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Figure 10.3 (a) An atomic-resolution STM image (2.5 nm 2.5 nm) of a Pt skin layer
formed on Pt 55 Co 45 after electrochemical stabilization. The image was observed at 0.05 V in
N 2 -purged 0.1 M HClO 4 . The tunneling current and bias voltage were 42.6 nA and - 4 mV,
respectively. (b) Scheme of formation of Pt skin layer on Pt alloyed with nonprecious metal.
10.2.2.3 Thickness of Pt Skin Layer Analyzed by ICP and EQCM We
determined the thickness of the Pt skin layer formed on the alloy, based on the
mass changes from EQCM and the amount of each ion dissolved in the electrolyte
solution from an inductively coupled plasma (ICP) analysis. The amount of Pt
ions detected by ICP remained at a very low level compared with those of Fe or Co
ions. This indicates that most of the Pt ions that might have dissolved out
from the alloy were re-deposited on the surface, and the Pt atoms were rearranged
during the potential sweeps (Fig. 10.3b). The thickness of the Pt skin layer
calculated was about 1 - 3 nm, corresponding to 4 - 11 monolayers, respectively,
if the layer is oriented preferentially to Pt(111). This is consistent with the order
of the value estimated from XPS measurements [Watanabe et al., 1999; Igarashi
et al., 2001].
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