Structural and Visible-Near Infrared Optical Properties of Cr-Doped TiO₂ for Colored Cool Pigments

Chromium-doped TiO₂ pigments were synthesized via a solid-state reaction method and studied with X-ray diffraction, SEM, XPS, and UV-VIS-NIR reflectance spectroscopy. The incorporation of Cr³⁺ accelerates the transition from the anatase phase to the rutile phase and compresses the crystal lattice. Moreover, the particle morphology, energy gap, and reflectance spectrum of Cr-doped TiO₂ pigments is affected by the crystal structure and doping concentration. For the rutile samples, some of the Cr³⁺ ions are oxidized to Cr⁴⁺ after sintering at a high temperature, which leads to a strong near-infrared absorption band due to the ³A₂ → ³ T₁ electric dipole-allowed transitions of Cr⁴⁺. And the decrease of the band gap causes an obvious redshift of the optical absorption edges as the doping concentration increases. Thus, the VIS and near-infrared average reflectance of the rutile Ti_1 - x Cr _x O₂ sample decrease by 60.2 and 58%, respectively, when the Cr content increases to x = 0.0375. Meanwhile, the color changes to black brown. However, for the anatase Ti_1 - x Cr _x O₂ pigments, only the VIS reflection spectrum is inhibited by forming some characteristic visible light absorption peaks of Cr³⁺. The morphology, band gap, and NIR reflectance are not significantly affected. Finally, a Cr-doped anatase TiO₂ pigment with a brownish-yellow color and 90% near-infrared reflectance can be obtained.