Colloidal crystal-templating methods have been used to prepare inverse opal photonic crystals of silica, mercaptopropyl-functionalized silica, titania, and zirconia. Ordered arrays of uniformly sized polymer spheres were infiltrated with fluid precursors capable of condensation or crystallization. After solidification of the material in the void spaces between the spheres, the polymer templates were removed by calcination or solvent extraction, leaving inverse replicas of the template arrays. By carefully controlling the synthetic procedures, gram-scale quantities of powdered macroporous materials exhibiting photonic crystal properties were obtained. For materials with crystalline walls (titania and zirconia), this required minimization of the size of the nanocrystalline grains. Because the periodicity introduced into the wall structure by the colloidal crystal templates was on the order of optical wavelengths, Bragg diffractions from the planes produced photonic stop bands in the visible spectra of these materials. The stop bands were manifested as brightly colored reflections and an optical filtering behavior of the materials. A crystallographic indexing of the optical spectrum of a polycrystalline inverse opal confirmed the fcc ordering of the pores. The optical properties of these materials were modified in predictable manners by numerous methods, including tailoring the pore size, filling the pores with fluids of various refractive indices, and changing the compositions of the solid material. The wavelengths of the colorful reflections (stop bands) were found to be proportional to the pore size and to vary linearly with the refractive index of the fluid filling the pores. The physical and synthetic modifications reported here allowed for the preparation of powders with optical reflections and bright colors spanning the entire visible spectrum.

Naturally occurring gem opals would seem to be the inspiration for the recent explosion of work on three‐dimensionally ordered macroporous, or “3DOM” materials. Myriad techniques have been developed for the creation of 3DOM products. This work focuses on ordered metal oxides and the effects that the chemistry behind precursors and template removal have on the phase and composition of the end product. Structural features that affect the materials' photonic, optical, catalytic, and magnetic properties are discussed.

No abstract

Three-dimensionally ordered macroporous (3DOM) carbon/titania nanoparticle composites were prepared in a program aimed at developing methods for assembling integrated multifunctional porous materials. The host material, 3DOM carbon, was synthesized by colloidal crystal templating with poly(methyl methacrylate) spheres, using a resorcinol−formaldehyde sol as a carbon source. This 3DOM support was pretreated with nitric acid to enhance the surface charge, and surface functional groups were characterized by Fourier transform infrared spectroscopy and acid−base titration. The modified support was then precoated with multiple layers of polyelectrolytes and finally coated with TiO2 nanoparticles using the hydrothermal reaction of an aqueous solution of titanium(IV) bis(ammonium lactato) dihydroxide (TAL) at varying concentrations, temperatures, and reaction times. Higher hydrothermal reaction temperatures favored the formation of larger TiO2 crystallites. The coating thickness increased at higher titanium precursor concentrations. Powder X-ray diffraction patterns indicated that the phase composition of the TiO2 layer varied with different synthesis conditions. Scanning electron microscopy images revealed that the most uniform coating of TiO2 was obtained at a reaction temperature of 200 °C with a TAL concentration of 0.2 M. This sample was characterized in more detail, using transmission electron microscopy, thermogravimetric analysis, differential scanning calorimetry, and nitrogen-sorption techniques.

Striking color changes in the visible spectrumare produced when the voids of three‐dimensionally ordered macroporous (3DOM) silica, alumina, and zirconia (“inverse opals”) are filled with solvents, it is reported here (see Figure). The wavelength of these colors is linearly related to the refractive index of the fluid filling the pores and can be tuned by modifying the refractive index of the wall material as well as the size and spacing of the pores.