| Abstract
| - 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 electronmicroscopy, optical microscopy, optical reflectivity, and Raman spectroscopy. Care was takento make well-ordered templates, by slowly growing colloidal crystals from lightly chargedpolystyrene latex particles and carefully drying them to form opals. Solid material wasdeposited 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 macroporeswere close-packed and interconnected by windows, and small additional voids were locatedat interstices between the pores. The macropores were arranged on a face-centered cubiclattice with domains of more than 750 × 350 × 250 unit cells. The size polydispersity of theair spheres and mean-squared displacements from the lattice sites were studied for the firsttime. Both quantities were small, thus quantifying the excellent long-range order. The latticeparameter was shrunk by about 30% relative to the template, irrespective of the radii of thepores, while the long-range order persisted. The surfaces of the macropores appeared to berough up to length scales of ∼10 nm, the sizes of the largest TiO2 crystallites, with aroughness distribution following a power law. The volume fraction of the solid backbonewas determined for the first time in situ by X-ray absorption and was found to be between5 and 12 vol % TiO2. The wavelengths of the optical Bragg reflections were proportional tothe radii of the air spheres. By the use of the measured TiO2 fraction, the refractive indexof the solid matrix was found to be 2.7 ± 0.4, in good agreement with the refractive indexof massive anatase TiO2. We briefly explored the assembly of macropores in rutile-TiO2 andin rock salt. It was concluded that macroporous crystals are highly effective three-dimensionalphotonic crystals.
|
