Synthesis of 3D Hierarchical Self-Assembled Microstructures Formed from α-MnO2 Nanotubes and Their Conducting and Magnetic Properties

Abstract

Highly crystalline α-MnO2 nanotubes with outer diameters between 25 and 40 nm are formed by a hydrothermal decomposition of KMnO4 precursor in an acidic environment. For nanotube formation, it is essential to include an addition of Fe3+ ions in the reaction mixture. SEM and TEM studies reveal that nanotubes are self-assembled into three-dimensional hollow microstructures where the shell thickness corresponds to the average nanotube length (∼370 nm). The intercalation of K+ and Fe3+ ions into the α-MnO2 structure determines the local Mn3+/Mn4+ ratio, and with this, influences their electric conductivity and magnetic properties. At high temperatures, the small polaron hopping of the eg electron from a Jahn−Teller-active Mn3+ (t2g3eg1) to a non-active Mn4+ (t2g3eg0) site is responsible for the activated type of conductivity in mixed Mn3+/Mn4+ regions with the activation energy Ea/kB = 2200 K. Magnetic susceptibility and EPR measurements show two distinct behaviors, suggesting nanometric phase segregation int...