Electrocatalysis at Platinum and Bimetallic Alloys - Fuel Cell Catalysis: A Surface Science Approach - page 317

Figure 10.2 (a) Cyclic voltammograms and (b) the simultaneous changes in mass during
repetitive potential scans on Pt 49 Co 51 alloy EQCM at 25 8C in 0.1 M HClO 4 purged with He.
Sweep rate 20 mV s 21 .
10.2.2.2 EC-STM Observation This puzzle of the change in the surface
state of Pt alloys during the potential sweeps was clearly resolved by EC-STM
[Wan et al., 2002]. When 0.1 M HClO 4 solution was injected into the EC-STM
cell, the surface morphology of the Pt-Co or Pt-Fe film was found to change, resulting
in a rough surface with cauliflower-like features that indicated dissolution of the
surface alloy layer. However, after several potential sweeps, we observed STM
images of a terrace - step structure, distinct from the rough surface. The step lines
were found to cross each other at 608 or 1208. The width of the terraces was from
several nanometers to more than 10 nm. Figure 10.3a shows an example of an atomic
resolution STM image on the terrace. A hexagonal structure can be seen in the image.
The interatomic distance was measured to be approximately 0.28+0.02 nm, which
is close to the lattice spacing of Pt(111). Hence, it is clear that the dissolution of surface
alloy layer was followed by a rearrangement of the Pt layer, resulting in well-ordered
Pt(111)-(1 1) structural domains of the Pt skin layer, which can protect underlying
bulk alloy.