Making the most of a scarce platinum-group metal: conductive ruthenia nanoskins on insulating silica paper

Abnormal Optoelectric Properties of Two-Dimensional Protonic Ruthenium Oxide with a Hexagonal Structure

Two-dimensional structures can potentially lead to not only modulation of electron transport but also the variations of optical property. Protonic ruthenium oxide, a two-dimensional atomic sheet material, has been synthesized, and its optoelectric properties have been investigated. The results indicate that protonic ruthenium oxide is an excellent candidate for use as a flexible, transparent conducting material. A hydrated-ruthenium-oxide sheet has been first prepared via the chemical exfoliation of sodium intercalated ruthenium oxide (NaRuO₂) and, subsequently, converted into a protonic ruthenium oxide sheet using thermal treatment. A thermally activated transport mechanism is dominant in hydrated ruthenium oxide but diminishes in protonic ruthenium oxide; this resulted in a high electrical conductivity of ∼200 S/cm of the protonic sheet. Because of the unique interband and intraband structure, protonic ruthenium oxide has a small optical absorption coefficient of ∼1.62%/L. Consequently, such high conductivity and low absorption coefficient of protonic ruthenium oxide results in excellent transparent conducting properties.

Rewriting Electron-Transfer Kinetics at Pyrolytic Carbon Electrodes Decorated with Nanometric Ruthenium Oxide

Platinum is state-of-the-art for fast electron transfer whereas carbon electrodes, which have semimetal electronic character, typically exhibit slow electron-transfer kinetics. But when we turn to practical electrochemical devices, we turn to carbon. To move energy devices and electro(bio)analytical measurements to a new performance curve requires improved electron-transfer rates at carbon. We approach this challenge with electroless deposition of disordered, nanoscopic anhydrous ruthenium oxide at pyrolytic carbon prepared by thermal decomposition of benzene (RuOx@CVD-C). We assessed traditionally fast, chloride-assisted ([Fe(CN)₆]^3-/4-) and notoriously slow ([Fe(H₂O)₆]^3+/2+) electron-transfer redox probes at CVD-C and RuOx@CVD-C electrodes and calculated standard heterogeneous rate constants as a function of heat treatment to crystallize the disordered RuOx domains to their rutile form. For the fast electron-transfer probe, [Fe(CN)₆]^3-/4-, the rate increases by 34× over CVD-C once the RuOx is calcined to form crystalline rutile RuO₂. For the classically outer-sphere [Fe(H₂O)₆]^3+/2+, electron-transfer rates increase by an even greater degree over CVD-C (55×). The standard heterogeneous rate constant for each probe approaches that observed at Pt but does so using only minimal loadings of RuOx.

Transient Optical and Terahertz Spectroscopy of Nanoscale Films of RuO2

Solution-deposited nanoscale films of RuO₂ ("nanoskins") are effective transparent conductors once calcined to 200 °C. Upon heating the nanoskins to higher temperature the nanoskins show increased transmission at 550 nm. Electronic microscopy and X-ray diffraction show that the changes in the optical spectrum are accompanied by the formation of rutile RuO₂ nanoparticles. The mechanism for the spectral evolution is clearly observed with ultrafast optical measurements. Following excitation at 400 nm, nanoskins calcined at higher temperatures show increased transmission above 650 nm, consistent with the photobleaching of a surface-plasmon resonance (SPR) band. Calculations based on the optical constants of RuO₂ substantiate the presence of SPR absorption. Sheet resistance and transient terahertz photoconductivity measurements establish that the nanoskins electrically de-wire into separated particles. The plasmonic behavior of the nanoskins has implications their use in a range of optical and electrochemical applications.

Static and Time-Resolved Terahertz Measurements of Photoconductivity in Solution-Deposited Ruthenium Dioxide Nanofilms

Thin-film ruthenium dioxide (RuO2) is a promising alternative material as a conductive electrode in electronic applications because its rutile crystalline form is metallic and highly conductive. Herein, a solution-deposition multi-layer technique is employed to fabricate ca. 70 ± 20 nm thick films (nanoskins) and terahertz spectroscopy is used to determine their photoconductive properties. Upon calcining at temperatures ranging from 373 K to 773 K, nanoskins undergo a transformation from insulating (localized charge transport) behavior to metallic behavior. Terahertz time-domain spectroscopy (THz-TDS) indicates that nanoskins attain maximum static conductivity when calcined at 673 K (σ = 1030 ± 330 S·cm-1). Picosecond time-resolved Terahertz spectroscopy (TRTS) using 400 nm and 800 nm excitation reveals a transition to metallic behavior when calcined at 523 K. For calcine temperatures less than 523 K, the conductivity increases following photoexcitation (ΔE < 0) while higher calcine temperatures yield films composed of crystalline, rutile RuO2 and the conductivity decreases (ΔE > 0) following photoexcitation.

