Well-defined synthesis of crystalline MnO, Mn2O3, and Mn3O4 phases by anodic electrodeposition and calcination

Tailoring of the stoichiometry, crystallinity, and microstructure of manganese oxides (MnO_x) is of utmost importance for technological applications in the field of catalysis, energy storage, and water splitting. In this work, α-Mn₂O₃, α-Mn₃O₄, and MnO thin films with defined stoichiometric compositions and crystal structures were prepared by calcination of an anodically electrodeposited Mn-oxyhydroxide precursor film in different gas atmospheres (air, inert, or reducing gas). The crystal structure and composition of the precursor and product films were determined by combining X-ray diffraction, transmission electron microscopy, Raman spectroscopy, Rutherford backscattering spectrometry, and elastic recoil detection analysis. The anodically electrodeposited precursor film consists of nanocrystals of α-Mn₃O₄ dispersed in an amorphous MnOOH matrix phase, and can be fully transformed into either crystalline α-Mn₂O₃, α-Mn₃O₄, or MnO upon calcination in an oxidizing, inert or reducing atmosphere, respectively. In situ high-temperature X-ray diffraction was applied to derive the phase transformation kinetics, resulting in a corresponding activation energy which decreases in the order α-Mn₂O₃ (268 kJ/mole) > MnO (102 kJ/mole) > α-Mn₃O₄ (60 kJ/mole). The disclosed synthesis routes for the preparation of single-phase MnO_x films with a defined crystal structure and stoichiometry can be exploited for a wealth of applications.