Iodine adlayer on Au

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. 

Yamada, T. Batina, N. Itaya, K. 1995. Structure of electrochemicaUy deposited iodine adlayer on Au(lll) studied by ultrahigh-vacuum instrumentation and in situ STM. J. Phys. Chem. 99 8817-8823. 

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 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... 

Iodine and bromine adsorb onto Au(l 11) from sodium iodide or sodium bromide solutions under an applied surface potential with the surface structure formed being dependent on the applied potential [166]. The iodine adsorbate can also affect gold step edge mobility and diffusion of the Au surface. Upon deposition of a layer of disordered surface iodine atoms, the movement of gold atoms (assisted by the 2-dimensional iodine gas on the terrace) from step edges out onto terraces occurs. However, this diffusion occurs only at the step edge when an ordered adlayer is formed [167]. 

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]. 

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. 

It was shown by in situ STM that highly ordered SAMs of the porphyrin compound 217 in face-on disposition were formed on iodine-modified Au(lll) surfaces, whereas disordered adlayers were formed on bare Au(lll) surfaces314. Two periodicities were found by AFM for the SAM on Au(lll) of a thioether derivative of calix[4]resorcinarene (218), one with lattice constant of ca 0.42 nm, attributed to the alkyl chain packing of the four thioether anchors on the metal surface, and the second one with lattice constant of ca 1.16 nm, attributed to the calixarene macrocycles315,316. 

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... 

A double role was proposed for iodine oxidant of the monomer to form thiophene radical cations which couple to produce short oligothiophenes, and glue by means of an adlayer on the Au(lll) surface such that the short oligothiophenes adhere on the surface, acting as nuclei for polymerization. 

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... 

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 "rosette" LEED patterns were successfully observed on Ag(lll) at potentials between the pea 2-2 and 3-3. The observation of the "rosette" patterns in LEED suggests that the iodine adlayer is more strongly attached on Ag( 111) than on Au( 111). It is now clear that the peaks 2-2 in Figure 4a correspond to the transition between the... 

---