Tunable Biogenic Manganese Oxides

Influence of the conditions for aerobic oxidation of Mn-(aq)(2+) catalysed by the MnxEFG protein complex on the morphology, structure and reactivity of the resulting biogenic manganese oxides (MnOx) is explored. Physical characterisation of MnOx includes scanning and transmission electron microscopy, and X-ray photoelectron and K-edge Mn, Fe X-ray absorption spectroscopy. This characterisation reveals that the MnOx materials share the structural features of birnessite, yet differ in the degree of structural disorder. Importantly, these biogenic products exhibit strikingly different morphologies that can be easily controlled. Changing the substrate-to-protein ratio produces MnOx either as nm-thin sheets, or rods with diameters below 20 nm, or a combination of the two. Mineralisation in solutions that contain Fe-(aq)(2+) makes solids with significant disorder in the structure, while the presence of Ca-(aq)(2+) facilitates formation of more ordered materials. The (photo)oxidation and (photo)electrocatalytic capacity of the MnOx minerals is examined and correlated with their structural properties.