Hong-Baek Cho

The Effects of Impeller Characteristics in the Hydrogenation of Aniline on Ru/C Catalyst

Effects of impeller characteristics have been studied in the hydrogenation of aniline on Ru/C catalyst. Dual impellers were employed and the experiments were performed at 300 rpm using a lab-scale reactor. Reaction with disk turbines (DT) resulted in the higher cyclohexylamine (CHA) selectivity and higher reaction rate compared to that with pitched blade turbines (PBT). When a combination of PBT and DT impellers was employed, high product selectivity and reaction rate were obtained and the selectivity was maintained constant. Changes in the product selectivity with the impeller geometry were explained in terms of the relative rates of the side reactions depending on the hydrogen concentration in the reaction mixture.

Room-Temperature H2 Gas Sensing Characterization of Graphene-Doped Porous Silicon via a Facile Solution Dropping Method

In this study, a graphene-doped porous silicon (G-doped/p-Si) substrate for low ppm H2 gas detection by an inexpensive synthesis route was proposed as a potential noble graphene-based gas sensor material, and to understand the sensing mechanism. The G-doped/p-Si gas sensor was synthesized by a simple capillary force-assisted solution dropping method on p-Si substrates, whose porosity was generated through an electrochemical etching process. G-doped/p-Si was fabricated with various graphene concentrations and exploited as a H2 sensor that was operated at room temperature. The sensing mechanism of the sensor with/without graphene decoration on p-Si was proposed to elucidate the synergetic gas sensing effect that is generated from the interface between the graphene and p-type silicon.

Synthesis of Samarium-Cobalt Sub-micron Fibers and Their Excellent Hard Magnetic Properties

High-throughput synthesis of Samarium-Cobalt sub-micron fibers with controlled composition and dimension was demonstrated by combining electrospinning and reduction-diffusion processes. The composition of fibers was readily varied (8 < Sm < 20 at.%) by adjusting precursor composition whereas the diameter of fibers was precisely controlled by varying electrospinning parameters (e.g., applied voltage, solution feed rate, temperature, and humidity) to reach single-domain size. X-ray diffraction patterns confirmed that single phase Sm2Co17 fibers were synthesized when the metal precursor ratio (Sm3+/(Sm3++Co2+)) was precisely controlled at 10.6%, whereas mixed phases (i.e., Co-Sm2Co17 or Sm2Co17-Sm2Co7) were observed when the ratio is deviated from the stoichiometric. Magnetic saturation (Ms) of the synthesized fibers monotonically decreased with an increased in Sm content. In contrast, coercivity (Hci) monotonically increased with an increase in Sm content.

A noble gas sensor platform: linear dense assemblies of single-walled carbon nanotubes (LACNTs) in a multi-layered ceramic/metal electrode system (MLES)

Monodispersed Pt nanocatalyst-doped and undoped assemblies of single-walled carbon nanotubes (SWCNTs) were aligned with high anisotropy on a multi-layered ceramic/metal electrode system (MLES), which was used as a cheap and cost-effective electrode system, by dielectrophoretic orientation to form linear dense assemblies of SWCNTs (LACNTs). It is envisaged that they can be used as a potentially inexpensive platform for multi-gas sensing nano-devices. To form homogeneously dispersed Pt nanocatalysts (NCs), an aqueous chloroplatinic acid solution (H2PtCl6) was deposited on the surface of acid-treated SWCNTs. This was followed by alignment under the application of alternative currents ranging from 5 kHz to 50 MHz to create electrically conducting LACNT bundles on the MLES platform and reduction under hydrogen plasma. The analysis showed that the Pt nanocatalysts (2 nmav, deviation: ±1 nm) are monodispersed on the SWCNTs, which contributed to a noticeable enhancement in the gas sensing properties towards H2, NO2, H2S and NH3 gases. In addition, the principal component analysis results showed fairly good discrimination of the gases by employing a sensor array composed of Pt/LACNTs and SWCNTs, and the hydrogen sensing performance was high in terms of sensitivity compared to other literature studies. This gas sensing device based on an MLES may pave the way for extended applications of new multi-gas sensing devices as a low-cost and life-long gas sensing platform.

