Young-Sang Cho

Fabrication of superhydrophobic surfaces using structured colloids

We have introduced water-in-oil emulsion systems to generate confining geometries for the self-organization of monodisperse silica nanospheres as building block particles. Then, through the slow evaporation of emulsion phases by heating, these nanospheres were packed into structural colloids such as raspberry-shaped micro-particles. The suspension of colloidal clusters was deposited onto glass substrate followed by surface modification of fluorinecontaining silane coupling agent to produce superhydrophobic surface with dual scale roughness. Similar self-assembly approach was employed to fabricate macroporous micro-particles from composite micro-particles of polystyrene microspheres and antimony-doped tin oxide nanoparticles by calcination. After deposition of the porous particles and fluorine treatment with silane coupling agent, superhydrophobic surfaces which have potential applications such as self-cleaning property could be obtained with contact angle of water larger than 150°.

Dispersion Polymerization of Polystyrene Particles Using Alcohol as Reaction Medium

In this study, monodisperse polystyrene nanospheres were prepared by dispersion polymerization using alcohol as reaction medium to prepare colloidal clusters of the latex beads. Polyvinylpyrrolidone (PVP) and 2-(methacryloyloxy)ethyltrimethylammonium chloride (MTC) were used as dispersion stabilizer and comonomer, respectively. The particle size could be controlled by adjusting the reactant compositions such as the amount of stabilizer, comonomer, and water in the reactant mixture. The size and monodispersity of the polymeric particles could be also controlled by changing the reaction medium with different alcohols other than ethanol or adjusting the polymerization temperature. The synthesized particles could be self-organized inside water-in-oil emulsion droplets by evaporation-driven self-assembly to produce colloidal clusters of the polymeric nanospheres.

Fabrication of Porous Titania Particles from Water-in-Oil Emulsions for the Applications of Photocatalyst

In this study, polystyrene nanospheres were synthesized by dispersion polymerization using batch-type reactor for the self-organization with precursor materials inside emulsion droplets. Mechanical homogenization was applied for the emulsification of the polymeric nanospheres and titanium diisopropoxide bis (acetylacetonate) to produce the macroporous titania particles by evaporation-driven self-assembly. Similarly, spherical titania crystallites could be synthesized via self-organization using triblock copolymer instead of polymeric latex beads after successive calcination. The morphologies of the porous particles were observed using electron microscope, and the crystallinity of the porous titania particles was analyzed by powder x-ray diffraction. As a demonstrative application, the macroporous titania microparticles with anatase phase were adopted as photocatalyst for the decomposition of Rhodamine B, and excellent catalytic performance was observed with higher rate constant compared to the result from commercial titania nanocolloid. Collectively, our macroporous titania microparticles were found to be safe catalytic materials for human body minimizing the skin toxicity since the size of the catalysts is in the micron-range. GRAPHICAL ABSTRACT

Fabrication of Hollow or Macroporous Silica Particles by Spray Drying of Colloidal Dispersion

In this study, hollow silica particles were fabricated by atomizing the dispersion of silica nanocolloids synthesized by modified Stober method and self-organization of the particles by spray drying technique. Rapid evaporation of the droplet containing the silica nanoparticles resulted in the formation of hollow microparticles at high evaporation temperature due to hydrodynamic instability of the droplet. Similar strategy was adopted for the fabrication of macroporous silica particles by the sol spray drying of the hetero-colloidal dispersion of polystyrene nanospheres and commercial silica nanoparticles. The morphologies of the porous particles were observed by scanning electron microscope with varying drying temperature. As a demonstrative purpose, the results using emulsion droplets as confining geometry was compared with the porous particles obtained from spray dryer. Collectively, spray drying was found to be more efficient manner to prepare the porous materials with continuous way in the view of production efficiency and time. GRAPHICAL ABSTRACT

Synthesis of Latex Particles with Surface Functional Groups and Their Applications for the Fabrication of Porous Materials

In this study, monodisperse latex particles with specific surface functional groups were synthesized by emulsifier-free emulsion polymerization. Amidine or carboxylated polystyrene nanospheres with narrow size distribution were prepared by emulsion polymerization using AIBA (α,α′-zodiisobutyramidine dihydrochloride) as amine-containing initiator or acrylic acid as carboxyl-containing comonomer, respectively. Factors affecting the particle size and distribution were systemically studied by changing the amount of initiator or monomer, the polymerization temperature, and the stirring speed of emulsion polymerization reactor. Monodisperse polymethylmethacrylate beads were also synthesized by soapless emulsion polymerization using methacrylic acid or aminoethylmethacrylate hydrogen hydrochloride as comonomer for the surface functionalization of the particles. As applications of the latex beads, the polymeric particles were adopted as templating materials for the fabrication of macroporous titania film and meso-macroporous silica particles by colloidal templating method. GRAPHICAL ABSTRACT

Synthesis of polymeric particles by seeded growth technique for the applications of particle self-assembly

