Morphology and phase tuning of α- and β-MnO₂ nanocacti evolved at varying modes of acid count for their well-coordinated energy storage and visible-light-driven photocatalytic behaviour

A simple hydrothermal method is developed to synthesize two different phases, α and β of MnO₂ nanocacti (comprising nanowires with 1–10 nm diameter self assembled by ultrathin sheets) as well as MnO₂ nanorods (10–40 nm diameter) without any seed or template. Sudden addition of concentrated H₂SO₄ (0.3–0.4 μL) results in the formation of nanocacti while gradual addition (dropwise) of H₂SO₄ solution (0.3–0.4 M) results in nanorods. Besides, the α phase of MnO₂ exists at relatively high acidic strength (4 pH) compared to the β phase, which is consistent at 5 pH. Thus this could be the first report exploring the possibilities of tuning morphology as well as the phase of MnO₂ through simple optimizations in acidic content. We find that polymorphic MnO₂ nanocacti exhibit superior photocatalytic activity and high energy capacity as an anode in Li-ion batteries than polymorphic MnO₂ nanorods. The α phase of MnO₂ performs better than the β phase. α-MnO₂ nanocacti demonstrate high visible light driven photocatalytic activity by degrading >90% of congo red and methyl orange dyes in 40 mg L⁻¹ organic dye aqueous solution with 0.1 g of the as-prepared sample within 25 and 70 min, respectively. We highlight the differences between the photocatalytic activities of different phases, α and β of MnO₂ nanostructures, depending on the charge transport through different dimensions of the same pristine MnO₂. The constant cycling stability of α-MnO₂ nanocacti with capacities as low as 300 mA h g⁻¹ at 1C rate after 50 cycles as an anode makes it a promising material for energy storage applications. We attribute the high electro- and photo-chemical activity for α-MnO₂ nanocacti to their highly mesoporous structure making this one of the highest specific surface areas (271 m² g⁻¹) possibly ever reported for pristine MnO₂.