Polyol synthesis of cobalt doped zinc oxide green nano-pigments: Colorimetry assessments and chemical synthesis parameters optimization

Cobalt-doped zinc oxide nanoparticles (CoxZn1-xO) were synthesized as inorganic nano-pigments by the polyol method, which is a simple and cost-effective method. Results showed that ethylene glycol (EG) should be utilized for synthesis of nanoparticles with smaller average crystallite size and better crystallinity. Other factors such as molar ratio of cobalt nitrate(Co(No3)2) to zinc acetate(Zn(Ac)2, reaction time, molar ratio of water and sodium hydroxide(NaOH) to the zinc acetate, surfactant and its molar ratio to the zinc were optimized via Taguchi design. The optimum sample was prepared under conditions such as the cobalt nitrate to zinc molar ratio of 0.01, reaction time of 5h, and Triton X100 (molar ratio of 0.2). X-Ray diffraction analysis demonstrated that average crystallite size of the optimum sample was 11 nm. FESEM revealed the uniformity of the powders having rod-like structures. Besides, the impacts of the effective synthesis factors on the colorimetric properties were optimized. Accordingly, the optimum sample revealed a green region with brightness of 58.89(L*), color purity (a* = −22.82), and color hue of 166.89(h°). Average crystallite size, amorphous phase percentage and colorimetric properties correlations have been investigated for the first time. Placement of more cobalt atoms in the ZnO structure raises the brightness coordinate and results more greenness in the color and increase of the crystal size does not directly affect the brightness coordinate.