Phase transitions in adsrbate layers

It has been realized that the phase behavior on the liquid side of the interface is often much more complex than in the traditional view, where ions are supposed to be randomly adsorbed at the electrode surface.

Fig. 2. Sulfate superstructure on Au(l 11) (a) at 0.85 Vsce in 0.1 M H2S04 (58 x 44 A2) and (b) at the transition potential 0.7 Vsce in 1 M H2S04 (185 x 205 A2) [13].

As an example, the ordered sulfate superstructure formed on Au(lll) in sulfuric acid at potentials «0.5 V higher than the potential of zero charge is shown in Fig. 2(a) [13]. Although a ^ unit cell can be assigned to this structure, in agreement with the coverage of 0.2 monolayers found in radiotracer and coulometric experiments [14], the precise composition of this structure is still unknown. This structure is only observed above a critical potential; at lower potentials no ordered superstructure is observed.

The disorder-order transition is marked by a sharp peak with only minimal hysteresis in the corresponding cyclic voltammogram. This indicates that the transition is fast and requires only very small changes in the sulfate coverage. Time-resolved STM observations of the corresponding, fast nucleation and growth processes turned out to be impossible. However, by keeping the potential very close to the transition potential, coexistent disordered and ordered areas with highly mobile domain boundaries in between, are observed. An example is shown in Fig.2(b) with islands of the disordered phase predominantly in the upper left and lower right corner of the image. In these disordered areas the (lxl) lattice of the underlying Au(lll) substrate is faintly visible. The observed two-phase regime either may indicate that the ordered sulfate adlayer forms by a nucleation and growth mechanism or may correspond to the critical fluctuations near a second-order phase transition. An unambiguous interpretation requires further detailed investigations of these phenomena.

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