Local Coordination and Valence States of Cobalt in Sodium Titanate Nanoribbons

Co2+-doped sodium titanate nanoribbons were grown under hydrothermal conditions from Co2+-doped TiO2 and NaOH(aq) at 175 °C. The obtained nanoribbons, with diameters of 30–150 nm and length up to several micrometers, were determined to have a trititanate structure ((Na,H)2Ti3O7). Transmission electron microscopy revealed nanoparticles, not exceeding 15–20 nm in length, as well as hexagonal nanoflakes located on the surface of the nanoribbons. These nanoflakes are most likely originating from a β-Co(OH)2 side-product, according to the X-ray diffraction investigation. High angle annular dark filed scanning transmission electron microscopy combined with electron energy loss spectroscopy showed that the amount of cobalt in the surface nanoparticles is much higher than that in the nanoribbons. X-ray photoelectron spectroscopy (XPS) revealed that atomic concentration of cobalt in the sample is 1.5 wt %, of which a small amount is in the oxidation state 3+. A detailed electron paramagnetic resonance characterization of this sample proved that Co2+ ions occupy octahedral sites with rhombic distortion in a high-spin S = 3/2 state. Temperature-dependent susceptibility measurement revealed the prevailing paramagnetic behavior from which a mass ratio 1.3 wt % of Co2+ was obtained and which is in agreement with the elemental analysis results and the value extracted from the XPS measurements. A weak antiferromagnetic transition at 12 K is associated with β-Co(OH)2 nanoflakes.