Morphological and structural evolution of α-MnO2 nanorods synthesized via an aqueous route through MnO4−/Mn2+ reaction

An aqueous route through MnO4−/Mn2+ reaction under mild conditions was used to synthesize α-MnO2 nanorods. The morphological and structural evolution of α-MnO2 nanorods during their growth were tracked by Powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and BET analysis. The crystallization of α-MnO2 nanorods was found to proceed through three steps: (1) Amorphous or poorly ordered nuclei formed first. (2) Then hollow nanoshperes consisting of γ-MnO2 nanorods formed via the Ostwald ripening process. (3) The hollow nanospheres broke down and the γ-MnO2 nanorods finally transformed into the α-MnO2 nanorods with increasing temperature or reaction time. The phase transformation from γ-MnO2 to α-MnO2 nanorods was accomplished by a short-range rearrangement of MnO6 octahedra. In addition, the performance of the MnO2 materials as a catalyst was evaluated in the aerobic oxidation of benzyl alcohol, showing that their catalytic activities were mainly dependent on their BET surface areas.