Monodisperse Silica Nanoparticle–Carbon Black Composite Microspheres as Photonic Pigments

The development of nonbleachable colorants made of safe and inexpensive materials is a scientifically important task in view of future environmental and biological impacts. In this study, the conditions under which spherical colloidal crystals (photonic balls) formed mainly of sub-micron-sized monodispersed silica fine particles and carbon black exhibit vivid structural coloring were systematically investigated. The (111) plane of the face-centered cubic colloidal crystal formed by the silica particles is mainly oriented on the surface of the photonic balls formed from monodispersed silica particles. As a result, light in a specific wavelength region is reflected from the photonic balls according to the Bragg condition. When silica particles with diameters of 221, 249, and 291 nm are used, the peaks of the Bragg reflections generated from the photonic balls occur at 495, 562, and 647 nm, respectively; each photonic ball exhibits the ability to produce blue, green, and red colors. In particular, when a black background is used, a vivid structural color is observed from each photonic ball, and it is possible to reproduce all colors using the three primary colors of light by changing the mixture ratio of these photonic balls. The introduction of a small amount of carbon black into the photonic balls makes it possible to reproduce the additive color mixture by the three primary colors of light even when the background color is white. We report that safe and nonbleachable coloring materials with controlled nanosized periodic structures and micrometer-sized geometric structures can be developed using three types of photonic balls consisting of safe and inexpensive silica fine particles with/without carbon black.