Structural properties and thermal stability of cobalt- and chromium-doped α-MnO₂ nanorods

α-MnO₂ nanorods were synthesized via the hydrothermal decomposition of KMnO₄ in an acidic environment in the presence of Co²⁺ and Cr³⁺ ions. Reactions were carried out at three different temperatures: 90, 130 and 170 °C. All prepared samples exhibit a tetragonal MnO₂ crystalline phase. SEM-EDS analysis shows that cobalt cations are incorporated to a higher degree into the MnO₂ framework than chromium ions, and that the content of the dopant ions decreases with increasing reaction temperature. The oxidation of Co²⁺ to Co³⁺ during the reaction was proved by an XANES study, while EXAFS results confirm that both dopant ions substitute Mn⁴⁺ 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-MnO₂ 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 MnO₂ phase at higher temperatures nor do they destabilize the cryptomelane structure.