Structures of iodine adlayers

In this chapter, we first briefly summarize the structures of iodine adlayers on Au(l 11) and Ag(l 11) based on our recent results obtained by using in situ STM and ex situ LEED (77-75), and then in situ STM of organic molecules ad rbed on the iodine-modified Au(lll), Ag(lll), and Pt(lll) (22-25), demonstrating the extremely important new aspect of iodine-modified electrc es. 

TMPyP on I-Ag(lll). The I-Ag(lll) electrode should be one of the most interesting iodine-modified electrodes, because the structures of iodine adlayers on Ag(lll) have been well characterized as described above, and also because bare Ag electrodes have frequently been used as the substrate for the investigation of the... 

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. 

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].

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... 

Because of publication Kmitations, the present chapter describes only a few topics for adlayer structures of iodine, sulfate/bisulfate, and simple aromatic molecules on well-defined single-crystal electrodes. Experimental procedures are only briefly described, because detailed reviews on this aspect have aheady been written [5]. 

Adlayer structures of iodine on Au(lll) and Ag(lll) single crystal electrodes are briefly described based mainly on our recent in situ STM and ex situ LEED studies. It is concluded that complimentary use of these two techniques is a powerful method for characterizing the iodine adalyers with an atomic s e. It is also shown that the iodine-modified electrodes are ideal substrates for the investigation of adsorption processes of various organic molecules on the electrode surface in solution. The internal molecular structure, orientation, and packing arrangement of ordered molecular layers can be determined with near-atomic resdution under electrochemical conditions. 

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. 

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... 

Figure 5 summarizes the structures found on Ag(lll) in acidic and alkaline solutions. It is noteworthy that all iodine atoms can be accomodated in the (V xv )R30, c(/7x 6R-30 ), and also square (v xv R-30 ) adlattices regardless of the value of p on the unit cell bisectors, which was defined by the traces across 2-fold bridge sites as shown in Figure 5. The in situ STM images of these structures look always very flat, indicating that all iodine atoms are locati on physically equivalent positions. The model structures (a, b, and d) shown in Figure 5 can explain the flat adlayers of iodine observed by STM (72,75).

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