Young Soo Kang

Organic–inorganic nanocomposite bilayers with triple shape memory effect

Various amounts of silica nanoparticles were chemically incorporated into amorphous polyurethanes (PU) of two different molecular weights by sol–gel reactions, and the effects were studied in terms of mechanical, dynamic mechanical, dual, and triple shape memory effects (DSME and TSME) of the nanocomposite films. It was found that the silica particles act as multifunctional cross-links as well as reinforcing fillers and significantly augmented the glassy and rubbery state moduli, yield strength, break strength, glass transition temperature, and dual shape memory properties. A cohesive bilayer of the two films fabricated from an interpenetrating polymer network (IPN) exhibited synergistic mechanical properties in the glassy and rubbery states along with two undisturbed glass transitions by which an intermediate plateau region and TSME were demonstrated.

Copper nanoparticles incorporated with conducting polymer: effects of copper concentration and surfactants on the stability and conductivity.

Copper nanoparticles are prepared in aqueous solution by reducing copper ions with hydrazine hydrate in the presence of cetyl trimethylammonium bromide (CTAB) and polyvinylpyrrolydone (PVP) as stabilizers. With only CTAB was used as stabilizer, copper nanoparticles are aggregated and partially oxidized to Cu(2)O. When both PVP and CTAB were used, dispersed copper nanoparticles with 56 nm diameter were obtained. Copper nanoparticles are simply mixed with poly (3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) in aqueous solution to form conducting composite. The effect of copper weight percent and surfactants on the conductivity and stability of the composite has been investigated.

Encapsulated-dye all-organic charged colored ink nanoparticles for electrophoretic image display.

Electrophoretic ink nanoparticles with high mobility are successfully fabricated by dispersion polymerization. The color of test cells can be changed by applying a bias voltage, as shown in the figure: the lower row shows the same cells as the upper row but with an applied voltage. These all-organic, encapsulated-dye, electrophoretic ink particles are expected to reduce the fabrication cost of e-ink in electrophoretic image display cells.

Morphological Transformation of Co(OH)2 Microspheres from Solid to Flowerlike Hollow Core–Shell Structures

We report, for the first time, a detailed investigation into the formation of highly uniform, 3D, flowerlike, hollow, spherical architectures of cobalt hydroxide through a facile solvothermal process. Various controlling parameters were examined, such as water content in starting materials, reaction time, cobalt(II) precursor concentration, and reaction temperature. On the basis of the experimental results, the formation mechanism of these flowerlike cobalt hydroxide hollow spheres involves aggregation of cobalt hydroxide building clusters into solid spheres and hollowing effect through subsequent dissolution, diffusion, and re-deposition of the smaller crystallites under the surface layer driven by an Ostwald ripening process. Metallic cobalt hollow spheres have also been obtained by thermal decomposition of cobalt hydroxide flowers in a mixed gas of Ar+4 % H(2) at 400 degrees C. The morphology and composition of the products were characterized by X-ray diffraction, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, and (high resolution) transmission electron microscopy.

Fabrication of SrTiO3–TiO2 heterojunction photoanode with enlarged pore diameter for dye-sensitized solar cells

We have demonstrated the N719 dye-SrTiO3–TiO2 system for the application in dye-sensitized solar cells, which is thermodynamically more favorable for electron transport and charge separation efficiency. The heterojunction SrTiO3–TiO2 photoanode with an enlarged pore diameter was fabricated by anodization of a Ti foil substrate and sequential hydrothermal reaction. Their photoelectrochemical properties were investigated as dye-sensitized solar cells for the first time. The optimal conditions for enlarged pore diameter of anodized TiO2 nanotube arrays (TNTAs) were suggested using a tetraethylene glycol based electrolyte, and SrTiO3 for the heterojunction photoanode was hydrothermally reacted on TNTAs. The structure and pore size of the hetero-structured SrTiO3–TiO2 nanotubes arrays (ST-TNTAs) were studied comparatively for their photocatalytic activity in parallel with various SrTiO3 morphologies. The enhanced photocatalytic property of ST-TNTAs compared with TNTAs could be attributed to the enhanced suppression of charge recombination by the heterojunction of ST-TNTAs due to easier crossing of the interface of ST-TNTAs by more the negative conduction band potential of SrTiO3. This is also possible because of the optimized interfacial area and the distance between SrTiO3 nanoparticles and TNTAs. With electrochemical impedance spectroscopy, we report promising applications of dye-sensitized solar cells with the hetero-structured photoanode of ST-TNTAs for the first time.

Tuning of the crystal engineering and photoelectrochemical properties of crystalline tungsten oxide for optoelectronic device applications

The photocatalysis, chromism, and sensing capabilities of nanostructured tungsten oxides, such as tungsten trioxide (WO3), its suboxides (WOx, 0 < x < 3) and hydrates (WO3·nH2O, n = 1/3 (0.33), 0.5, 0.75, 1, 2, etc.), tungsten bronzes MxWO3 (M = Li, Na, K, Rb, Cs and NH4), and metal tungstates (such as Bi2WO6 and CuWO4) have attracted much attention. To improve these properties, many strategies have been pursued, such as morphology control, doping, and heterostructuring. Crystal facet engineering has recently become a very important method of fine-tuning the physicochemical properties of semiconductors. The photocatalytic reactivity of a photocatalyst is significantly affected by its surface environment, including its surface electronic and atomic structures, which strongly depend on the crystal facets. It is believed that crystals with different exposed facets will have different properties, with the exposure of highly activated facets enhancing the photocatalytic and sensing properties. This article describes the syntheses of 2D WO3 crystals with the {002} facet primarily exposed, octahedral WO3 or WO3·nH2O with exposed {111} facets, and WO3 films with dominant orientations, such as orientation along the {002} facet. WO3 doping, WO3-based heterostructuring and their applications are also presented in this paper.

