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 Ir_<i>x</i>Co_1–<i>x</i>O_<i>y</i>, where <i>x</i> is the relative Ir atomic content to the overall metal content. The formation of a variety of Ir_<i>x</i>Co_1–<i>x</i>O_<i>y</i> (0 ≤ <i>x</i> ≤ 1) crystalline tubular nanocomposites was readily achieved by electrospinning and subsequent calcination process. Structural characterization clearly confirmed that Ir_<i>x</i>Co_1–<i>x</i>O_<i>y</i> 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 Ir_<i>x</i>Co_1–<i>x</i>O_<i>y</i> 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 Ir_<i>x</i>Co_1–<i>x</i>O_<i>y</i> nanotubes was characterized for oxygen evolution reaction (OER) with rotating disk electrode voltammetry in 1 M NaOH aqueous solution. Among diverse Ir_<i>x</i>Co_1–<i>x</i>O_<i>y</i>, Ir_0.46Co_0.54O_<i>y</i> 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 Ir_0.46Co_0.54O_<i>y</i> 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

Highly Efficient Silver–Cobalt Composite Nanotube Electrocatalysts for Favorable Oxygen Reduction Reaction

This paper reports the synthesis and characterization of silver–cobalt (AgCo) bimetallic composite nanotubes. Cobalt oxide (Co₃O₄) nanotubes were fabricated by electrospinning and subsequent calcination in air and then reduced to cobalt (Co) metal nanotubes via further calcination under a H₂/Ar atmosphere. As-prepared Co nanotubes were then employed as templates for the following galvanic replacement reaction (GRR) with silver (Ag) precursor (AgNO₃), which produced AgCo composite nanotubes. Various AgCo nanotubes were readily synthesized with applying different reaction times for the reduction of Co₃O₄ nanotubes and GRR. One hour reduction was sufficiently long to convert Co₃O₄ to Co metal, and 3 h GRR was enough to deposit Ag layer on Co nanotubes. The tube morphology and copresence of Ag and Co in AgCo composite nanotubes were confirmed with SEM, HRTEM, XPS, and XRD analyses. Electroactivity of as-prepared AgCo composite nanotubes was characterized for ORR with rotating disk electrode (RDE) voltammetry. Among differently synthesized AgCo composite nanotubes, the one synthesized via 1 h reduction and 3 h GRR showed the best ORR activity (the most positive onset potential, greatest limiting current density, and highest number of electrons transferred). Furthermore, the ORR performance of the optimized AgCo composite nanotubes was superior compared to pure Co nanotubes, pure Ag nanowires, and bare platinum (Pt). High ethanol tolerance of AgCo composite nanotubes was also compared with the commercial Pt/C and then verified its excellent resistance to ethanol contamination

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, Ir_<i>x</i>Ru_1–<i>x</i>O_<i>y</i> (<i>y</i> = 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 Ir_<i>x</i>Ru_1–<i>x</i>O_<i>y</i> 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 Ir_<i>x</i>Ru_1–<i>x</i>O_<i>y</i> nanofibers, the cathodically activated Ir_0.80Ru_0.20O_<i>y</i> shows the best HER activity and stability even compared with IrO_<i>y</i> and RuO_<i>y</i>, 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 Ir_0.80Ru_0.20 are close to those at Pt. This suggests that mixed metallic Ir and Ru are significant contributors to the improved HER activity of Ir_0.80Ru_0.20O_<i>y</i> 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

Polydiacetylene-Based Colorimetric and Fluorescent Chemosensor for the Detection of Carbon Dioxide

We developed a colorimetric and fluorescent turn-on carbon dioxide sensor that relies on a polydiacetylene, <b>PDA-1</b>, functionalized with amines and imidazolium groups. The pendant amines react with CO₂ under basic conditions to form carbamoate anions, which partially neutralize the polymer’s positive charges, inducing a phase transition. <b>PDA-1</b> allows for the selective sensing of CO₂ with high sensitivity, down to atmospheric concentrations. Naked-eye detection of CO₂ is accomplished either in water solutions of <b>PDA-1</b> or in the solid state with electrospun coatings of <b>PDA-1</b> nanofibers

