Results And Discussion 31 Electrocapillary Curves

The electrocapillary curve of a solid metal electrode is more difficult to measure than that of a liquid electrode, because of problems of surface cleanliness. The most widely used approach has been the bending-beam method, which was originally developed by Fredlein et al. [28] using large samples. More recently Raiteri and Butt [29] have used gold electrodes deposited on an AFM cantilever to record electrocapillary curves.

As a calibration of our instrument we have recorded electrocapillary curves for a gold (111) electrode surface (Fig.2), vacuum-evaporated onto a standard silicon nitride AFM cantilever.

Fig. 2. Cyclic voltammogram/stress curves of electrocapillary effects on a gold(lll)-coated cantilever in 0.1 M KC1. Scan rate lOmVs"1.

Potential vs. Ag/AgCI N

Fig. 2. Cyclic voltammogram/stress curves of electrocapillary effects on a gold(lll)-coated cantilever in 0.1 M KC1. Scan rate lOmVs"1.

The cantilever deflection results from a change in the surface stress as the electrode potential is varied. The surface stress is related to the surface energy by the Shuttleworth equation:

as where cris the surface stress, / the surface energy and sthe surface strain. For liquid electrodes the second term is identically zero. However, this is not the case for solid surfaces because the surface energy required to form unit area of a strained surface is different from that required to form unit area of an unstrained surface. Despite this complication, Mohilner and Beck [40] have shown that typical values of this term are several orders of magnitude smaller (10"4 Nm"1) than the measured surface stress changes associated with electrocapillary curves or UPD processes, which are greater than 0.1 Nm"1. So for the purposes of this work the surface stress and surface energy can be regarded as approximately equal.

There is considerable variation in the quoted values of the potential of zero charge (pzc) or electrocapillary maximum for gold. For instance the quoted pzc values for gold in 0.1 M KC1 range from 0 to -0.6 V [28 - 29] versus the standard hydrogen electrode (SHE). This must be due to the sensitivity of the pzc to the surface crystallinity and the fact that these studies were carried out on polycrystalline gold surfaces . The pzc is also known to be highly sensitive to surface contaminants.

The pzc of our well-defined gold (111) surface was in all cases between -0.2 and -0.3 V with respect to the SHE (Fig.2). Despite the wide range of pzc values, the relative stress changes in this work and that in the literature are all in good agreement. That is to say that the measured stress changes between the pzc and some other potential are similar. The electrocapillary curve is therefore a good diagnostic test of the stress sensor and our calibration procedure.

There is some hysteresis associated with the electrocapillary curves. This is shown in the difference between the pzc values for cathodic and anodic scan directions (scanning to more negative and positive values respectively). This has been observed in other stress-voltage measurements. This hysteresis shows some scan rate dependence and is reduced at lower scan rates, suggesting that some kinetic factor may be involved.

3.2 Silver Underpotential Deposition

The stress-voltage curve for the silver UPD process in sulfuric acid (Fig. 3) shows a clear change in the lever deflection signal corresponding to the first silver monolayer peak. This is apparent in both deposition and stripping. This indicates that there is a change in surface stress which occurs when a silver monolayer is deposited onto or stripped from the gold surface. The direction of the lever deflection shows that as the silver monolayer is deposited there is a reduction in the compressive surface stress. Then as the potential is scanned in the cathodic direction, the compressive stress is gradually restored. The general form of the variation shows several similarities to that shown for copper UPD on Au (111) in sulfuric acid [30], a comparable UPD system.

At high positive potentials prior to the deposition of the first silver monolayer, the gold surface is covered with an ordered sulfate lattice [41]. Repulsive anion-anion interactions give rise to a compressive surface stress. On formation of the first silver monolayer some of the sulfate species must be displaced from the surface. According to AFM studies [13], the resulting adlattice consists of an open (3x3) arrangement of the silver atoms with some remaining co-adsorbed sulphate, and STM studies find a (a/3 x V3)R30° adlayer [15]. Irrespective of the apparent disagreement between these AFM and STM studies, this phase is under less compressive stress than the sulfate arrangement, because of the reduction in anion-anion repulsions and the presence of partially discharged cations.

There are three possible explanations for the gradual increase in the compressive stress as the potential is scanned towards the bulk silver deposition region. The first is that the silver monolayer enhances further adsorption of sulphate anions (this has been demonstrated on polycrystalline gold by radiotracer measurements [42 - 43]). Consequently there is a gradual build-up in compressive stress which continues until further silver deposition occurs near the bulk silver deposition region.

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