Sang-Hoon Yoo

Wet-Chemical Synthesis of Palladium Nanosprings

We report a methodology for synthesis of palladium (Pd) nanospring structures using an anodic aluminum oxide (AAO) membrane template and facile electrochemical deposition. The hydroxyl-terminated surfaces of alumina nanochannels and localized hydrogen evolution contribute to the growth of Pd atoms at peripheral positions of the alumina nanochannels in the presence of an effectual electric potential and a plating solution consisting of PdCl(2), CuCl(2), and HCl. Structural characterization including EDS line analysis and element mapping revealed Pd nanodomains curling up on the Cu nanorods. A clear Pd nanospring shape was observed after selectively removing Cu. The lengths of the nanosprings were dictated by the charges transported through electrodeposition, and the diameters of the nanosprings were tunable by altering the diameter of the alumina nanochannels. Screw dislocation is the most probable crystallographic defect responsible for the formation of coiled Pd nanostructures. Pd nanosprings have potential applications in nanomachines, nanosensors, nanoinductors, and metamaterials. We anticipate that our synthesis method will motivate and inform the synthesis of more advanced nanomaterials.

Synthesis of perpendicular nanorod arrays with hierarchical architecture and water slipping superhydrophobic properties.

The utilization of vertically aligned smooth gold nanorod arrays with and without nanoporous tip architectures as superhydrophobic surfaces is described. Nanoporous architecture was produced on the tips of nanorods by selectively dissolving less noble components from the alloy nanorods. The resulting nanoscopic dual-size roughness features enhanced the surface dewettability after surface modification with low-surface-energy materials such as long-chain normal alkanethiols and fluorinated organic compounds. The surface cleaning properties were also tested with a rolling water droplet.

Three-Dimensional Pt-Coated Au Nanoparticle Arrays : Applications for Electrocatalysis and Surface-Enhanced Raman Scattering

This study demonstrates a novel approach to synthesis methods for core-shell nanoparticle assembly using nanoparticle trapping at an interface and subsequent transfer onto a substrate for electrochemical ultrathin layer coating. The transferred nanoparticle array can have a tunable surface area depending on the number of transferred layers. Subsequently coating the surface with Pt-group metals that behave as an ultrathin film provides electrocatalytic activities with respect to a variety of chemical reactions, depending on the properties of the selected coating materials. The transferred 3D Au nanoparticle arrays act as a high-surface-area platform for the diversity of overlayer materials. The resulting 3D core-shell nanoparticle films could be utilized as a highly active electrocatalysis and Raman scattering substrate. The approach provides a versatile and convenient synthesis route to new nanoporous material with tailorable pore structure and material properties through bottom-up assembly.

Nanoparticle films as a conducting layer for anodic aluminum oxide template-assisted nanorod synthesis

A simple, inexpensive, and robust methodology was developed to fabricate conductive film substrates by mechanically packing nanoparticles (NPs) on one side of anodic aluminum oxide (AAO). Gold, silver NPs, and carbon nanotubes were used as building blocks in the synthesis of conductive film substrates, upon which perpendicular nanorod arrays and colloidal free-standing nanorods were easily constructed. Characterizations by field emission scanning electron microscopy (FE-SEM) and optical dark-field microscopy confirmed the validity of the conductive NP film substrates on the AAO template. This contribution could provide a convenient and low-cost means for the fabrication of various conductive substrates on AAO.

Platinum Films with Controlled 3-Dimensional Nanoscopic Morphologies and Their Effects on Surface Enhanced Raman Scattering

The synthesis of Pt thin films with a controlled nanoscopic architecture that can support surface enhanced Raman scattering (SERS) is reported. The syntheses are achieved by replicating the pores of a type of mesoporous silica thin film whose pore structure could be described as a regular array of vertical channels of approximately 9 nm in diameter and their interconnections, forming a 3-dimensional pore network. Electrochemical deposition into the pores followed by the removal of the templates produced Pt films composed of arrays of vertically standing Pt nanorods with narrow gaps between them. The 3-dimensional nanostructure increases the surface area and enables the Pt film to absorb visible light. SERS studies of rhodamine 6G and benzenethiol on such Pt films as substrates reveals that the control of the nanostructure is critical for the SERS effect.

