Chongmok Lee

Size quantization effects in cadmium sulfide layers formed by a Langmuir-Blodgett technique

Abstract Layers of Q-state particles ( 2 S to yield cadmium sulfide particles in a lattice of arachidic acid. The onset of absorption of this layer is blue-shifted by 0.38 eV with respect to bulk CdS. The adherence to Beers law and insensitivity of the absorbance edge to the number of LB layers deposited (up to seven) suggest no strong electronic interactions between successive layers. Ellipsometric measurements show that exposure of the LB films of cadmium arachidic acid to H 2 S gas results in an increase in thickness of about 3 A per layer.

Polymer Films on Electrodes XXIV . Ellipsometric Study of the Electrochemical Redox Processes of a Polypyrrole Film on a Platinum Electrode

Ellipsometry was used to observe the in situ growth and electrochemical conversion redox processes of a polypyrrole film, which was electrochemically deposited to a thickness of ca. 100 nm on a platinum electrode in solution containing 50 mM pyrrole. To determine the spatial distribution of oxidized and reduced states during the reduction and oxidation of the film, the conversion process was simulated by a multilayer film model. Such simulations of the ellipsometric results show that the redox conversion of a polypyrrole film proceeds from the solution toward the electrode and is controlled by counterion movement. These results contrast with previous work on a thionine film where conversion proceeds from electrode to solution. The effective diffusion constant for charge transport based on ellipsometric simulation was estimated as .

Hierarchically Driven IrO2 Nanowire Electrocatalysts for Direct Sensing of Biomolecules

Applying nanoscale device fabrications toward biomolecules, ultra sensitive, selective, robust, and reliable chemical or biological microsensors have been one of the most fascinating research directions in our life science. Here we introduce hierarchically driven iridium dioxide (IrO(2)) nanowires directly on a platinum (Pt) microwire, which allows a simple fabrication of the amperometric sensor and shows a favorable electronic property desired for sensing of hydrogen peroxide (H(2)O(2)) and dihydronicotinamide adenine dinucleotide (NADH) without the aid of enzymes. This rational engineering of a nanoscale architecture based on the direct formation of the hierarchical 1-dimensional (1-D) nanostructures on an electrode can offer a useful platform for high-performance electrochemical biosensors, enabling the efficient, ultrasensitive detection of biologically important molecules.

Single carbon fiber decorated with RuO2 nanorods as a highly electrocatalytic sensing element.

We demonstrate highly efficient electocatalytic activities of single crystalline RuO(2) nanorods grown on carbon fiber (CF), i.e., RuO(2) nanorod-CF hybrid microelectrode, prepared by a simple thermal annealing process from the Ru(OH)(3) precursor at 300 °C. The general electrochemical activity of a RuO(2) nanorod-CF microelectrode represents faster electron transfer for the [Fe(CN)(6)](3-/4-) couple than that of the bare CF microelectrode which are confirmed from the cyclic voltammetry (CV) measurement. Also, the amperometric response for the H(2)O(2) oxidation is remarkably facilitated at the RuO(2) nanorod-CF microelectrode by not only the enlarged surface area but the high electrocatalytic activity of the RuO(2) nanorod material itself. Furthermore, a single microelectrode of RuO(2) nanorod-CF exhibits the superior tolerance to Cl(-) ion poisoning unlike Pt-based electrocatalysts, indicating the promising sensor candidate in physiological conditions.

Spongelike nanoporous Pd and Pd/Au structures: facile synthesis and enhanced electrocatalytic activity.

