Hydrogen production by solar water splitting (SWS) using abundant and eco-friendly photoelectrode materials is very appealing nowadays. The overall efficiency of SWS reaction is directly related to the photoelectrochemical (PEC) activity including both oxidation and reduction reactions that occur at the interface between photoelectrodes and the aqueous electrolyte. Hematite is the archetype semiconducting material used as photoanode, presenting a band gap of 2.15 eV, perfectly matching the solar spectrum for optimized absorption and thus for direct SWS applications. Compared to state-of-the-art oxide semiconductors used for solar water splitting, pristine hematite presents low efficiency because of the reduced hole mean free path (~ 2 -4 nm) and poor surface kinetics related to a complex oxygen evolution reaction (OER). It was demonstrated that surface kinetics during SWS can be tuned by controlling the surface states nature using various approaches: doping, hetero-and nano-structuring, annealing, catalyst film coating, etc. In particular the synergic effect between Ti-doping and induced oxygen vacancies through thermal treatments during or post growth under oxygen depleted atmosphere has shown its efficiency. Various mechanisms were suggested to contribute to the enhancement of PEC activity. Zhao et al. have shown that by annealing pristine and Ti-doped hematite photoanodes in Nitrogen gas at 600°C the photocurrent is increased by 200 % and 67 % respectively. They assigned this enhancement to oxygen vacancies generated by nitrogen treatment, leading to carrier density increase. Opposite behaviors were found for the charge transfer processes, comparing pristine hematite and Ti-doped hematite, suggesting increased transfer resistance for the later one. To the contrary, Wang et al. demonstrated strong electrocatalytic surface contribution of the induced oxygen vacancies along to the expected improvement of the bulk conductivity. Such divergences arise from the variety of the employed sample preparation methods, leading to different sensitivities to subtle surface effects. These conclusions are obtained from PEC macroscopic measurements only, nanoscale information being absent. A perfect demonstration is given by Zhang et al., in the case of highly ordered attached Ti-modified hematite mesocrystals. Their study presents strongly enhanced PEC properties promoted by a double contribution: first, the formation of interfacial oxygen vacancies yielding high carrier density, and second, shorter depletion width (< 10 nm) over large regions through formation of rutile TiO2 at the mesocrystals surface, as determined by high resolution electron microscopy. Indeed, complex hematite-Ti based heterostructures (Fe2O3/Fe2TiO5 or Fe2O3/FeTiO3) are known to be responsible for PEC activity enhancement owing to strongly increased surface charge separation. This leads to increased number of holes injection at the interface with the electrolyte.