Jeseung Yoo

Competitive Adsorption of Metals onto Magnetic Graphene Oxide: Comparison with Other Carbonaceous Adsorbents

Competitive adsorption isotherms of Cu(II), Pb(II), and Cd(II) were examined on a magnetic graphene oxide (GO), multiwalled carbon nanotubes (MWCNTs), and powered activated carbon (PAC). A series of analyses confirmed the successful synthesis of the magnetic GO based on a simple ultrasonification method. Irrespective of the adsorbents, the adsorption was highly dependent on pH, and the adsorption was well described by the Langmuir isotherm model. The maximum adsorption capacities of the adsorbents were generally higher in the order of Pb(II) > Cu(II) > Cd(II), which is the same as the degree of the electronegativity and the hydrated radius of the metals, suggesting that the metal adsorption may be governed by an ion exchange between positively charged metals and negatively charged surfaces, as well as diffusion of metals into the surface layer. The adsorption of each metal was mostly lower for multi- versus single-metal systems. The antagonistic effects were influenced by solution pH as well as the type of metals, and they were higher in the order of the magnetic GO > MWCNT > PAC. Dissolved HS played a greater role than HS adsorbed onto the adsorbents, competing with the adsorption sites for metal complexation.

Morphology Control of Surfactant-Assisted Graphene Oxide Films at the Liquid–Gas Interface

Control of a two-dimensional (2D) structure of assembled graphene oxide (GO) sheets is highly desirable for fundamental research and potential applications of graphene devices. We show that an alkylamine surfactant, i.e., octadecylamine (ODA), Langmuir monolayer can be utilized as a template for adsorbing highly hydrophilic GO sheets in an aqueous subphase at the liquid-gas interface. The densely packed 2-D monolayer of such complex films was obtained on arbitrary substrates by applying Langmuir-Schaefer or Langmuir-Blodgett technique. Morphology control of GO sheets was also achieved upon compression by tuning the amount of spread ODA molecules. We found that ODA surfactant monolayers prevent GO sheets from sliding, resulting in formation of wrinkling rather than overlapping at the liquid-gas interface during the compression. The morphology structures did not change after a graphitization procedure of chemical hydrazine reduction and thermal annealing treatments. Since morphologies of graphene films are closely correlated to the performance of graphene-based materials, the technique employed in this study can provide a route for applications requiring wrinkled graphenes, ranging from nanoelectronic devices to energy storage materials, such as supercapacitors and fuel cell electrodes.

Long-term stability of CdSe/CdZnS quantum dot encapsulated in a multi-lamellar microcapsule

We developed a novel and easy encapsulation method for quantum dots (QDs) using a partially oxidized semi-crystalline polymeric material which forms a micron-sized granule with a multi-lamellar structure from a dilute solution. The QDs were highly dispersed in the granule in such a way that they were adsorbed on the lamella with ∼12 nm spacing followed by lamellar stacking. The QDs were heavily loaded into the granule to 16.7 wt% without aggregation, a process which took only a few minutes. We found that the quantum yield of the QDs was not degraded after the encapsulation. The encapsulated QD-silicone composite exhibited excellent long-term photo- and thermal stability with its initial photoluminescence intensity maintained after blue LED light radiation for 67 days and storage at 85 °C and 85% relative humidity for 119 days.

Fabrication of Hollow Silica Particles Using a Self-Assembled Polyethylene Granule as a Template

We developed a novel method preparing nonspherical hollow silica particles (HSP) using a micron-sized granule self-assembled from partially oxidized PE wax. The morphology of the granule was closely investigated in terms of concentration and acid value of PE wax and cooling rate. Due to the oxidized unit in PE wax, magnetic nanoparticle was incorporated into the granule during the self-assembly, and silica was coated on the granule surface via the self-assembly. Silica-coating condition was studied by varying water content and reaction time. After the PE wax was removed by calcination, nonspherical HSP or magnetic HSP was obtained. This cost-effective HSP is expected to be useful for practical applications.

