Dong Kyun Seo

Spectroelectrochemistry of cytochrome c and azurin immobilized in nanoporous antimony-doped tin oxide

Stable immobilization of two redox proteins, cytochrome c and azurin, in a thin film of highly mesoporous antimony-doped tin oxide is demonstrated via UV-vis spectroscopic and electrochemical investigation.

Calcium-modified hierarchically porous aluminosilicate geopolymer as a highly efficient regenerable catalyst for biodiesel production

A new class of highly active solid base catalysts for biodiesel production was developed by creating hierarchically porous aluminosilicate geopolymer with affordable precursors and modifying the material with varying amounts of calcium. For the catalysts containing ≥8 wt% Ca, almost 100% conversion has been achieved in one hour under refluxing conditions with methanol solvent, and the high catalytic activity was preserved for multiple regeneration cycles. Temperature-programed desorption studies of CO2 indicate that the new base catalyst has three different types of base sites on its surface whose strengths are intermediate between MgO and CaO. The detailed powder X-ray diffraction (PXRD) and X-ray photoelectron spectroscopic (XPS) studies show that the calcium ions were incorporated into the aluminosilicate network of the geopolymer structure, resulting in a very strong ionicity of the calcium and thus the strong basicity of the catalysts. Little presence of CaCO3 in the catalysts was indicated from the thermogravimetric analysis (TGA), XPS and Fourier transform infrared spectroscopy (FT-IR) studies, which may contribute to the observed high catalytic activity and regenerability. The results indicate that new geopolymer-based catalysts can be developed for cost-effective biodiesel production.

Preparation and electrochemical properties of nanoporous transparent antimony-doped tin oxide (ATO) coatings

Nanoporous antimony-doped tin oxide (ATO) coatings with high surface area, optical transparency and electron transfer properties have been prepared using an interpenetrating inorganic–organic hybrid sol–gel approach. UV-Vis and X-ray photoelectron spectroscopic studies were carried out on the prepared materials in addition to the characterization of their microstructures with scanning electron microscopy, transmission electron microscopy, and nitrogen sorption experiments. Cyclic voltammetry (CV) and impedance spectroscopy were employed to characterize the electrochemical and electron transfer properties of the coatings in both acidic and neutral media with an Fe3+/Fe2+ redox couple. The material had a specific surface area of over 90 m2 g−1 with a bimodal pore distribution with two distinctive peaks at ca. 8 and 40 nm. The electrochemical capacitance of the material was about 100 times as high as the value obtained for a commercially available nonporous fluorine-doped tin oxide (FTO) electrode. The estimated electron transfer rate of the former was three times larger than that of the FTO. The optical transparency, high surface area and high electron transfer rate of the nanoporous ATO coatings make the material well suited for diverse photoelectrochemical applications.

Blue-silica by Eu2+-activator occupied in interstitial sites

A blue-emitting SiO2:Eu2+ compound has been successfully synthesized and characterized. The PL intensity of SiO2:Eu0.0022+ compound is about 24 times higher than that of the O-defective SiO2 compound (without activators), which emits blue light. The valence state of the Eu ions responsible for the highly enhanced blue emission was determined to be Eu2+ using reference materials (EuCl2 and EuCl3) and XPS measurements. The Eu2+-activator ions occupied in the interstitial sites of the SiO2 matrix were confirmed by FT-IR, XPS, and 29Si MAS-NMR spectroscopy. Even though the void spaces formed structurally in both α-quartz and α-cristobalite can accommodate Eu2+ ions (ionic radius = 1.25 A at CN = 8), SiO2:Eu2+ compound fired at 1300 °C under a hydrogen atmosphere is destined to be deficient in O or Si atoms, indicating the formation of the wider void spaces in the SiO2 crystal lattice. A sputtered depth profile of SiO2-related compounds obtained by time-of-flight secondary ion mass spectrometry (TOF-SIMS) corroborates the O-defective SiO2 induced by hydrogen. In particular, the interatomic potentials, depending on the interstitial positions of Eu atoms in α-cristobalite and α-quartz, are calculated based on Lennard-Jones and coulomb potentials. For α-cristobalite, the minimum potential value is −51.47 eV and for α-quartz, the value is 221.8 eV, which reveals that the Eu2+-activator ions more preferably enter the interstitial sites of α-cristobalite than those of α-quartz. Thanks to the stable Eu2+-activator ions enclosed by Si–O linkages, the SiO2:Eu0.0022+ compound emits blue light and its PL emission intensity is about 24 times higher than that of the O-defective SiO2 compound. This phosphor material could be a platform for modeling a new phosphor and for application in the solid-state lighting field.

Equipment-Free Deposition of Graphene-Based Molybdenum Oxide Nanohybrid Langmuir–Blodgett Films for Flexible Electrochromic Panel Application

The potential electrochromic application of graphene-based nanohybrids is hampered by the challenges in interfacing the electrochromic nanoparticles with graphene at atomic scale and in fabricating their thin film on the substrate through a scalable method. In an effort to overcome these challenges, we demonstrate a highly dispersible graphene-based molybdenum oxide nanohybrid (mRGO-MoO3-x) for flexible electrochromic application. With only a squeeze pipet, mRGO-MoO3-x could be deposited with a high coverage on various substrates through a scalable equipment-free Langmuir-Blodgett film deposition method. By taking advantage of high transmittance benefited from its remarkable thinness, the mRGO-MoO3-x Langmuir-Blodgett film shows a superior reversible electrochromic property with high coloration efficiency on both hard and flexible substrates.

