Junko Konishi

Sol–gel synthesis of macro–mesoporous titania monoliths and their applications to chromatographic separation media for organophosphate compounds

We have developed a method of independently tailoring the macro- and mesoporous structures in titania (TiO2) monoliths in order to achieve liquid chromatographic separations of phosphorous-containing compounds. Anatase TiO2 monolithic gels with well-defined bicontinuous macropores and microstructured skeletons are obtained via the sol-gel process in strongly acidic conditions using poly(ethylene oxide) as a phase separator and N-methylformamide as a proton scavenger. Aging treatment of the wet gels in the mother liquor at temperatures of 100-200 degrees C and subsequent heat treatment at 400 degrees C allow the formation and control of mesoporous structures with uniform pore size distributions in the gel skeletons, without disturbing the preformed macroporous morphology. The monolithic TiO2 rod columns with bimodal macro-mesoporous structures possess the phospho-sensitivity and exhibit excellent chromatographic separations of phosphorus-containing compounds.

Strong light scattering in macroporous TiO2 monoliths induced by phase separation

Macroporous titania (TiO2) monoliths have been prepared from systems containing titania colloid and poly(ethylene oxide) using a sol-gel method, and the light-scattering properties have been investigated by means of coherent backscattering. Three-dimensionally interconnected macroporous morphology is obtained by inducing the phase separation parallel to the sol-gel transition. The crystal structure of TiO2 gel is transformed from anatase to rutile through the heat treatment above 900°C, while the macroporous morphology remains unchanged. We show that the rutile-type TiO2-based macropurous monoliths are strongly scattering media for visible light.

Random lasing from localized modes in strongly scattering systems consisting of macroporous titania monoliths infiltrated with dye solution

We have prepared random lasers that consist of macroporous titania monoliths infiltrated with dye solution and that operate close to light localization regime. When the excitation pulse energy exceeds a threshold, discrete spectral lines ascribed to laser oscillation appear on a featureless emission peak. No pulse-to-pulse variation was observed in the spectral positions of lasing lines, indicating that the lasing modes are localized in the medium. We demonstrated selective excitation of lasing modes by pumping at different positions on the sample.

Random lasing in ballistic and diffusive regimes for macroporous silica-based systems with tunable scattering strength

We have systematically investigated random lasing properties in weakly scattering systems composed of a macroporous silica disk immersed in a dye solution where the solvent is a mixture of two alcohols. Controlling the refractive index of the mixed solvent allows us to vary the scattering strength over a wide range. We have found two different scattering regimes where sharp spectral spikes with linewidth less than 1.0 nm, i.e., random laser with coherent feedback, appear in emission spectra. When the refractive index contrast between the solvent and the silica is very small, random lasing with coherent feedback is observed although the system appears nearly transparent. The coherent feedback vanishes when the refractive index contrast is increased up to a critical value, while further increase in the refractive index contrast results in the revival of the coherent feedback. We suggest that the existence of underlying microcavities plays an important role in the very weakly scattering regime (ballistic) while other mechanisms such as amplified extended modes may lead to the coherent feedback in lasing oscillation when the scattering strength increases.

Morphological control and strong light scattering in macroporous TiO2 monoliths prepared via a colloid-derived sol?gel route

Abstract Macroporous titania (TiO2) monoliths have been prepared via the sol–gel route started from aqueous anatase-type titania colloid in the presence of poly(ethylene oxide) (PEO), and the light-scattering properties have been investigated by means of coherent backscattering. Well-defined macroporous bicontinuous structures are formed when the transient structure of phase separation is fixed as the permanent morphology by the sol–gel transition. The macroporous morphology, i.e., the size and volume fraction of continuous macropores, can be tailored by adjusting the amount and/or molecular weight of PEO and the TiO2 concentration in the starting solution. During the heat treatment at temperatures above 1000 1C, the skeleton is sintered into fully dense body, and the crystalline structure is transformed from anatase to rutile phases, while keeping the macroporous morphology. We show that the rutile-type TiO2-based macroporous monoliths are strongly scattering media for visible light and that the scattering strength can be controlled by the macroporous morphology.

Fabrication of macroporous TiO2 monoliths for photonic applications

Macroporous titania (TiO2) monoliths have been prepared by the sol-gel method including phase separation, and the light-scattering properties have been investigated by means of coherent backscattering. Macroporous TiO2 gels are obtained in the systems containing aqueous titania colloid and poly(ethylene oxide)(PEO). Threedimensionally interconnected macroporous structure is formed when the transient structure of phase separation is fixed as the permanent morphology by the sol-gel transition. The domain size of macroporous TiO2 gels can be controlled reproducibly by adjusting the concentration of PEO. During the heat treatment above 1000 °C, the TiO2 skeleton is sintered into fully dense body and the crystalline structure is transformed from anatase to rutile, while maintaining macroporous morphology. We show that the rutile-type TiO2 -based macroporous monoliths are strongly scattering media for visible light.