Study of CeO2 XPS spectra by factor analysis: reduction of CeO2

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. This 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 problem using factor analysis (FA), a method that has been applied with success to other spectroscopic problems. In order to obtain confident results for the individual shapes of the components, (the basis for a correct quantification) we have combined target testing (TT) and iterative target transformation factor analysis (ITTFA). We have shown that the combined use of these two methods results in a more accurate evaluation (ca. ±2%) of the reduction state of partially reduced CeO2. The election of the “real factors” corresponding to Ce4+ and Ce3+ species is discussed in detail.