Recent advances in low-cobalt substrate-based cobalt blue composite pigments: Structural engineering, sustainable synthesis, and multifunctional applications

Cobalt blue (CoAl 2 O 4 ) is a spinel-structured inorganic pigment characterized by high chemical and thermal stability. However, its broader industrial application is constrained primarily by the high cost and supply volatility of cobalt and by the energy demand of conventional high-temperature production. Substrate-based composite pigments provide a practical strategy to reduce cobalt consumption while maintaining desirable optical performance. This review critically examines recent advances in the preparation and application of these composite colorants. The crystal structure and coloration mechanism of CoAl 2 O 4 are first discussed, with particular emphasis on the ligand field of tetrahedrally coordinated Co 2+ . Conventional solid-state and liquid-phase routes are then compared with emerging approaches like thermal decomposition and mechanochemical strategies in terms of process characteristics, cost, scalability, and environmental implications. Representative substrates, including high-purity synthetic oxides (TiO 2 , SiO 2 and Al 2 O 3 ) and naturally abundant minerals (kaolinite, palygorskite, and quartz-rich sands), are further evaluated in terms of their surface oxygen-donor chemistry, spatial localization of Co–Al precursors, and potential participation in spinel formation, together with their resource availability and processing requirements. A dedicated mechanistic discussion then addresses the coordination and phase evolution of Co and Al species, substrate-mediated interfacial interactions, and the formation of tetrahedral [CoO 4 ] chromophores. Furthermore, recent advances in ion modification are analyzed in relation to cation-site occupancy, local ligand-field regulation, and secondary-phase formation. Finally, multifunctional applications in inks, ceramics, polymers, and near-infrared reflective coatings are assessed, followed by an outline of future challenges and opportunities for the scalable and sustainable production of next-generation cobalt blue pigments.