B-site doping effects of NdBa0.75Ca0.25Co2O5+δ double perovskite catalysts for oxygen evolution and reduction reactions

As bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), we introduce an economical, particularly active, stable, and completely noble-metal-free catalyst based on CaO-doped NdBaCo2O5+δ with the aim of exploiting its high catalytic activity for the oxygen electrode. Further, the electrochemical performances of the NdBa0.75Ca0.25Co2O5+δ catalyst are improved by doping transition-metal oxides (Fe2+, Ni2+, Cu2+, and Mn2+) into the B-sites of double perovskite oxides, facilitating the movement of electrons and oxygen-ions through oxygen vacancies. Among these, NdBa0.75Ca0.25Co1.5Fe0.5O5+δ (NBCCFe) shows the highest electrocatalytic oxygen electrode activity for the OER and ORR, and NBCCFe thus is hybridized with nitrogen-reduced graphene oxide (N-rGO) to further improve its catalytic activity with excellent durability. Hybridization of NBCCFe with N-rGO further boosts bifunctional oxygen electrode activity (0.761 V) with better durability, which surpasses those of Pt/C (0.815 V) and Ir/C (0.768 V) catalysts. A rechargeable lithium–air battery assembled with the NBCCFe/N-rGO catalyst exhibits remarkably improved discharge capacity, reduced charge overpotential and an extended cycle life, corroborating the excellent bifunctional catalytic activity of NBCCFe/N-rGO. The NBCCFe/N-rGO catalyst also displays the most improved polarization during both discharge and charge, further confirming the highest bifunctional catalytic activity of NBCCFe/N-rGO.

Surge-Resistant Nanocomposite Enameled Wire Using Silica Nanoparticles with Binary Chemical Compositions on the Surface

We developed polyesterimide (PEI) nanocomposite enameled wires using surface-modified silica nanoparticles with binary chemical compositions on the surface. The modification was done using silanes assisted by ultrasound, which facilitated high density modification. Two different trimethoxysilanes were chosen for the modification on the basis of resemblance of chemical compositions on the silica surface to PEI varnish. The surface-modified silica was well dispersed in PEI varnish, which was confirmed by optical observation and viscosity measurement. The glass transition temperature of the silica-PEI nanocomposite increased with the silica content. The silica-dispersed PEI varnish was then used for enameled wire fabrication. The silica-PEI nanocomposite enameled wire exhibited a much longer lifetime compared to that of neat PEI enameled wire in partial discharge conditions.

Development of Greenhouse Gas (CO2) Emission Factor for Korean Coal Briquettes

Korea adheres to the basic emission factor proposed by the Intergovernmental Panel on Climate Change Guidelines to calculate the greenhouse gas emission. However, the Intergovernmental Panel on Climate Change Guidelines recommends applying an emission factor unique to a country, which reflects its features in fuel consumption. Especially, there has been little research in emission factors and in the amount of Korean coal briquettes used in Korea, and even worse, the Intergovernmental Panel on Climate Change Guidelines has not proposed the basic emission factor for such briquettes. Recently, the Korean government has made efforts to calculate greenhouse gas emission factors for each sector, and it is requiring research and development be conducted in the greenhouse gas emission factors of Korean coal briquettes in household and commercial sectors. The amount of carbon, hydrogen, and calorific values in the input fuel was measured using a calorimeter, elemental analyzer and proximate analyzer. For fuel analysis, the CO2 emission factor for Korean coal briquettes is 95,558 CO2 Kg/TJ. It is lower than the local anthracite CO2 emission factor, 111,100 CO2 Kg/TJ and the 2006 IPCC Guidelines, 87,399 CO2 Kg/TJ by about 13.99 and 2.79%, respectively.

High Purity and Yield of Boron Nitride Nanotubes Using Amorphous Boron and a Nozzle-Type Reactor

Enhancement of the production yield of boron nitride nanotubes (BNNTs) with high purity was achieved using an amorphous boron-based precursor and a nozzle-type reactor. Use of a mixture of amorphous boron and Fe decreases the milling time for the preparation of the precursor for BNNTs synthesis, as well as the Fe impurity contained in the B/Fe interdiffused precursor nanoparticles by using a simple purification process. We also explored a nozzle-type reactor that increased the production yield of BNNTs compared to a conventional flow-through reactor. By using a nozzle-type reactor with amorphous boron-based precursor, the weight of the BNNTs sample after annealing was increased as much as 2.5-times with much less impurities compared to the case for the flow-through reactor with the crystalline boron-based precursor. Under the same experimental conditions, the yield and quantity of BNNTs were estimated as much as ~70% and ~1.15 g/batch for the former, while they are ~54% and 0.78 g/batch for the latter.