Au surfaces, reconstruction

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. 

Au(111)/Br X-ray diffraction and STM studies of the adsorption of bromide (and other halides) onto Au single-crystal electrodes have been reported in a series of papers from the Brookhaven group [55-57]. Figure 11(a) shows a CV for a Au(lll) electrode in deaerated 0.1 M HCIO4 + 0.1 M NaBr solution. As discussed by Kolb [9], the peaks at —0.12 V (anodic sweep) and —0.18 V (cathodic sweep) are related to the lifting/forming of the Au surface reconstruction, which is... 

Bonig L, Liu S and Metiu FI 1996 An effective medium theory study of Au islands on the Au(IOO) surface reconstruction, adatom diffusion, and island formation Surf. Sot 365 87... 

Surface reconstructions have been observed by STM in many systems, and the teclmique has, indeed, been used to confmn the missing row structure in the 1 x 2 reconstruction of Au(l 10) [28]. As the temperature was increased within 10 K of the transition to the disordered 1 1 phase (700 K), a drastic reduction in domain size to -20-40 A (i.e. less than the coherence width of LEED) was observed. In this way, the STM has been used to help explain and extend many observations previously made by diffraction methods. 

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... 

The surface reconstruction of Au(110) is more rapid than that of Au(l 1 l)andAu(100).257,467,504-514,516-518Au(533)andAu(311), localized in the [(110)-(100)] zone, and Au(221) and (331), localized in the [(111)-(110)] zone, exhibit stable terrace step structural arrangements largely free from disordering and facetting 485 Au(210) and (410), localized in the [(100)-(110)] zone, display only a short-range structural order related to the especially open nature of these faces. 

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... 

Kolb and Franke have demonstrated how surface reconstruction phenomena can be studied in situ with the help of potential-induced surface states using electroreflectance (ER) spectroscopy.449,488,543,544 The optical properties of reconstructed and unreconstructed Au(100) have been found to be remarkably different. In recent model calculations it was shown that the accumulation of negative charges at a metal surface favors surface reconstruction because the increased sp-electron density at the surface gives rise to an increased compressive stress between surface atoms, forcing them into a densely packed structure.532... 

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 second procedure is different from the previous one in several aspects. First, the metallic substrate employed is Au, which does not show a remarkable dissolution under the experimental conditions chosen, so that no faradaic processes are involved at either the substrate or the tip. Second, the tip is polarized negatively with respect to the surface. Third, the potential bias between the tip and the substrate must be extremely small (e.g., -2 mV) otherwise, no nanocavity formation is observed. Fourth, the potential of the substrate must be in a region where reconstruction of the Au(lll) surface occurs. Thus, when the bias potential is stepped from a significant positive value (typically, 200 mV) to a small negative value and kept there for a period of several seconds, individual pits of about 40 nm result, with a depth of two to four atomic layers. According to the authors, this nanostructuring procedure is initiated by an important electronic (but not mechanical) contact between tip and substrate. As a consequence of this interaction, and stimulated by an enhanced local reconstruction of the surface, some Au atoms are mobilized from the Au surface to the tip, where they are adhered. When the tip is pulled out of the surface, a pit with a mound beside it is left on the surface. The formation of the connecting neck between the tip and surface is similar to the TILMD technique described above but with a different hnal result a hole instead of a cluster on the surface (Chi et al., 2000). 

The abilities of this approach will then be illustrated with two examples (i) the potential-induced lifting of the Au(lOO) surface reconstruction and (ii) the electrochemical oxidation of Pt(l 11). 

Besides surface reconstructions induced by heat treatment, potential-induced reconstruction has recently become a topic of interest in electrochemistry. It has been observed that at potentials negative with respect to the potential of zero surface charge, [Kolb, 1996, 2002 Dakkouri, 1997], the reconstructions found under UHV conditions are also stable in contact with an electrolyte. Although aU low index faces of Au and Pt undergo potential-induced reconstruction, it has been particularly well characterized for Au(lOO) (Fig. 5.5). 

Figure 5.6 Capacity versus potential measurements on the lifting of surface reconstruction of Au(lOO) in 0.1 M H2SO4 [Kolb, 1996]. Whereas below 0.55 V the sohd curve of Au(100)-hex more or less coincides with separate measurements on Au(l 11) (dashed curve), increasing the potential above +0.55 V lifts the reconstruction and gives Au(100)-(1 x 1) (dotted curve).

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. 

Jacob T. 2007a. Potential-induced lifting of the Au(100)-surface reconstruction studied with DFT. Electrochim Acta 52 2229-2235. 

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. 

Given the efforts in this group and others (Table 1) to form the Cd based II-VI compounds, studies of the formation of Cd atomic layers are of great interest. The most detailed structural studies of Cd UPD have, thus far, been published by Gewirth et al. [270-272]. They have obtained in-situ STM images of uniaxial structures formed during the UPD of Cd on Au(lll), from 0.1 M sulfuric acid solutions. They have also performed extensive chronocoulometric and quartz crystal microbalance (QCM) studies of Cd UPD from sulfate. They have concluded that the structures observed with STM were the result of interactions between deposited Cd and the sulfate electrolyte. However, they do not rule out a contribution from surface reconstructions in accounting for the observed structures. 

A mechanism which proceeds through surface reconstruction of the substrate has been identified for Ni deposition on Au(lll) [120, 121]. The process begins with place exchange of nickel into a particular position in the reconstructed Au(lll) surface, followed by deposition of Ni islands on top of the imbedded atom. At higher overpotentials, nucleation occurs instead at step edges, so that control of the potential allows control of the nucleation process and the distribution of Ni in the early stages of growth. In this instance, the nucleation process has been captured by STM on the atomic scale. 

Surface reconstruction on metal crystals depends on the interior lattice as well as on the nature of the metal, such as Au (100)-(5 x20), Au (lll)-(l x 23) and Pt (llOHl X 2) [Kolb, 1993]. In general, the activation energy of surface reconstruction is relatively great ( 1 eV) on clean metal surfaces so that the reconstruction is frequently suppressed at room temperature. Usually, surface adsorption changes the activation energy that catalyzes or inhibits surface reconstruction. 

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