Facile preparation of hierarchical TiO2 nano structures: growth mechanism and enhanced photocatalytic H2 production from water splitting using methanol as a sacrificial reagent.

Owing to unique features, hierarchical nanostructure of TiO2 has superior photocatalytic activity. In this work a facile hydrothermal route has been explored to prepare 3D hierarchical TiO2 (3D-HTiO2), 1D/3D hybrid hierarchical TiO2 composite (HHC), and 3D hierarchical protonated titanate microspheres H2Ti2O5·H2O (3DHPTMS) at the expense of free-standing titania nanotube membrane (TiO2-Memb). It proceeded through the formation of peroxotitanium complex, a water-soluble Ti complex as an intermediate. Mechanism of formation, role of membrane crystallinity, and reaction parameters giving fine control on tuning morphology and crystal structure have been investigated systematically. Photocatalytic activities were determined by measuring the amount hydrogen generated from water splitting under UV irradiation in the presence of methanol as a sacrificial reagent. Self-assembled hierarchical titania nanostructures exhibited much superior photocatalytic activity compared to that of starting material, i.e., TiO2-Memb. Enhanced photocatalytic activity is due to characteristic morphology, increased surface area, and enhanced production of photogenerated charge carriers.

A selectively exposed crystal facet-engineered TiO2 thin film photoanode for the higher performance of the photoelectrochemical water splitting reaction

In the present study, a selectively exposed (101)-crystal facet engineered TiO2 photoanode is investigated for the higher efficiency of the hydrogen evolution reaction. To date, even though the photoelectrochemical performance (PEC) dependent on exposed crystal facets has been calculated and demonstrated in semiconducting microcrystals, selectively exposed crystal facets of photocatalyst thin films have not been reported yet. Herein, we demonstrate a TiO2 thin film photoanode with 100%-exclusively exposed crystal facets and suggest a methodology to obtain metal oxide thin film photoanodes with selectively exposed crystal facets. A selectively exposed crystal facet-manipulated metal oxide thin film photoanode is fabricated over pre-synthesized microcrystals through a three-step strategy: (1) hydrothermal synthesis of microcrystals, (2) positioning of microcrystals via polymer-induced manual assembly, and (3) fabrication of selectively exposed crystal facets of a TiO2 thin film through a secondary growth hydrothermal reaction. Based on the synthesis of representative TiO2 microcrystals with dominantly exposed (101), (100) and (001) crystal facets, the selectively exposed crystal faceted TiO2 thin film photoanode is comparatively investigated for practical PEC performance. The photocurrent density of the selectively exposed (101) crystal faceted TiO2 thin film photoanode is determined as 0.13 mA cm−2 and has an 18% conversion efficiency of incident photon-to-current at a 0.65 V Ag/AgCl potential under AM 1.5G illumination. Its photoelectrochemical hydrogen production reached 0.07 mmol cm−2 for 12 h, which is higher than those of (100) and (001) faceted photoelectrodes.

Preparation of dendritic NiFe films by electrodeposition for oxygen evolution

Dendritic materials are attractive as catalysts due to their highly ordered structure and high surface area. Herein, we report a NiFe dendritic nanostructure obtained by a simple electrodeposition without template. The control of concentration, potential, and pH plays an important role in the formation of the dendritic nanostructures. The longer and thinner NiFe dendritic nanostructure was obtained by changing the potential from −1.1 to −1.3 V vs. Ag/AgCl and a highly ordered nanostructure was obtained at the low pH of 2. NiFe dendritic material obtained at a potential of −1.3 V in pH 2.00 at 0.025 M NiSO4 and 0.025 M FeSO4 has a three-dimensional nanostructure. The growth mechanism of the NiFe dendritic nanostructure was investigated by SEM and the ED pattern of TEM. EDS, XRD, and XPS were used to investigate elemental composition, crystalline structure, and the chemical state of the as-obtained NiFe dendritic material. The efficiency of water-oxidation catalysts is critically influenced by the surface conditions and controlling the surface state is a major factor in developing an artificial photosynthetic system which stores the sunlight energy by water-splitting and carbon dioxide reduction in aqueous solution. Hence, we tested the NiFe dendritic nanomaterial as an oxygen-evolving catalyst and found that it shows oxygen evolution at 0.44 V vs. Ag/AgCl in 1 M NaOH solution.

Red emitting Y2O3:Eu3+ nanophosphors with >80% down conversion efficiency

Obtaining nanophosphors of a controlled size and shape with a high quantum efficiency is the current challenge for display and imaging technologies. Although the surface state induced luminescence quenching in nanophosphors may be compensated to some extent by incorporating activators like rare-earth ions, or by exploiting their quantum size effect, obtaining nanophosphors with quantum efficiencies as high as their bulk counterparts remains elusive. In the present article, we report on the synthesis of uniform Eu-doped Y2O3 nanoparticles, with an average size of 20–53 nm and a down conversion efficiency as high as 85%, using a simple chemical precipitation technique. Along with size control, the effects of Eu3+ content on their emission behaviors have been discussed. We believe the low-cost synthesis process of these nanoparticles will greatly enhance their application potential in optical display and bio-imaging technologies.