The Effect of Structurally Well-Defined Pt Modification on the Electrochemical and Electrocatalytic Properties of Ru(0001) Electrodes - Fuel Cell Catalysis: A Surface Science Approach - page 468

14.3.1.2 Pt Film-Modified Ru(0001) Electrodes

Surface Structure and Adsorption Properties at the Solid / Vacuum
Interface For deposition at temperatures around room temperature, Pt growth on
Ru(0001) proceeds via nucleation and lateral growth of monolayer islands with a dis-
tinct triangular shape, which at higher coverages start to coalesce (Fig. 14.3a) [Buatier
de Mongeot et al., 1998; Diemant et al., 2003; K¨sberger et al., 2003]. The island
shape is indicative of a dendritic growth process [Hwang et al., 1991; Brune et al.,
1996]. At Pt coverages around 1 ML, essentially perfect monolayer films can be
produced by Pt deposition at 300 - 350K and subsequent annealing to 800K
(Fig. 14.4a) [Hoster et al., 2008; K¨sberger et al., 2003]. Up to film thicknesses of
at least four atomic layers, Pt grows pseudomorphically on Ru(0001), i.e., for films
in this thickness range, the Pt lattice is laterally compressed by 2.5% with respect to
Pt(111) to adapt to the lattice constant of the Ru(0001) surface [Buatier de

Mongeot et al., 1998; Schlapka et al., 2003].
The chemical properties of these bimetallic surfaces were characterized previously
by adsorption of CO and deuterium. The interaction of a Pt monolayer-covered
Ru(0001) surface with these adsorbates is very weak—significantly weaker than for
adsorption on the respective bulk substrates Ru(0001) or Pt(111) [Buatier de
Mongeot et al., 1998; Schlapka et al., 2002, 2003; Jakob and Schlapka, 2007;
Davies et al., 2005; Greeley and Mavrikakis, 2005; Groß, 2006]. With increasing Pt
film thickness, the binding energies of CO ad and O ad increase again and finally
approach constant values [Schlapka et al., 2003; Lischka et al., 2007]. The differences
between these latter values and the adsorption energies on Pt(111) were attributed to the
compressive strain in the pseudomorphic layers, while the additional destabilization
observed for very thin films was associated with an electronic modification of the Pt

Figure 14.3 (a) Deposition of 0.5 ML Pt vapor onto Ru(0001) at 40 8C (217 nm 217 nm,
I t ¼ 0.56 nA, U t ¼ 1 V). (b) The same surface after formation of a Pt 0.5 Ru 0.5 /Ru(0001)
surface alloy by flash annealing to 1350K; Pt and Ru atoms appear dark and bright, respectively
(11 nm 11 nm, I t ¼ 28 nA, U t ¼ 52 mV).