Single inorganic-organic hybrid photovoltaic nanorod

We demonstrate that single photovoltaic (PV) nanorods can be readily fabricated by electrochemical processing in solution-phase under ambient conditions. A porous Au nanorod electrode in the core of the PV nanorod was central to both its structural formation and superior performance. We examined an intrinsically conducting polymer (polypyrrole) and an inorganic semiconductor (cadmium selenide) as precursor materials. Through an extremely simple and cost-effective fashioning process (solution-phase, room temperature), unadorned PV nanorods with up to 1.1% power conversion efficiency were obtained.

Platinum-Coated Porous Gold Nanorods in Methanol Electrooxidation: Dependence of Catalytic Activity on Ligament Size

Here we demonstrate that, in the dealloying process of Au-Ag nanorods, temperature is the key parameter for producing porous Au nanorods with tunable ligament sizes. The vertically aligned Au-Ag alloy nanorods were first synthesized by the electrochemical co-deposition of Au and Ag onto anodic aluminum oxide (AAO) membrane templates. Porous Au nanorods were then obtained by selectively etching Ag away from the precursor Au-Ag alloy nanorods. Control of the ligament size was achieved by controlling the dealloying temperature. Pt deposited on the porous Au nanorods with smaller ligaments exhibited a higher catalytic activity during methanol electrooxidation than those deposited on nanorods with larger ligaments produced by dealloying at higher temperatures. The strong dependence of the catalytic activity on the ligament size of porous Au is principally due to different amounts of carbon monoxide (CO) generated during methanol electrooxidation. Less CO was generated as the ligament size decreased. This finding is of importance for developing highly efficient cathode materials for carrying out methanol electrooxidation in practical applications in which porous Au with a large surface area is used as a supporting substrate.

Electrochemical Synthesis of Dumbbell-like Au-Ni-Au Nanorods and Their Surface Plasmon Resonance

In this report, we demonstrate that the longitudinal localized surface plasmon resonance mode can be suppressed when the nanorods were in dumbbell shape. The seed nanorods were synthesized by electrochemical deposition of metals into the pores of anodic aluminum oxide templates. The dumbbell-like nanorods were grown from seed Au-Ni-Au nanorods by a rate-controlled seed-mediated growth strategy. The selective deposition of Au atoms onto Au blocks of Au-Ni-Au nanorods produced larger diameter of Au nanorods with bumpy surface resulting in dumbbell-like nanorods. The morphology of nanorods depended on the reduction rate of , slow rate producing smooth surface of Au nanorods, but high reduction rate producing bumpy surface morphology. Through systematic investigation into the UV-Vis-NIR spectroscopy, we found that the multiple localized surface plasmon resonance (LSPR) modes were available from single-component Au nanorods. And, their LSPR modes of Au NRs with bumpy surface, compared to the smooth seed Au NRs, were red-shifted, which was obviously attributed to the increased electron oscillation pathways. While the longitudinal LSPR modes of smoothly grown Au NRs were blue-shifted except for a dipole transverse LSPR mode, which can be interpreted by decreased aspect ratio. In addition, dumbbell-like nanorods showed an almost disappeared longitudinal LSPR mode. It reflects that the plasmonic properties can be engineered using complex nanorods structure.

Composite Materials with MWCNTs and Conducting Polymer Nanorods and their Application as Supercapacitors

This study demonstrated the synthesis of high-surface-area metal-free carbonaceous electrodes (CE) from anodic aluminum oxide (AAO) templates, and their application as supercapacitors. Multi-walled Carbon nanotubes (MWCNTs) were interwoven into a porous network sheet that was attached to one side of AAO template through a vacuum filtration of the homogeneously dispersed MWCNT toluene solution. Subsequently, the conducting polymer was electrochemically grown into the porous MWCNT network and nanochannels of AAO, leading to the formation of a carbonaceous metal-free film electrode with a high surface area in the given geometrical surface area. Typical conducting polymers such as polypyrrole (PPY) and poly(3,4-ethylenedioxythiophene) (PEDOT) were examined as model systems, and the resulting electrodes were investigated as supercapacitors (SCs). These SCs exhibited stable, high capacitances, with values as high as 554 F/g, 1.08 F/ for PPY and 237 F/g, 0.98 F/ for PEDOT, that were normalized by both the mass and geometric area.