Papers by Author: Jeung Soo Huh

Abstract: A novel and simple approach has been developed for a large-scale synthesis of ZnO nanorods using ultrasound radiation. Double-pod-like ZnO nanorods were directly grown from the starting precursors of zinc nitrate hydrate [Zn(NO3)26H2O] and hexamethylene tetramine (HMT) in deionized water without any templates or seeds. The obtained double-pod-like product had a diameter ranging from 80 to 150 nm and length ranging from 1 to 5 um. Scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscope (TEM) revealed that these products are of high purity, hexagonal and of single-crystalline structure. The ZnO nanorods synthesized were utilized for gas sensing application. Alumina plate consists of Pt interdigitated electrode was used as sensor substrate. The as-prepared products were transferred to the surface of substrate by screen printing method to fabricate the gas sensor. The sensing properties of the sensor were investigated toward methyl mercaptan (CH3SH). The sensor shows good and fast response towards methyl mercaptan at a fixed operating temperature. The high sensitivity may be ascribed to the high surface area and aspect ration of the double-pod-like nanomaterial.

Abstract: Ultrafine TiO2 powders as rutile and anatase phase were simply precipitated at room temperature for only tens of hours by simply controlling the pH value and Ti4+ concentration via aqueous TiCl4 solution. Under the optimal pH value and Ti4+ concentration, the average particle size of powders with rutile phase was 3.7nm, while that of powders with anatase phase was 3.0nm. The average particle size was calculated from the broadening of corresponding X-ray spectral peaks by Scherrer formula. In addition, 3.0 mol.l-1 are suggested to be used as concentration of stock solutions instead of the current concentration 2.0 mol.l-1.

Abstract: The paper presents the effect of acid treatment on the structure of single-walled carbon nanotube (SWNT) bundles and on the characteristics of SWNT-based gas sensors. The commercial SWNT powder was treated with a mixture of concentrated H2SO4:HNO3 (3:1 in volume) before used to fabricate sensors for ammonia (NH3) detection at room temperature. The Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) images indicated that the acid treatment not only removed most of catalytic particles from the SWNT bundles, but also caused SWNT bundles to be fragile. The fracture of the SWNT facilitated for gas molecules to adsorb in the SWNT sites, leading to an enhancement in sensitivity of the sensors. Upon exposing to 50 ppm NH3 in 450 sccm of nitrogen flowing rate at room temperature, the resistance of the 2-h-stirred sensors increased to 38% compared to 22% of the 1-h-stirred sensor. The recovery of the SWNT sensor was also accelerated owing to the treatment. These findings opened a new direction to improve the characteristics of SWNT-based gas sensors.

Abstract: The nano-CeO2 was synthesized by two-step solid-phase reaction. The image of TEM showed that nano-CeO2 with an average size of about 70 nm. The series of polyaniline/nano-CeO2 composites with different PANi: CeO2 ratios were prepared by in-situ polymerization in the presence of hydrochloric acid (HCl) as dopant by adding nano-CeO2 into the polymerization reaction mixture of aniline. The composites obtained were characterized by FT-IR and UV-vis spectroscopy analysis. The FT-IR spectra of nanocomposites indicate different blue-shifts, attributed to C–N stretching mode for benzenoid unit. The UV-vis spectra of nanocomposites display einstein-shifts compared with PANi at 620nm. The conductivity properties of the composites are also changed compare to the pure PANi. These results suggest that the interactions between the polymer matrix and nanoparticles take place in polyaniline/nano- CeO2 composites.

Abstract: The influence of oxidant on surface morphology and sensing property of polypyrrole (PPy) – poly (vinyl alcohol) (PVA) double layer sensors was investigated. PPy–PVA double layer sensors were prepared by vapor polymerization using FeCl3 as oxidizing agent and PVA as adhesive agent. The surface of PPy–PVA layer was observed on a scanning electron microscope (SEM). The surface morphology of sensor showed homogeneous layer and thickness of 1µm. And the sensing properties were investigated using a flow measuring system. The maximum sensitivity to methanol 1000ppm was proportional to increasing 12wt % FeCl3 concentration at 25°C. The response time was not affected by FeCl3 concentration, but the recovery time increased according to the amount of FeCl3. The best amount of FeCl3 was observed at 12wt%.

