Is important to mention that we did not find any Sn diffusion, as often reported for the hematite based photoanodes deposited on FTO substrates. At the O K-edge (~533 eV) we would expect a characteristic spectral feature related to electronic transitions in the Sn 5p orbitals. This is not the case for any of the samples studied here. It is generally accepted that Sn diffusion scales with the annealing temperature. Since both samples were annealed at the same temperature (i.e. 600 °C), if any Sn diffusion is present (not detectable by our measurements), it should be of the same amount and therefore affect the PEC results at same level. In addition, we tested the possible formation of nitride phases at the N K-edge (for Fe-N and/or Ti-N coordination). Nothing detectable was measured. We want to stress that, as for the Sn diffusion, NEXAFS loses its sensitivity for concentration values below ~0.1 %. While we cannot rule out definitively the formation of nitride phases, that would be certainly below the needed quantity to drive the PEC enhancement upon N2 annealing. In addition, we recall that the probability of obtaining nitrides by exposing an oxide in N2 atmosphere, even at higher temperatures, is very low (related to the high activation energy barrier of the N-N triple bond of 940.95 kJ/mol). Indeed, the studies reporting metal nitrides synthesis show different methods than high temperature annealing. 2.3.2. CORRELATIVE MEASUREMENTS: STXM, SPECTRO-PTYCHOGRAHY AND STEM S-XPEEM offers high surface selectivity, but its spatial resolution is altered due to space charge effects, occurring particularly in 3D-shaped samples, like the ones measured here. Space charge effect arises from Coulomb collective repulsion of the photoelectrons generated in the surface region of our samples, resulting in a spatial broadening and therefore in resolution loss. To improve the spatial resolution associated with the spectral features discussed above, we measured the samples using STXM and spectroptychography. These are photon-in/photon-out techniques, exempted thus from space charge effects.