A novel method for the clean synthesis of nano-sized cobalt based blue pigments

Abstract

Cobalt based pigments are categorized as hazardous materials because they contain a high level of environmentally toxic metals. Therefore, the aim of the present investigation is to partially replace cobalt by magnesium and zinc. The powder compositions are prepared according to the statistical mixture design methodology and Co_1−(x+y)Mg_xZn_yAl₂O₄ is synthesized via sol–gel self-combustion in which glycine is employed as the fuel because it produces nontoxic gases. This study considers a wide range of synthetic factors, including composition (x + y ≤ 0.50), calcination temperature (900–1000 °C) and pH (2.5–10.5). The products are characterized via Fourier transform infrared spectroscopy, X-ray diffraction, Brunauer–Emmett–Teller surface area measurement, and scanning and transmission electron microscopy. The Co_0.75Zn_0.25Al₂O₄ and Co_0.67Mg_0.16Zn_0.16Al₂O₄ pigments have a bright blue color and are approximately identical to the reference composition (CoAl₂O₄) when synthesized under the neutral conditions. The colorimetric data indicated the formation of blue pigment, corresponding to highly negative b* value, ∼−30, for the Co_0.67Mg_0.16Zn_0.16Al₂O₄ composition prepared in the acidic condition. A safe and reliable synthetic method was introduced for the synthesis of nano-sized spinel ≤30 nm by the substitution of magnesium and zinc for cobalt in the structure of the pigment. In comparison to cobalt aluminate oxide, the modified composition, which contains 33.33 mol% magnesium and zinc, can effectively improve chromatic performance, reduce environmental risk and be employed for the economic production of nano-sized pigments.