Yun Ho Jin

Highly Reversible Lithium Storage in Bacillus subtilis-Directed Porous Co3O4 Nanostructures

In this work, a simple, high-yield biomineralization process is reported for cobalt oxide nanostructures using Gram-positive bacteria, Bacillus subtilis , as the soft templates. Rod-type cobalt oxide is prepared at room temperature through an electrostatic interaction between the functional surface structures of the bacteria and the cobalt ions in an aqueous solution. Additionally, porous Co₃O₄ hollow rods are formed through a subsequent heat treatment at 300 °C. These rods have a high surface area and exhibited an excellent electrochemical performance for rechargeable Li-ion batteries. This facile, inexpensive, and environmentally benign synthesis for transition metal oxides with unique nanostructures can be used for several practical applications, such as batteries, catalysts, sensors, and supercapacitors.

RbBaPO4:Eu2+: a new alternative blue-emitting phosphor for UV-based white light-emitting diodes

A novel blue-emitting phosphor on a phosphate-based host matrix, RbBaPO4:Eu2+ (RBP), was synthesized by a solid-state reaction. This phosphor could be excited efficiently by ultra violet-visible light in the 220–420 nm range to exhibit a highly efficient emission peak in the range of 420–440 nm. To investigate the possibility of commercial use, it was compared with the commercial blue phosphors of BaMgAl10O17:Eu2+ (BAM) and Sr3MgSi2O8:Eu2+ (SMS) to assess the photoluminescence properties. Compared to commercial phosphors, RBP displayed enhanced photoluminescence properties, in this case a high quantum efficiency and excellent thermal stability under near-Ultra Violet (n-UV) excitation. Furthermore, bright white light-emitting diodes (LEDs) were fabricated by integrating the RBP phosphor, along with commercial green/red phosphors, into n-UV LEDs. These results strongly suggest that the newly discovered RBP phosphor can be commercially utilized in UV-based white LEDs.

Sn-induced low-temperature growth of Ge nanowire electrodes with a large lithium storage capacity

We herein present the synthesis of germanium (Ge) nanowires on Au-catalyzed low-temperature substrates using a simple thermal Ge/Sn co-evaporation method. Incorporation of a low-melting point metal (Sn) enables the efficient delivery of Ge vapor to the substrate, even at a source temperature below 600 °C. The as-synthesized nanowires were found to be a core/shell heterostructure, exhibiting a uniform single crystalline Ge sheathed within a thin amorphous germanium suboxide (GeO(x)) layer. Furthermore, these high-density Ge nanowires grown directly on metal current collectors can offer good electrical connection and easy strain relaxation due to huge volume expansion during Li ion insertion/extraction. Therefore, the self-supported Ge nanowire electrodes provided excellent large capacity with little fading upon cycling (a capacity of ∼900 mA h g(-1) at 1C rate).

Synthesis of core/shell spinel ferrite/carbon nanoparticles with enhanced cycling stability for lithium ion battery anodes

Monodispersed core/shell spinel ferrite/carbon nanoparticles are formed by thermolysis of metal (Fe3+, Co2+) oleates followed by carbon coating. The phase and morphology of nanoparticles are characterized by x-ray diffraction and transmission electron microscopy. Pure Fe3O4 and CoFe2O4 nanoparticles are initially prepared through thermal decomposition of metal–oleate precursors at 310 degrees C and they are found to exhibit poor electrochemical performance because of the easy aggregation of nanoparticles and the resulting increase in the interparticle contact resistance. In contrast, uniform carbon coating of Fe3O4 and CoFe2O4 nanoparticles by low-temperature (180 degrees C) decomposition of malic acid allowed each nanoparticle to be electrically wired to a current collector through a conducting percolative path. Core/shell Fe3O4/C and CoFe2O4/C nanocomposite electrodes show a high specific capacity that can exceed 700 mAh g(-1) after 200 cycles, along with enhanced cycling stability.

