Structure and optical absorption of combustion-synthesized nanocrystalline LiCoO₂

Nanocrystalline LiCoO₂ powders (10–50 nm) were synthesized by a citrate-nitrate combustion process followed by calcination at different temperatures (300–800 °C) in air. Thermogravimetric analyses indicated a sharp combustion at a low temperature of 225 °C, producing fine crystallites. Quantitative phase analyses from the x-ray diffractograms showed that while annealing at 500 °C produced mixed phases of cubic and rhombohedral LiCoO₂, annealing at 800 °C resulted in single-phase rhombohedral LiCoO₂. Electronic transitions related to the Co 3d bands were investigated by ultraviolet-visible reflectance spectra in absorbance mode and were ascribed to the Co 3d intra-band transition involving t_2g and e_g orbitals. The d-d transitions underwent a blue shift of about 0.3 eV as the cubic LiCoO₂ transformed into the rhombohedral structure with band gap values of about 1.4 and 1.7 eV.