Integrative structural and advanced imaging characterization of manganese oxide nanotubes doped with cobaltite

Abstract

Manganese oxide nanotubes (MnO₂) were efficiently produced through a hydrothermal method, using SiO₂ powder as nucleation points, then doped with cobaltite (Co₃O₄) nanoparticles uniformly deposited along the surface of the MnO₂ nanotubes. An integrative approach using advanced analytical electron microscopy techniques (UHR FE-SEM, HR-TEM, and BF/HAADF-STEM, coupled with EDX) in combination with spectroscopy allowed the determination of the structural characteristics of this composite nanomaterial. Advanced imaging clearly revealed the tubular structure of the MnO₂ nanotubes (diameter of 30–80 nm and length of 3–5 μm) and the arrangement of the discrete Co₃O₄ deposits (10–40 nm). Remarkably, high-resolution and spherical aberration-corrected STEM imaging allowed for the determination of the crystalline arrangement of the nanomaterials, particularly at the interface between MnO₂ and Co₃O₄ particles with high spatial sub-Angstrom resolution, revealing the distribution and high structural consistency of the novel composite materials produced. Furthermore, X-ray diffraction and Raman spectroscopy confirmed that MnO₂ corresponded to the crystallographic phase cryptomelane (K₂-xMn₈O₁₆), while the dopant cobalt nanoparticles adopted a cobaltite (Co₃O₄) phase. We demonstrated the catalytic properties of the composite MnO₂–Co₃O₄ nanotubes as an electrocatalyst material for oxygen evolution, where it showed superior behaviour, with a significantly higher catalytic activity (6.8 times) than pure MnO₂ in the OER region.