Yong-Ho Choa

Tailored Synthesis of Photoactive TiO2 Nanofibers and Au/TiO2 Nanofiber Composites: Structure and Reactivity Optimization for Water Treatment Applications

Titanium dioxide (TiO2) nanofibers with tailored structure and composition were synthesized by electrospinning to optimize photocatalytic treatment efficiency. Nanofibers of controlled diameter (30-210 nm), crystal structure (anatase, rutile, mixed phases), and grain size (20-50 nm) were developed along with composite nanofibers with either surface-deposited or bulk-integrated Au nanoparticle cocatalysts. Their reactivity was then examined in batch suspensions toward model (phenol) and emerging (pharmaceuticals, personal care products) pollutants across various water qualities. Optimized TiO2 nanofibers meet or exceed the performance of traditional nanoparticulate photocatalysts (e.g., Aeroxide P25) with the greatest reactivity enhancements arising from (i) decreasing diameter (i.e., increasing surface area), (ii) mixed phase composition [74/26 (±0.5) % anatase/rutile], and (iii) small amounts (1.5 wt %) of surface-deposited, more so than bulk-integrated, Au nanoparticles. Surface Au deposition consistently enhanced photoactivity by 5- to 10-fold across our micropollutant suite independent of their solution concentration, behavior that we attribute to higher photocatalytic efficiency from improved charge separation. However, the practical value of Au/TiO2 nanofibers was limited by their greater degree of inhibition by solution-phase radical scavengers and higher rate of reactivity loss from surface fouling in nonidealized matrixes (e.g., partially treated surface water). Ultimately, unmodified TiO2 nanofibers appear most promising for use as reactive filtration materials because their performance was less influenced by water quality, although future efforts must increase the strength of TiO2 nanofiber mats to realize such applications.

Adhesion enhancement of ink-jet printed conductive copper patterns on a flexible substrate

Ink-jet printed conductive copper patterns with enhanced substrate adhesion were fabricated using a conductive copper ink containing a silane coupling agent as an adhesion promoter. The effect of the silane coupling agent on the copper complex ion ink properties, including viscosity and surface tension, was systematically investigated. The copper complex ion ink that was ink-jet printed on a polyimide film was transformed to copper films by thermal treatment at 200 °C for 2 h in H2. The phase, microstructure, resistivity and peel strength were examined by X-ray diffraction, field emission scanning electron microscopy, the four-point probe technique, the 90° peel test and the ASTM D3359 tape test. The proper amount of silane coupling agent was determined according to the electrical conductivities and adhesive strengths of the ink-jet printed copper patterns containing varied amounts of adhesion promoter. As a result, the patterns formed from copper complex ion ink containing 3 wt% silane coupling agent exhibited not only the highest peel strength (240.3 gf mm−1 and 4B) but also low resistivity (approx. 20 μΩ cm). The mechanism of adhesion promotion via the silane coupling agent was also suggested.

Nanopeapods by Galvanic Displacement Reaction

complex configurationswith discretely positioned particles within a wire or tubestructure, deemed inorganic nanopeapods, have proved mostchallenging. Nanopeapods are comprised of a discontinuousinterface system that has recently attracted attention forenhancement of thermoelectric, sensing, and photovoltaiccharacteristics with similar platforms. These features are aconsequence of the difference in physical properties of thematerials at the interfaces and confinement effects of thenanoparticles, which have the ability to cause biased scatter-ing of photons and phonons, modulation of charge carriermobility/concentration, and surface plasmon enhanced pho-tocurrent.

Synthesis of LiFexMn2−xO4 cathode materials by emulsion method and their electrochemical properties

The cathode material, LiFexMn2 xO4 (0.0VxV0.2) was prepared by the emulsion method and calcined at 800 jC for 48 h in air. The calcined powder had a spinel structure and was of spherical shape. The average particle size and the specific surface area were 0.20 Am and 3.4 m 2 /g, respectively. The composition LiFe0.05Mn1.95O4 showed the largest discharge capacity, 105 mA h/g, and 9% of capacity deterioration after 30 charge–discharge cycles. As the content of Fe increased, the discharge capacity decreased, and the two plateaus at 4.1 and 3.9 V seen in LiMn2O4 disappeared. It is considered that Fe addition into LiMn2O4 inhibited the formation of Li0.5Mn2O4 phase. D 2002 Elsevier Science B.V. All rights reserved.

Enhancement of electroconductivity of polyaniline/graphene oxide nanocomposites through in situ emulsion polymerization

The present study introduces a systematic approach to disperse graphene oxide (GO) during emulsion polymerization (EP) of Polyaniline (PANI) to form nanocomposites with improved electrical conductivities. PANI/GO samples were fabricated by loading different weight percents (wt%) of GO through modified in situ EP of the aniline monomer. The polymerization process was carried out in the presence of a functionalized protonic acid such as dodecyl benzene sulfonic acid, which acts both as an emulsifier and protonating agent. The microstructure of the PANI/GO nanocomposites was studied by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, UV–Vis spectrometry, Fourier transform infrared, differential thermal, and thermogravimetric analyses. The formed nanocomposites exhibited superior morphology and thermal stability. Meanwhile, the electrical conductivities of the nanocomposite pellets pressed at different applied pressures were determined using the four-probe analyzer. It was observed that the addition of GO was an essential component to improving the thermal stability and electrical conductivities of the PANI/GO nanocomposites. The electrical conductivities of the nanocomposites were considerably enhanced as compared to those of the individual PANI samples pressed at the same pressures. An enhanced conductivity of 474 S/m was observed at 5 wt% GO loading and an applied pressure of 6 t. Therefore, PANI/GO composites with desirable properties for various semiconductor applications can be obtained by in situ addition of GO during the polymerization process.

