Synthesis of hexagonal Al_2−xCr_xO₃ nanodisks by a thermal decomposition of mesoporous Cr⁴⁺:Al₂O₃ powders

Monodispersed hexagonal Al_2−xCr_xO₃ nanodisks are synthesized through a reactive doping of Cr⁶⁺ cations in a hydrogenated mesoporous AlO(OH)·αH₂O powder followed by annealing at 1,200 °C in air. The reaction was carried out by a drop wise addition of an aqueous Cr⁶⁺ solution (0.5–1.0 M) to AlO(OH)·αH₂O, at room temperature. Al_2−xCr_xO₃ nanostructure formation was controlled by the nucleation and growth from the intermediate amorphous mesoporous Cr⁴⁺:Al₂O₃ composites in the temperature range 400–1,000 °C. The nanodisks of ∼50 nm diameter and thickness of ∼16 nm is observed in the sample with x of 0.2 and similar nanodisks with a low dimension is observed at a higher value of x of 1.6 (after 2 h of heating at 1,200 °C). The Cr³⁺ ↔ Al³⁺ substitution, x ≤ 1.2, inhibits grain growth in small crystallites. The crystallites in x = 0.2 composition have 43 nm diameter while it is 15 nm in those with x = 1.2 composition.