Bromide adlayer

The continuous change of the bromide coverage with potential upon the transition c(.y2 X 2. 2) R45° —> c(. 2 x p)R45 can be approximated by the power law 6 = 0.122 (E - Ecf + 0.50 with p = 0.40 and Ec = 0.375 V. The measured exponent is smaller than the theoretical value p = 0.50 predicted by Pokrovsky and Talapov for the C UIC transition within a model of noninteracting domain walls [70]. The theoretical prediction is only supposed to apply close to the transition, where the domain walls are narrow relative to their separation. The experimental precision does not permit to quantitatively extract the exponent close to the transition, in which the theoretical prediction could be unambiguously tested. At higher incommensurabilities the bromide coverage is not only determined by the proximity to the C/UIC phase transition, but rather by the compressibility of the bromide adlayer, which may explain the deviations in the exponent. The width of the X-ray diffraction (XRD) peaks in the incommensurate direction scale quadratically with the wave vector component in this direction, and increase continuously by an order of magnitude as the C/UIC transition is approached from the UIC side. 

Cu-UPD on Pt(lll) electrodes in the presence of bromide anions proceeds also in two steps An incommensurate bromide adlayer transforms first into a CuBr-bilayer... 

Fig. 12 The CV for a Au (001) electrode in 0.05 M NaBr solution and the corresponding X-ray intensity at the position (0, 1, 0.1) in which scattering from a bromide adlayer was observed. (Taken from Ref [57].)...

In a preliminary report of the results for bromide adsorption onto Pt(lll), it was shown that an incommensurate (3 X 3) hexagonal bromide adlayer is present on the Pt(lll) surface in the potential range 0.05-0.7 V [60]. From fits to the scattering profile at the lowest-order diffraction peak, the peak position... 

The isotherm for bromide adsorption (Fig. 14c) shows that increasing the potential in the double-layer region causes a continuous increase in the bromide coverage. The data shown in Fig. 15 and in other experiments, however, are not consistent with the notion that the additionally adsorbed bromide leads to a continuous compression of the incommensurate bromide adlayer, as this would cause the sharp peak at/ = 0.67 to shift gradually to higher wave vector. An alternative explanation of our results is that bromide forms a series of high-order commensurate (HOC) structures on the Pt(lll) surface, that is, at all potentials the structure corresponds to a close-packed monolayer but that the periodicity depends critically on the ratio of the bromide and Pt lattice parameters. At 0.2 V, the unit cell corresponds to a (3 x 3) structure with a basis of four Br atoms. At 0.6 V, the unit cell is a (7 x 7) structure containing 25 Br atoms, that is, with a 7 5 ratio of the Br and Pt lattice parameters. 

Magnussen et al. studied the adsorption of bromide adlayers on Au lll electrodes in 0.1 mol dm HCIO4 + 0.001 to 0.1 mol dm NaBr. In this case, Br ions were selected as a model system, in contrast to the more usually studied I , as Br ions are less strongly adsorbed but are still classed as specifically adsorbed ions [19]. At potentials more positive than a critical potential, which would depend on the NaBr concentration, it was found that Br ions would form a hexagonal adlayer rotated relative to the 3 direction of the Au l 11 substrate, with the extent of the rotation and adlayer density being dependent on the potential and NaBr concentration. The Br -Br spacing within the layer was found to vary continuously, from 4.24 A at the critical potential to 4.03 A at a potential 0.3 V more positive. 

Figure 3. Top A representation of the in-plane scattering in reciprocal sjpace. The location of CTR s and Pt Bragg reflections are shown by filled circles. The open circles are at position where diffraction from the bromide adlayer was observed. Bottom Proposed structure in which the Pt atoms are shown by open circles and the bromide overlayer atoms by filled circles.

In order to ascertain the surface normal structure of the bromide adlayer and determine the coverage to confirm our structural model, we carried out specular x-ray ray reflectivity measurements. The interpretation of CTR measurements have also been described in several papers and readers are refereed to these papers for details [24, 25, 27]. Briefly sununarizing, the CTR analyses gave the surface coverage of bromide, 0g = 0.43 ML at 0.2 V and Gg = 0.48 ML at 0.7 V. The platinum-bromide layer spacing ( 2.7 A) suggest that bromide is covalently bonded to the platinum (the covalent radii for Pt and Br are 1.30 and 1.14 A, respectively). 

FIGURE 20.13 (a) STM top view of a bromide adlayer on the Cu(llO) surface, recorded with the sample... 

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