This paper reports the facile synthesis and characterization of spongelike nanoporous Pd (snPd) and Pd/Au (snPd/Au) prepared by a tailored galvanic replacement reaction (GRR). Initially, a large amount of Co particles as sacrificial templates was electrodeposited onto the glassy carbon surface using a cyclic voltammetric method. This is the key step to the subsequent fabrication of the snPd/Au (or snPd) architectures by a surface replacement reaction. Using Co films as sacrificial templates, snPd/Au catalysts were prepared through a two-step GRR technique. In the first step, the Pd metal precursor (at different concentrations), K2PdCl4, reacted spontaneously to the formed Co frames through the GRR, resulting in a snPd series. snPd/Au was then prepared via the second GRR between snPd (prepared with 27.5 mM Pd precursor) and Au precursor (10 mM HAuCl4). The morphology and surface area of the prepared snPd series and snPd/Au were characterized using spectroscopic and electrochemical methods. Rotating disk electrode (RDE) experiments for oxygen reduction in 0.1 M NaOH showed that the snPd/Au has higher catalytic activity than snPd and the commercial Pd-20/C and Pt-20/C catalysts. Rotating ring-disk electrode (RRDE) experiments reconfirmed that four electrons were involved in the electrocatalytic reduction of oxygen at the snPd/Au. Furthermore, RDE voltammetry for the H2O2 oxidation/reduction was used to monitor the catalytic activity of snPd/Au. The amperometric i-t curves of the snPd/Au catalyst for a H2O2 electrochemical reaction revealed the possibility of applications as a H2O2 oxidation/reduction sensor with high sensitivity (0.98 mA mM(-1) cm(-2) (r = 0.9997) for H2O2 oxidation and -0.95 mA mM(-1) cm(-2) (r = 0.9997) for H2O2 reduction), low detection limit (1.0 μM), and a rapid response (<∼1.5 s).

Highly Branched RuO2 Nanoneedles on Electrospun TiO2 Nanofibers as an Efficient Electrocatalytic Platform

Highly single-crystalline ruthenium dioxide (RuO2) nanoneedles were successfully grown on polycrystalline electrospun titanium dioxide (TiO2) nanofibers for the first time by a combination of thermal annealing and electrospinning from RuO2 and TiO2 precursors. Single-crystalline RuO2 nanoneedles with relatively small dimensions and a high density on electrospun TiO2 nanofibers are the key feature. The general electrochemical activities of RuO2 nanoneedles-TiO2 nanofibers and Ru(OH)3-TiO2 nanofibers toward the reduction of [Fe(CN)6](3-) were carefully examined by cyclic voltammetry carried out at various scan rates; the results indicated favorable charge-transfer kinetics of [Fe(CN)6](3-) reduction via a diffusion-controlled process. Additionally, a test of the analytical performance of the RuO2 nanoneedles-TiO2 nanofibers for the detection of a biologically important molecule, hydrogen peroxide (H2O2), indicated a high sensitivity (390.1 ± 14.9 μA mM(-1) cm(-2) for H2O2 oxidation and 53.8 ± 1.07 μA mM(-1) cm(-2) for the reduction), a low detection limit (1 μM), and a wide linear range (1-1000 μM), indicating H2O2 detection performance better than or comparable to that of other sensing systems.

Highly efficient electrochemical responses on single crystalline ruthenium-vanadium mixed metal oxide nanowires.

Highly efficient single crystalline ruthenium-vanadium mixed metal oxide (Ru1-xVxO2, 0≤x≤1) nanowires were prepared on a SiO2 substrate and a commercial Au microelectrode for the first time through a vapor-phase transport process by adjusting the mixing ratios of RuO2 and VO2 precursors. Single crystalline Ru1-xVxO2 nanowires show homogeneous solid-solution characteristics as well as the distinct feature of having remarkably narrow dimensional distributions. The electrochemical observations of a Ru1-xVxO2 (x=0.28 and 0.66)-decorated Au microelectrode using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) demonstrate favorable charge-transfer kinetics of [Fe(CN)6]3-/4- and Ru(NH3)6(3+/2+) couples compared to that of a bare Au microelectrode. The catalytic activity of Ru1-xVxO2 for oxygen and H2O2 reduction at neutral pH increases as the fraction of vanadium increases within our experimental conditions, which might be useful in the area of biofuel cells and biosensors.

