2178 Bull. Korean Chem. Soc. 2011, Vol. 32, No. 7 Hye Min Yu et al. DOI 10.5012/bkcs.2011.32.7.2178 Stability of PS Opals in Supercritical Carbon Dioxide and Synthesis of Silica Inverse Opals Hye Min Yu, Ah Ram Kim, Jun Hyuk Moon, Jong Sung Lim, * and Kyu Yong Choi † Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 121-742, Korea. * E-mail: limjs@sogang.ac.kr † Department of Chemical and Biomolecular Engineering, University of Maryland, MD 20742, USA Received October 19, 2011, Accepted May 9, 2011 Recently, the synthesis of ordered macroporous materials has received much attention due to its potential use as photonic band gap materials. 1 In this study, we have used the three-dimensional (3D) latex array template impregnated with benzenesulfonic acid (BSA), which is capable of catalyzing the reaction using tetraethyl orthosilicate (TEOS) as a precursor and distilled water. The polystyrene (PS) templates were reacted with TEOS in scCO 2 at 40 °C and at 80 bar. In the reactor, TEOS was filtrated into the PS particle lattice. After the reaction, porous silica materials were obtained by calcinations of the template. The stability test of the PS template in pure CO 2 was conducted before the main experiment. Scanning electron microscopy (SEM) images showed that the reaction in scCO 2 takes place only on the particle surface. This new method using scCO 2 has advantages over conventional sol-gel processes in its capability to control the fluid properties such as viscosity and interfacial tension. It has been found that the reaction in scCO 2 occurs only on the particle surface, making the proposed technique as more rapid and sustainable method of synthesizing inverse opal materials than conventional coating processes in the liquid phase and in the vapor phase. Key Words : Supercritical carbon dioxide, Silica inverse opals, TEOS Introduction Three-dimensional (3D) photonic crystals (PCs) are crystalline materials in which the refractive index is per- iodically modulated on a length scale that is comparable to the light wavelength of interest. 2 These materials share the unique photonic band gap (PBG), which prohibits the propa- gation of a particular range of electromagnetic frequencies through the structure. 3 Therefore, it is possible to tailor the emissive properties of internal light sources by controlling the PBG. To have a complete PBG, the structure must be extremely porous with a specific sub-micrometer 3D peri- odicity, and the materials must have a high refractive index and negligible absorption at the desired wavelengths. 4 There have been many approaches to fabricate 3D PCs to satisfy this requirement. 5,6 Of these, colloidal crystals have been found to be one of the simplest and fastest chemical assembly methods. Colloidal crystals are regular crystalline arrays of highly monodisperse colloids of dielectric materials, such as silica or polymer. One natural example is the gemstone opal. 7 However, natural opals are expensive and have a charac- teristic in which different frequencies of light are reflected against the incident light with a specific direction. Colloidal crystals provide an ideal scaffold for the creation of a more interesting photonic crystal-inverted opal, 8 a material with complex porous structures. Inverse opals have an inverted structure that is obtained by filling the voids of the opal structure with materials of high refractive index and then removing the original opal materials. Thus, inverse opals consist of a regular arrangement of spherical void spaces surrounded by solid walls rather than a regular arrangement of uniform spherical particles. Because of this special struc- ture, inverse opals, as with porous ceramic materials, can be used in a variety of applications, including photonic crystal devices, catalysts, sorbents, chromatographic materials, bio- materials, and microelectronics. 8 Since the late 1990’s, a variety of methods 9 have been designed to synthesize inverse opals. Of those methods, we chose supercritical deposition. In supercritical deposition, the precursor is deposited onto the substrates using super- critical-state fluids. Supercritical fluids have attracted much attention in the synthesis of ceramic materials. If a super- critical fluid is used as a reaction medium, the interfacial tension of the reactants and the fluid density are more readily controlled because the high-density liquid state is uniform and can be continuously changed. Specifically, supercritical carbon dioxide (scCO 2 ) is most frequently used as a medium because of its cost effectiveness, nontoxic characteristics, and pressure requirements (T c = 31 °C, P c = 73.8 bar). 2,10 Compared to liquid-phase deposition, scCO 2 possesses low viscosity, high diffusivity, and very low surface tension; therefore, the infiltration of precursors can be improved. 2,11 In particular, 3D materials are coated, and the reactants can diffuse deep inside the particle easily. In this paper, we investigate the fabrication of SiO 2 in- verse opals using scCO 2 . For this purpose, a 3D latex template infiltrated with tetraethyl orthosilicate (TEOS) was used. We conducted the experiments under different condi- tions, varying temperature, pressure, and reaction time. Sample characteristics were evaluated using scanning elec- tron microscopy.