Synthesis and characterization of inorganic double helices of cryptomelane nanomaterials

For the past century, researchers have invested great efforts in the synthesis of materials with various shapes and sizes. We have developed a method to prepare what appear to be inorganic double helices of cryptomelane nanomaterial with the highest surface area ever reported for this class of materials. The synthesis is based on the oxidation of MnSO4 by permanganate ion in acidic conditions at 65 °C and wet-aging at 7 °C for 48 h and then lyophilized for 8 days. The particle size, crystal structure, and properties of this nanomaterial have been studied by X-ray diffraction (XRD), high-resolution electron microscopy (HRTEM), thermogravimetric analysis (TGA), and N2 adsorption (BET) analysis. The wet-aging time, and exposure to air during reaction affect the size of the crystals formed in the final product. The XRD pattern showed a single phase corresponding to the pure cryptomelane-type manganese oxide. TEM studies suggest a fibrous structure. In the HRTEM images, however, at 5 nm length scale, spiraling along the radial direction is clearly observed suggesting helices. The BET surface area of this nanomaterial was found to be 150 m2/g which is higher than the previous reported surface area (130 m2/g) [J.C. Villegas, L.J. Garces, S. Gomes, J.P. Durand, S.L. Suib Chem. Mater. 17 (2005) 1910] for this class of materials. The TGA measurement demonstrated that it possessed an excellent thermal stability up to 400 °C. We show here that inorganic helices of cryptomelane nanomaterial may be produced at low temperature, without the use of template or cross-linking reagents.