Abstract: The effect of operating temperature on characteristics of single-walled carbon nanotubes (SWNT) based gas sensor was investigated. SWNT-based sensor was fabricated from SWNT powder (Iljin Nanotech, Korea) by screen-printing method. SWNT powder (30 mg, AP grade) was dispersed into 0.78 gram a-terpineol (Aldrich) by ultrasonic vibration for 1 hour then stirred manually for 1 hour to increase adhesion. From this condensed solution, a thick film of SWNT was printed onto alumina substrates. The film then was sintered at 300oC for 2 hours to remove residual impurities. Upon exposure to some gases such as nitrogen, ammonia or nitric oxide, resistance of the sensor dramatically changes due to gas adsorption. In our experiments, SWNT-based sensor was employed to detect NH3 gas in N2 ambience. After saturated of N2, the sensor exposes to NH3 with various concentrations (from 5 ppm to 100 ppm, diluted by N2 as carrier gas). This sensor exhibits a fast response, high sensitivity but slow recovery at room temperature. By heating at high temperature and increasing the flow-rate of carrier gas, NH3 gas desorbs easily and recovery of the sensor improved. The heating also influenced the characteristics of sensors such as response and reproducibility. Other special changes in electric property of SWNT-based sensor caused by heating are also discussed.

Abstract: The polyaniline (PAn) polymer was synthesized by a chemical method using aniline (AN) as monomer, ammonium persulfate (APS) as initiator, and dodecylbenzene sulfonic acid (DBSA) as dopant at 0ı. Conducting polymer films were prepared on the alumina substrate with an interdigitated electrode by using the dip coating method. These films were soaked in methanol solvent for 1h at room temperature to remove excessive DBSA and then were heated at 70ı for 4h in N2. The detecting materials having a liquid phase at one atmospheric were precisely controlled by the system having a mass flow controller (MFC), temperature controller and measuring chamber. The sensitivity was estimated by heating from 70ı. The sensitivity observed in PAn sensor was lower than PAn-DBSA sensor at 1000 ppm methanol vapors under N2. because the additional thermal doping effect was resulted from increasing molecular motion in polymer chain to 70ı but DBSA used as dopant was decomposed by heating over 120ı. It caused the melting point of DBSA to be lower than 120ı. So the thermal properties of PAn and PAn-DBSA were investigated by DSC (differential scanning calorimetry). The DSC was scanned with a heating rate of 10ı/min. The PAn sample did not show the melting peak but PAn-DBSA sample showed the melting peak at 190ı. From this result, the new crystalline-region was evaluated in PAn-DBSA and increased of crystallinity of PAn polymer. And FE-SEM studies showed that the presence of the dopant strongly influenced the morphology of the polymer.

Abstract: After the 9∙11 terrorism and America-Iraq war, apprehension of mass destruction weapons such as bio-chemical agents calls for much more sensitive sensors which can detect toxic gases. In this study, semiconductor gas sensors based on tin oxide were examined to detect chemical agent simulants: dimethyl-methyl-phosponate (DMMP), di(propylene glycol) methyl ether (DPGME), acetonitrile, and dichloromethane. DMMP((CH3O)3), DPGME(C7H16O3), acetonitrile(CH3CN), and dichloromethane(CH2Cl2) gases are the simulants of nerve agent-sarin, vesicant agent-HN(N(CH2CH2Cl)3), blood agent-AC(HCN), and choking agent-CG(COCl2) gases, repectively. The SnO2 powder was prepared by a coprecipitation method from the mixture of tin chloride and zinc acetate dihydrate. Zinc oxide was doped into SnO2 from 1 wt% to 4 wt% to improve its reliability and sensitivity. To fabricate a thick film, powders were made into paste with organic binder of ethyl cellulose and screen-printed on the alumina substrate. The phase development and morphology of ZnO-doped SnO2 film were investigated by XRD (X-ray diffraction analysis), BET (surface and pore size analyzer), and SEM (scanning electron microscope). The gas sensing characteristics for target gases were examined with a flow-type measurement system. The concentrations of simulants were controlled from 500 ppb to 1500 ppb, and working temperatures were regulated from 250  to 400 .