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

Ultraviolet and Visible Photochemistry of Methanol at 3D Mesoporous Networks: TiO₂ and Au–TiO₂

Comparison of methanol photochemistry at three-dimensionally (3D) networked aerogels of TiO₂ or Au–TiO₂ reveals that incorporated Au nanoparticles strongly sensitize the oxide nanoarchitecture to visible light. Methanol dissociatively adsorbs at the surfaces of TiO₂ and Au–TiO₂ aerogels under dark, high-vacuum conditions. Upon irradiation of either ultraporous material with broadband UV light under anaerobic conditions, adsorbed methoxy groups act as hole-traps and extend conduction-band and shallow-trapped electron lifetimes. A higher excited-state electron density arises for UV-irradiated TiO₂ aerogel relative to commercial nanoparticulate TiO₂, indicating that 3D networked TiO₂ more efficiently separates electron–hole pairs. Upon excitation with narrow-band visible light centered at 550 nm, long-lived excited-state electrons are evident on CH₃OH-exposed Au–TiO₂ aerogelsbut not on identically dosed TiO₂ aerogelsverifying that incorporated Au nanoparticles sensitize the networked oxide to visible light. Under aerobic conditions (20 Torr O₂) and broadband UV illumination, surface-sited formates accumulate as adsorbed methoxy groups oxidize, at similar rates, on Au–TiO₂ and TiO₂ aerogels. Moving to excitation wavelengths longer than ∼400 nm (i.e., the low-energy range of UV light) dramatically decreases methoxy photoconversion for methanol-saturated TiO₂ aerogel, while Au–TiO₂ aerogel remains highly active for methanol photooxidation. The wavelength dependence of formate production on Au–TiO₂ tracks the absorbance spectrum for this material, which peaks at λ = 550 nm due to resonance with the surface plasmon in the Au particles. The photooxidation rate for Au–TiO₂ aerogel at 550 nm is comparable to that for TiO₂ aerogel under broadband UV illumination, indicating efficient energy transfer from Au to TiO₂ in the 3D mesoporous nanoarchitecture

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 (RuO<i>x@</i>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 RuO<i>x@</i>CVD-C electrodes and calculated standard heterogeneous rate constants as a function of heat treatment to crystallize the disordered RuO<i>x</i> 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 RuO<i>x</i> 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 RuO<i>x</i>

Electroanalytical Assessment of the Effect of Ni:Fe Stoichiometry and Architectural Expression on the Bifunctional Activity of Nanoscale Ni_<i>y</i>Fe_1–<i>y</i>O<i>x</i>

Electrocatalysis of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) was assessed for a series of Ni-substituted ferrites (Ni_<i>y</i>Fe_1–<i>y</i>O<i>x</i>, where <i>y</i> = 0.1 to 0.9) as expressed in porous, high-surface-area forms (ambigel and aerogel nanoarchitectures). We then correlate electrocatalytic activity with Ni:Fe stoichiometry as a function of surface area, crystallite size, and free volume. In order to ensure in-series comparisons, calcination at 350 °C/air was necessary to crystallize the respective Ni_<i>y</i>Fe_1–<i>y</i>O<i>x</i> nanoarchitectures, which index to the inverse spinel structure for Fe-rich materials (<i>y</i> ≤ 0.33), rock salt for the most Ni-rich material (<i>y</i> = 0.9), and biphasic for intermediate stoichiometry (0.5 ≤ <i>y</i> ≤ 0.67). In the intermediate Ni:Fe stoichiometric range (0.33 ≤ <i>y</i> ≤ 0.67), the OER current density at 390 mV increases monotonically with increasing Ni content and increasing surface area, but with different working curves for ambigels versus aerogels. At a common stoichiometry within this range, ambigels and aerogels yield comparable OER performance, but do so by expressing larger crystallite size (ambigel) versus higher surface area (aerogel). Effective OER activity can be achieved without requiring supercritical-fluid extraction as long as moderately high surface area, porous materials can be prepared. We find improved OER performance (η decreases from 390 to 373 mV) for Ni_0.67Fe_0.33O<i>x</i> aerogel heat-treated at 300 °C/Ar, owing to an increase in crystallite size (2.7 to 4.1 nm). For the ORR, electrocatalytic activity favors Fe-rich Ni_<i>y</i>Fe_1–<i>y</i>O<i>x</i> materials; however, as the Ni-content increases beyond <i>y</i> = 0.5, a two-electron reduction pathway is still exhibited, demonstrating that bifunctional OER and ORR activity may be possible by choosing a nickel ferrite nanoarchitecture that provides high OER activity with sufficient ORR activity. Assessing the catalytic activity requires an appreciation of the multivariate interplay among Ni:Fe stoichiometry, surface area, crystallographic phase, and crystallite size

