Green nanoceramic pigments based on cobalt chromite doped with Al: Synthesis, characterisation and colour performance

Abstract Nano-sized spinel cobalt chromite oxide CoCr2O4 and its substituted forms CoCr2−xAlxO4 (0≤x≤2) have been successfully synthesised by wet chemical route. The prepared powders were characterised by means of thermal analysis (DTA/TGA), X-ray diffraction (XRD), Field emission scanning electron microscopy (FE-SEM), Brunauer–Emmett–Teller analysis (BET), UV-visible techniques (UV-Vis) and colorimetric analyses (L* a* b*). The results show that the CoCr2−xAlxO4 (0≤x≤2) samples are in the crystalline phase at 900°C with a particle size range of 20–200 nm. The colouration of the synthesised pigments is sensitive to the distribution of doping trivalent ions in the spinel lattice. The Al-enrichment leads to a colour shift from green to blue and the grains become more and more spherical and less agglomerated. Green cobalt chromite powders were substituted with 50% aluminium (CoCrAlO4), allowing the specific surface area of green nanometric powders to be increased from 7.4 to 12.4 m2/g.