Morphological control and strong light scattering in macroporous TiO2 monoliths prepared via a colloid-derived sol-gel route

Macroporous titania (TiO2) monoliths have been prepared via the sol-gel route started from aqueous anatase-type titania colloid in the presence of poly(ethylene oxide) (PEO), and the light-scattering properties have been investigated by means of coherent backscattering. Well-defined macroporous bicontinuous structures are formed when the transient structure of phase separation is fixed as the permanent morphology by the sol-gel transition. The macroporous morphology, i.e., the size and volume fraction of continuous macropores, can be tailored by adjusting the amount and/or molecular weight of PEO and the TiO2 concentration in the starting solution. During the heat treatment at temperatures above 1000 °C, the skeleton is sintered into fully dense body, and the crystalline structure is transformed from anatase to rutile phases, while keeping the macroporous morphology. We show that the rutile-type TiO2-based macroporous monoliths are strongly scattering media for visible light and that the scattering strength can be controlled by the macroporous morphology.