ABSTRACT In this study, shape-anisotropic polymeric particles were synthesized by seeded growth technique for the applications of particle self-assembly. First, cross-linked seed particle dispersion was prepared by emulsifier-free emulsion polymerization with divinylbenzene as cross-linker. Then, seeded growth scheme was applied to the seeds by swelling the particles with monomer and subsequent polymerization. The morphologies of the nonspherical particles could be controlled by adjusting the size and the amount of monomer during the swelling step or the cross-linking density of the seed particles, enabling the synthesis of prolate ellipsoids, lobed spheres with triangular shape, snowman-shaped particles, and dumbbell particles. As a demonstrative application, the cross-linked particles could be used as templates for the synthesis of porous materials, whereas dumbbell-shaped particles could be self-organized into colloidal clusters using toluene emulsions as confining geometries. Collectively, shape-anisotropic particles were found to be efficient building blocks to prepare the unique packing structures other than simple spherical colloids. GRAPHICAL ABSTRACT

Sol–gel synthesis of porous titania fibers by electro-spinning for water purification

ABSTRACT In this study, porous ceramic fibers were prepared by the sol–gel-assisted electro-spinning process using colloidal dispersion of complex fluids for the application of phtotocatalysts. First, polystyrene nanospheres were synthesized by dispersion polymerization as sacrificial templates for porous fibers. Then, the mixture of polyvinylpyrrolidone and the ceramic precursor with the polymeric nanospheres was prepared as the spinning solution and self-organized by electro-spinning, followed by calcination of the electrospun composite fibers. The morphologies of the porous fibers could be controlled according to the size of the templates and the amount of the ceramic precursor. The nano-structure of the pores in the fibrous materials could also be adjusted as open or closed cavities with various potential applications. As a demonstrative application, the macroporous titania fibers could be utilized as photocatalysts for the removal of organic dyes dissolved in water. A better photocatalytic activity of the macroporous fibers with 700-nm pore diameter was observed compared to the result of nonporous titania fibers due to the increased porosity. Collectively, the macroporous ceramic fibers were found to be efficient functional materials to prepare the unique nano-structured materials other than simple nonporous fibers. GRAPHICAL ABSTRACT

Successive growth and applications of polymeric particles with controllable size and shapes

Nonspherical particles resembling sea pineapples were synthesized by successive growth technique during soapless emulsion polymerization for various applications. First, highly cross-linked seed particle dispersion was synthesized by emulsifier-free emulsion polymerization with acrylic acid as co-monomer for the formation of surface carboxylic groups. Then, a successive growth scheme was applied to the seeds by swelling the particles with monomer droplets, followed by polymerization. The sea pineapple-shaped particles could be produced by adjusting the amount of monomer during the swelling step of the third growth. As a demonstrative application, the seed or sea pineapple-shaped particles could be used as templates for the synthesis of porous inorganic particles by spray drying technique. The resulting porous particles could be adopted as photocatalyst for the decomposition of organic molecules such as methylene blue. As another application, the dye molecules could be adsorbed onto the second grown particles to produce dye-doped nanospheres. Finally, the sea pineapple-shaped particles could be self-organized into supra-aggregates using toluene emulsions as confining geometries. Collectively, successively grown particles were found to be efficient building blocks to prepare the unusually packed structures or functionalized into colored products.

Synthesis of Macroporous Silica Particles by Continuous Generation of Droplets for Insulating Materials

We report on the synthesis of porous silica particles by self-assembly routes in a continuous manner for application to thermal insulators. A continuous process was employed to produce tiny droplets containing precursor materials such as silica and organic templates for self-organization to fabricate particles with well defined pores. A rotating cylinder system or a spray drying process was adopted to form emulsions or aerosol droplets as micro-reactors for self-assembly, and the physical properties including the thermal conductivity of the resulting porous particles were compared between the two methods. The porous particles could be coated as a thick film by solution dripping, and the fluorination treatment using a silane coupling agent was performed to produce superhydrophobic surfaces of insulating layers by a lotus effect.

Finite Packing of Shape-Anisotropic Colloidal Particles

Small aggregates (or clusters) of silica doublet or dumbbell particles were produced inside oil-in-water or water-in-oil emulsions. First, forced aggregation of silica microsphere suspension was induced under a high concentration of ammonia, followed by the addition of tetraethylorthosilicate for bonding and fixation of the particle aggregates. Then, density gradient centrifugation was applied for the fractionation of silica doublets or dumbbell particles. The resultant shape-anisotropic particles, which were re-dispersed in oil phase such as hexane or toluene after surface modification with octadecyltrmethoxysilane, were emulsified and small clusters of silica doublets or dumbbells were obtained by evaporation-driven self-assembly. Most configurations of the doublet clusters are similar to minimal second-moment clusters from singlet particles. However, for a few particular cases of n = 6, 8, and 12, we observed some isomeric structures. In addition, the pear-shaped microparticles as asymmetric dimers were also assembled by self-organization of the shape-anisotropic particles inside aerosol droplets by electrospray-assisted atomization, and the resultant configurations were quite similar to the clusters of simple microspheres. GRAPHICAL ABSTRACT