Fabrication and Characterization of Large Macroporous Photonic Crystals in Titania

We made large, highly ordered structures consisting of crystals of macropores in titania (TiO2), by template-assisted growth. The crystals were characterized by synchrotron small-angle X-ray diffraction, X-ray absorption, wide-angle X-ray diffraction, scanning electron microscopy, optical microscopy, optical reflectivity, and Raman spectroscopy. Care was taken to make well-ordered templates, by slowly growing colloidal crystals from lightly charged polystyrene latex particles and carefully drying them to form opals. Solid material was deposited in the opal template by precipitation from an alkoxide hydrolysis. Subsequently, the samples were heated to 450 °C to form anatase TiO2 and to remove the latex template, which resulted in a macroporous crystal, inverse opal, or air-sphere crystal. The macropores were close-packed and interconnected by windows, and small additional voids were located at interstices between the pores. The macropores were arranged on a face-centered cubic lattice with domains of more than 750 × 350 × 250 unit cells. The size polydispersity of the air spheres and mean-squared displacements from the lattice sites were studied for the first time. Both quantities were small, thus quantifying the excellent long-range order. The lattice parameter was shrunk by about 30% relative to the template, irrespective of the radii of the pores, while the long-range order persisted. The surfaces of the macropores appeared to be rough up to length scales of ∼10 nm, the sizes of the largest TiO2 crystallites, with a roughness distribution following a power law. The volume fraction of the solid backbone was determined for the first time in situ by X-ray absorption and was found to be between 5 and 12 vol % TiO2. The wavelengths of the optical Bragg reflections were proportional to the radii of the air spheres. By the use of the measured TiO2 fraction, the refractive index of the solid matrix was found to be 2.7 ± 0.4, in good agreement with the refractive index of massive anatase TiO2. We briefly explored the assembly of macropores in rutile-TiO2 and in rock salt. It was concluded that macroporous crystals are highly effective three-dimensional photonic crystals.