Artificial black opal fabricated from nanoporous carbon spheres

A nanocasting method via chemical vapor deposition of acetonitrile was successfully employed to fabricate porous carbon colloidal crystal using colloidal crystal from monodispersed mesoporous silica spheres (MMSS) as a sacrificial scaffold. The mesostructure as well as periodic arrays within (111) plane of MMSS were replicated for the carbon colloidal crystal (black opal) with the length scale in the centimeter range. Brilliant iridescent colors were clearly observed for the first time on the black carbon colloidal crystal fabricated from porous carbon spheres, and they changed dramatically in accordance with the observation angle, like natural black opals. Reflection spectra measurements based on 2D surface diffraction and Bragg diffraction in the mirror mode were conducted for the fabricated carbon periodic arrays. The periodicity in the (111) plane as well as in the direction perpendicular to the (111) plane of the colloidal crystal was evaluated by comparing the results obtained from these two measurements. It was found that the periodicity in the direction perpendicular to the (111) surface is not high for the obtained black carbon opal. On the other hand, the relationship between the incident angles and the peak wavelengths of the reflection spectra, collected in the condition where the incident light and the reflected light pass through in the same direction, is governed by an approximation based on 2D surface diffraction. The results imply that the origin of the iridescent colors on the fabricated black carbon opal is derived from the periodicity not in the direction perpendicular to the (111) plane but within the (111) plane.

New strategy using glycol-modified silane to synthesize monodispersed mesoporous silica spheres applicable to colloidal photonic crystals

A new approach focusing on the reactivity of a silica source was developed for the particle size control of monodispersed mesoporous silica spheres (MMSSs). A glycol-modified silanes, tetrakis(2-hydroxyethyl) orthosilicate was chosen as the silica source and successfully applied in the surfactant-templated synthesis of MMSSs in a very dilute alkaline alcohol-water mixture. Due to its higher hydrolysis rate compared with tetraalkyl orthosilicates, it took less time for the primary particles to come out, resulting in the formation of small particles with diameters falling in the low submicrometer range. The resultant spheres possessed a well-ordered mesoporous structure, which was typically MCM-41-type hexagonal. The MCM-48-type cubic spheres could also be obtained by changing the reaction condition. The monodispersity and particle size of the spheres were precisely controlled by the adjustment of the solvent composition in the methanol-ethanol-water system. Furthermore, ionic colloidal crystals exhibiting a well-defined stop band in the visible light region could be fabricated from the resultant MMSSs for the first time. This work encourages the further utilization of MMSSs in photonics as well as in catalysis or biochemistry.

Nanoscale control over phase separation in conjugated polymer blends using mesoporous silica spheres

A method of preparing blended conjugated polymer microparticles using mesoporous silica spheres is described. Poly(3,4-ethylenedioxythiophene) (PEDOT) was blended with poly(furfuryl alcohol) (PFA) by a sequential infiltration-polymerization approach. The materials were evaluated by both scanning and transmission electron microscopy and are shown to retain the overall spherical structure of the silica template. The filling of the mesopores and the polymer distribution within individual particles were determined by a combination of energy-dispersive X-ray microanalysis, X-ray photoelectron spectroscopy, and nitrogen adsorption. The results suggest that when PEDOT is added to the silica host, followed by PFA, the phase separation of the two immiscible polymers is constrained by the dimensions of the silica mesopores, ensuring nanoscale contact between the two phases. The silica template can be removed by etching with 25% hydrofluoric acid, leaving behind a blended polymer microparticle. The etched microparticles exhibit macroporous morphologies different from that of pure PEDOT particles prepared by a similar route. The blended microparticles also appear to undergo limited phase separation; no evidence for distinct polymer domains was observed. Conductivity measurements indicate that the blended particles are above the percolation threshold and support the conclusion that the phase domains are extremely small. Importantly, when PFA is added to the host first, followed by PEDOT, there is a striking difference to the final composition and morphology of the particles. This reversal of the blending order results in a more amorphous, phase-separated material. These results demonstrate the preparation of conjugated polymer blends with engineered nanoscale phase separation and may allow for future improvements in organic device architecture and performance.