Drawing on nature, synthetic opals (e.g., gilsonite) consist of uniformly sized ceramic spheres ordered into large close-packed domains. In order to improve the toughness or appearance of these chemically bonded ceramics, a polymer is often infiltrated into the interconnected space between the packed spheres. In a similar fashion, colloidal crystal arrays of submicrometer silica or polymer spheres have been employed as templates for periodic porous solids. All the methods for preparing periodic macroporous materials share a common synthetic thread: first, the colloidal crystals are formed from monodisperse spheres; next, the void spaces are filled and solidified; finally, the template is removed by heat, by refluxing in a solvent such as THF (in the case of polymer spheres), or by soaking in hydrofluoric acid (in the case of silica spheres). The product, which can be thought of as an “inverse opal,” often exhibits the same iridescence as a natural opal due to the similarity between the periodicity of the wall structure and the wavelength of light.
Synthesis of Metal Oxide Inverse Opals from Metal Nitrates by PMMA Colloidal Crystal Templating