Keunsu Choi

Gas sensing properties of defect-controlled ZnO-nanowire gas sensor

The effect of oxygen-vacancy-related defects on gas-sensing properties of ZnO-nanowire gas sensors was investigated. Gas sensors were fabricated by growing ZnO nanowires bridging the gap between two prepatterned Au catalysts. The sensor displayed fast response and recovery behavior with a maximum sensitivity to NO2 gas at 225u2009°C. Gas sensitivity was found to be linearly proportional to the photoluminescence intensity of oxygen-vacancy-related defects in both as-fabricated and defect-controlled gas sensors by postannealing in Ar and H2 atmosphere. This result agrees well with previous theoretical prediction that oxygen vacancies play a role of preferential adsorption sites for NO2 molecules.

Effect of nitrogen doping on titanium carbonitride-derived adsorbents used for arsenic removal.

Arsenic in water and wastewater is considered to be a critical contaminant as it poses harmful health risks. In this regard, to meet the stringent regulation of arsenic in aqueous solutions, nitrogen doped carbon-based materials (CN) were prepared as adsorbents and tested for the removal of arsenic ion from aqueous solutions. Nitrogen-doped carbon (CNs) synthesized by chlorination exhibited well-developed micro- and small meso-pores with uniform pore structures. The structure and characteristics of the adsorbents thus developed were confirmed by field-emission scanning electron microscopy, transmission electron microscopy, Brunauer-Emmett-Teller analysis, X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. Among the CNs developed, CN700 exhibited high adsorption capacity for arsenic (31.08 mg/g). The adsorption efficiency for arsenic ion was confirmed to be affected by pyrrolic nitrogen and micro-pores. These results suggest that CNs are useful adsorbents for the treatment of arsenic, and in particular, CN700 demonstrates potential for application as an adsorbent for the removal of anionic heavy metals from wastewater and sewage.

Spatially resolved photoluminescence mapping of single CdS nanosheets

We have utilized spatially resolved low temperature photoluminescence to probe the electronic states and structural symmetries of individual single crystalline CdS nanosheets. Spatially resolved photoluminescence imaging of a single CdS nanosheet reveals a distinctive spectral variation across the nanosheet. We observe A-like exciton states which emit most strongly near the outer edges of the 5μm wide nanosheet, while B-like exciton states emit most strongly along the center region of the 30μm long axis of the nanosheet.

Polarized photoluminescence and time-resolved photoluminescence from single CdS nanosheets

We have utilized polarized low temperature photoluminescence (PL) to probe the electronic states and structural symmetries of individual CdS nanosheets. High resolution transmission electron microscopy measurements indicate highly crystalline material with different nanosheets exhibiting significant variations of the direction of the c axis, which are consistent with polarization measurements of PL from single CdS nanosheets. The quality of the nanosheets is reflected in measurements of exciton lifetimes of ∼200ps, a value significantly longer than observed for CdS nanowires whose diameter is the same as the thickness of these nanosheets, but shorter than that observed in bulk crystals.

Chromate adsorption mechanism on nanodiamond-derived onion-like carbon

The onion-like carbon (OLC) was prepared as adsorbent and tested for the removal of chromate ions from aqueous solutions. The OLC was thermally derived from nanodiamond by vacuum annealing at 1000-2000°C. An investigation was conducted the chromate adsorption mechanism of OLC, by analysing the temperature-dependent evolution of the various oxygen-carbon bonds and the chemisorbed water by X-ray photo electron spectroscopy, as well as by the first principle calculation of the bond energies for relevant bond configurations. The present work demonstrated the importance of the carbon-oxygen bond type and carbon dangling bonds for chromate adsorption, as well as for other anionic heavy metals adsorbed from wastewater and sewage.

Chromium removal from aqueous solution by a PEI-silica nanocomposite

It is essential and important to determine the adsorption mechanism as well as removal efficiency when using an adsorption technique to remove toxic heavy metals from wastewater. In this research, the removal efficiency and mechanism of chromium removal by a silica-based nanoparticle were investigated. A PEI-silica nanoparticle was synthesized by a one-pot technique and exhibited uniformly well-dispersed PEI polymers in silica particles. The adsorption capacity of chromium ions was determined by a batch adsorption test, with the PEI-silica nanoparticle having a value of 183.7u2009mg/g and monolayer sorption. Adsorption of chromium ions was affected by the solution pH and altered the nanoparticle surface chemically. First principles calculations of the adsorption energies for the relevant adsorption configurations and XPS peaks of Cr and N showed that Cr(VI), [HCrO4]− is reduced to two species, Cr(III), CrOH2+ and Cr3+, by an amine group and that Cr(III) and Cr(VI) ions are adsorbed on different functional groups, oxidized N and NH3+.

