A second notable difference appeared in the Si2p region indicating oxidized species. In the Si2p region, peaks around 99.7 eV and103.5 eV indicate the Si0 substrate and completely oxidized Si4+ respectively. In between both extremes, underoxidized Si (SiOx) and/or silicates (Y/Zr-O-Si) can appear26. In principle, at least one monolayer of silicate bonds is expected due to the interface between YSZ and Si or SiO2, which contributes significantly to the XPS spectra due to the surface sensitivity of XPS. As visible in Fig. 1, the region indicating SiO2 increased with respect to the region indicating silicates and SiOx species when the pO2 was increased or the laser repetition rate was decreased. Although any quantification cannot be performed without knowledge about the morphology and distribution of the different species, the measurements suggest increasing regrowth of SiO2 with increasing pO2 or decreasing laser repetition rate. Quantification of the SiO2 thickness will be discussed later.