For electron microscopy characterization, a variety of equipment was used. The morphological study was carried out on a JEOL JSM 7600F scanning electron microscope operating at an acceleration voltage of 15 kV. Atomic resolution characterization was performed on a JEOL JEM-ARM200cF electron microscope (Cold Emission Gun) operating at120 kV provided with a spherical aberration corrector in probe (current emission density ∼1.4 × 10 -9 Ă and probe size ∼0.08 nm), a GIF-QuantumER spectrometer and an Oxford INCA-350 detector. High-angle annular dark-field (HAADF) images were recorded using a nominal camera length of 60 mm and inner and outer collection semi angles of 68 and 280 mrad, respectively. Electron Energy Loss Spectroscopy (EELS) experiments were acquired using a collection semi-angle of 18 mrad and a convergence semi-angle of 20.3 mrad. The local iron oxidation state was analyzed from the Energy-Loss Near-Edge Fine Structure (ELNES) of Fe-L2,3 signals.