Aerosol Synthesis and Optical Characterization of Co-Doped MgTiO₃ Blue Pigments with High NIR Reflectance

Near-infrared (NIR) reflective pigments are considered as energy-efficient cooling materials to mitigate the urban heat island (UHI) effect and reduce energy consumption. In this study, a novel blue pigment based on Mg_1-xCo_xTiO₃ with high NIR reflectance was synthesized by spray pyrolysis, and its crystallographic and optical properties were investigated by varying the Co content and the calcination temperature. X-ray diffractometer (XRD) and TEM analyses revealed that the prepared Mg_1-xCo_xTiO₃ pigments possessed an impurity-free ilmenite crystal structure and a dense spherical shape with a uniform distribution of Co. The intense blue color is due to the ⁴T_1g (F) → ⁴T_1g (P) transition of Co²⁺ in the octahedral ligand field. The optimal Co content and calcination temperature for obtaining the highest NIR reflectance with excellent blue characteristics were determined to be 20% (x = 0.2) and 800 °C, respectively. As a result, the Mg_0.8Co_0.2TiO₃ blue pigment (L* = 59.8, a* = -1.23, b* = -36.8) shows a high NIR solar reflectance of 78.9%, which is 34% higher than that of CoAl₂O₄. The color difference (ΔE*) values before and after immersed in acid/alkaline solution are less than 1.0, indicating that Mg_0.8Co_0.2TiO₃ exhibits excellent chemical stability. The temperature rise test under NIR irradiation demonstrated that the synthesized Mg_0.8Co_0.2TiO₃ blue pigment exhibited improved heat shielding performance compared to that of CoAl₂O₄. Therefore, Mg_0.8Co_0.2TiO₃ could be used as a promising cool blue pigment for sustainable solutions to alleviate heat accumulation due to sunlight.