Iodine adlayer

The platinum electrode was modified with an iodine adlayer of the specified symmetry. ) AC = acetate anion. 

S oriaga and coworkers [111-113] discovered that the anodic dissolution Pd(lll) and Pd(lOO) single-crystal electrodes in pure H2SO4 solutions was catalyzed by the presence of monolayers of iodine chemisorbed on these surfaces. Large anodic peaks were found for the dissolution even in noncorrosive electrolyte solutions only when the surfaces of Pd were modified by the iodine adlayer. 

Iodine adlayers covering an Au(lll) electrode have been characterized [66] and shown to be convenient substrates for the studies on adsorption of various organic molecules [66, 67]. 

Shi et al. [68] have studied iodine adsorption on Ag using atomic-resolution electrochemical scanning turmehng microscopy (ECSTM) method. Distinctly different iodine adlayer structures and surface diffusion behavior were observed on mechanically polished pc-Ag in comparison with those obtained on single-crystal electrodes. 

Iodine adlayers can provide a protective layer against oxidation and contamination while single-crystal surfaces are handled in ambient atmospheres. Also, the adsorbed iodine can be replaced by CO, which in turn can be electrochemically oxidized in solution to yield a clean metal surface. Anodic dissolution of various metals occurs at step edges under carefully adjusted electrochemical conditions, and this is a promising method for in-situ preparation of atomically flat terraces. Such layer-by-layer dissolution has been demonstrated for Ni, Ag, Co, and iodine-modified Pd and Cu surfaces. 

On lodine-Au(lll) [161] a centered rectangular c(p x y3R-30 ) phase and a rotated hexagonal phase has been found depending on the potential. Again atomic resolution was achieved on the iodine adlayer, and then TMPyP was injected. With time the iodine adlayer blurred and adsorbed TMPyP molecules became visible in STM images, in several ordered structures depending on the applied potential. 

After achieving atomic resolution on the iodine adlayer, again TMPyP was injected under potential control. Within the first few minutes no significant changes were observed with only the iodine adlayer structure being resolved (more see text). 

Summary. Structures and properties of iodine adlayers on Pt(l 11), Au(l 11), and other sLugle-crystal electrodes are described, based mainly on our recent STM studies. The imderpotential deposition of Ag on Pt(l 11) in the absence and presence of the iodine adlayer is also briefly described. It is shown that the iodine-modified electrodes are promising substrates for the investigation of the adsorption of organic molecules. High-resolution STM allows us to determine molecular arrangements and internal structures of molecules adsorbed on the iodine-modified electrodes in solution. 

The adsorption of anions such as halides, cyanide, and sulfate/bisulfate on electrode surfaces is currently one of the most important subjects in electrochemistry [1 - 3]. It is well known that various electrochemical surface processes such as underpotential deposition of hydrogen and metal ions are strongly affected by co-adsorbed anions. Particularly, structures of the iodine adlayers on Pt, Rh, Pd, Au, and Ag surfaces have... 

The objective of this paper is to describe structures and properties of the iodine-modified Pt(l 11), Rh(l 11), Pd(l 11), and Au(l 11) based on our recent results obtained by using in-situ STM. The structure of the iodine adlayers, particularly on the Pt and Au electrodes is described first, followed by the rmderpotential deposition (UPD) of Ag on Pt(lll) in the absence and presence of the iodine adlayer with the (V x V7)R19.1° structure, and finally the adsorption of organic molecules on the iodine-modified electrodes. 

Fig. 1. Cyclic voltammogram ofPt(lll) inO.l mM KI (pH 4) reported in [4] (a) and atomic STM image of iodine adlayer obtained at 0 V vs. Ag/AgCl. The (3 x 3) and (V x V7)R19.1° structures co-existed even after 30 min at the electrode potential of 0 V [19].

On the other hand, the structure of the iodine adlayer is more complicated on Au(l 11). Indeed, various structures were reported for I/Au(l 11). Bravo et al. using the electrochemical UHV technique foimd that the iodine adlayer observed upon emersion fi-om Csl solution possessed a (VS x /3)R30° lattice at a low iodine coverage [20]. A (5 X V3) structure was also found at more positive potentials (high coverage). McCarley and Bard found only the (V3 x V3)R30 structure in their STM studies in air [21]. Haiss et al. reported several structures such as (V3 x V3)R30°, (5 x V3) and (7 x 7)R21.8 in air and in a nonaqueous solvent [22]. These discrepancies strongly suggest that the structure of I/Au(l 11) is sensitive to electrochemical parameters such as electrode... 

Although in-situ STM provided overall information of the structural changes in both phases and helped to obtain accurate lattice parameters in the LEED analysis [13, 14], the STM technique alone could not capture such small variations in the adlattice. Our work clearly demonstrates that complementary use of LEED and STM is a powerful technique more easily available in ordinary laboratories compared with surface X-ray scattering to determine accurate structural parameters of adlayers on electrode surfaces. We have recently found that the iodine adlayers on Ag(lll) were also continuously compressed with changing electrode potential [15], which is in contrast to the result reported previously [25]. 

The UPD of Ag on Au and Pt is also an interesting reaction to investigate with surface structure-sensitive techniques. It has clearly been demonstrated that the iodine adlayers on Pt(lll) and Au(lll) strongly affect the UPD of Ag [1, 8, 36]. For example, Fig. 3 illustrates a clear difference in the electrochemical response of the UPD of Ag on a well-defined Pt(l 11) in sulfuric acid (a) and on a Pt(l 11) with the (V X V7)R19.1° iodine adlayer (b), respectively. Two sets of well-defined UPD peaks in the cyclic voltammogram were observed on a well-ordered Pt(lll) in sulfuric acid... 

Therefore, the anodic dissolution of palladium was enhanced significantly when the iodine adlayer was adsorbed on the palladium surface. The rate of the dissolution is directly proportional to the coverage of iodine on the surface, and the iodine... 

The ex situ LEED [113-116] and in situ STM measurements with high resolution [105, 117] demonstrated that the ordered iodine adlayers were formed on the palladium surfaces as (V3 x V3/ 30°)-I-Pd(lll),c(2 X 2)-I-Pd(100)and pseudohexagonal-I-Pd-(llO). Itis noteworthy that the same adlattice structure of iodine was observed on the palladium terrace after anodic dissolution. The ordered... 

Electrochemistry of LB films of fullerenes has been widely studied and remains the subject of much research effort from both theoretical and experimental approaches. Bard etal. have studied basic electrochemistry of Ceo fullerene LB films on an electrode in acetonitrile solutions [23]. The study indicated that reduction of the fullerene films could form insoluble films with incorporated electrolyte cations or lead to dissolution. The study on Cgo LB films has become a focus of considerable interest however, it is difficult to fabricate high-quality LB films of pure Cgo due to its intrinsic hydropho-bicity. Kajiyama et al. applied a multistep creep method as an LB technique for constructing a fairly homogeneous Ceo monolayer, which is regularly packed in a hexagonal array [44]. Kunitake etal. developed the electrochemical replacement method to form epitaxial adlayers of fullerenes on Au(lll) surfaces [45]. The wet process method consists of the transfer of Langmuir films of fullerene onto iodine-modified Au(lll) surfaces at an air-water interface followed by the electrochemical removal and replacement of iodine adlayers with fullerene adlayers in solution. The fullerene adlayers prepared by this method showed excellent quality and uniformity. A visuahzing... 

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