Generating Color from Polydisperse, Near Micron-Sized TiO₂ Particles

Single particle Mie calculations of near micron-sized TiO₂ particles predict strong light scattering dominating the visible range that would give rise to a white appearance. We demonstrate that a polydisperse collection of these "white" particles can result in the generation of visible colors through ensemble scattering. The weighted averaging of the scattering over the particle size distribution modifies the sharp, multiple, high order scattering modes from individual particles into broad variations in the collective extinction. These extinction variations are apparent as visible colors for particles suspended in organic solvent at low concentration, or for a monolayer of particles supported on a transparent substrate viewed in front of a white light source. We further exploit the color variations on optical sensitivity to the surrounding environment to promote micron-sized TiO₂ particles as stable and robust agents for detecting the optical index of homogeneous media with high contrast sensitivities. Such distribution-modulated scattering properties provide TiO₂ particles an intriguing opportunity to impart color and optical sensitivity to their widespread electronic and chemical platforms such as antibacterial windows, catalysis, photocatalysis, optical sensors, and photovoltaics.