Preparation and optical properties of titanium dioxide iridescent pigment based on black titanium oxynitride

Titanium dioxide flakes, serving as a starting material, were prepared by peeling off titanium dioxide coated on a mica substrate. Black substrates were prepared by reducing the titanium dioxide flakes, followed by coating the black substrates with a predetermined amount of titanium dioxide. Various pigments, each exhibiting colors due to optical interference even on white paper, could be obtained. By preparing black substrates in this way, we could obtain an iridescent pigment that absorbed transmitted light in the same way as a coating film formed on a black base layer. The hue of the resulting pigment could also be controlled. As the amount of a titanium tetrachloride solution added dropwise to the black substrates was increased, that is, as the amount of titanium dioxide coated on the surfaces of the black substrates was increased, the reflection peak of the spectral reflectance curve shifted to longer wavelengths. When black substrates having a thickness of 140 nm were used as the starting material, the hue changed from purple, to blue, green, yellow, gold, and red as the amount of coated titanium dioxide coated was increased. For black substrates having a thickness of 220 nm, the hue changed from magenta, to purple and green. The results of measuring the thicknesses of the pigment flakes showed a correlation between the total thickness and the hue produced by optical interference. In addition, the black titanium dioxide substrates used as the starting material were strongly photocatalytic. However, we also found that the photocatalytic properties could be decreased by the reduction process and by a silica-coating process performed in the production of the black substrates.