Colorimetric and Fluorescent Detecting Phosgene by a Second-Generation Chemosensor

Because of the current shortage of first-generation phosgene sensors, increased attention has been given to the development of fluorescent and colorimetric based methods for detecting this toxic substance. In an effort focusing on this issue, we designed the new, second-generation phosgene chemosensor <b>1</b> and demonstrated that it undergoes a ring-opening reaction with phosgene in association with color and fluorescent changes with a detection limit of 3.2 ppb. Notably, in comparison with the first-generation sensor RB-OPD, <b>1</b> not only undergoes a much faster response toward phosgene with an overall response time within 2 min, but it also generates no byproducts during the sensing process. Finally, sensor <b>1</b> embedded nanofibers were successfully fabricated and used for accurate and sensitive detection of phosgene

Electrothermally Induced Highly Responsive and Highly Selective Vanadium Oxide Hydrogen Sensor Based on Metal–Insulator Transition

We report highly effective hydrogen gas detection based on the metal–insulator transition (MIT) by the electrothermally induced Pd-nanoparticles-decorated vanadium oxide (VO₂) nanowire prepared by the efficient and size-controllable growth method originating from V₂O₅ thin film driven by supercooled liquid nanodroplets. By irradiating a well-defined electron beam into the nanowires, we could significantly increase the conductivity up to four times with only a modest change in the semiconductor-to-metal transition temperature (<2 °C). When exposed to trace amounts of hydrogen gas in a single nanowire configuration, the enhanced conductivity gave rise to about a two times as fast transition to metallic phase even near room temperature (∼35 °C), by reaching much faster (∼3×) a critical current density at which the self-heating initiates. Consequently, we achieved the greatly shorter response time as well as lower operating temperature and voltage for the detection of hydrogen gas in a single VO₂ nanowire device, which can be attributed to the self-heating effect accelerated by the increase in the conductivity. The single nanowire sensor also shows the capability of detecting selectively hydrogen of different three gases (O₂, CO, and ethylene)

Self-Assembled and Highly Selective Sensors Based on Air-Bridge-Structured Nanowire Junction Arrays

We describe a strategy for creating an air-bridge-structured nanowire junction array platform that capable of reliably discriminating between three gases (hydrogen, carbon monoxide, and nitrogen dioxide) in air. Alternatively driven dual nanowire species of ZnO and CuO with the average diameter of ∼30 nm on a single substrate are used and decorated with metallic nanoparticles to form two-dimensional microarray, which do not need to consider the post fabrications. Each individual nanowires in the array form n–n, p–p, and p–n junctions at the micro/nanoscale on single substrate and the junctions act as electrical conducting path for carriers. The adsorption of gas molecules to the surface changes the potential barrier height formed at the junctions and the carrier transport inside the straight semiconductors, which provide the ability of a given sensor array to differentiate among the junctions. The sensors were tested for their ability to distinguish three gases (H₂, CO, and NO₂), which they were able to do unequivocally when the data was classified using linear discriminant analysis

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 (IrO₂) 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 (H₂O₂) 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

Growth of Highly Single Crystalline IrO₂ Nanowires and Their Electrochemical Applications

We present the facile growth of highly single crystalline iridium dioxide (IrO₂) nanowires on SiO₂/Si and Au substrates via a simple vapor phase transport process under atmospheric pressure without any catalyst. Particularly, high-density needle-like IrO₂ nanowires were readily obtained on a single Au microwire, suggesting that the melted surface layer of Au might effectively enhance the nucleation of gaseous IrO₃ precursors at the growth temperature. In addition, all the electrochemical observations of the directly grown IrO₂ nanowires on a single Au microwire support favorable electron-transfer kinetics of [Fe­(CN₆)]^4–/3– as well as Ru­(NH₃)₆^3+/2+ at the highly oriented crystalline IrO₂ nanowire surface. Furthermore, stable pH response is shown, revealing potential for use as a miniaturized pH sensor, confirmed by the calibration curve exhibiting super-Nernstian behavior with a slope of 71.6 mV pH^–1

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