Photocurrent generation by photosynthetic purple bacterial reaction centers interfaced with a porous antimony-doped tin oxide (ATO) electrode

The ability to exchange energy and information between biological and electronic materials is critical in the development of hybrid electronic systems in biomedicine, environmental sensing, and energy applications. While sensor technology has been extensively developed to collect detailed molecular information, less work has been done on systems that can specifically modulate the chemistry of the environment with temporal and spatial control. The bacterial photosynthetic reaction center represents an ideal photonic component of such a system in that it is capable of modifying local chemistry via light-driven redox reactions with quantitative control over reaction rates and has inherent spectroscopic probes for monitoring function. Here a well-characterized model system is presented, consisting of a transparent, porous electrode (antimony-doped tin oxide) which is electrochemically coupled to the reaction center via a cytochrome c molecule. Upon illumination, the reaction center performs the 2-step, 2-electron reduction of a ubiquinone derivative which exchanges with oxidized quinone in solution. Electrons from the electrode then move through the cytochrome to reoxidize the reaction center electron donor. The result is a facile platform for performing redox chemistry that can be optically and electronically controlled in time and space.

Remarkable flux effect of Li-codoping on highly enhanced luminescence of orthosilicate Ba2SiO4:Eu2+ phosphors for NUV-LEDs: autonomous impurity purification by eutectic Li2CO3 melts

We report large photoluminescence (PL) enhancement of green-emitting Ba2SiO4:Eu2+ phosphors prepared in an eutectic Li2CO3 melt as a flux. Among the phosphor materials synthesized using low-purity precursors (99%-pure BaCO3 and 99.6%-pure SiO2), the emission intensity of Ba2SiO4:(Li0.02,Eu0.02) (Li = 200% excess) is found to be 470% as high as that of Ba2SiO4:Eu0.02 (no-Li) and in fact is almost equivalent to that of Li-undoped Ba2SiO4:Eu2+ synthesized using ultrapure precursors (99.98%-pure BaCO3 and 99.995%-pure SiO2). In combination with the results from PL measurements and inductively coupled plasma mass spectrometry (ICP-MS), the elemental distribution of the products and melts found from time-of-flight secondary ion mass spectrometry (TOF-SIMS) directly indicates that the excess Li2CO3 autonomously removes the impurities that were originally contained in the low-purity precursors, in particular SiO2, and thus critically improves the photoluminescence efficiency of the material. The newly found Li flux effect on large PL enhancement may not only contribute to more economic production of phosphors but provide a platform for discovery of new efficient phosphors for solid state lighting.

Nanoporous Delafossite CuAlO2 from Inorganic/Polymer Double Gels: A Desirable High-Surface-Area p-Type Transparent Electrode Material

Nanoporous structures of a p-type semiconductor, delafossite CuAlO(2), with a high crystallinity have been fabricated through an inorganic/polymer double-gel process and characterized for the first time via Mott-Schottky measurements. The effect of the precursor concentration, calcination temperature, and atmosphere were examined to achieve high crystallinity and photoelectrochemical properties while maximizing the porosity. The optical properties of the nanoporous CuAlO(2) are in good agreement with the literature with an optical band gap of 3.9 eV, and the observed high electrical conductivity and hole concentrations conform to highly crystalline and well-sintered nanoparticles observed in the product. The Mott-Schottky plot from the electrochemical impedance spectroscopy studies indicates a flat-band potential of 0.49 V versus Ag/AgCl. It is concluded that CuAlO(2) exhibits band energies very close to those of NiO but with electrical properties very desirable in the fabrication of photoelectrochemical devices including dye-sensitized solar cells.

Concomitant Thionation and Reduction of Graphene Oxide Through Solid/Gas Metathetical Sulfidation Reactions at High Temperatures

Abstract We show that thionation of graphene oxide sheets is possible concomitantly with reduction of graphene oxide through a high-temperature (up to 500°C) solid/gas metathetical reaction process by employing gaseous boron sulfides as a thionation reagent which can effectively prevent the graphene oxide sheets from restacking during the heating. The X-ray photoelectron (XPS) and energy dispersive X-ray (EDS) spectroscopic studies revealed that reaction products have the S:O atomic ratios of about 2:1. The deconvolution of the high-resolution S2p XPS spectra of the product showed that about 90% of the sulfur atoms in the products are present in the form of C-SH. Ellman assay results indicated that practically all the thiols act as a free thiol which can participate in disulfide bond formation. From the thermogravimetric analysis (TGA) studies up to 600°C, the thiol functional groups in the products was found to be more thermally stable than hydroxyl groups. The estimated Tauc optical gap of the products is about 0.03 eV, which corroborates the reduced nature of the products. GRAPHICAL ABSTRACT

A highly stable and scalable photosynthetic reaction center-graphene hybrid electrode system for biomimetic solar energy transduction

A photosynthetic reaction center (RC)-based electrode system is one of the most promising biomimetic approaches for solar energy transduction which is a renewable and environment-friendly source of energy. However, the instability of RCs in a non-cellular environment and the unfeasible scalability of electrode materials hamper the promising application of these systems. Herein, we report a highly stable and scalable RC-electrode system in which RCs are directly immobilized on a flexible and transparent mercapto reduced graphene oxide (mRGO) electrode. RCs immobilized on a mRGO film retain their photoactivity after twenty-week storage under darkness and even after 24 h continuous illumination at room temperature under aerobic conditions. The remarkable stability and mechanical flexibility of our system offer great potential for the development of a flexible RC-based biomimetic device for solar energy transduction.