Photochemically-Assisted Synthesis of Birnessite Nanosheets and Their Structural Alteration in the Presence of Pyrophosphate

Recent findings have showed the environmental abundance of soluble Mn(III)-complexes (e.g., Mn-siderophore and Mn(III)-pyrophosphate (Mn(III)-PP)). Specifically, Mn(III)-PP is of emerging interest in understanding Mn redox cycling and phosphorus cycling in catalysis and environmental systems. Curiously, however, further oxidation of Mn(III)-PP and consequent formation of a MnO₂ solid phase have not been considered yet. Here, using photochemically-assisted fast oxidation of Mn²⁺ (aq) by the reaction with superoxide, generated from nitrate photolysis, we report the role of PP in formation of disordered δ-MnO₂ nanosheets and changes in their stacking and structures. With increasing PP concentrations from 0.3 to 2 mM, δ-MnO₂ nanosheets showed better stacking (thicker) layers and an alteration from hexagonal to orthogonal structure (more Mn(III) in layers). We found that the amount of Mn(III)-PP complex and the formation rate of δ-MnO₂ nanosheets under the different PP concentrations affected both their layer structures and stacking behaviors. These findings open a new possibility of using photochemically-assisted green chemistry to control the properties of disordered δ-MnO₂ birnessite nanosheets for use in more effective catalytic reactions, such as water oxidation, and suggest the importance of Mn(III)-PP complexes for the formation of δ-MnO₂ nanosheets in nature.