Sang Hoon Nam

Fabrication of an Efficient Light‐Scattering Functionalized Photoanode Using Periodically Aligned ZnO Hemisphere Crystals for Dye‐Sensitized Solar Cells

Since the pioneering work of O’Regan and Grätzel in 1991[1] numerous studies have investigated dye-sensitized solar cells (DSSCs) as an alternative next generation solar cell. This evolution has continued to progress, and solar light-to-electricity conversion efficiencies (power conversion efficiency, PCE) have now exceeded 11%, which was attained with a 12 μm thick TiO2 nanoparticulate photoanode.[2] DSSCs have recently attracted increasing attention as an ideal photovoltaic concept; the advantages of DSSCs are their low-cost, transparency, color rendition, eco-friendly process, biocompatibility and simplicity.[3,4] Generally, improvements in overall PCE have focused on increasing the photovoltage through the modification of the oxide layer, improving the photocurrent with new dye molecules, and increasing the stability by controlling the cell configurations.[4,5] A transparent mesoporous TiO2 nanoparticulate layer is a well-known photoanode material used in conventional DSSCs. However, the small size of TiO2 nanoparticles (diameter ∼ 20 nm) makes this layer transparent to visible light, and thus weakly light scattering due to the small particle size. As a result, a substantial amount of the incident light passes through the TiO2 nanoparticulate layer without being captured and utilized to produce photocurrent. Many studies have focused on capturing more light from the photoanode layer by using sub-micron poly-dispersed oxide particle aggregates, which act as effective scattering centers,[6,7] and/or by using gradient scattering layers consisting of TiO2 nano-particles with different radii along the light path.[8] Although the utilization of the large size aggregates within the photoanode film with a thickness of ∼9 μm and a cell area of

Efficient photovoltaic device fashioned of highly aligned multilayers of electrospun TiO2 nanowire array with conjugated polymer

We report here a simple and easy method of fabricating arranged inorganic nanowire architecture via electrospinning method equipped with a devised collector and demonstrate hybrid photovoltaic cells that are fashioned of planar-aligned TiO2 nanowire architectures such as uniaxially aligned nanowires and multiple layers of cross-aligned nanowire arrays with poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene]. The power conversion efficiency can be significantly improved by at least 70% under 1sun condition depending on the degree of aligning TiO2 nanowire arrays through enhancing charge collection and transport rate, as well as facilitating the polymer infiltration as compared to a randomly collected TiO2 nanowire electrode.

SnO2 Nanorod-Planted Graphite: An Effective Nanostructure Configuration for Reversible Lithium Ion Storage

We report a novel architecture of SnO(2) nanorod-planted graphite particles for an efficient Li ion storage material that can be prepared by a simple catalyst-assisted hydrothermal process. Rectangular-shaped SnO(2) nanorods are highly crystalline with a tetragonal rutile phase and distributed uniformly over the surface of micrometer-sized graphite particles. In addition, the size dimensions of grown SnO(2) nanorods can be controlled by varying the synthesis conditions. The diameter can be engineered to a sub-100 nm range, and the length can be controlled to up to several hundred nanometers. Significantly, the SnO(2) nanorod-planted graphite demonstrates an initial Li ion storage capacity of about 1010 mAh g(-1) during the first cycle. Also, these SnO(2)-graphite composites show high Coulombic efficiency and cycle stability in comparison with SnO(2) nanomaterials that are not combined with graphite. The enhanced electrochemical properties of SnO(2) nanorod-planted graphite, as compared with bare SnO(2) materials, inspire better design of composite materials with effective nanostructural configurations for advanced electrodes in lithium ion batteries.

Probing the Lithium Ion Storage Properties of Positively and Negatively Carved Silicon

Here, we report Si pillar and well arrays as tailored electrode materials for advanced Li ion storage devices. The well-ordered and periodic morphologies were formed on a Si electrode thin film via laser interference lithography followed by a dry etch process. Two different patterns of negatively or positively carved Si electrodes exhibited highly improved cycle performance as a consequence of the enlarged surface area and the nanoscale pattern effects. The Si well arrays showed the highest energy density, rate capability, and cycling retention among the prepared Si electrodes. This tailored electrode platform demonstrates that these design principles could be applied to future developments in Si electrodes.

