Photochemically Facilitated Todorokite Nanoparticle Formation in Circumneutral pH Environments

Abstract

The formation of Mn oxides and their reactivities affect electron flows and important redox-sensitive element cycles in nature. Thus, understanding the formation of nano-Mn oxides provides critical information about Earth’s redox systems. Natural Mn oxides mostly occur with both layered and large tunneled (todorokite) nanostructures. However, while biotic/abiotic processes well explain how the layered structures form, the widespread occurrence of tunneled todorokite nanomaterials in nature remains puzzling owing to the difficulty in reproducing their formation under environmentally relevant conditions. Here, we show that todorokite forms directly via the oxidation of Mn2+(aq) by superoxide generated from photochemical reactions under circumneutral aqueous conditions. We also explore that Mn(III) plays a key role by accommodating Mg(OH)+ through charge compensation, with Mg(OH)+ acting as a framework unit in the formation of nanoscale todorokite. This study suggests a new paradigm for the homogeneous formation of todorokite, which has been hitherto believed to be a secondary mineral phase resulting from diagenesis of layer-structured Mn oxides, under environmentally relevant conditions. In addition, our findings suggest a sustainable and facile pathway for the synthesis of todorokite nanostructures with potential applications in energy storage and catalysis.