Fabrication, In-Depth Characterization, and Formation Mechanism of Crystalline Porous Birnessite MnO2 Film with Amorphous Bottom Layers by Hydrothermal Method

Hydrated porous birnessite MnO 2 films were prepared on Si substrates and characterized in-depth by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT-IR). The as-prepared film is composed of a well-crystallized porous surface layer and one or more amorphous bottom layers. The morphology and structure of the film can be understood by the combined action of internal stress and conversion of the porous surface layer. The dynamic process of formation and conversion in the growth of porous surface layer is pointed out, which shows the transformation of reactants to amorphous bottom layers via the formation and conversion process of the porous surface layer. This result is of significant importance for increasing the utilization ratio of materials and for understanding basic physical and chemical processes of manganese oxide film growth under hydrothermal conditions.