Polyacrylamide gel synthesis, characterization, and optical properties of Co1-xNixCr2O4 spinel nanopigment

In the present study, Co1-xNixCr2O4 spinel nanopigment was synthesized by the polyacrylamide gel method. The effects of nickel substitution on morphology, crystal structure, and optical properties were investigated. The powders were characterized using X-ray diffraction, field emission scanning electron microscope, differential thermal analysis, Fourier transform infrared spectroscopy, and ultraviolet-visible spectroscopy. XRD patterns showed characteristic peaks of the spinel structure corresponding to the cobalt chromate phase. The ultraviolet-visible spectrum showed two absorption bands at 613 and 656 nm related to the charge transfer of spin ion Co2+ at the tetrahedral position and a change in the absorption spectrum with the increasing nickel content. Besides, TEM results revealed an average particle size of 22 nm for the Co0.6Ni0.4Cr2O4 sample. CIE-Lab was used to evaluate the color characteristics of the pigments. In this evaluation, L* depicts lightness. The a* and b* parameters are the four colors of human vision include red, green, blue, and yellow. Colorimetric results indicated with the substitution of nickel in the structure of cobalt chromate, its brightness (L*) increased, a* became more negative (more green) and b* tended to be more positive (more yellow).