Atomic Structures of Iodine Adlayers

Hubbard and co-workers investigated the structure of adsorbed iodine on a well-defined Pt(l 11) using a electrochemical UHV technique, demonstrating that the iodine adlayer structure was potential dependent [1,4]. The adlattices of (3 x 3) (1=4/9) and ("V7 x V7)R19.1° (1=3/7) were found to form at relatively positive and negative potentials, respectively, in the featureless double-layer region. They also found a (V3 x V3)R30° (1=1/3) adlattice at relatively negative potentials where a partial reductive desorption of the adsorbed iodine took place. Figure 1(a) shows a replica of one of then-results obtained in a solution (pH 4) containing 0.1 mM KI (Fig.4 in [4]). It has long been expected, at least by us, that the transformation between those structures should take place reversibly when the electrode potential was changed in the potential range shown in Fig. 1(a). Although previous in-situ STM studies revealed atomic structures on Pt(l 11) in air and in solution [5-8], no direct in-situ STM investigation of the potential dependence expected from Fig. 1(a) has hitherto been carried out in solutions containing KI. We recently found that two structures of (3 x 3) and (V7 x V7)R19.1° always coexisted even after the electrode potential was changed greatly in the double-layer region. Figure 1(b) shows an example of STM images acquired in a 1 mM KI solution (pH 4). It can be clearly seen that domains with two different structures form on the terrace of Pt(lll). Time-dependent STM images acquired consecutively in the same area after a potential step indicated surprisingly that the structural transformation did occur but was very slow. Two structures could be seen even after 30 min at each potential where only one structure was expected to appear according to the result shown in Fig. 1(a) [4]. The result shown in Fig. 1(b) directly indicates that the surface

Fig. 1. Cyclic voltammogram of Pt(l 11) in 0.1 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 (a/7 x V7)R19.1° structures co-existed even after 30 min at the electrode potential of 0 V [19].

Fig. 1. Cyclic voltammogram of Pt(l 11) in 0.1 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 (a/7 x V7)R19.1° structures co-existed even after 30 min at the electrode potential of 0 V [19].

mobility of adsorbed iodine atoms is very low on Pt(l 11), in contrast to that of the iodine on Au(lll) [14]. Further investigations [19] are needed to reveal dynamic processes in the iodine adlayers formed on Pt in addition to the simple determination of the structure, even though the I/Pt(l 11) system has been thought to be well defined and well characterized by many previous investigators.

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 found that the iodine adlayer observed upon emersion from Csl solution possessed a (V3 x V3)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(lll) is sensitive to electrochemical parameters such as electrode potential. Using in-situ STM under potential control, Gao and Weaver reported potential-dependent structures on Au(l 11) in KI solution including (5 x V3) and (7 x 7)R21.8° [23]. Under a very similar set of experimental conditions, Tao and Lindsay found a potential-dependent transition only from (V3 x V3)R30° to (3 x 3) [24].

However, the recent in-situ surface X-ray scattering studies by Ocko et al. revealed a series of I/Au(lll) adlattices [12], good agreement with our results obtained by complementary use of LEED and in-situ STM [13, 14]. The adlattice constants varied continuously with the electrode potential in each of the two-dimensional phases designated by Ocko et al., the rectangular (p x V3) phase and the rotated hexagonal phase[12]. Figure 2(a) shows a cyclic voltammogram for a well-defined Au(lll) in 1 mM KI at a scan rate of 5 mV/s in which the peaks are sharper than those obtained at 20 mV/s as reported in our previous paper [13]. The peaks observed in the potential range between 0 V and -0.5 V correspond to the adsorption-desorption reactions of iodide on Au(lll). The small peaks at ca. 0.4 V are associated with the structural transformation of two different phases. The LEED pattern shown in Fig. 2(b) was obtained at an emersion potential of 0.3 V [13]. Three split subspots were clearly seen to move away further from the center and to increase in distance between the split spots when the potential was scanned in the positive direction. The values of p in the rectangular (p x V3) phase shown in Fig. 2(c) could be fairly accurately determined by the analysis of the LEED pattern as shown in Fig. 2(b). The p-values obtained as a function of the electrode potential using LEED and in-situ STM [13] were in good agreement with the in-situ surface X-ray scattering results reported by Ocko et al. [12]. The applicability of LEED and in-situ STM was demonstrated in our previous papers for the investigation of continuous structural change [13, 14], LEED provided the p-values in the (p x V3) adlayer precisely enough to describe the structural variation in the electrocompression.

Fig. 2. Cyclic voltammogram of Au(l 11) in 1 mM KI at 5 mV/s (a), LEED (b), and real-space structure of (p x V3) (c) [13].

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].

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