Hyun Woo Shim

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.

Scalable One-pot Bacteria-templating Synthesis Route toward Hierarchical, Porous-Co3O4 Superstructures for Supercapacitor Electrodes

Template-driven strategy has been widely used to synthesize inorganic nano/micro materials. Here, we used a bottom-up controlled synthesis route to develop a powerful solution-based method of fabricating three-dimensional (3D), hierarchical, porous-Co3O4 superstructures that exhibit the morphology of flower-like microspheres (hereafter, RT-Co3O4). The gram-scale RT-Co3O4 was facilely prepared using one-pot synthesis with bacterial templating at room temperature. Large-surface-area RT-Co3O4 also has a noticeable pseudocapacitive performance because of its high mass loading per area (~10 mg cm−2), indicating a high capacitance of 214 F g−1 (2.04 F cm−2) at 2 A g−1 (19.02 mA cm−2), a Coulombic efficiency averaging over 95%, and an excellent cycling stability that shows a capacitance retention of about 95% after 4,000 cycles.

Facile hydrothermal synthesis of porous TiO2 nanowire electrodes with high-rate capability for Li ion batteries

Anatase TiO(2) nanowires were successfully synthesized using a low-temperature hydrothermal treatment on as-prepared one-dimensional (1D) hydrogen titanate nanowires (H(2)Ti(3)O(7)) at 180 degrees C. The anatase TiO(2) nanowires were porous in nature with a high specific surface area. These nanowires were characterized using transmission electron microscopy (TEM), high-resolution TEM, x-ray powder diffraction, Raman spectroscopy, and Brunauer-Emmett-Teller (BET) measurements. The topochemical phase transformation mechanism from H(2)Ti(3)O(7) to anatase TiO(2) is discussed. The porous anatase TiO(2) nanowire electrodes demonstrated an excellent cycling performance and superior rate capabilities compared with the H(2)Ti(3)O(7) nanowires and the anatase TiO(2) nanowires that were prepared through calcination at 700 degrees C. The porous anatase TiO(2) nanowires exhibited a capacity of approximately 145 mA h g(-1) at 1 C after 500 cycles and 115 mA h g(-1) at 20 C. This improvement in the long-term cycle stability and outstanding rate capability was explained by various microscopic observations of the porous 1D nanostructured nature of the nanowires during the Li intercalation/deintercalation cycles.

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.

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.

Synthesis of Heterogeneous Li4Ti5O12 Nanostructured Anodes with Long-Term Cycle Stability

The 0D-1D Lithium titanate (Li4Ti5O12) heterogeneous nanostructures were synthesized through the solvothermal reaction using lithium hydroxide monohydrate (Li(OH)·H2O) and protonated trititanate (H2Ti3O7) nanowires as the templates in an ethanol/water mixed solvent with subsequent heat treatment. A scanning electron microscope (SEM) and a high resolution transmission electron microscope (HRTEM) were used to reveal that the Li4Ti5O12 powders had 0D-1D heterogeneous nanostructures with nanoparticles (0D) on the surface of wires (1D). The composition of the mixed solvents and the volume ratio of ethanol modulated the primary particle size of the Li4Ti5O12 nanoparticles. The Li4Ti5O12 heterogeneous nanostructures exhibited good capacity retention of 125 mAh/g after 500 cycles at 1C and a superior high-rate performance of 114 mAh/g at 20C.

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.

Electrochemical performance of NixCo1-xMoO4 (0 ≤ x ≤ 1) nanowire anodes for lithium-ion batteries.

NixCo1-xMoO4 (0 ≤ x ≤ 1) nanowire electrodes for lithium-ion rechargeable batteries have been synthesized via a hydrothermal method, followed by thermal post-annealing at 500°C for 2 h. The chemical composition of the nanowires was varied, and their morphological features and crystalline structures were characterized using field-emission scanning electron microscopy and X-ray powder diffraction. The reversible capacity of NiMoO4 and Ni0.75Co0.25MoO4 nanowire electrodes was larger (≈520 mA h/g after 20 cycles at a rate of 196 mA/g) than that of the other nanowires. This enhanced electrochemical performance of NixCo1-xMoO4 nanowires with high Ni content was ascribed to their larger surface area and efficient electron transport path facilitated by their one-dimensional nanostructure.

Direct assembly of tin–MWCNT 3D-networked anode for rechargeable lithium ion batteries

Nanocomposites of Sn nanoparticle–multiwalled carbon nanotubes (MWCNTs) were prepared by simple two-step electrochemical processes. First, acid-functionalized MWCNTs were electrophoretically deposited on a stainless steel substrate. Sn nanoparticles decorated on the MWCNTs were prepared by the electrodeposition of a SnCl4 aqueous solution on the pre-deposited MWCNT-film layer. The structure of the newly fabricated Sn-MWCNT composite was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). The electrochemical performance of the composite was evaluated by cyclic voltammetry and galvanostatic cycling. The as-deposited composite film shows the size of the uniformly dispersed Sn particles, ranging from few to several tens of nanometres, as a function of the deposition time. The composite shows electrochemical performance that is superior to that of a pure Sn nanoparticle anode.