Seung-Yeop Kwak

Carbon quantum dots embedded with mesoporous hematite nanospheres as efficient visible light-active photocatalysts

Carbon quantum dots (CQDs) and mesoporous hematite (α-Fe2O3) complex photocatalysts were successfully prepared using a facile solvent-thermal process in an aqueous solution. Mesostructured α-Fe2O3 clusters with a high surface area and a porous framework were an important consideration in the design of the photocatalysts because such structures enhance the absorption of photons and promote the decomposition of organic pollutants. More significantly, the CQDs in this catalyst play a pivotal role in improving the photocatalytic activity under visible light irradiation. The nanocomposites were characterized using X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, energy-dispersive spectroscopy, field-emission scanning electron microscopy, transmission electron microscopy, dynamic light scattering methods, ultraviolet-visible spectroscopy, and other techniques. The results confirmed the formation of CQD/mesoporous α-Fe2O3 hybrid clusters with a uniform size (about 700 nm), three-dimensional spherical morphologies, a large internal surface area (up to 187 m2 g−1), and a wormhole-like mesopore structure. Moreover, these novel composite catalysts displayed a continuous absorption band in the visible region. Photocatalytic studies of the CQD-embedded mesoporous α-Fe2O3 showed excellent photocatalytic efficiency (up to 97% capacity retention after three cycles) toward the degradation of organic compounds in aqueous media under visible light irradiation. The relationship between the physicochemical properties and the photocatalytic performance in our system is described and discussed on the basis of the results.

POLYETHYLENE/CLAY NANOCOMPOSITE BY IN SITU EXFOLIATION OF MONTMORILLONITE DURING ZIEGLER–NATTA POLYMERIZATION OF ETHYLENE

Complete exfoliation of montmorillonite during Ti-based Ziegler-Natta polymerization of ethylene has been successfully carried out by using montmorillonite (MMT-OH) modified with intercalation agents containing hydroxy groups. Hydroxyl groups in intercalation agents offer facile reactive sites for anchor ing catalysts in between silicate layers. Comparison of exfoliation characteristics between MMT-OH and non-intercalated montmorillonite showed that the feasibility of exfoliation during ethylene polymerization was highly dependent on the catalyst fixation method.

Details of Surface Features in Aromatic Polyamide Reverse Osmosis Membranes Characterized by Scanning Electron and Atomic Force Microscopy

In the present article, some new events on the surface morphology of the aromatic polyamide thin-film-composite (TFC) membranes were demonstrated in con- junction with their inherent chemical nature. In addition, the detailed, quantitative understanding of the microscopic surface features was shown to be essential in con- trolling the water permeability and eventually developing the high performance mem- branes. The surface roughness and the surface area were mainly affected by the existence or nonexistence of the crosslinking and/or the free amide groups not pertinent to the formation of the hydrogen bonding, which in turn contributed to the water permeability.

Use of atomic force microscopy and solid-state NMR spectroscopy to characterize structure-property-performance correlation in high-flux reverse osmosis (RO) membranes

Abstract Morphology and relaxation studies were very effective in understanding of the reverse osmosis (RO) permeation for the high-flux reverse osmosis (RO) membranes which were the thin-film-composite (TFC) type based on aromatic polyamide of m-phenylene diamine (MPD)/trimesoyl chloride (TMC). Microscopic morphology analyzed by atomic force microscopy (AFM) together with field-emission scanning electron microscopy (FE-SEM) and molecular relaxation characterized by solid-state 1 H nuclear magnetic resonance (NMR) spectroscopy revealed an important factor crucially affecting the enhancement of RO permeability. The proton spin-lattice relaxation in the rotating frame for the aromatic polyamides in their wet state (i.e., saturated with D2O) has been shown to be sensitive to the water flux and played a significant role in enhancing the membrane permeability, regardless of the surface features. The aromatic polyamide possessing relatively shorter spin-lattice relaxation times in the rotating frame, T1ρ, provided a TFC membrane with higher RO permeation, and vice versa.

