Figure 3: Narrow scans over the O 1s region (a) sample as introduced (b) after 450s Ar+ sputtering

The O 1s photoemission peak, shown in Figure 3, is composed of two peaks due to the nonequivalence of the surface oxygen chemical environments. Previously, the high binding energy feature has been assigned to extraneous surface materials remaining from sample preparation, such as a surface carbonate species. A further suggestion presented by Pfau et al.² is the formation of Ce(III) related surface defects, where oxygen occupies additional lattice sites, resulting in an O 1s core level peak at 2.4eV higher binding energy. This suggestion is supported by this work as the proportion of the O 1s high binding energy feature increases relative to the primary O 1s peak as the Ce(III) surface content increases with increasing exposure of the surface to Ar⁺ ion sputtering. Further evidence that the O 1s high binding energy peak is not due to surface carbonate is demonstrated by considering the C 1s region. With each sputter cycle the C 1s signal is seen to decrease such that after 750 seconds sputtering the surface, the C 1s becomes negligible, whereas the high binding energy O 1s peak increases in intensity and is at a maximum after 750 seconds sputtering.



Acknowledgement

Samples were provided by Prof. Wendy Flavell, UMIST.



References

1. P. Burroughs, A. Hamnett, A.F. Orchard and G. Thornton, J. Chem. Soc. Dalton Trans., (1976) 1686.
2. A. Pfau and K.D. Schierbaum,Surf. Sci., 321 (1994) 71.


Summary

The surface characterisation the Ce_1-x Zr_x O₂ car exhaust catalysts has shown that the AMICUS instrument can easily be employed to provide quantitative information on sample composition.

The features of AMICUS demonstrated in this application include:
•     A high intensity X-ray source allowing good quality photoemission data to be collected within a few minutes
•     Excellent energy resolution of the analyser allowing chemical state information to be determined
•     Quantitative nature of X-ray photoelectron spectroscopy enabling surface composition to be deduced.