Early stage oxynitridation process of Si(001) surface by NO gas: Reactive molecular dynamics simulation study

Initial stage of oxynitridation process of Si substrate is of crucial importance in fabricating the ultrathin gate dielectric layer of high quality in advanced MOSFET devices. The oxynitridation reaction on a relaxed Si(001) surface is investigated via reactive molecular dynamics (MD) simulation. A total of 1120 events of a single nitric oxide (NO) molecule reaction at temperatures ranging from 300 to 1000u2009K are statistically analyzed. The observed reaction kinetics are consistent with the previous experimental or calculation results, which show the viability of the reactive MD technique to study the NO dissociation reaction on Si. We suggest the reaction pathway for NO dissociation that is characterized by the inter-dimer bridge of a NO molecule as the intermediate state prior to NO dissociation. Although the energy of the inter-dimer bridge is higher than that of the intra-dimer one, our suggestion is supported by the ab initio nudged elastic band calculations showing that the energy barrier for the int...

Synthesis of amine-functionalized ZIF-8 with 3-amino-1,2,4-triazole by postsynthetic modification for efficient CO2-selective adsorbents and beyond

The facile tuning of the gate size and the chemical functionalization of zeolitic imidazolate frameworks (ZIFs) have been considered efficient strategies for various potential applications including gas membranes, sensors, and catalysts. Herein, we demonstrate the synthesis of amine-functionalized ZIF-8 (ZIF8-A) with 3-amino-1,2,4-triazole (Atz) by postsynthetic modification (PSM) towards two objectives: (1) CO2 selective adsorption by a combination of chemical interactions and controlled gate sizes and (2) potential for further chemical modifications. The acquired ZIF8-A substantially enhanced CO2/N2 and CO2/CH4 selectivity at 35 °C compared to ZIF-8 since the Atz conversion enhanced chemical interactions with CO2 due to the introduction of amine moieties while reducing both the surface area and pore volume. The gate size control of ZIF-8 by the replacement of Atz was thoroughly investigated by extensive transport experiments and density functional theory (DFT)-based computational simulations. In addition, the vinyl-functionalized ZIF-8, another versatile starting material, was successfully prepared by further chemical modifications of ZIF8-A with either methacrylic anhydride or glycidyl methacrylate through nucleophilic substitution reactions. As such, we believe that our current work can provide promising platforms for designing ZIF-based materials with versatile properties including precise control of the gate size and the incorporation of various functional groups.

Strong chromate-adsorbent based on pyrrolic nitrogen structure: An experimental and theoretical study on the adsorption mechanism

Chromate is considered a toxic contaminant in various water sources because it poses a risk to animal and human health. To meet the stringent limits for chromium in water and wastewater, pyrrolic nitrogen structure was investigated as a chromate adsorbent for aqueous solutions, employing a polypyrrole coating on carbon black. The characteristics of the adsorbent were analyzed by high-resolution transmission electron microscopy, energy-filtered transmission electron microscopy, and X-ray photoelectron spectroscopy. Chromate was adsorbed as both Cr(III) and Cr(VI). The chromate adsorption capacity increased (from 50.84 to 174.81u202fmg/g) with increasing amounts of pyrrole monomers (from 50 to 86%) in the adsorbent. The adsorption capacity was well-correlated with the pyrrolic nitrogen content (from 2.06 to 6.57u202fat%) in the adsorbent, rather than other types of nitrogen. The optimized adsorption capacity (174.81u202fmg/g in the equilibrium batch experiment and 211.10u202fmg/g at an initial pH of 3) was far superior to those of conventional adsorbents. We investigated the mechanism behind this powerful chromate adsorption on pyrrolic nitrogen via physical/chemical analyses of the pH-dependent adsorption behavior, supported by first-principles calculation based on density functional theory. We found that Cr(III) and Cr(VI) adsorption followed different reaction paths. Cr(III) adsorption occurred in two sequential steps: 1) A Jones oxidation reaction (JOR)-like reaction of Cr(VI) with pyrrolic N that generates Cr(III), and 2) Cr(III) adsorption on the deprotonated pyrrolic N through Cr(III)-N covalent bonding. Cr(VI) adsorption followed an alternative path: hydrogen-bonding to the deprotonation-free pyrrolic N sites. The pH-dependent fractional deprotonation of the pyrrolic N sites by the JOR-like reaction in the presence of chromate played an important role in the adsorption.

A Tailored Bifunctional Electrocatalyst: Boosting Oxygen Reduction/Evolution Catalysis via Electron Transfer Between N-Doped Graphene and Perovskite Oxides

Fabricating perovskite oxide/carbon material composite catalysts is a widely accepted strategy to enhance oxygen reduction reaction/oxygen evolution reaction (ORR and OER) catalytic activities. Herein, synthesized, porous, perovskite-type Sm0.5 Sr0.5 CoO3-δ hollow nanofibers (SSC-HF) are hybridized with cross-linked, 3D, N-doped graphene (3DNG). This rationally designed hybrid catalyst, SSC-HF-3DNG (SSC-HG), exhibits a remarkable enhancement in ORR/OER activity in alkaline media. The synergistic effects between SSC and 3DNG during their ORR and OER processes are firstly revealed by density functional theory calculations. It suggests that electron transport from 3DNG to O2 and SSC increases the activity of electrocatalytic reactions (ORR and OER) by activating O2 , increasing the covalent bonding of lattice oxygen. This electron transfer-accelerated catalysis behavior in SSC-HG will provide design guidelines for composites of perovskite and carbon with bifunctional catalysts.