70 Trends in activity and stability
electrochemical measurements a pure Pt overlayer of thickness d is imposed over
a bulk alloy indicated in general as PtnMm: where M will typically stand for one of
the lanthanide metals of this chapter. Organic contaminations mainly consisting of
C, O and H are taken into account as a further overlayer of thickness t. All three
layers are assumed to be uniform and flat as illustrated in Figure 4.7.
Figure 4.7: Sketch of typical layered structure of a PtnMm alloy after ORR activity test. A
layer of C, O and H contaminations (C-O-H layer) of thickness t lies on top of all samples.
This is followed by a pure Pt overlayer of thickness d over an infinitely thick bulk alloy with
stoichiometry PtnMm. The emitted photoelectrons are emitted along straight lines with an
angle with respect to the surface normal.
The information about the thickness of the Pt overlayer d will be provided by
the signals of Pt and M. These will be expressed as:
IP t =
P t (EP t ) cos ()
where the terms I1
P t,allo y and I1
M,allo y represent the XPS intensities of Pt and M
respectively from an infinitely thick PtnMm alloy; both these terms are attenuated
in parentheses by the presence of the Pt overlayer. The second term in the sum
of 4.7a represents the contribution to the Pt signal from the Pt overlayer. The
constants A and B take into account the attenuation of the two signals in the C-O-H