Monoatomic island

Thus, in summarizing this STM data, unlike the electrolytic deposition discussed in the previous section, where up to 0.7 monolayer (ML) coverage of ruthenium is deposited as mainly monoatomic islands with a tendency to create three-dimensional deposits as the coverage increases, when spontaneous deposition is used, about 10% of the islands are no longer monoatomic. Instead, such islands have a bilayer character, displaying a second monolayer deposit over the first monolayer. The result that such bilayer islands are formed at low coverage is related to the composition and morphology of the ruthenium deposits formed under a variety of electrochemical conditions. 

Fig. 1. STM image of a chemically polished Ag(l 11) electrode in 0.01 M HCIO4, showing stepped terrace domains with monoatomic steps, a monoatomic island, and a monoatomic pit [4].

Monoatomic islands and monoatomic pits are observed regularly, with typical average... 

Fig.2. Schematic presentation of the local progress of Pb underpotential deposition at monoatomic pits, monoatomic islands, and stepped terrace domains of non-ideal chemically polished Ag(l 11) electrodes [4]. For fiarther explanation see the text.

On the monoatomic islands, no adsorbate layer growth has been observed up to now after step polarization into the potential range of peak A2. However, in one experiment a sequence of local formation and subsequent disappearance of a cluster-like adsorbate domain has been observed within peak A2 on an island. 

On the monoatomic islands, step polarization into the range of peak A3 leads to... 

Desorption of the complete Pb adlayer within the three distinct desorption peaks D3, D2 and D1 (see Fig. 2) by step polarization proceeds in an analogous way to the adsorption sequence, except on the monoatomic islands in contrast to the complete adsorbate formation at the islands in peak A3, desorption in peak D3 only involves the outermost part of the monolayer at the island periphery, whereas the remaining adsorbate coverage is completely desorbed in peak D2. Desorption on the monoatomic islands occurs thus in the same way as at the stepped terrace domains, except for the missing step decoration coverage desorbed in D1. 

The progress of adsorbate formation in the monoatomic pits and at monoatomic islands has not been investigated yet. In contrast to the system Pb/Ag(lll), a higher-periodicity superstructure imagii the adsorbate-substrate registry has been resolved only feintly [18]. 

Waszczuk et al., 2001b Tong et al., 2002]. Because Ru is deposited as nanosized Ru islands of monoatomic height, the Ru coverage of Pt could be determined accurately. In that case, the best activity with regard to methanol oxidation was found for a Ru coverage close to 40-50% at 0.3 and 0.5 V vs. RHE. However, the structure of such catalysts and the conditions of smdy are far from those used in DMFCs. Moreover, the surface composition of a bimetallic catalyst likely depends on the method of preparation of the catalyst [Caillard et al., 2006] and on the potential [Blasini et al., 2006]. 

Schmuld and coworkers [422] have studied initial stages of Cd electrodeposition on the Au(lll) surface from H2SO4 solutions. The results have shown that in such solutions (Cd(II) in H2SO4), the Au(lll) surface starts to reconstruct at potentials around 850 mV versus bulk formal potential of cadmium electrode. Nucle-ation of Cd started in the potential range 350-300 mV as the formation of separated islands of monoatomic height. Further, cathodic polarization led to the formation of strings and finally to the cadmium layer... 

Figure 2.7 In situ STM images illustrating the influence of the electrode potential on the surface structure of a Au(lOO) substrate 12.10]. System Au(100)/10 M H2SO4 at T = 298 K (a) and (c) = - 200 mV vs. SCE (b) and (d) E = 300 mV. Images (a) and (b) show atomic resolution of reconstructed (rows) and unreconstructed surfeces (c) and (d) represent larger scans emphasizing reconstruction rows and monoatomically thick islands, respectively. Reprintetl by permission of Kluwer Academic Publishers.

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