Competitive Oxygen Evolution in Acid Electrolyte Catalyzed at Technologically Relevant Electrodes Painted with Nanoscale RuO2

Using a solution-based, non-line-of sight synthesis, we electrolessly deposit ultrathin films of RuO₂ ("nanoskins") on planar and 3D substrates and benchmark their activity and stability for oxygen-evolution reaction (OER) in acid electrolyte under device-relevant conditions. When an electrically contiguous ∼9 nm thick RuO₂ nanoskin is expressed on commercially available, insulating SiO₂ fiber paper, the RuO₂@SiO₂ electrode exhibits high current density at low overpotential (10 mA cm^-2 @ η = 280 mV), courtesy of a catalyst amplified in 3D; however, the mass-normalized activity falls short of that achieved for films deposited on planar, metallic substrates (Ti foil). By wrapping the fibers with a <100 nm thick graphitic carbon layer prior to RuO₂ deposition (RuO₂@C@SiO₂), we retain the high mass activity of the RuO₂ (40-60 mA mg^-1 @ η = 330 mV) and preserve the desirable macroscale properties of the 3D scaffold: porous, lightweight, flexible, and inexpensive. The RuO₂@C@SiO₂ anodes not only achieve the 10 mA cm^-2 figure of merit at a low overpotential (η = ∼270 mV), but more importantly they do so while (1) minimizing the mass of catalyst needed to achieve this metric, (2) incorporating the catalyst into a practical electrode design, and (3) improving the long-term stability of the catalyst. Our best-performing anodes achieve state-of-the-art or better performance on the basis of area and mass, and do so with a catalyst density 300-580× less than that of bulk RuO₂. By limiting the oxidizing potential required to evolve O₂ at the electrode, even at 10 mA cm^-2, we achieve stable activity for 100+ h.

Synthesis and electrochemical evaluation of the PANI/δ-MnO2 electrode for high performing asymmetric supercapacitors

Structure–property correlation and electrochemistry of in situ polymerized PANI on MnO₂ and an asymmetric supercapacitor using them are presented.

Disordered RuO2 exhibits two dimensional, low-mobility transport and a metal-insulator transition

The discovery of low-dimensional metallic systems such as high-mobility metal oxide field-effect transistors, the cuprate superconductors, and conducting oxide interfaces (e.g., LaAlO₃/SrTiO₃) has stimulated research into the nature of electronic transport in two-dimensional systems given that the seminal theory for transport in disordered metals predicts that the metallic state cannot exist in two dimensions (2D). In this report, we demonstrate the existence of a metal–insulator transition (MIT) in highly disordered RuO₂ nanoskins with carrier concentrations that are one-to-six orders of magnitude higher and with mobilities that are one-to-six orders of magnitude lower than those reported previously for 2D oxides. The presence of an MIT and the accompanying atypical electronic characteristics place this form of the oxide in a highly diffusive, strong disorder regime and establishes the existence of a metallic state in 2D that is analogous to the three-dimensional case.

Metallicity enhancement in core–shell SiO2@RuO2nanowires

SiO₂@RuO₂composite nanowires exhibit an enhanced metallic nature of the external RuO₂shell, reflected in a significantly improved electronic conductance.

Preparation and characterization of conductive and transparent ruthenium dioxide sol-gel films

RuO2 conductive thin films were synthesized using the sol-gel method and deposited onto transparent insulating substrates. The optical transmission, film thickness, surface morphology and composition, resistivity, and spectroelectrochemical performance have been characterized. The optical transmission values of these films ranged from 70 to 89% in the visible region and from 56 to 88% in the infrared region. Resistivity values of the RuO2 sol-gel films varied from 1.02 × 10(-3) to 1.13 Ω cm and are highly dependent on the initial solution concentration of RuO2 in the sol-gel. The RuO2 sol-gel films were used as electrodes for the electrochemical oxidation and reduction of ferrocenemethanol. The electrochemical behavior of our novel RuO2 sol-gel films was compared to that of a standard platinum disk electrode and showed no appreciable differences in the half-wave potential (E1/2). The mechanical and chemical stability of the coatings was tested by physical abrasion and exposure to highly acidic, oxidizing Piranha solution. Repeated exposure to these extreme conditions did not result in any appreciable decline in electrochemical performance. Finally, the use of the novel RuO2 sol-gel conductive and transparent films was demonstrated in a spectroelectrochemistry experiment in which the oxidation and reduction of ferrocenemethanol was monitored via UV-vis spectroscopy as the applied potential was cycled.

Tailoring structural and electronic properties of RuO2 nanotubes: a many-body approach and electronic transport

The electrical conduction properties of ruthenium oxide nanocables are of high interest. These cables can be built as thin shells of RuO2 surrounding an inner solid nanowire of a dielectric insulating silica material. With this motivation we have investigated the structural, electronic and transport properties of RuO2 nanotubes using the density functional formalism, and applying many-body corrections to the electronic band structure. The structures obtained for the thinnest nanotubes are of the rutile type. The structures of nanotubes with larger diameters deviate from the rutile structure and have in common the formation of dimerized Ru-Ru rows along the axial direction. The cohesive energy shows an oscillating behavior as a function of the tube diameter. With the exception of the thinnest nanotubes, there is a correlation such that the electronic band structures of tubes with high cohesive energies show small gaps at the Fermi energy, whereas the less stable nanotubes exhibit metallic behavior, with bands crossing the Fermi surface. The electronic conductance of nanotubes of finite length connected to gold electrodes has been calculated using a Green-function formalism, and correlations have been established between the electronic band structure and the conductance at zero bias.