Polymer films on electrodes: Part 23. Ellipsometric study of the electrochemical redox processes of a thionine film on a glassy carbon electrode

Abstract Ellipsometry was used to observe the in situ electrochemical redox processes of a thionine film that was electrochernically deposited (thickness > 70 nm) on a glassy carbon electrode at 1.2 V from a 0.05 M H2SO4 solution containing 50 μM thionine. To determine the spatial distribution of the oxidized and reduced states during the reduction and oxidation of the film, the conversion process was simulated by a multilayer film model, where 8 or 9 discrete film layers were considered. Such simulations of the ellipsometric results show that the redox conversion of a thionine film proceeds from the electrode/film interface towards the solution and is controlled by electron hopping.

Nanotubular Iridium–Cobalt Mixed Oxide Crystalline Architectures Inherited from Cobalt Oxide for Highly Efficient Oxygen Evolution Reaction Catalysis

Here, we report the unique transformation of one-dimensional tubular mixed oxide nanocomposites of iridium (Ir) and cobalt (Co) denoted as IrxCo1-xOy, where x is the relative Ir atomic content to the overall metal content. The formation of a variety of IrxCo1-xOy (0 ≤ x ≤ 1) crystalline tubular nanocomposites was readily achieved by electrospinning and subsequent calcination process. Structural characterization clearly confirmed that IrxCo1-xOy polycrystalline nanocomposites had a tubular morphology consisting of Ir/IrO2 and Co3O4, where Ir, Co, and O were homogeneously distributed throughout the entire nanostructures. The facile formation of IrxCo1-xOy nanotubes was mainly ascribed to the inclination of Co3O4 to form the nanotubes during the calcination process, which could play a critical role in providing a template of tubular structure and facilitating the formation of IrO2 by being incorporated with Ir precursors. Furthermore, the electroactivity of obtained IrxCo1-xOy nanotubes was characterized for oxygen evolution reaction (OER) with rotating disk electrode voltammetry in 1 M NaOH aqueous solution. Among diverse IrxCo1-xOy, Ir0.46Co0.54Oy nanotubes showed the best OER activity (the least-positive onset potential, greatest current density, and low Tafel slope), which was even better than that of commercial Ir/C. The Ir0.46Co0.54Oy nanotubes also exhibited a high stability in alkaline electrolyte. Expensive Ir mixed with cheap Co at an optimum ratio showed a greater OER catalytic activity than pure Ir oxide, one of the most efficient OER catalysts.

Fundamental Study of Facile and Stable Hydrogen Evolution Reaction at Electrospun Ir and Ru Mixed Oxide Nanofibers

Electrochemical hydrogen evolution reaction (HER) has been an interesting research topic in terms of the increasing need of renewable and alternative energy conversion devices. In this article, IrxRu1-xOy (y = 0 or 2) nanofibers with diverse compositions of Ir/IrO2 and RuO2 are synthesized by electrospinning and calcination procedures. Their HER activities are measured in 1.0 M NaOH. Interestingly, the HER activities of IrxRu1-xOy nanofibers improve gradually during repetitive cathodic potential scans for HER, and then eventually reach the steady-state consistencies. This cathodic activation is attributed to the transformation of the nanofiber surface oxides to the metallic alloy. Among a series of IrxRu1-xOy nanofibers, the cathodically activated Ir0.80Ru0.20Oy shows the best HER activity and stability even compared with IrOy and RuOy, commercial Pt and commercial Ir (20 wt % each metal loading on Vulcan carbon), where a superior stability is possibly ascribed to the instant generation of active Ir and Ru metals on the catalyst surface upon HER. Density functional theory calculation results for hydrogen adsorption show that the energy and adsorbate-catalyst distance at metallic Ir0.80Ru0.20 are close to those at Pt. This suggests that mixed metallic Ir and Ru are significant contributors to the improved HER activity of Ir0.80Ru0.20Oy after the cathodic activation. The present findings clearly demonstrate that the mixed oxide of Ir and Ru is a very effective electrocatalytic system for HER.