Underpotential features

Auother uauostructuriug techuique shares some commou features with that described above and is shown in Fig. 36.3. A polymer-coated Pt ultramicroelectrode is used as the tip of a STM, and graphite is used as substrate. Ag atoms are deposited on the tip at underpotentials, so that approximately one atomic monolayer is deposited on the tip. After this, a first bias pulse is applied to the tip, causing the formation of a shallow pit. Then a second bias pulse with a smaller amplitude is applied to cause the desorption of silver from the tip. The silver ions desorbed diffuse aud migrate across the tip-sample gap aud deposit ou the high coordiuatiou sites preseut... 

Atomically resolved STM images of underpotential deposition on Au(lOO) and Au(lll) in sulfuric acid solution were first observed by Magnussen et al. in 1990, features of which are shown in Figs. 19, 20,... 

An ex situ EXAFS study was conducted for Cu underpotential deposition on polycrystalline Pt in 0.2mM CUSO4 + 0.5M H2SO4. The iodine-treated Pt was subjected to Cu underpotential deposition, the features of which were observed by EXAFS. ° An EXAFS study of Cu... 

For example, the voltammogram in Fig. 1 depicts Ag UPD on an I-coated Pt(lll) electrode [26], Three features can be attributed to the UPD of Ag, each of which results in the formation of a new structure on the surface, as indicated by the FEED patterns diagrammed in the circles. It was concluded in that work that UPD involved more than a single monolayer of Ag. Ag depositing at underpotential reacted with the Pt substrate as well as with the adsorbed I atom layer. It is also interesting to note that Ag underpotentially deposited in Fig. 1 reacted with the adsorbed atomic layer of I atoms to form a monolayer of the I-VII compound Agl on the Pt surface. 

Swathirajan and Bruckenstein [113] have recently discussed the thermodynamics and kinetics of underpotential deposition phenomena on polycrystalline electrodes. Since work functions and surface structural features vary in different crystal faces, upd phenomena on single crystals have shown distinctive differences [109]. 

The specific features of underpotential deposition vary from system to system but some general trends can be mentioned on a general basis. 

A unique feature observed in metal deposition on a foreign substrate is underpotential deposition. It is found that a metal can be deposited on a foreign substrate at potentials positive with respect to the reversible potential for deposition of the metal in the same solution. Considering, for example, the deposition of silver on platinum, which can be written as... 

The most remarkable feature is that the peaks representing copper underpotential adsorption disappear in the presence of iodide in solution when its concentration reaches values higher than 0.5 mM, but the anodic peak is still present due to the bulk deposition of copper. However, the... 

Several other features can be seen in tlie Figure 2 voltammogram. The first is the small cathodic current observed between 0.1 and 0.5V. This can be attributed to the underpotential deposition (upd) of aluminum onto polycrystalline tungsten (51). The second is that, other than aluminum upd, no electrochemical activity is observed between the aluminum deposition potential and 1.5 V. Thus, a fairly large electrochemical window exists for the examination of niobium in the 52 48 electrolyte. 

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