The evaluation of the reduction state of CeO2 by XPS is still controversial
because of the presence of more than 10 peaks in the Ce(3d) photoemission
spectra due to Ce3+ and Ce4+, some of them at similar binding energies. Thi
s makes difficult conventional fitting using nonlinear least-square (NLLSF)
methods with Gaussian-Lorentzian curves that require the knowledge of the
number of bands. In the present work, we have tried to overcome this proble
m using factor analysis (FA), a method that, has been applied with success
to other spectroscopic problems. In order to obtain confident results for t
he individual shapes of the components, (the basis for a correct quantifica
tion) we have combined target testing (TT) and iterative target transformat
ion factor analysis (ITTFA). We have shown that the combined use of these t
wo methods results in a more accurate evaluation (ca. +/- 2%) of the reduct
ion state of partially reduced CeO2. The election of the "real factors" cor
responding to Ce4+ and Ce3+ species is discussed in detail. (C) 2000 Elsevi
er Science B.V. All rights reserved.