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

Gd 3+ -doped cerium vanadates, Ce 1- x Gd x VO 4 (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- x Gd x VO 4 (0 ≤ x ≤ 0.30) samples were obtained in a single-phase form and the lattice volume decreased with increasing Gd 3+ 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- x Gd x VO 4 was shifted to the longer wavelength side as the Gd 3+ content increased, because of the increase in the crystal field around V 5+ due to the lattice shrinkage. As a result, the sample color gradually changed from dark brown to black with increasing Gd 3+ content. Among the samples synthesized in this study, Ce 0.80 Gd 0.20 VO 4 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%).