Facile route to control the surface morphologies of 3D hierarchical MnO₂ and its Al self-doping phenomenon

In this article, a catalytic-ion assisted hydrothermal method has recently been developed for synthesis of hierarchical manganese dioxide nanostructures. In this study, various shapes of hierarchical MnO₂ (nanorod, nanothorn sphere, sphere) were successfully synthesized using the hydrothermal method with quantitative control of Al³⁺ in solution. The aspect ratio of aligned nanorods on the nanothorn sphere was easily controlled by the amount of Al³⁺ in the MnSO₄ and (NH₄) ₂S₂O₈ aqueous solution. Furthermore, we found that Al³⁺ species in the solution acted as a functional doping species into 2 × 2 tunnels of α-MnO₂ and also as a catalyst. The formation mechanism of hierarchical MnO₂ structures as a function of Al³⁺ concentration during hydrothermal reaction was sufficiently investigated, and the role of Al³⁺ as catalyst and doping species in the solution was discussed.