Enhancement of cyclability of urchin-like rutile TiO2 submicron spheres by nanopainting with carbon

Phase-pure urchin-like rutile TiO2 (U-TiO2) submicron (<1 μm) spheres composed of numerous single-crystalline nanorods are successfully synthesized using a surfactant-free wet-chemical route. In addition, a reliable mechanism for the formation of U-TiO2, different from the well-known “growth-then-assembly” mode, is suggested. To provide a highly electron-conducting network, the U-TiO2 submicron spheres are nanopainted with a conductive amorphous carbon layer. As anodes for Li-ion batteries, the carbon-coated U-TiO2 submicron sphere electrodes show enhanced cycling performance, maintaining a reversible capacity of 165.7 mA h g−1 after 100 cycles at a rate of 0.2 C; this is attributed to the provision of an efficient electron-transport path by the conductive carbon.

Low-temperature synthesis of CuO-interlaced nanodiscs for lithium ion battery electrodes

In this study, we report the high-yield synthesis of 2-dimensional cupric oxide (CuO) nanodiscs through dehydrogenation of 1-dimensional Cu(OH)2 nanowires at 60°C. Most of the nanodiscs had a diameter of approximately 500 nm and a thickness of approximately 50 nm. After further prolonged reaction times, secondary irregular nanodiscs gradually grew vertically into regular nanodiscs. These CuO nanostructures were characterized using X-ray diffraction, transmission electron microscopy, and Brunauer-Emmett-Teller measurements. The possible growth mechanism of the interlaced disc CuO nanostructures is systematically discussed. The electrochemical performances of the CuO nanodisc electrodes were evaluated in detail using cyclic voltammetry and galvanostatic cycling. Furthermore, we demonstrate that the incorporation of multiwalled carbon nanotubes enables the enhanced reversible capacities and capacity retention of CuO nanodisc electrodes on cycling by offering more efficient electron transport paths.

Facile synthesis of nano-Li4 Ti5O12 for high-rate Li-ion battery anodes

One of the most promising anode materials for Li-ion batteries, Li4Ti5O12, has attracted attention because it is a zero-strain Li insertion host having a stable insertion potential. In this study, we suggest two different synthetic processes to prepare Li4Ti5O12 using anatase TiO2 nanoprecursors. TiO2 powders, which have extraordinarily large surface areas of more than 250 m2 g-1, were initially prepared through the urea-forced hydrolysis/precipitation route below 100°C. For the synthesis of Li4Ti5O12, LiOH and Li2CO3 were added to TiO2 solutions prepared in water and ethanol media, respectively. The powders were subsequently dried and calcined at various temperatures. The phase and morphological transitions from TiO2 to Li4Ti5O12 were characterized using X-ray powder diffraction and transmission electron microscopy. The electrochemical performance of nanosized Li4Ti5O12 was evaluated in detail by cyclic voltammetry and galvanostatic cycling. Furthermore, the high-rate performance and long-term cycle stability of Li4Ti5O12 anodes for use in Li-ion batteries were discussed.

Synthesis of cuprous oxide nanocomposite electrodes by room-temperature chemical partial reduction

We demonstrate a template-free synthetic approach for the preparation of a highly conductive Cu/Cu(2)O nanocomposite electrode by a chemical reduction process. Cu(2)O octahedra were prepared through chemical dehydrogenation of as-synthesized Cu(OH)(2) nanowire precursors. To provide a sufficiently electron-conducting network, the Cu(2)O particles were transformed into Cu/Cu(2)O nanocomposites by an intentional reduction process. The Cu/Cu(2)O nanocomposite electrodes showed enhanced cycling performance compared to Cu(2)O particles. Furthermore, their rate capabilities were superior to those of their mechanically mixed Cu/Cu(2)O counterparts. This enhanced electrochemical performance of the hybrid Cu/Cu(2)O nanocomposites was ascribed to the formation of homogeneous nanostructures, offering an efficient electron-transport path provided by the presence of highly dispersed Cu nanoparticles.