Assembly of magnetite nanocrystals into spherical mesoporous aggregates with a 3-D wormhole-like pore structure†

Spherical mesoporous magnetite (Fe3O4) aggregates with a wormhole-like pore structure were successfully synthesized for the first time using a single iron precursor (iron(III) ethoxide) and an amphiphilic poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) triblock copolymer (PEO100–PPO65–PEO100) as a soft template. In this synthesis, the interaction between the iron precursor and the triblock copolymer self-assemblies in ethanol leads to the assembly of magnetite nanocrystals into spherical mesoporous aggregates. These aggregates were characterized using Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, field-emission scanning electron microscopy, standard and high-resolution transmission electron microscopy, 57Fe Mossbauer spectroscopy, and X-ray diffraction, confirming the formation of pure-phase Fe3O4 particles with monodisperse morphology (about 130 nm in diameter), three-dimensional wormhole-like mesopores, and highly crystalline spinel structure. In addition, a formation mechanism for this material in the present system is proposed, based on the analysis of results. The mesoporous magnetite has a high specific surface area of 165.6 m2 g−1, and relatively large pores with a mean size of 5.2 nm. The magnetic susceptibility data demonstrate that this material exhibits superparamagnetic behavior.

Relationship of relaxation property to reverse osmosis permeability in aromatic polyamide thin-film-composite membranes

We proposed a new approach to characterize the reverse osmosis (RO) permeability in conjunction with the macromolecular structures and inherent polymer properties for crosslinked and linear model aromatic polyamides. Aromatic polyamides were synthesized via interfacial reaction of phenylene diamines (p- and m-phenylene diamines) with either tri-functional (trimesoyl chloride) or di-functional (terephthaloyl and isophthaloyl chlorides) acyl halides.The relaxation properties obtained by cross polarization/magic angle spinning (CP/MAS) 13C NMR spectroscopy, in conjunction with the chemical structures, built a bridge between the specific polymer properties and the RO performance of the aromatic polyamides. The spin-lattice relaxation time in the rotating frame, T1ρ and hence chain mobility seemed to be an important parameter to control the RO membrane permeability. The longer T1ρ resulted from the crosslinked aromatic polyamides, in which presence of the crosslinking retarded local polymer motion, and a lower water flux resulted. In contrast, the shorter T1ρ for the linear crosslinked polyamides played a significant role for the higher water flux.

Amphiphilic Thiol Functional Linker Mediated Sustainable Anti-Biofouling Ultrafiltration Nanocomposite Comprising a Silver Nanoparticles and Poly(vinylidene fluoride) Membrane

We develop sustainable anti-biofouling ultrafiltration membrane nanocomposites by covalently immobilizing silver nanoparticles (AgNPs) onto poly(vinylidene fluoride) (PVDF) membrane mediated by a thiol-end functional amphiphilic block copolymer linker. Field emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectroscopy (EDXS) measurements reveal that the AgNPs are highly bound and dispersed to the PVDF membrane due to the strong affinity of the AgNPs with the thiol-modified block copolymeric linkers, which have been anchored to the PVDF membrane. The membrane performs well under water permeability and particle rejection measurements, despite the high deposition of AgNPs on the surface of membrane. The Ag-PVDF membrane nanocomposite significantly inhibits the growth of bacteria on the membrane surface, resulting in enhanced anti-biofouling property. Importantly, the AgNPs are not released from the membrane surface due to the robust covalent bond between the AgNPs and the thiolated PVDF membrane. The stability of the membrane nanocomposite ensures a sustainable anti-biofouling activity of the membrane.

Self-assembled mesoporous Co and Ni-ferrite spherical clusters consisting of spinel nanocrystals prepared using a template-free approach

