it spans from 200 to 500 nm, taking into account the 16 degrees incidence angle of the impinging X-rays with respect to the sample surface. To compare the chemical composition between the samples, i.e. the elemental Ti/O/Fe ratio, the spectra are normalized in two steps. First, a normalization is applied with respect to the incoming X-ray beam intensity, 𝐼 0 (Figure ). To fulfil the requirement of a flat spectral response over the useful energy range, gold-plated substrates were used to obtain the 𝐼 0 . Since we record two kinds of signals on the sample, i.e. photoemission (surface -blue) and transmission (bulk -red), different 𝐼 0 normalization are required. Thus, for the surface signal, we used 𝐼 𝐼 0 ⁄ (Figure ). The optical density of the bulk signal was extracted as 𝑂𝐷 = 𝑙𝑜𝑔(𝐼 0 𝐼 ⁄ ) (Figure ). A second normalization was applied, considering that the chemical formula includes only Ti, O and Fe. A linear background is subtracted from the Ti L 2,3 pre-edge region (450 -455 eV) bringing this region to zero. In this way we remove spectral contribution arising from thickness and density. Then the spectra are matched over the Fe L2,3 post-edge (735 -740 eV) region, by applying a multiplicative factor. As a result, the elemental chemical ratio is directly proportional to the edge jumps. As it can be observed in Figure , the spectra corresponding to the bulk regions are very similar for the S1 and S2 samples. Indeed, the edge jumps are almost identical suggesting that there is no difference in the bulk elemental composition between S1 and S2. Across the Fe L2,3 absorption edges, the signal from bulk is saturated due to the thickness of the particles (i.e. larger than 200 nm) and the strong X-ray attenuation length that varies between 80 and 120 nm in this spectral region.