Morphology Control of Cryptomelane Type MnO2 Nanowires by Soft Chemistry. Growth Mechanisms in Aqueous Medium

Nanowires of one-dimensional manganese oxide (cryptomelane) have been synthesized using reaction between Mn II (MnSO 4 ) and Mn VII (KMnO 4 ) in water by a low-temperature route at 60 °C and 95 °C. Characterization of the nanoparticles was carried out using powder X-ray diffraction, transmission electron microscopy, electron diffraction, and nitrogen adsorption−desorption. Two synthesis methods were developed depending whether the pH was initially adjusted or fixed all along the solid formation. Both methods exhibited variation of the nanowire morphology with diameter, length, and specific area varying in the 15−40 nm, 0.1−1.1 μm, and 35−110 m 2 ·g -1 ranges, respectively, depending on the growth conditions (acidity and temperature). The growth mechanism of the nanowires was proved to proceed by oriented attachment of primary 10 nm width nanorods for the lateral direction and dissolution−crystallization for the longitudinal direction. The diameter variation with acidic conditions was interpreted in terms of electrostatic interactions between primary nanorods.