Novel near-infrared reflective black inorganic pigment based on cerium vanadate

Gd³⁺-doped cerium vanadates, Ce_1- Gd VO₄ (0 ≤ x ≤ 0.30), were synthesized as near-infrared (NIR) reflective black pigments by a conventional solid-state reaction method. Crystal structure, particle size, optical properties, and color of the samples were characterized. The Ce_1- Gd VO₄ (0 ≤ x ≤ 0.30) samples were obtained in a single-phase form and the lattice volume decreased with increasing Gd³⁺ concentration. Optical absorption below 630 nm was observed in all samples, which corresponded to the charge-transfer transition between Ce_4f and V_3d orbitals. The absorption spectrum of Ce_1- Gd VO₄ was shifted to the longer wavelength side as the Gd³⁺ content increased, because of the increase in the crystal field around V⁵⁺ due to the lattice shrinkage. As a result, the sample color gradually changed from dark brown to black with increasing Gd³⁺ content. Among the samples synthesized in this study, Ce_0.80Gd_0.20VO₄ absorbed visible light with wavelengths shorter than 650 nm and exhibited the darkest color. Furthermore, this black pigment showed a sufficient NIR reflectance value (R = 66.3%), which was higher than those of the commercially available products (R < 50%).