Transient Optical and Terahertz Spectroscopy of Nanoscale Films of RuO2

Abstract

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.

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

“…Scherrer analysis using the full width at halfmaximum of the (101) reflection yields an average crystallite diameter of ∼9 nm, consistent with the feature sizes observed via SEM. Similar trends were previously reported for ruthenia nanoskins deposited at planar silica, 30 titanium, 24 and silica fiber papers. 22,23 Chemical-state analysis of ruthenia by X-ray photoelectron spectroscopy is an effective method to assess the chemical and electronic states of this oxide, as shown previously for RuOxcoated silica fiber papers 23 and hydrous RuOx.…”

“…Ruthenium dioxide (RuO 2 ), while still an oxide of a platinum-group metal, has been used extensively for industrial-scale electrochemical applications, where it exhibits outstanding electrocatalytic activity for such reactions as the chlor-alkali process . Inspired by the innately attractive electrochemical properties of RuO 2 , including Pt-like electron-transfer kinetics in nonaqueous electrolytes, we have developed protocols that use low-temperature decomposition of RuO 4 from alkane-based solutions to deposit nanoscale ruthenium oxide, RuO x , on a wide range of substrates, including silica aerogels, native- , and carbon-wrapped silica fiber paper, carbon nanofoams, polymers, , and planar fused-silica and CaF 2 plates. ,− The key advantage of this electroless route is its self-limiting nature, such that the thickness of the RuO x coating is ∼3 nm on porous objects and ∼10 nm on planar substrates. The expression of ultrathin RuO x “nanoskins” at a thickness of only 1–3 crystal units affords high electrochemical utilization of this otherwise expensive platinum-group metal oxide.…”

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

“…10,15,17 The nanoskins studied here, however, achieve maximum conductivity at a higher calcination temperature of 673 K before the onset of dewiring effects take place. This maximum can be explained by considering two ways in which the calcination procedure tends to improve conductivity.…”

“…17 The procedure for generating RuO 2 nanoskins is a layer-by-layer technique involving 373 K heating steps between depositing each ~10 nm thick layer from solution at sub ambient temperature. For this work, seven layers (7×) were deposited) the coated substrate was then sectioned into individual pieces that were calcined at temperatures ranging from 373 K to 773 K to generate the final sample set.…”

“…However, the lack of electrical conductivity that occurred upon dewiring at T cal > 473 K prevented us from observing the same TRTS sign change for the 20 nm thick solution-deposited nanoskins. 17 Herein, we use TRTS to investigate the insulator-to-metallic transition in solution-deposited RuO 2 nanoskins but now focus on thicker (70 ± 20 nm) films. This improved film growth allows the nanoskins to retain a sufficient degree of particle-to-particle contiguity despite the required thermal ripening when heating the nanoskins to T cal > 473 K such that improved conductivity is achieved compared to the thinner films calcined above this temperature.…”