48 Pt-La and Pt-Ce alloys: the active chemical phase
Pt overlayer. For both alloys, and in particular for Pt5La, these changes are very
modest, demonstrating that these surfaces are highly stable under these cycling
conditions and no bulk de-alloying occurs.
3.4 Ion scattering spectroscopy
18 min sputt.
51 min sputt.
84 min sputt.
Intensity (arbitrary units)
EK / eV
Figure 3.6: ISS spectra on polycrystalline Pt5Ce after ORR testing for different sputtering
times. The ISS spectrum acquired on a sputter-cleaned (as-prepared) sample is also reported
The AR-XPS depth profiles of Figure 3.4 seem to indicate that, after ORR testing,
the surfaces of Pt5La and Pt5Ce consist of pure Pt. As it will be better discussed
in Chapter 4, their significance is mostly qualitative: the XPS measurements represent
an average over a 400 m wide region of the surface and over a depth of a few
electron mean free paths ( 10 Å 111). This means that they don’t necessarily
give a comprehensive description of the surface structure that could in principle
be inhomogeneous due to the polycrystalline nature of the samples. Therefore,
ion scattering spectroscopy (ISS) was employed as an even more surface sensitive
tool to further investigate the surface structure of these catalysts. By only probing
the composition of the topmost layer, it allows to exclude the presence of residual
La and Ce atoms on the surface after ORR testing: in this case the possibility of