A

Fig. 4. STM image of an Ag(l 11) surface in 0.2 M NaOH + 0.8 mM Na2S. E = -1.03 V/SCE and £tip = 250 mV with It = 1.5 nA and a W tip.

The chemisorption of sulfide ions involves a total charge transfer, as indicated by the perfect correspondence between the ordered structures revealed by the STM images and the flow of charge which accompanies their formation and which was estimated both by cyclic voltammetry and by chronocoulometry [12].

In investigating the behavior of silver electrodes in sulfide solutions, it was observed that the aging of the solution is accompanied by a progressive irreproducibility of the cyclic voltammograms. Thus, if the sulfide solution is exposed to the light for a sufficently long time, long polysulfide chains are observed on the Ag(lll) surface; these are strongly adsorbed even at far negative potentials and are clearly revealed STM, as shown in Fig. 5.

The underpotential deposition of sulfur on Ag(l 11) is just the first step towards the realization of mono- and multilayers of semiconducting sulfides, such as CdS and PbS, by the alternated underpotential deposition of S and Cd (or Pb) atomic monolayers on the low-index faces of silver. This procedure, called electrochemical atomic layer

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