Redox deposition of nanoscale metal oxides on carbon for next-generation electrochemical capacitors

Transition metal oxides that mix electronic and ionic conductivity are essential active components of many electrochemical charge-storage devices, ranging from primary alkaline cells to more advanced rechargeable Li-ion batteries. In these devices, charge storage occurs via cation-insertion/deinsertion mechanisms in conjunction with the reduction/oxidation of metal sites in the oxide. Batteries that incorporate such metal oxides are typically designed for high specific energy, but not necessarily for high specific power. Electrochemical capacitors (ECs), which are typically composed of symmetric high-surface-area carbon electrodes that store charge via double-layer capacitance, deliver their energy in time scales of seconds, but at much lower specific energy than batteries. The fast, reversible faradaic reactions (typically described as "pseudocapacitance") of particular nanoscale metal oxides (e.g., ruthenium and manganese oxides) provide a strategy for bridging the power/energy performance gap between batteries and conventional ECs. These processes enhance charge-storage capacity to boost specific energy, while maintaining the few-second timescale of the charge-discharge response of carbon-based ECs. In this Account, we describe three examples of redox-based deposition of EC-relevant metal oxides (MnO2, FeOx, and RuO2) and discuss their potential deployment in next-generation ECs that use aqueous electrolytes. To extract the maximum pseudocapacitance functionality of metal oxides, one must carefully consider how they are synthesized and subsequently integrated into practical electrode structures. Expressing the metal oxide in a nanoscale form often enhances electrochemical utilization (maximizing specific capacitance) and facilitates high-rate operation for both charge and discharge. The "wiring" of the metal oxide, in terms of both electron and ion transport, when fabricated into a practical electrode architecture, is also a critical design parameter for achieving characteristic EC charge-discharge timescales. For example, conductive carbon must often be combined with the poorly conductive metal oxides to provide long-range electron pathways through the electrode. However, the ad hoc mixing of discrete carbon and oxide powders into composite electrodes may not support optimal utilization or rate performance. As an alternative, nanoscale metal oxides of interest for ECs can be synthesized directly on the surfaces of nanostructured carbons, with the carbon surface acting as a sacrificial reductant when exposed to a solution-phase, oxidizing precursor of the desired metal oxide (e.g., MnO4(-) for MnO2). These redox deposition methods can be applied to advanced carbon nanoarchitectures with well-designed pore structures. These architectures promote effective electrolyte infiltration and ion transport to the nanoscale metal oxide domains within the electrode architecture, which further enhances high-rate operation.

Stable anatase TiO₂ coating on quartz fibers by atomic layer deposition for photoactive light-scattering in dye-sensitized solar cells

Quartz fibers provide a unique high surface-area substrate suitable for conformal coating using atomic layer deposition (ALD), and are compatible with high temperature annealing. This paper shows that the quartz fiber composition stabilizes ALD TiO(2) in the anatase phase through TiO(2)-SiO(2) interface formation, even after annealing at 1050 °C. When integrated into a dye-sensitized solar cell, the TiO(2)-coated quartz fiber mat improves light scattering performance. Results also confirm that annealing at high temperature is necessary for better photoactivity of ALD TiO(2), which highlights the significance of quartz fibers as a substrate. The ALD TiO(2) coating on quartz fibers also boosts dye adsorption and photocurrent response, pushing the overall efficiency of the dye-cells from 6.5 to 7.4%. The mechanisms for improved cell performance are confirmed using wavelength-dependent incident photon to current efficiency and diffuse light scattering results. The combination of ALD and thermal processing on quartz fibers may enable other device structures for energy conversion and catalytic reaction applications.

Architectural integration of the components necessary for electrical energy storage on the nanoscale and in 3D

We describe fabrication of three-dimensional (3D) multifunctional nanoarchitectures in which the three critical components of a battery--cathode, separator/electrolyte, and anode--are internally assembled as tricontinuous nanoscopic phases. The architecture is initiated using sol-gel chemistry and processing to erect a 3D self-wired nanoparticulate scaffold of manganese oxide (>200 m(2) g(-1)) with a continuous, open, and mesoporous void volume. The integrated 3D system is generated by exhaustive coverage of the oxide network by an ultrathin, conformal layer of insulating polymer that forms via self-limiting electrodeposition of poly(phenylene oxide). The remaining interconnected void volume is then wired with RuO(2) nanowebs using subambient thermal decomposition of RuO(4). Transmission electron microscopy demonstrates that the three nanoscopic charge-transfer functional components--manganese oxide, polymer separator/cation conductor, and RuO(2)--exhibit the stratified, tricontinuous design of the phase-by-phase construction. This architecture contains all three components required for a solid-state energy storage device within a void volume sized at tens of nanometres such that nanometre-thick distances are established between the opposing electrodes. We have now demonstrated the ability to assemble multifunctional energy-storage nanoarchitectures on the nanoscale and in three dimensions.