To quantify the carbon component, we performed an energy stack across the C K-edge absorption edge, from 270 to 320 eV. The inset in Fig. shows the difference between images recorded post-(320 eV) and pre-(270 eV) C K-edge. The C K-edge contrast in this difference image (a carbon map) is due to the X-ray induced carbon layer deposition on the nanorod. Since we use OD1 units reference spectra, the RGB mapping is quantified in terms of the thickness, i.e. nm units, and thus the color-bar scale is given in nm units and the color intensity of each component in the map indicates the thickness of that component. Hematite has a maximum intensity of 105 nm in the left center of the core of the nanorod, while hexahydrate iron chloride has a maximum intensity of 51 nm at the interface of the nanorod and the FTO support layer. To calculate the deposited carbon contribution, the SVD was determined in OD units and then converted to thickness using the elemental response for pure C and a density of 2 g/cm 3 density (amorphous carbon). The extracted maximum value for carbon in the map is 85 nm, a value that corresponds to the size of the sides of the nanorod. Comparing this value to the expected width of the nanorods of ~100 nm as determined from SEM, most probably the C layer is a few nanometers thick with a density that is < 2 g/cm 3 . In this color-coded composite image the astonishing 650 nm long and 110 nm wide hematite nanocrystal with well-defined rectangular shape, is seen clearly. At both ends, the hexahydrate iron chloride phase is present in larger quantity, characterized by a visibly brighter green component. starts from the tip. Furthermore, the segmented nanorod morphology implies that during the phase transformation hematite crystals form by an aggregation mechanism as evidenced using in-situ X-ray diffraction . Additional useful results about the nanoscale mechanism driving the phase transformation could be obtained by in-situ STXM . It is also interesting that, despite the initial spectral approximation (i.e. considering the linear combination of hematite and hexahydrate iron chloride), the SVD approach using the references in OD1 units produces thickness values that are in perfect agreement with the nanorod dimensions from SEM measurements. Finally, there are obvious implications to the PEC activity of the distribution of the specific iron phases. Even considering the "simple" case of an ideal hematite surface, the OER driving the PEC activity is a very complex process . From Fig. it appears that a major part of the surface exposed to the electrolyte is the hexahydrate iron chloride phase, and only a small part of the surface is hematite. We assume that FeCl3.6H2O acts as a Brønsted Lowry acid, presenting slightly increased ℎ + concentration at the interface with the electrolyte, i.e. acting as a proton donor. Indeed, similarly to hexa-aqua complex ions, the Fe 3+ cations will cause the electrons to be pushed away from the O 2-. As a result, the electron pairs from the O-H bonds will be pulled toward the O 2-creating an apparent overall positive charge.