Microstructure - Electrochemical Surface Reactivity Relationship of Electrochemically Grown Manganese Oxides Films

This Thesis provides a comprehensive and correlative investigation of the microstructure, chemical state, and electrochemical reactivity of manganese oxide (MnOx) films, aiming to gain a deeper understanding of the deposition and dissolution mechanism of MnOx films.Two distinct methods were employed for preparing MnOx films: anodic electrodeposition from Mn2+ solution and anodic oxidation of metallic Mn. In the case of anodic electrodeposition, an extensive inspection of the as-deposited MnOx films was conducted using a combination of Secondary and Transmission Electron Microscopies, Rutherford Backscattering Spectrometry, Elastic Recoil Detection Analysis, Raman and X-ray Photoemission Spectroscopies characterization techniques. The findings revealed that the electrodeposited precursor film consists of nanocrystals of α-Mn3O4 dispersed in an amorphous MnOOH matrix phase under de-aerated electrolyte conditions. In contrast, the Electrochemical Quartz Crystal Microbalance setup allowed for the identification of Mn3O4 and Mn2O3 phases formation in an aerated electrolyte.Post-processing annealing treatments were applied to broaden the range of MnOx stoichiometric variants and to improve the intrinsic stability of the deposited MnOx films. α-Mn2O3, α-Mn3O4, and MnO phases were successfully obtained through the calcination of the electrodeposited Mn-(oxy)hydroxide precursor film in different gas atmospheres (air, inert, or reducing gas). The photocatalytic activity of selected MnOx phases was assessed for the removal and degradation of Tetracycline antibiotics. As-deposited and annealed MnOx exhibited significantly different efficiency under LED visible illumination at varying pH values, with the best-performing α-Mn2O3 phase achieving a 92.4% Tetracycline mineralization efficiency after 180 min. The study of MnOx photocorrosion highlighted that the degradation of the oxide surface structure is a key factor limiting the photocatalytic activity of MnOx films. In this context, the development of a crystalline structure upon annealing proved to be beneficial to increase the microstructural and chemical stability of the MnOx films during the photocatalytic experiment.To provide further insights into the chemical stability of manganese oxides, anodic oxidation processes were implemented. The alkaline "pH effect", with solid products formation predicted from the Pourbaix Diagram, proved insufficient for forming a protective oxide/hydroxide layer on metallic Mn. The addition of phosphate was crucial for achieving this protective passive layer. Potentiostatic growth in the presence of phosphates demonstrated progressive stabilization of the protective layer, consisting of a mix of manganese(II) oxide, hydroxide, and phosphate. Concurrently, the application of higher potentials during the anodic oxidation of metallic Mn resulted in the growth of thicker and more porous oxides, with an in-average higher Mn oxidation state (Mn(II) → Mn(III)).Similar trends of impedance phase shifts were observed during the Electrochemical Impedance Spectroscopy characterization of both anodically electrodeposited and oxidized MnOx films. This comprehensive understanding of the microstructure – electrochemical reactivity relationship extends to distinguishing between porous and catalytic oxides like Mn3O4 and Mn2O3, presenting higher cation mobility, and the more compact MnO film that prevent ionic transportation through its denser structure.