Synthesis and optical properties of cobalt-doped zinc aluminate nano pigments  with an average crystallite size of 8 nm

In this work, we report the synthesis of Zn 1 − x Co x Al 2 O 4 x = 0, 0.01, 0.03, 0.06 nano pigments which were prepared by the coprecipitation method and heat treatment of precipitates. The samples were characterized using X-ray powder diffraction associated with the Rietveld refinement, UV-Vis spectroscopy, and transmission electron microscopy. The optical properties of the nano pigments, color development, crystal field strength, Racah parameters, and nephelauxetic parameters were investigated using spectroscopic data. The results show that sky blue ceramic nano pigments with normal spinel structure are obtained in samples annealed at 700°C for 2h, with an average crystallite size of 8 nm, with the incorporation of a very low concentration of Co 2+ in the host lattice. The band gap of the nano pigments was calculated using Kubelka–Munk derived from the diffuse reflectance spectra. These nano pigments show high thermal stability and retain their blue color in the temperature range from 700 to 1300°C. Optical absorption spectra confirmed the substitution of zinc for cobalt in the ZnAl 2 O 4 spinel, three absorption bands in the visible region and two intense bands are observed in the ultraviolet region. Zn 1 − x Co x Al 2 O 4 x = 0, 0.01, 0.03, 0.06 nano pigments are insoluble in water, acids and bases. Results suggest that these nano pigments are candidates for use in cosmetics.