Electrode surface reconstruction

Pettinger B, Lipkowski J and Mirwald S 1995 In situ SFIG studies of adsorption induced surface reconstruction of Au (111 )-electrodes Electrochim. Acta 40 133-42... 

Surface SHG [4.307] produces frequency-doubled radiation from a single pulsed laser beam. Intensity, polarization dependence, and rotational anisotropy of the SHG provide information about the surface concentration and orientation of adsorbed molecules and on the symmetry of surface structures. SHG has been successfully used for analysis of adsorption kinetics and ordering effects at surfaces and interfaces, reconstruction of solid surfaces and other surface phase transitions, and potential-induced phenomena at electrode surfaces. For example, orientation measurements were used to probe the intermolecular structure at air-methanol, air-water, and alkane-water interfaces and within mono- and multilayer molecular films. Time-resolved investigations have revealed the orientational dynamics at liquid-liquid, liquid-solid, liquid-air, and air-solid interfaces [4.307]. 

According to the data obtained with SXRS in salt solutions,519 520 at a < 0 the surface of Au(lll) forms a ( 3 x 22) structure as in a vacuum. At a > 0 the reconstruction disappears and the (1 x 1) structure is observed. On the reconstructed Au(l 11) surface there are 4.4% more atoms than on the (1 x 1) structure and on the reconstructed Au( 100) there are 24% more atoms than on the (1 x 1) structure.506,519 This phase transition shifts in the negative direction with the adsorbability of the anion. The adsorption-induced surface reconstruction of Au(l 11) electrodes has been studied in situ by second harmonic generation by Pettinger et al.521... 

The specific adsorption of OH" ions depends on the electrode surface structure increasing in the order Au(l 11) < Au(100) < Au(311).391 The similarity of the results obtained in alkaline solutions and those observed in acid and neutral media have led the authors of many papers to conclude that surface reconstruction occurs at a < 0 and is removed at 0. 

The term G T, a,, A/, ) is the Gibbs free energy of the full electrochemical system x < x < X2 in Fig. 5.4). It includes the electrode surface, which is influenced by possible reconstructions, adsorption, and charging, and the part of the electrolyte that deviates from the uniform ion distribution of the bulk electrolyte. The importance of these requirements becomes evident if we consider the theoretical modeling. If the interface model is chosen too small, then the excess charges on the electrode are not fuUy considered and/or, within the interface only part of the total potential drop is included, resulting in an electrostatic potential value at X = X2 that differs from the requited bulk electrolyte value < s-However, if we constrain such a model to reproduce the electrostatic potential... 

Figure 5.8 Calculated y versus electrode potential A< sce (referenced to an SCE) curves for Au(lOO) in 0.01 M HCIO4. The crossing between the curves indicates the electrode potential at which the surface reconstruction is lifted [Au(100)-hex Au(100)-(1 x 1)].

We have also discussed two applications of the extended ab initio atomistic thermodynamics approach. The first example is the potential-induced lifting of Au(lOO) surface reconstmction, where we have focused on the electronic effects arising from the potential-dependent surface excess charge. We have found that these are already sufficient to cause lifting of the Au(lOO) surface reconstruction, but contributions from specific electrolyte ion adsorption might also play a role. With the second example, the electro-oxidation of a platinum electrode, we have discussed a system where specific adsorption on the surface changes the surface structure and composition as the electrode potential is varied. 

The majority of deposits formed in this group have been on Au electrodes, as they are robust, easy to clean, have a well characterized electrochemical behavior, and reasonable quality films can be formed by a number of methodologies. However, Au is a soft metal, there is significant surface mobility for the atoms, which can lead to surface reconstructions, and alloying with depositing elements. In addition, Au it is not well lattice-matched to most of the compounds being formed by EC-ALE. 

The studies under ultrahigh vacuum have shown that adsorption and surface charging influence the stability of the reconstructed surfaces. A similar influence has been observed for metal surfaces in contact with electrolyte solutions [336]. In this case, the separation of these two influences is not simple, since the surface charging and adsorption processes are interdependent. Generally, it has been concluded [4] that Au surface reconstruction occurs for negative electrode charges and disappears for positive surface charges. It is noteworthy that as early as in 1984, Kolb and coworkers [339, 340], who carried out systematic study on all three low-index faces Au electrodes, showed that the reconstructed surfaces can be stable in electrolyte solutions. 

The half-cell enclosed by the dashed line in Figure 15-1 is called a silver-silver chloride electrode. Figure 15-3 shows how the electrode is reconstructed as a thin tube that can be dipped into an analyte solution. Figure 15-4 shows a double-junction electrode that minimizes contact between analyte solution and KCI from the electrode. The silver-silver chloride and calomel reference electrodes (described soon) are used because they are convenient. A standard hydrogen electrode (S.H.E.) is difficult to use because it requires H2 gas and a freshly prepared catalytic Pt surface that is easily poisoned in many solutions. 

Au(100) offers another interesting case for reconstruction [156]. Previous studies on this electrode surface with LEED and RHEED have shown that the reconstructed Au(100)-(5x20) surface is stable in a potential range where no anion adsorption is present [104]. In the presence of anions, the reconstruction is presumed to be lifted to a (lxl) structure. However, at negative potentials, the (5x20) structure is regenerated. The authors observed a difference in the SH rotational anisotropy at these two potentials and attributed it to the reconstruction and lifting of the reconstruction [156]. 

The additional charge and the corresponding additional surface tension are time-dependent quantities in which the equilibrium between the bulk and the interface is not established. The irreversible contribution can be separated from the reversible by considering the time dependence, if the experimental time scale allows for such a test. Time-dependent effects can be observed by impedance measurements at different frequencies. For gold, as an example, impedance measurements showed spectra characteristic for equilibration processes at least over a time scale of 0.1 ms to 100 s. Gold also shows a surface reconstruction depending on the potential [148]. Fortunately, the variation of the interfacial strain with potential is usually so small that the original Lippmann equation (41) for a solid is practically the same as for a liquid electrode 1149]. 

The reason for surface relaxation and surface reconstruction is the minimization of the specific surface energy of the system. Surface reconstruction can be induced either thermally or by the electrode potential. 

Surface relaxation and surface reconstruction phenomena can be lifted thermally, by adsorption of foreign species, or by changing the electrode potential in the case of electrochemical systems [2.10]. In the case of AuQikl) surfaces, the influence of the electrode potential on the structure of the surface top layer is illustrated in Fig. 2.7. The transition from the reconstructed to the unreconstructed surface top layer represents a first order phase transition process in many cases. 

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