Ordered adlayers

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]. 

STM has been used to examine the surface processes accompanying the near reversible wave (Ic, la) shown in Figure 16 [104]. High resolution imaging at 0.455 V revealed an ordered adlayer on the Cu(l 11) surface as shown in Figure 17. A possible model representing this structure corresponds to a layer of tetrachloroaluminate ions... 

Fig. 17. A 30 x 30 nm image of an ordered adlayer on Cu(lll) at 0.455 V vs. A1 in 55.0 m/o AICI3-EtMelmCl. A schematic of the proposed tetrachloroaluminate adlayer structure is shown. Reproduced from Stafford et al. [104] by permission of The Electrochemical Society.

Zhang et al. [197] have studied adsorption of DL-homocysteine and L-homo-cysteine thiolactone on Au(lll) electrode in 0.1 M HCIO4 using CV and STM. Both compounds formed highly ordered adlayer on Au(lll). For both adlayers, structural models have been proposed. 

In another work from this series [433], deposition of Pd on the unreconstructed Au(llO) has been studied. An ordered adlayer of PdCfi " was imaged with atomic resolution. Pd deposition started at monoatomic high steps. From the coulo-metric data, it follows that approximately three monolayer equivalents are deposited in the UPD range, what may, in turn, be a result of the surface alloy formation. 

Ordered Adlayers. Lateral interactions for chemisorbed adsorbates are generally repulsive, although there have been reports of attractive interactions for adsorbates at distances of about twice the distance between neighboring sites.At low coverage repulsive interactions simply keep the adsorbates apart. The adlayer is disordered. At intermediate and high coverages a rich field of possible adlayer structures can be observed even when there is only one type of adsorbate. 

On S-Au(lll) [162] The STM image indicates that the sulphur adlayer (without TMPyP) obeys a ( /3 x v )R30° structure and after injection of TmPyP shows a highly ordered adlayer of TMPyP molecules showing a reduced mobility compared to on iodine-Au(lll). 

The quantity ruling the adsorption-site preference and the formation of ordered adlayers is the adatom binding energy ... 

The formation of ordered adlayers of CN" on Pt(lll) was reported by the Weaver and Itaya groups."" """ Both groups observed the same overlayer symmetry, which consists of anions adsorbed in atop sites. Sawaguchi et al. studied the structures wliich form on Au(lll) in the presence of Au(CN)2 anions and reported two different adlayers depending on the electrode potential. 

The stability and equilibrium curvature of a given orientation are functions of and not of a alone. Adsorbates reduce a on any surface to which they spontaneously attach. However, to produce infinite stiffness and facets, adsorption must be narrowly focused on particular orientations. The formation of such ordered adlayers has been well documented for chloride on the Cu(100)21-25 and Cu(lll)21,26 27 surfaces. 

The adsorption of these molecules has also been investigated on various iodine-modified electrodes such as I/Ag(lll), I/Pt(lll) and I/Rh(lll). Highly ordered adlayers of TMPyP were found to form on I/Au(l 11) and I/Ag(l 11), whereas less-ordered adlayers formed on I/Pt(lll) and I/Rh(lll) [41]. The surface mobility of iodine on Pt(l 11) is very slow, as shown in Fig. 1(b). TMPyP is adsorbed on I/Pt(l 11) as isolated molecules with a disordered arrangement These results suggest that the surface mobility of the iodine adatoms also plays an inqmrtant role in the ordering process of the organic adlayer. 

Liquid-phase adsorption characteristics examined by atomic force microscopy (AFM) were compared for two pyridine base molecules, pyridine and d-picoline. on (010) surfaces of two natural zeolites, heulandite and stilbite. These adsorption systems formed well-ordered. two-dimensioncJ (quasi-)hexagonal adlayers. The 2D lattice structures of the ordered adlayers w ere dependent on the adsorbate/substrate combinations. Although there existed certain habit in the orientation of the 2D lattice unit vector of the adsorbed phase with respect to the substrate(OlO) lattice vectors, the molecular arrays w ere incommensurate with the substrate atomic arrangements. 

Figure 3 shows an AFM image of /3-picoline molecules adsorbed on stilbite(OlO). From the obtained images the 2D unit cell dimension of the ordered adlayer phase was calculated to be 0.5o 0.04 nm. Molecular orientations were also determined using computer graphics as described above. 

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