Scalable synthesis of carbon-embedded ordered macroporous titania spheres with structural colors

Carbon-embedded ordered macroporous titania (C-MAC TiO₂) spheres are prepared in solution by the cooperative self-assembly of polymer beads and a titania precursor within evaporative emulsions and subsequent direct carbonization. Because the highly reactive titania precursors are easily crosslinked to form gels early in evaporation before the polymer beads are self-organized, non-reactive toluene-in-formamide emulsions are used. These non-aqueous emulsions should be stable at relatively high temperatures (~80 °C) for the evaporation process. We found that amphiphilic triblock copolymers of poly(ethylene oxide) (PEO) and poly(phenylene oxide) (PPO) with longer PEO chains (Pluronic® F108 (EO₁₂₅-b-PO₆₄-b-EO₁₂₅) are required to stabilize those non-aqueous emulsions, and become more important at higher concentrations used for bulk fabrication. The carbon inside our C-MAC TiO₂ significantly suppresses strong multiple scattering from structural defects or imperfections, thus emphasizing their Bragg reflection colors.