Synthesis and characterization of layered sodium cobaltates

The layered sodium cobaltates γ-NaxCoO₂ show low electrical conductivity andlarge thermopower. These features are required for candidate materials in thermoelectricapplications. Superconductive sodium cobalt oxyhydrates can be obtained by topotactictransformation of γ-NaxCoO₂ via oxidative de-intercalation and hydration. This chapterpresents the synthesis of γ-Na0.7CoO₂ in the form of sol-gel bulk, films, nanotubules andsubmicron tubules. Materials characterization of γ-NaxCoO₂ and cobalt oxyhydrates willbe also presented. According to thermopower analysis, band narrowing is observed when γ-Na0.7CoO₂ is transformed to potassium sodium cobaltoxyhydrates. According to theCo K-edge x-ray absorption spectra, the Co oxidation number for bi-layer hydrateNa0.33K0.02(H2O)1.33CoO₂ is higher than that of mono-layer hydrate Na0.07K0.21(H2O)0.63CoO₂ with a chemical shift of 3.5 eV, which is in sharp contrast with Co Kedgex-ray absorption spectroscopy and cerimetric titration results for Nax(H2O)yCoO₂,where there is essentially no difference of the oxidation number of Co between bi-layerand mono-layer sodium cobalt oxyhydrates.