Formation and properties of nanostructured colloidal manganese oxide particles obtained through the thermally controlled transformation of manganese carbonate precursor phase.

Structurally and morphologically different colloidal manganese oxide solids, including manganosite (MnO), bixbyite (Mn2O3) and hausmannite (Mn(2+)[Mn(3+)]2O4), were obtained through the initial biomimetically induced precipitation of a uniform, nanostructured and micron-sized rhodochrosite (MnCO3) precursor phase and their subsequent thermally controlled transformation into oxide structures in air and Ar/H2 atmospheres. The structures and morphology of the obtained precipitates were investigated using X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM). Their surface properties were investigated by electrophoretic mobilities (EPM) and specific surface area (SSA) measurements. The results showed that the structurally diverse, micron-sized, spherical manganese oxide particles exhibit unusual and fascinating nanostructured surface morphologies. These were developed through the coalescence of an initially formed, nanosized, crystalline, manganese carbonate precursor phase which, during the heating, transformed into coarser, irregular, elongated, micron-sized, manganese oxide solids. It was also shown that structural transformations and morphological tailoring were followed by significant changes in the physico-chemical properties of the obtained solids. Their SSA values were drastically reduced as a result of the progressive coalescence at the particle surfaces occurring at higher temperatures. The isoelectric points (IEPs) of the obtained manganese oxides were diverse. This is the consequence of their range of crystal-chemical properties that governed the complex physico-chemical processes at the interface of the manganese oxide solid and the aqueous solution. The results of this study may lead to a conceptually new method for the synthesis of high-performance, nanostructured, manganese oxide solids with desirable structural, morphological and surface properties.