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 (MnOx) is of utmost importance for technological applications in the field of catalysis, energy storage, and water splitting. In this work, alpha-Mn2O3, alpha-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 alpha-Mn3O4 dispersed in an amorphous MnOOH matrix phase, and can be fully transformed into either crystalline alpha-Mn2O3, alpha-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 alpha-Mn2O3 (268 kJ/mole) > MnO (102 kJ/mole) > alpha-Mn3O4 (60 kJ/mole). The disclosed synthesis routes for the preparation of single-phase MnOx films with a defined crystal structure and stoichiometry can be exploited for a wealth of applications.