Based on a self-assembly strategy, spherical mesoporous cobalt and nickel ferrite nanocrystal clusters with a large surface area and narrow size distribution were successfully synthesized for the first time via a template-free solvothermal process in ethylene glycol and subsequent heat treatment. In this work, the mesopores in the ferrite clusters were derived mainly from interior voids between aggregated primary nanoparticles (with crystallite size of less than 7 nm) and disordered particle packing domains. The concentration of sodium acetate is shown herein to play a crucial role in the formation of mesoporous ferrite spherical clusters. These ferrite clusters were characterized in detail using wide-angle X-ray diffraction, thermogravimetric-differential thermal analysis, (57)Fe Mössbauer spectroscopy, X-ray photoelectron spectroscopy, field-emission scanning electron microscopy, standard and high-resolution transmission electron microscopy, and other techniques. The results confirmed the formation of both pure-phase ferrite clusters with highly crystalline spinel structure, uniform size (about 160 nm) and spherical morphology, and worm-like mesopore structures. The BET specific surface areas and mean pore sizes of the mesoporous Co and Ni-ferrite clusters were as high as 160 m(2) g(-1) and 182 m(2) g(-1), and 7.91 nm and 6.87 nm, respectively. A model for the formation of the spherical clusters in our system is proposed on the basis of the results. The magnetic properties of both samples were investigated at 300 K, and it was found that these materials are superparamagnetic.

Unentangled Star-Shape Poly(ε-caprolactone)s as Phthalate-Free PVC Plasticizers Designed for Non-Toxicity and Improved Migration Resistance

We develop a nontoxic unentangled star-shape poly(ε-caprolactone) (UESPCL) plasticizer with excellent migration resistance for the production of phthalate-free flexible poly(vinyl chloride) (PVC) by means of the ring-opening polymerization of ε-caprolactone, initiated from the multifunctional core, combined with end-capping, and vacuum purification processes. UESPCL is a transparent liquid at room temperature and exhibits unentangled Newtonian behavior because of its extremely short branched segments. UESPCL is biologically safe without producing an acute toxicity response. Torque analysis measurements reveals that UESPCL offers a faster fusion rate and a higher miscibility with PVC compared to a typical plasticizer, di(2-ethylhexyl) phthalate (DEHP). The solid-state (1)H nuclear magnetic resonance (NMR) spectrum reveals that PVC and UESPCL are miscible with an average domain size of less than 8 nm. The flexibility and transparency of the PVC/UESPCL mixture, that is, phthalate-free flexible PVC, are comparable to the corresponding properties of the PVC/DEHP mixture, and the stretchability and fracture toughness of PVC/UESPCL are superior to the corresponding properties of the PVC/DEHP system. Most of all, PVC/UESPCL shows excellent migration resistance with a weight loss of less than 0.6% in a liquid phase, whereas DEHP migrated out of PVC/DEHP into a liquid phase with a weight loss of about 10%.

Solvent-Free Polymer Electrolytes I. Preparation and Characterization of Polymer Electrolytes Having Pores Filled with Viscous P ( EO ‐ EC ) ∕ Li C F 3 S O 3

We have attempted to combine a typical gel (hybrid liquid-solid system) and a typical solvent-free polymer electrolyte (solid system) through the concept of the pore-filling polymer electrolyte; instead of organic solvents, a viscous polymer complexed with a Li salt is filled into the pores of a porous membrane. The viscous polymer, poly(ethylene oxide-ethylene carbonate) copolymer [P(EO-EC)] with a low molecular weight (Mw = 1800) and amorphous nature is synthesized using ethylene carbonate (EC) and its viscosity is found to be independent of shear rate, which is a characteristic of a Newtonian fluid. Porous membranes consisting of poly(vinylidene fluoride-co-hexafluoropropylene) [P(VdF-HFP)] and P(EO-EC) are prepared by a phase inversion method and examined by means of 1 H solid-state nuclear magnetic resonance and field emission-scanning electron microscopy. From these results, it is confirmed that they are homogenous on a scale of a few tens of nanometers and the existence of P(EO-EC) in membranes makes much larger pore size and higher porosity than the pure P(VdF-HFP) membrane. This implies that the relative composition of P(EO-EC) plays a critical role in determining the membranes morphology and porosity. The temperature dependence of the ionic conductivity for the polymer electrolytes is well fitted to the Arrhenius equation. The highest conductivity of 3.7 X 10 - 5 S cm - 1 at 25°C and 1.6 X 10 - 4 S cm - 1 at 55°C is obtained for E-V6E4. Cyclic voltammetry on stainless steel electrodes shows that the polymer electrolyte is electrochemically stable up to at least 5.0 V vs Li/Li + .