Adsorption

3.1 Atoms

The atomic resolution of STM is widely exploited for the structural investigation of overlayers on metal substrates. Achieving atomic resolution requires not only a good instrument, but also a careful elimination of vibrations and thermal gradients as well as the preparation of single crystals with a minimum of surface defects.

Figure 2 shows the stage immediately preceding the formation of an iodine overlayer on an Ag(ll 1) surface [8, 9] . This STM image, which reveals the presence of the first few isolated iodine atoms, was obtained at polarization potentials positive to the most negative adsorption peak, as recorded by cyclic voltammetry. This permits us to locate the most negative potential at which partial charge transfer between the adsorbed anion and the metal substrate starts to become strong enough as to immobilize the anion, making it "visible" by STM: only at more negative potentials does iodide adsorption become reversible enough to allow the determination of the relative adsorption isotherm via a thermodynamic analysis of capacitive charge data obtained by chronocoulometry [10].

7 01 nm

Fig. 2. STM image of an Ag(lll) surface in 10 mM NaF + 0.1 mM Nal. E = 0.835 V/SCE and Etip = +50 mV with I,= 1.5 nA and a W tip.

Fig. 2. STM image of an Ag(lll) surface in 10 mM NaF + 0.1 mM Nal. E = 0.835 V/SCE and Etip = +50 mV with I,= 1.5 nA and a W tip.

To the authors' knowledge this is the first case in which isolated atoms have been imaged on an electrode surface: at submonolayer coverages only ordered islands of adatoms are normally imaged on the substrate, as in the case of bromide adsorption on Ag(lll) [8, 9]. At more positive potentials iodide anions also give rise to the formation of islands with a (V3 x V3)R30° symmetry with respect to the substrate, as shown in Fig. 3. Sulfide ion is chemisorbed on Ag(lll) from neutral and alkaline solutions starting from the most negative accessible potentials [11]. Proceeding towards less negative potentials, it is first chemisorbed randomly, then undergoes a disorder/order two-dimensional (2D) phase transition whose kinetics is controlled by nucleation and growth, and finally it undergoes a further 2D phase transition yielding a more compact overlayer, before giving rise to a bulk deposition of elemental sulfur. The first 2D phase transition takes place at about -1.0 V/SCE and yields a (V3 x V3)R30° overlayer, as shown in Fig. 4.

3.26 nm

6.53 nm

3.26 nm

6.53 nm

Fig. 3. STM image of an Ag(l 11) surface in 10 mM NaF + 0.1 mM Nal. E = -0.74 V/SCE and £„p = +100 mV with /, = 0.9 nA and a W tip.

Fig. 3. STM image of an Ag(l 11) surface in 10 mM NaF + 0.1 mM Nal. E = -0.74 V/SCE and £„p = +100 mV with /, = 0.9 nA and a W tip.

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