Preparation and formation mechanism of three-dimensionally ordered macroporous (3DOM) MgO, MgSO4, CaCO3, and SrCO3, and photonic stop band properties of 3DOM CaCO3

Three-dimensionally ordered macroporous (3DOM) magnesium (Mg) oxide (MgO), MgSO4, calcium (Ca) carbonate (CaCO3), and strontium (Sr) carbonate (SrCO3) were prepared using a colloidal crystal of polymer spheres as a template. Ethanol or ethanol–water solution of metal salts (acetate or nitrate) and citric acid was infiltrated into the void of the colloidal crystal template of a monodispersed poly(methyl methacrylate) (PMMA) sphere. Heating of this PMMA–metal salt–citric acid composite produced the desired well-ordered 3DOM materials with a high pore fraction, which was confirmed by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and ultraviolet-visible (UV–vis) diffuse reflectance spectra. The presence of citric acid is crucial for production of the 3DOM structures. Reaction of citric acid with metal salt produces metal citrate solid in the void of PMMA spheres, which is necessary to maintain the 3DOM structure during the calcination process. 3DOM CaCO3 shows opalescent colors because of itʹs photonic stop band properties.