Structural transitions of the electrode surface

It has been shown previously by ex-situ and spectroscopic techniques that after transfer into an electrochemical environment the reconstruction of Au single crystals, well known from UHV studies, is preserved at negative potentials but is lifted upon raising the potential above a critical limit [2, 3]. Furthermore, the transition between the reconstructed and unreconstructed surface is reversible, with the transition potential depending on the electrolyte composition. These results have been recently confirmed by in-situ STM [4 - 10]. Particularly well studied is the hexagonal reconstructed

Au(100) electrode surface [4, 5, 9, 10], where the "hex" <-ยป (lxl) transition is strongly kinetically hindered, leading to a large hysteresis in the transition potentials.

The onset of this structural transition from the "hex" reconstructed to the unreconstructed (lxl) phase is illustrated in the STM images presented in Fig. 1 [9]. At potentials below the transition potential (Fig. 1(a)) an atomically flat Au(100> terrace with several domains of the "hex" reconstruction is visible. The reconstructed domains consist of integer multiples of individual, row-like elements, which in this case are terminated at a step edge running horizontally. These domains are separated here by unreconstructed areas, which appear darker in the STM images. Raising the potential above the transition potential (Fig. 1(b)) a slow removal of the reconstruction sets in. This process starts in the direct vicinity of the step edge. In the transition from the higher-density reconstructed phase to the (lxl) phase surplus Au atoms are expelled

Fig. 1. Initial stages of the "hex" (lxl) transition on Au(100) in 0.01 M H2S04; (a) -0.15 Vsce and (b) 0.3 VSCE (1300 x 1060 A2) [9].

onto the surface, where they coalesce into small islands that are plainly visible in the STM experiments. With time, the unreconstructed areas grow along the reconstruction rows towards the center of the terrace. As evident from the unchanged appearance of the domain boundaries in the center of the terrace, the (lxl) areas do not grow perpendicular to the reconstruction rows (Fig. 1(b)). Hence, the lifting of the reconstruction, as well as the reverse process of the potential-induced growth of the "hex" reconstruction, proceed via heterogeneous nucleation at defects (e.g., steps) and subsequent quasi one-dimensional growth. This is in good agreement with the zero-order kinetics found in spectroscopic experiments [2]. The growth and removal of individual quasi one-dimensional reconstruction elements is the microscopic mechanism of all transitions between the reconstructed and the unreconstructed phases on the three low-index Au surfaces [5, 7-10]. A similar growth behavior was observed in UHV studies for the removal of the structurally identical "hex" reconstruction of Pt(100) by CO adsorption [11, 12].

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