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%).