Slightly doped hydroxyapatite pigments of subtractive color with high near-infrared reflectance

Abstract

A novel series of high near-infrared (NIR) reflective Ca5(PO4)3OH based pigments were synthesized by a solid-phase method. Slight single doping of Fe, Mn, and Cu can change white hydroxyapatite into yellow-orange, cyan, and red-violet, respectively. In the apatite-type structure, Fe ions would occupy the site of the Ca2+, Mn ions preferred to form MnO43− in place of PO43−, and Cu ions tended to stay in the hexagonal channel substituting the protons of OH−. When two of the elements were introduced into hydroxyapatite, the color of the co-doped materials followed the principle of subtractive color mixture. Because of the extremely low concentration of dopant, the structure of hydroxyapatite was kept undisturbed, and its high NIR reflectance was also maintained. This paper discussed chromatic properties, spectral analysis, crystalline structure, and morphology of both single-doped and co-doped samples. The economics, environment-friendly, and colorful Ca5(PO4)3OH based material is a potential cool pigment.