Structural properties and thermal stability of cobalt- and chromium-doped α-MnO(2) nanorods

α-MnO(2) nanorods were synthesized via the hydrothermal decomposition of KMnO(4) in an acidic environment in the presence of Co(2+) and Cr(3+) ions. Reactions were carried out at three different temperatures: 90, 130 and 170 °C. All prepared samples exhibit a tetragonal MnO(2) crystalline phase. SEM–EDS analysis shows that cobalt cations are incorporated to a higher degree into the MnO(2) framework than chromium ions, and that the content of the dopant ions decreases with increasing reaction temperature. The oxidation of Co(2+) to Co(3+) during the reaction was proved by an XANES study, while EXAFS results confirm that both dopant ions substitute Mn(4+) in the center of an octahedron. The K/Mn ratio in the doped samples synthesized at 170 °C is significantly lower than in the undoped samples. Analysis of an individual cobalt-doped α-MnO(2) nanorod with HAADF-STEM reveals that the distribution of cobalt through the cross-section of the nanorod is uniform. The course of thermal decomposition of the doped nanorods is similar to that of the undoped ones. Dopant ions do not preserve the MnO(2) phase at higher temperatures nor do they destabilize the cryptomelane structure.