Standing pillar arrays of C-coated hollow SnO2 mesoscale tubules for a highly stable lithium ion storage electrode

This work reports the direct growth of hollow one-dimensional nanostructure arrays on conducting substrates for use as efficient electrodes in Li-ion batteries. The C-coated hollow SnO2 pillar array structures can be prepared by template-directed synthesis, selective wet etching, and a carbonization route. The well-oriented ZnO nanorod arrays, which are grown on titanium substrates, are used as a sacrificial template for the deposition of SnO2 layers through a simple hydrothermal process. The ZnO portions are selectively removed by wet etching, producing hollow SnO2 arrays that are consecutively covered with carbon layers via the carbonization of glucose. The lithium storage performance of the synthesized C-coated hollow SnO2 pillar array structures is demonstrated by applying them directly to a working electrode without additive materials. The standing pillar array electrode, consisting of C-coated hollow SnO2, exhibits an excellent discharge capacity of ca. 1251.9 mA h g−1 on the first cycle, and it also shows promising cyclability, rate capability, and coulombic efficiency, indicating that C-coated hollow SnO2 arrays fabricated on the current collector can be powerful candidates for a highly stable lithium storage electrode platform.

Magnetic Response of Hydrothermally Prepared Self-Assembled Co 3 O 4 Nano-platelets

In the present communication, we report a strong ferrimagnetic behavior of self-assembled Co3O4 nano-platelets, which most likely originates from the intrinsic spin structure of the unique Co3O4 structure. The microsphere-like structures are composed of nano-platelets that are entangled together to form the organized network. These anomalous ferrimagnetic properties can be rationalized by supposing that one of the Co3+ and one of the Co2+ ions are switched between the octahedral and tetrahedral sites. The powder sample was also characterized by x-ray diffraction and superconducting quantum interface device magnetometry.

Honeycomb pattern array of vertically standing core-shell nanorods: Its application to Li energy electrodes

An energy storage electrode system is fabricated via a template method with one-dimensional nanostructures that are hexagonally patterned in a honeycomblike fashion and vertically standing nanorods made of a gold-coated carbon nanotube core and a V2O5 shell layer. The performance of this system for Li insertion and extraction shows an increased capacity along with an enhanced rate performance, which could be attributed to the aligned nanostructures having increased reaction sites, facilitated charge transport, and improved stability in the face of mechanical stress.

Copper nanofiber-networked cobalt oxide composites for high performance Li-ion batteries.

We prepared a composite electrode structure consisting of copper nanofiber-networked cobalt oxide (CuNFs@CoOx). The copper nanofibers (CuNFs) were fabricated on a substrate with formation of a network structure, which may have potential for improving electron percolation and retarding film deformation during the discharging/charging process over the electroactive cobalt oxide. Compared to bare CoOx thin-film (CoOx TF) electrodes, the CuNFs@CoOx electrodes exhibited a significant enhancement of rate performance by at least six-fold at an input current density of 3C-rate. Such enhanced Li-ion storage performance may be associated with modified electrode structure at the nanoscale, improved charge transfer, and facile stress relaxation from the embedded CuNF network. Consequently, the CuNFs@CoOx composite structure demonstrated here can be used as a promising high-performance electrode for Li-ion batteries.

Pt-Embedded MoO3 Electrodes for Rechargeable Lithium Batteries

Effects of Pt nanoparticle incorporation into a MoO 3 thin film, which was prepared via cosputtering of a Pt and MoO 3 target, were investigated for a cathode electrode of the rechargeable Li batteries. The incorporation of the Pt with size of ca. 1.5 nm formed a heterogeneous film morphology with nanophases of Pt and MoO 3 domains, leading to a better cyclic performance than the electrode of MoO 3 without Pt. These enhancements with Pt in the MoO 3 electrode could be ascribed to a combined effect of substantial decrease in the sheet resistance and amelioration of the mechanical stability of the film itself.