Gwang Hee Lee

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.

Enhanced cycling performance of an Fe0/Fe3O4 nanocomposite electrode for lithium-ion batteries

We demonstrate the formation of a highly conductive, Fe0/Fe3O4 nanocomposite electrode by the hydrogen reduction process. Fe2O3 nanobundles composed of one-dimensional nanowires were initially prepared through thermal dehydrogenation of hydrothermally synthesized FeOOH. The systematic phase and morphological evolutions from Fe2O3 to Fe2O3/Fe3O4, Fe3O4, and finally to Fe/Fe3O4 by the controlled thermochemical reduction at 300 degrees C in H2 were characterized using x-ray diffraction (XRD) and transmission electron microscopy (TEM). The Fe/Fe3O4 nanocomposite electrode shows excellent capacity retention ( approximately 540 mA h g(-1) after 100 cycles at a rate of 185 mA g(-1)), compared to that of Fe2O3 nanobundles. This enhanced electrochemical performance in Fe/Fe3O4 composites was attributed to the formation of unique, core-shell nanostructures offering an efficient electron transport path to the current collector.

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.

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.

Three-dimensional hierarchical self-supported multi-walled carbon nanotubes/tin(iv) disulfide nanosheets heterostructure electrodes for high power Li ion batteries

It is of great significance to improve the power density of Li ion batteries (LIBs) in pursuit of high-end products and technologies. Herein, we investigated the three-dimensional (3D) hierarchical self-supported multi-walled carbon nanotubes (MWCNTs)/tin(IV) disulfide nanosheets (SnS2 NS) heterostructured electrodes, demonstrating superior rate capabilities of 480 and 420 mAh g−1 even at the very high c-rates of 5C and 10C (charging in 6 min), respectively. The origins of the enhancement of the rate capabilities were discussed in detail by focusing on the roles of MWCNTs, which were directly grown on the metallic current collector. Furthermore, we have delicately dealt with the in-plane and plane-normal growth mechanisms of hexagonal SnS2 NS from the crystallographic point of view.

Superior long-term cycling stability of SnO2 nanoparticle/multiwalled carbon nanotube heterostructured electrodes for Li-ion rechargeable batteries

We demonstrate the fabrication of hybrid nanocomposite electrodes with a combination of SnO(2) nanoparticles (NPs) and conducting multiwalled carbon nanotube (MWCNT) anodes (SnO(2)@CNT) through the direct anchoring of SnO(2) NPs on the surface of electrophoretically pre-deposited MWCNT (EPD-CNT) networks via a metal-organic chemical vapor deposition process. This SnO(2)@CNT nanocomposite displays large reversible capacities of over 780, 510, and 470 mA h g(-1) at 1 C after 100, 500, and 1000 cycles, respectively. This outstanding long-term cycling stability is a result of the uniform distribution of SnO(2) NPs (~8.5 nm), a nanoscale EPD-CNT network with good electrical conductivity, and the creation of open spaces that buffer a large volume change during the Li-alloying/dealloying reaction of SnO(2).

Biomineralized Sn-based multiphasic nanostructures for Li-ion battery electrodes

A method for preparing multiphasic hollow rods consisting of nanoscale Sn-based materials through a thermochemical reduction process involving bacteria and Sn oxides is reported. This facile process involves the bacteria-mediated synthesis of SnO(2) nanoparticles that form on bacterial surfaces used as templates at room temperature. The subsequent template removal proceeds via a reduction of the heat-treated SnO(2) nanoparticles at 400 °C under reduction atmosphere, leaving free-standing hollow nanocomposite rods. These unique hollow nanocomposite rods have multiple components, including amorphous carbon, metal oxides (SnO(2) and SnO), and metallic Sn, and retain the original rod shapes. The systematic phase and morphological evolutions of the bacteria@SnO(2) composite rods are investigated by performing controlled thermochemical reduction at various temperatures. In addition, the application of multiphasic hollow nanocomposite rods as anode materials for rechargeable Li-ion batteries is evaluated. These materials exhibit excellent electrochemical performance, with capacities of about 505 and 350 mA h g(-1) at current densities of 157 and 392 mA g(-1), respectively.

