The effect of annealing temperature on the structure and optical properties of sol-gel derived nanocrystalline cobalt aluminate spinel

► Sol-gel derived spinel-type CoAl2O4 thermal evolution was monitored. ► Below 700 °C part of Co exists in 3+ state (octahedral sites), yielding green color. ► Above 700 °C Co3+ reduction to Co2+ (tetrahedral sites) is promoted, changing to blue. ► Treatment at 500-600 °C resulted in crystal spinel-type phase Co2AlO4. ► At 700-900 °C Co2AlO4 and amorphous alumina react, producing CoAl2O4.