XPS spectral analysis for a multiple oxide comprising NiO, TiO2, and NiTiO3

XPS spectra collected from a mixture of NiO, TiO2, and NiTiO3 cannot be analyzed using a standard curve-fitting method; thus, principal component analysis (PCA) was conducted to accurately determine the ratio of various oxides in the mixture. Model samples comprising NiO, TiO2, and NiTiO3 exhibited a complex-shaped Ni 2p spectrum with a satellite peak, from which the atomic ratio of NiO and NiTiO3 could be accurately quantified using the PCA. In contrast, the Ti 2p spectrum exhibited a symmetric shape despite comprising both NiTiO3 and TiO2 states; thus, the PCA results predicted the ratio inaccurately. However, the PCA results showed linear correlation with the theoretical values over an entire range. Thus, accurate ratios were obtained using a calibration curve. The PCA procedure for multiple oxides was applied to angle-resolved XPS spectra for thermally oxidized nickel titanium alloy. The results revealed the in-depth distribution of NiO, TiO2, and NiTiO3 on the surface of the alloy.