Fabrication of core/shell ZnWO4/carbon nanorods and their Li electroactivity.

Carbon-coated ZnWO4 [C-ZW] nanorods with a one-dimensional core/shell structure were synthesised using hydrothermally prepared ZnWO4 and malic acid as precursors. The effects of the carbon coating on the ZnWO4 nanorods are investigated by thermogravimetry, high-resolution transmission electron microscopy, and Raman spectroscopy. The coating layer was found to be in uniform thickness of approximately 3 nm. Moreover, the D and G bands of carbon were clearly observed at around 1,350 and 1,600 cm-1, respectively, in the Raman spectra of the C-ZW nanorods. Furthermore, lithium electroactivities of the C-ZW nanorods were evaluated using cyclic voltammetry and galvanostatic cycling. In particular, the formed C-ZW nanorods exhibited excellent electrochemical performances, with rate capabilities better than those of bare ZnWO4 nanorods at different current rates, as well as a coulombic efficiency exceeding 98%. The specific capacity of the C-ZW nanorods maintained itself at approximately 170 mAh g-1, even at a high current rate of 3 C, which is much higher than pure ZnWO4 nanorods.

Germanium microflower-on-nanostem as a high-performance lithium ion battery electrode

We demonstrate a new design of Ge-based electrodes comprising three-dimensional (3-D) spherical microflowers containing crystalline nanorod networks on sturdy 1-D nanostems directly grown on a metallic current collector by facile thermal evaporation. The Ge nanorod networks were observed to self-replicate their tetrahedron structures and form a diamond cubic lattice-like inner network. After etching and subsequent carbon coating, the treated Ge nanostructures provide good electrical conductivity and are resistant to gradual deterioration, resulting in superior electrochemical performance as anode materials for LIBs, with a charge capacity retention of 96% after 100 cycles and a high specific capacity of 1360 mA h g−1 at 1 C and a high-rate capability with reversible capacities of 1080 and 850 mA h g−1 at the rates of 5 and 10 C, respectively. The improved electrochemical performance can be attributed to the fast electron transport and good strain accommodation of the carbon-filled Ge microflower-on-nanostem hybrid electrode.

Comparison of toxicity of different nanorod‐type TiO2 polymorphs in vivo and in vitro

It is predicted that the toxicity of nanoparticles may be different depending on the properties of the nanoparticles and biological system being tested. However, the factors that influence the toxicity of nanoparticles have not been adequately investigated. In this study, we characterized two types of TiO2 nanorods, anatase (ATO) and brookite (BTO), and compared their toxicity in vivo and in vitro. ATO and BTO differed from each other most notably in their surface areas. Treatment with the two TiO2 nanorods (10 µg ml–1) produced similar effects on the cell cycle in eight cell lines which are derived from potential target organs of nanoparticles, with the BTO eliciting stronger responses than ATO in all cell lines, among the cell lines, H9C2 showed the maximal change. Similarly, when mice were exposed to two TiO2 nanorods (1 mg kg–1), BTO induced clearer histopathological lesions and triggered a more robust secretion of inflammatory cytokines than ATO. Furthermore, we compared the cellular response of both TiO2 nanorods using BEAS‐2B cells, the human bronchial epithelial cell line. Both nanorods induced cell death by increasing the formation of autophagosome‐like vacuoles. The mitochondrial calcium concentration decreased by exposure of both types, but the distribution of lysosome and endoplasmic reticulum (ER) showed a clear difference between the two nanorods. Thus, we conclude that the surface area acts as an important factor which depends on toxicity of nanorod type‐TiO2 nanoparticles. Furthermore, the toxicity of nanoparticles varies according to the type of cells tested, and that the assembly of autophagosome‐like vacuoles is a critical part of the cellular response to nanoparticle exposure. Copyright