Taegyung Ko

Ferroelectric behavior of orientation-controlled PbBi4Ti4O15 thin films

Ferroelectric lead bismuth titanate (PbBi4Ti4O15) thin films, selectively controlled in c-axis and off-c-axis orientation, were fabricated on a Pt layer by a chemical solution deposition method. The off-c-axis oriented PbBi4Ti4O15 films demonstrated much higher remanent polarization (8.7 μC/cm2) than those of c-axis oriented films (3.7 μC/cm2). Regardless of grain orientation, PbBi4Ti4O15 films were not fatigued up to 1010 cycles under 9-V application. It is deduced that the role of Bi2O22+ layer in inducing fatigue-free property for this Bi-layered perovskite structure is the self-regulation of space charge.

The microstructure and characteristics of magnetite thin films prepared by ultrasound-enhanced ferrite plating

Magnetite thin films were produced using the ultrasound-enhanced ferrite plating method. The effect of ferrite plating conditions on the microstructure and magnetic properties was investigated. The saturation magnetization (Ms) and the coercive force (Hc) of the magnetic thin films were 465-475 emu/cm/sup 3/ and 60-65 Oe, respectively. Then, the applicability of the magnetite thin films as a CO gas sensor was investigated. The magnetite thin films were annealed in the air to be converted to maghemite ones through oxidation, which manifested CO gas sensitivity. Rair/Rgas=3.5 was obtained in the CO gas sensitivity at the operation temperature of 85/spl deg/C, when the thin films were annealed at 300/spl deg/C.

Photoluminescence and Fabrication of Zirconia Nanofibers from Electrospinning an Alkoxide Sol Templated on a Polyvinyl Butyral

A zirconia gel/polymer hybrid nanofiber was produced in a nonwoven fabric mode by electrospinning a sol derived from hydrolysis of zirconium butoxide with a polyvinyl butyral. Results indicated that the hydroxyl groups on the vinyl alcohol units in the backbone of the polymer were involved in the hydrolysis as well as grafting the hydrolyzed zirconium butoxide. In addition, use of acetic acid as a catalyst resulted in further hydrolysis and condensation in the sol, which led to the growth of -Zr-O-Zr- networks among the polymer chains. These networks gradually transformed into a crystalline zirconia structure upon heating. The as-spun fiber was smooth but partially wrinkled on the surface. The average fiber diameter was 690±110 ㎚. The fiber exhibited a strong but broad blue photoluminescence with its maximum intensity at a wavelength of ~410 ㎚ at room temperature. When the fiber was heat-treated at 400℃, the fiber diameter shrunk to 250±60 ㎚. Nanocrystals which belonged to a tetragonal zirconia phase and were ~5 ㎚ in size appeared. A strong white photoluminescence was observed in this fiber. This suggests that oxygen or carbon defects associated with the formation of the nanocrystals play a role in generating the photoluminescence. Further heating to 800℃ resulted in a monoclinic phase beginning to form In the heat-treated fibers, coloring occurred but varied depending on the heating temperature. Crystallization, coloring, and phase transition to the monoclinic structure influenced the photoluminescence. At 600℃, the fiber appeared to be fully crystallized to a tetragonal zirconia phase.

Ethanol sensing properties and dominant sensing mechanism of NiO-decorated SnO2 nanorod sensors

NiO-decorated SnO2 nanorods were synthesized by the thermal evaporation of Sn powders followed by the solvothermal deposition of NiO. A multi-networked p-n heterostructured nanorod sensor was fabricated by dropping the p-NiO-decorated n-SnO2 nanorods onto the interdigited electrode pattern and then annealing. The multi-networked p-n heterostructured nanorod sensor exhibited enhanced response to ethanol compared with the pristine SnO2 nanorod and NiO nanoparticle sensors. The former also exhibited a shorter sensing time for ethanol. Both sensors exhibited selectivity for ethanol over other volatile organic compounds (VOCs) such as HCHO, methanol, benzene and toluene and the decorated sensor exhibited superior selectivity to the other two sensors. In addition, the dominant sensing mechanism is discussed in detail by comparing the sensing properties and current-voltage characteristics of a p-NiO/n-SnO2 heterostructured nanorod sensor with those of a pristine SnO2 nanorod sensor and a pristine NiO nanoparticle sensor. Of the two competing electronic mechanisms: a potential barrier-controlled carrier transport mechanism at a NiO-SnO2p-n junction and a surface-depletion-controlled carrier transport mechanism, the former has some contribution to the enhanced gas sensing performance of the p-n heterostructured nanorod sensor, however, its contribution is not as significant as that of the latter.

Room-temperature hydrogen gas sensing properties of the networked Cr2O3-functionalized Nb2O5 nanostructured sensor

Cr2O3-functionalized Nb2O5 nanoparticles were synthesized via a facile hydrothermal route. The multiple-networked Cr2O3-functionalized Nb2O5 nanostructured sensor showed enhanced H2 gas sensing performance compared to its pristine Nb2O5 nanostructure counterpart. The Cr2O3-functionalized Nb2O5 nanostructure sensor showed responses of 5.24 to 2 ppm of H2 at room temperature, whereas the pristine Nb2O5 nanoparticle sensors showed responses of 2.29. The former also exhibited a faster response to H2. The multiple-networked pristine and Cr2O3-functionalized Nb2O5 nanostructured sensors were stronger and much shorter, respectively, than other nanomaterial-based Schottky diode-type sensors and Nb2O5-based Schottky diode-type sensors. The underlying mechanism for the enhanced sensing performance of the Cr2O3-functionalized Nb2O5 nanostructured sensor towards H2 gas is discussed in detail. Particular emphasis is placed on the role of the Cr2O3-Nb2O5 p-n junction in the Cr2O3-functionalized Nb2O5 nanostructure sensor.

