and 7.

The chemisorption of benzene on a Pd(l 11) Clavilier-bead facet yielded two stable stmctures, as depicted by the EC-STM images in Fig. 8, that depended upon the applied potential within the... 

Figure 7. High-resolution EC-STM images of a Pd(l 11) facet on a Clavilier bead. Bias voltage 60 mV tunneling current 20 nA. (A) In an environment of high-purity argon. (B) In 0.01 M H2SO4 at potentials in the double-layer region.

In contrast to the successful implementation of the bead method in studying the anomalous features, the contributions from studies with UHV-electrochemical systems has been limited to just a few. Subsequent work from our apparatus following corroboration of Clavilier s results concentrated on the effect of potential cycling through "oxide formation potential on the surface structure (19). and later on the effect of pH and type of anion (Wagner, F.T. Ross, P.N., J. Electroanal. Chem.. in press) on the anomalous features. Using the system in Yeager s laboratory, Hanson (20) was able to reproduced Clavilier s voltammetry not only for the (111) surface, but also the (100) and (110) surfaces as well. In spite of the relatively small number of contributions to the literature that have come from the UHV-electrochemical systems, they have made and essential validation of the bead method of surface preparation, and have verified the structure sensitivity of the anomalous features inferred from purely electrochemical observations. 

The Au(l 11) is often employed in air and ECSTM due to its inertness. The Au(l 11) can be prepared as a thin film grown on mica or glass.Gold beads can also be used as substrates. They are formed by melting the end of a wire on a torch following the method of Clavilier. The beads present flat areas suitable for molecular adsorption. The Au surfaces are cleaned by thermal annealing in a flame before molecular adsorption. When well prepared, the Au(lll) surface presents a reconstruction known as 22 xJ3. This structure is characterized by a pair of dislocation lines and is sometimes preserved upon molecular adsorption. Other compounds promote the lifting of the reconstruction. The other low Miller index faces of Au. namely, Au(l 10) and Au(lOO), normally employed as monocrystals, were also studied. 

Polycrystalline gold and platinum electrodes can be converted into clean, well-ordered single-crystal electrodes by heating the respective wires in an Hj h- O2 flame. This is the so-called Clavilier method (134-137). Such electrodes are prepared by melting one end of a pure wire (1 mm diameter)—a zone-refining process. The crystallinity of the bead is usually... 

FIGURE 20.6 Cross section illustration of an EC-STM cell constructed to accommodate a Clavilier-type Au bead crystal as the sample. The volume of liquid used with a cell of this design is around 0.8 mL. 

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