Synthesis of Co-lean and Co-rich Li2CoTi3O8-based pigments: Potential Co reduction and blue-green dichroism

Abstract

Abstract Li2CoTi3O8 pigment is a cobalt-based blue pigment and is highly safe as it is actually used for cosmetics such as an eye shadow and a lipstick. Despite its excellent features, available reports on Li2CoTi3O8 blue pigment are quite limited. In this study, we demonstrate that the colors of CoO-lean compositions, cyanic blue, are substantially same as that of stoichiometric Li2CoTi3O8, which contributes further reduction of Co resources. In addition, we also demonstrate that green color pigments can be synthesized just by increasing the CoO compositions. Li2CO3, CoO and TiO2 (anatase) powders were used as starting materials. The mixed powders were heat-treated at 1100 °C in air for 2 h to obtain Li2CoTi3O8 powders, and they were characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and CIE1976-L*a *b* color coordinates measurement. Quantitative color measurement revealed that 10% reduction of CoO from stoichiometric Li2CoTi3O8 is possible without much affecting the human eye feeling. Green colors appeared for the CoO-rich compositions, and the dichroism of the Li2CoTi3O8 pigment was attributable to the valence change of Co ions from divalent (cyanic) to trivalent (greenish).