Acetone gas sensing properties of a multiple-networked Fe 2 O 3 -functionalized CuO nanorod sensor

Fe2O3-decorated CuO nanorods were prepared by Cu thermal oxidation followed by Fe2O3 decoration via a solvothermal route. The acetone gas sensing properties of multiple-networked pristine and Fe2O3-decorated CuO nanorod sensors were examined. The optimal operating temperature of the sensors was found to be 240°C. The pristine and Fe2O3-decorated CuO nanorod sensors showed responses of 586 and 1,090%, respectively, to 1,000 ppm of acetone at 240°C. The Fe2O3-decorated CuO nanorod sensor also showed faster response and recovery than the latter sensor. The acetone gas sensing mechanism of the Fe2O3-decorated CuO nanorod sensor is discussed in detail. The origin of the enhanced sensing performance of the multiple-networked Fe2O3-decorated CuO nanorod sensor to acetone gas was explained by modulation of the potential barrier at the Fe2O3-CuO interface, highly catalytic activity of Fe2O3 for acetone oxidation, and the creation of active adsorption sites by Fe2O3 nanoparticles.

An Approach for Synthesis and Characterization of Zirconia Nanopowder via Microwave Thermal Process

We applied a simple, versatile and an efficient non-toxic methodology based on microwave irradiation process to prepare a gel from n-zirconium butoxide without using any solvent at ambient temperature to open new possibilities to control particle size distribution, surface chemistry and agglomeration. The alkoxy-derived gel changed to tetragonal zirconia powder on heating at 400°C with crystalline particles size (3 ∼ 5 nm). Zirconia exhibited strong luminescence under a UV laser. The presence of a organic moieties as impurities exhibit a strong luminescence in zirconia powder. From the structural analysis it is concluded that the morphology of the oxide strongly depends on the nature of the precursors and the synthetic methology.

공증발과 열산화로 제조한 Ag-CuO-SnO₂ 박막에서 미세조직과 CO 가스 감지특성

In this study, we investigated microstructure and the CO gas sensing properties of Ag-CuO-SnO₂ thin films prepared by coevaporation and subsequently thermal oxidation at air atmosphere. The sensitivity of a Cu-Sn films, thermally oxidized at 600℃, is strongly affected by the amount of Cu. At Cu:7 wt%-Sn:93 wt%, the film exhibited a maximum sensitivity of ~2.3 to CO gas of 1000 ppm at 300℃. In contrast, the sensitivity of a Sn-Ag film did not change significantly with the amount of Ag. An enhanced sensitivity of ~3.7 was observed in the film with a composition of Ag:3 wt%-Cu:4 wt%-Sn:93 wt%, when thermally oxidized at 600℃. In addition, this thin film shows a response time of ~80 sec and a recovery time of ~450 sec to 1000 ppm CO gas. The results demonstrate that the CO sensitivity of the Ag-CuO-SnO₂ thin films may be closely associated with coexistence of SnO₂ and SnO phase, decrease in average particle size, and a porous microstructure. We also suggest that co-evaporation and followed by thermal oxidation is a very simple and effective method to prepare oxide gas sensor thin films.

Preparation and Characterization of NiZn-Ferrite Nanofibers Fabricated by Electrospinning Process

Electrospinning process is the useful and unique method to produce nanofibers from metal precursor and polymer solution by controlled viscosity. In this study, the NiZn ferrite nanofibers were prepared by electrospinning with a aqueous metal salts/polymer solution that contained polyvinyl pyrrolidone and Fe (Ⅲ) chloride, Ni (Ⅱ) acetate tetrahydrate and zinc acetate dihydrate in N,Ndimethylformamide. The applied electric field and spurting rate for spinning conditions were 10 ㎸, 2 ㎖/h, respectively. The obtained fibers were treated at 250℃ for 1 h to remove the polymer. Finally, the NiZn ferrite fibers were calcined at 600℃ for 3 h and annealed at 900~1200℃ in air. By tuning the viscosity of batch solution before electrospinning, we were able to control the microstructure of NiZn ferrite fiber in the range of 150~500 ㎚ at 770 cP. The primary particle size in 600℃ calcined ferrite fiber was about 10 ㎚. The properties of those NiZn ferrite fibers were determined from X-ray diffraction analysis, electron microscopy, energy dispersive spectroscopy, Fourier transform infrared spectroscopy, thermal analysis, and magnetic measurement.

A Novel Approach for the Synthesis of Ti- and Zr-oxo Species as Precursors for Metal Oxide Nanopowder

ABSTRACT Thermal and microwave irradiation treatment of Ti(OPri)4 and Zr(OBu)4 leads to the formation of homogenous and transparent oxo-alkoxy derivatives of titanium and zirconium in a quantitative yield. The oxo-alkoxy derivatives were characterized with the help of FT-IR, NMR, UV, Raman, GC–mass spectroscopy, and elemental analysis. The physicochemical properties of gel differ considerably from their corresponding parent metal alkoxides. Hence, the gel can be considered as an immediate precursor for the preparation of functional metal oxide nanomaterials under mild conditions with control over particle size and its distribution, surface chemistry, and particle agglomeration. The morphology of the metal oxides strongly depends on the nature of the precursor and as well as synthetic methodology.