hematite phase, doesn't seem to visibly affect the morphology of the nanorods. We expected morphological changes during the phase transition from akaganeite to hematite since there should be mass reduction from water and chlorine loss with increasing temperature. PEC activity is characterized for all samples by photocurrent measurements. For this purpose, fast sweep (50 mV/sec) voltammetry is performed using a dedicated setup detailed elsewhere . Figure presents the photocurrent densities, 𝐽 𝑝ℎ , as a function of the applied potential V (vs. RHE) obtained from the ACG synthesized sample, in comparison with an epitaxial counterpart . All hematite photoanodes give low photocurrent density values, well below 1 mA/cm 2 . Miao et al. show that by increasing the open-circuit PEC voltage (ΔVF,OC), defined as the difference between the Fermi levels for electrons (EFn) and holes (EFp) upon illumination, the onset potential and, therefore, the input energy necessary to initiate the OER, decreases. Iandolo et al. , have shown that: i) complex charge transfer processes that occur show an onset potential lower than +0.6 V vs. RHE , while those obtained by AO-MBE exhibit values  1.0 V vs. RHE . At first sight, morphology accounts for this difference. Indeed, the photoanode surface exposed to the electrolyte is estimated to be more than 40 times larger for the ACG deposited nanorods than AO-MBE samples. The factor of 40 is derived using a simple model: each 2500 nm 2 unit surface is assumed to be replaced by a parallelepiped with 50 × 50 nm 2 base and 500 nm height. Nevertheless, the increase of the size of the active interface should lead to a proportional increase of the photocurrent. When comparing the two kinds of samples, an increase of the photocurrent value is observed, but not proportional to the increase in surface area. Charge transfer at the semiconductor/electrolyte interface, related to a specific chemistry, might account for the onset potential differences. To test this hypothesis, we studied the ACG hematite nanorods using the STXM at the HERMES beamline of the SOLEIL synchrotron facility.