The ACG synthesis method offers an easy, cheap, and versatile way of obtaining nanostructured photoanodes . Guided by a global low environmental impact approach, samples were synthesized from FeCl3 salt dissolved in water, with pH values controlled by adjusting the salt quantity. No additives were used to control solution pH value or growth rate. Nanostructured hematite layers prepared by the ACG method were deposited on Fluorine-doped Tin-Oxide (FTO) coated glass substrates. Thus, hydrothermal activation at 95°C for several hours was used to generate the β-FeOOH phase (akaganeite). Air annealing at 500 °C is employed in a second step to induce a phase transformation of β-FeOOH to hematite, α-Fe2O3. As reported in the literature for the ACG method , a strict protocol for the cleaning of the FTO substrates must be followed to get uniformly grown layers and thus reproducible PEC measurements. The detailed protocol used to prepare the samples studied in this work was reported in a previous publication . We remind here briefly that several growth parameters play an important role in determining the resulting morphology, especially regarding the length of nanorods, while their width remains constant. Thus, varying the deposition time from 4 to 24 hours, allow tuning the length of the nanorods, at fixed pH value. As an example, at pH = 1.4, we can vary the nanorods length from ~200 to 500 nm. This variation is limited though, and we observed that nanorods appear quite rapidly during the hydrothermal process. The samples we used in the present study were all grown for 24 hours. Nanostructured hematite films are thus obtained, composed of nanorods oriented perpendicularly to the substrate in a carpet or brush-like morphology (Figure ). It is worth mentioning here, without diving into details beyond the purpose of this report, that diffraction measurements showed the nanorods had the [110] hematite crystallographic axis orientation perpendicular to the FTO substrate. The [110] orientation in hematite is the electrical conduction easy axis and is thus beneficial to PEC applications. In addition, X-ray diffraction data collected using in-plane geometry, showed a powder like diffraction pattern, implying a fully isotropic (i.e. non-textured) in plane orientation of the hematite nanorods . Tuning the pH value leads to a variation of the nanorod length, from ~1 µm to ~300 nm in pH ranges from 1.0 to 1.4, respectively. Figures 1a, 1b and 1c are Scanning Electron Microscopy (SEM) images of samples grown in solutions with pH = 1.0, 1.2 and 1.4 respectively. In all cases uniform brush-like layer of hematite nanorods grow perpendicularly to the surface of the FTO substrates. By increasing the pH of the solution from 1.0 to 1.4, the number of irregularly oriented nanorods grown as overlayer is reduced. While the nanorod length varies when varying the pH value, their width (50-100 nm) remains relatively constant. An SEM image of a cross-section of the hematite "brush" fabricated by ultramicrotomy (Fig. ), clearly shows the ~80 nm thick, bright, FTO substrate layer and the welldefined hematite nanorods. Air-annealing at 500 °C, typically applied to transform the akaganeite to and), in akaganeite phase (1 st row) and hematite phase (2 nd row). We remark the same uniform underlayer composed of nanorods pointing out from the surface for all pH values. Decreasing the pH value increases the number of the irregularly oriented nanorods growing on the top of the well-defined, carpet-like hematite layer. g) ultramicrotome section through the nanostructured hematite layer, showing ~500 nm long nanorods. The bright, well-defined layer below the hematite nanorod "brush" is the FTO substrate on top of very thick glass support.