Mechanical properties and microstructure for 3 mol% yttria doped zirconia/silicon carbide nanocomposites

Abstract 3 Mol% yttria stabilized tetragonal zirconia polycrystalline (3Y-TZP) is well known as a transformation toughening material with excellent mechanical properties at ambient temperature. However, the properties of 3Y-TZP drop down with increasing temperature. In this study, nanocomposite techniques were applied in order to improve mechanical properties of 3Y-TZP. 3Y-TZP/SiC nanocomposites were fabricated by hot-pressing, and effects of SiC particles on microstructure, transformation from tetragonal zirconia (t-ZrO 2 ) to monoclinic ZrO 2 (m-ZrO 2 ) and its mechanical properties were investigated. Fracture toughness of the nanocomposite was improved without decrease of strength. This should be due to not only crack deflection by dispersed SiC particles with high Youngs modulus, but also the phase transformation of t-ZrO 2 accelerated by the residual stresses from coefficient of thermal expansion mismatch between 3Y-TZP and SiC.

Structural characteristics of diamond-like nanocomposite films grown by PECVD

Abstract Diamond-like nanocomposite (DLN) films have been deposited on Si substrates using CH4/(C2H5O)4Si/H2/Ar gas mixtures as source gases by conventional plasma-enhanced chemical vapor deposition (PECVD). The film structure was investigated by transmission electron microscope (TEM), Fourier transform infrared spectrometer (FT-IR) and Raman spectrometry. The DLN films deposited mainly consisted of diamond-like a-C:H and quartz-like a-Si:O networks. The mechanical and tribological properties as well as microstructural modifications of the grown films were investigated. In order to understand the characteristics of DLN films, the diamond-like carbon (DLC) films were also prepared using CH4/H2 gases by the same deposition process.

Synthesis and optimization of Fe2O3 nanofibers for chromate adsorption from contaminated water sources

In this work, α-Fe2O3 nanofibers were synthesized via electrospinning and characterized to observe optimal morphological and dimensional properties towards chromate removal. The Fe2O3 nanofiber samples were tested in aqueous solutions containing chromate (CrO4(2-)) to analyze their adsorption capabilities and compare them with commercially-available Fe2O3 nanoparticles. Synthesized Fe2O3 nanofibers were observed with a variety of different average diameters, ranging from 23 to 63 nm, while having a constant average grain size at 34 nm, point zero charge at pH 7.1, and band gap at 2.2 eV. BET analysis showed an increase in specific surface area with decreasing average diameter, from 7.2 to 59.2 m(2)/g, due to the increased surface area-to-volume ratio with decreasing nanofiber size. Based on CrO4(2-) adsorption isotherms at pH 6, adsorption capacity of the Fe2O3 nanofibers increased with decreasing diameter, with the 23 nm sized nanofibers having an adsorption capacity of 90.9 mg/g, outperforming the commercially-available Fe2O3 nanoparticles by nearly 2-fold. Additionally, adsorption kinetics was also analyzed, increasing with decreasing nanofiber diameter. The enhanced performance of the nanofiber is suggested to be caused solely due to the increased surface area, in part by its size and morphology. Electrospun Fe2O3 nanofibers provide a promising solution for effective heavy metal removal through nanotechnology-integrated treatment systems.

Palladium/single-walled carbon nanotube back-to-back Schottky contact-based hydrogen sensors and their sensing mechanism.

A Schottky contact-based hydrogen (H2) gas sensor operable at room temperature was constructed by assembling single-walled carbon nanotubes (SWNTs) on a Si/SiO2 substrate bridged by Pd microelectrodes in a chemiresistive/chemical field effect transistor (chemFET) configuration. The Schottky barrier (SB) is formed by exposing the Pd-SWNT interfacial contacts to H2 gas, the analyte it was designed to detect. Because a Schottky barrier height (SBH) acts as an exponential bottleneck to current flow, the electrical response of the sensor can be particularly sensitive to small changes in SBH, yielding an enhanced response to H2 gas. The sensing mechanism was analyzed by I-V and FET properties before and during H2 exposure. I-Vsd characteristics clearly displayed an equivalent back-to-back Schottky diode configuration and demonstrated the formation of a SB during H2 exposure. The I-Vg characteristics revealed a decrease in the carrier mobility without a change in carrier concentration; thus, it corroborates that modulation of a SB via H2 adsorption at the Pd-SWNT interface is the main sensing mechanism.

Effect of Complex Agent on Characteristics of Copper Conductive Pattern Formed by Ink-jet Printing

In this study, Cu ion complex ink was successfully synthesized by a modified electrolysis method in which the Cu ions generated from bulk metal plates by an electric field were coordinated with complex agents. The synthesized ink was ink-jet-printed on a flexible substrate and converted to a dense Cu pattern after sintering at 250 °C. The pattern was characterized by X-ray diffractometry, field emission scanning electron microscope, and four-point probe method to confirm the crystal structure, microstructure, and electrical conductivity, respectively. The effect of the type of complex agent on the characteristics of a Cu conductive pattern was also determined using the analysis results. Finally, we conducted the direct writing of conductive dots and lines using the Cu ion complex ink, and confirmed that fine patterning for application in electronics is possible with the Cu ion complex ink.