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 multilayer 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 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 using 400 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 (Δ<i>E</i> < 0) while higher calcine temperatures yield films composed of crystalline, rutile RuO₂ and the conductivity decreases (Δ<i>E</i> > 0) following photoexcitation

Plasmonic Aerogels as a Three-Dimensional Nanoscale Platform for Solar Fuel Photocatalysis

We use plasmonic Au–TiO₂ aerogels as a platform in which to marry synthetically thickened particle–particle junctions in TiO₂ aerogel networks to Au∥TiO₂ interfaces and then investigate their cooperative influence on photocatalytic hydrogen (H₂) generation under both broadband (i.e., UV + visible light) and visible-only excitation. In doing so, we elucidate the dual functions that incorporated Au can play as a water reduction cocatalyst and as a plasmonic sensitizer. We also photodeposit non-plasmonic Pt cocatalyst nanoparticles into our composite aerogels in order to leverage the catalytic water-reducing abilities of Pt. This Au–TiO₂/Pt arrangement in three dimensions effectively utilizes conduction−band electrons injected into the TiO₂ aerogel network upon exciting the Au SPR at the Au∥TiO₂ interface. The extensive nanostructured high surface-area oxide network in the aerogel provides a matrix that spatially separates yet electrochemically connects plasmonic nanoparticle sensitizers and metal nanoparticle catalysts, further enhancing solar-fuels photochemistry. We compare the photocatalytic rates of H₂ generation with and without Pt cocatalysts added to Au–TiO₂ aerogels and demonstrate electrochemical linkage of the SPR-generated carriers at the Au∥TiO₂ interfaces to downfield Pt nanoparticle cocatalysts. Finally, we investigate visible light–stimulated generation of conduction band electrons in Au–TiO₂ and TiO₂ aerogels using ultrafast visible pump/IR probe spectroscopy. Substantially more electrons are produced at Au–TiO₂ aerogels due to the incorporated SPR-active Au nanoparticle, whereas the smaller population of electrons generated at Au-free TiO₂ aerogels likely originate at shallow traps in the high surface-area mesoporous aerogel

Correlating Changes in Electron Lifetime and Mobility on Photocatalytic Activity at Network-Modified TiO₂ Aerogels

We use intensity-modulated photovoltage spectroscopy (IMVS) and intensity-modulated photocurrent spectroscopy (IMPS) to characterize carrier dynamics in titania (TiO₂) aerogels under photocatalytic conditions. By systematically increasing the weight fraction of the sol–gel precursor during TiO₂ sol–gel synthesis, we are able to impart drastic changes in carrier transport/trapping and improve the photocatalytic activity of TiO₂ aerogels for two mechanistically divergent photochemical reactions: reductive water splitting (H₂ generation) and oxidative degradation of dichloroacetate (DCA). The lifetimes of photogenerated electrons increase in going from lowest-to-highest precursor concentrations, as measured by IMVS, indicating increasing site density for electron trapsa trend that correlates with an 8× improvement for photocatalytic H₂ generation. Electron mobility in the aerogel films, as measured by IMPS, decreases with increasing trap density, further implicating the trapping sites as reactive sites. In contrast, photocatalytic DCA degradationdriven primarily by direct hole transfer to adsorbed DCAdepends only weakly on the electron dynamics in the film. Transient infrared spectroscopy shows no difference in carrier decay among the aerogel samples on picosecond time scales, indicating that changes in carrier dynamics within these networked nanomaterials are only observable at time scales measured by IMPV and IMPS. Correlating hole-mediated and electron-mediated photocatalytic activity with direct measurement of electron dynamics under photocatalytically relevant conditions and time scales comprises a powerful approach to determine how synthetic modifications to networked nanostructured photocatalysts affect the relevant physicochemical phenomena underlying their photocatalytic performance