Synthesis and thermoelectric characterization of bulk-type tellurium nanowire/polymer nanocomposites

A simple method to fabricate three-dimensionally (3-D) aligned thermoelectric nanowires attached polymer particle was demonstrated by combination of solution casting of thermoelectric nanostructures (e.g., tellurium nanowires (Te NWs)) on the surface of thermoplastic polymer (e.g., poly(methyl methacrylate (PMMA)) microbeads followed by hot compaction of thermoplastic matrix. The percolation threshold of composite with 3-D assembled Te NWs (i.e., 3.45xa0vol%) significantly was lower than that of a randomly dispersed Te NWs (i.e., 5.26xa0vol%), which resulted in an order of magnitude greater thermoelectric figure of merit (ZT of 2.8xa0×xa010−3) compared to randomly dispersed Te NWs in PMMA matrix (ZT of 6.4xa0×xa010−4) at room temperature by enhancing the electrical conductivity without increasing thermal conductivity.

Near theoretical ultra-high magnetic performance of rare-earth nanomagnets via the synergetic combination of calcium-reduction and chemoselective dissolution

Rare earth permanent magnets with superior magnetic performance have been generally synthesized through many chemical methods incorporating calcium thermal reduction. However, a large challenge still exists with regard to the removal of remaining reductants, byproducts, and trace impurities generated during the purifying process, which serve as inhibiting intermediates, inducing productivity and purity losses, and a reduction in magnetic properties. Nevertheless, the importance of a post-calciothermic reduction process has never been seriously investigated. Here, we introduce a novel approach for the synthesis of a highly pure samarium-cobalt (Sm-Co) rare earth nanomagnet with near theoretical ultra-high magnetic performance via consecutive calcium-assisted reduction and chemoselective dissolution. The chemoselective dissolution effect of various solution mixtures was evaluated by the purity, surface microstructure, and magnetic characteristics of the Sm-Co. As a result, NH4Cl/methanol solution mixture was only capable of selectively rinsing out impurities without damaging Sm-Co. Furthermore, treatment with NH4Cl led to substantially improved magnetic properties over 95.5% of the Ms for bulk Sm-Co. The mechanisms with regard to the enhanced phase-purity and magnetic performance were fully elucidated based on analytical results and statistical thermodynamics parameters. We further demonstrated the potential application of chemoselective dissolution to other intermetallic magnets.

Ultrasensitive detection of low-ppm H2S gases based on palladium-doped porous silicon sensors

In this study, the sensing properties of palladium-doped porous silicon (Pd/p-Si) substrates for low-ppm level detection of toxic H2S gas are investigated. A Si substrate with dead-end pores ranging from nano- to macroscale was generated by a combined process of metal-assisted chemical etching (MacE) and electrochemical etching with tuned reaction time, in which nano-Pd catalysts were decorated by E-beam sputtering deposition. The sensing properties of the Pd/p-Si were enhanced as the thickness of the substrate layer increased; along with the resulting variation in surface area, this resulted in superior H2S sensing performances in the low-ppm range (less than 3 ppm), with a detection limit of 300 ppb (sensitivity 30%) at room temperature. Furthermore, the sensor displayed excellent selectivity toward the hazardous H2S molecules in comparison with various other reducing gases, including NO2, CO2, NH3, and H2, showing its potential for application in workplaces or environments affected by other toxic gases. The enhancement in sensing performance was possibly due to the increased dispersion and surface area of Pd nano-catalysts, which led to an increase in chemisorption sites of adsorbate molecules.

A Novel Synthetic Method for N Doped TiO2 Nanoparticles Through Plasma-Assisted Electrolysis and Photocatalytic Activity in the Visible Region

Nitrogen doped TiO2 (N-TiO2) nanoparticles were synthesized via a novel plasma enhanced electrolysis method using bulk titanium (Ti) as a source material and nitric acid as the nitrogen dopant. This method possesses remarkable merits with regard to the direct-metal synthesis of nanoparticles with its one-step process, eco-friendliness, and its ability to be mass produced. The nanoparticles were synthesized from bulk Ti metal and dipped in 5–15 mmol of a nitric acid electrolyte under the application of AC 500 V, the minimum range of voltage to generate plasma. By controlling the electrolyte concentration, the nanoparticle size distribution could be tuned between 12.1 and 24.7 nm using repulsion forces via variations in pH. The prepared N-TiO2 nanoparticles were calcined at between 100 and 300°C to determine their photocatalytic efficiency within the visible-light region, which depended on their crystal structure and N doping content. Analysis showed that the temperature treatment yielded an anatase TiO2 crystalline structure when the N doping content was varied from 0.4 to 0.54 at.%. In particular, the 0.4 at.% N doped TiO2 catalyst exhibited the highest catalytic performance with quadruple efficiency compared to the P-25 standard TiO2 nanoparticles, which featured a 91% degradation of methyl orange organic dye within 300 min. This solid-liquid reaction based on plasma enhanced electrolysis could open new pathways with regard to high purity mass producible ceramic nanoparticles with advanced properties.