Transparent CoAl₂O₄ hybrid nano pigment by organic ligand-assisted supercritical water

Transparent types of inorganic pigments are important as they can be used in a variety of applications, such as metallic finishing, contrast enhancing luminescent pigments, high-end optical filters, and so on. Currently, the difficulty in producing monodisperse and stable binary metal oxide nano pigments at low temperature hampers the applicability and realization of transparent blue nano pigments. Here, for the first time, we report organic ligand capped CoAl₂O₄ hybrid transparent nano pigment, which has a particle size less than 8 nm with well-stabilized single nanocrystals, using organic ligand-assisted supercritical water as the reaction medium. The organic ligand capping could effectively inhibit the particle growth and also control the size of nanocrystals. This helps to diminish the scattering effect of the nano blue pigment, realizing a transparent cobalt blue nano pigment without any postheat treatment.