A Au-CeO(2) nanocomposite film has been investigated as a potential sensing element for high-temperature plasmonic sensing of H(2), CO, and NO(2) in an oxygen containing environment. The CeO(2) thin film was deposited by molecular beam epitaxy (MBE), and Au was implanted into the as-grown film at an elevated temperature followed by high temperature annealing to form well-defined Au nanoclusters. The Au-CeO(2) nanocomposite film was characterized by X-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS). For the gas sensing experiments, separate exposures to varying concentrations of H(2), CO, and NO(2) were performed at a temperature of 500 °C in oxygen backgrounds of 5.0, 10, and ∼21% O(2). Changes in the localized surface plasmon resonance (LSPR) absorption peak were monitored during gas exposures and are believed to be the result of oxidation-reduction processes that fill or create oxygen vacancies in the CeO(2). This process affects the LSPR peak position either by charge exchange with the Au nanoparticles (AuNPs) or by changes in the dielectric constant surrounding the particles. Spectral multivariate analysis was used to gauge the inherent selectivity of the film between the separate analytes. From principal component analysis (PCA), unique and identifiable responses were seen for each of the analytes. Linear discriminant analysis (LDA) was also used and showed separation between analytes as well as trends in gas concentration. Results indicate that the Au-CeO(2) thin film is selective to O(2), H(2), CO, and NO(2) in separate exposures. This, combined with the observed stability over long exposure periods, shows the Au-CeO(2) film has good potential as an optical sensing element for harsh environmental conditions.

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Raman spectra were measured for palladium oxide (PdO) at temperatures from 298 to 973 K. The first-order scattering B 1g and E g phonon modes show prominent frequency and line width dependencies on temperature. The phonon behaviors are described well by a model considering contributions from thermal expansion related quasiharmonicity and explicit lattice anharmonicity from both cubic and quartic phonon decay processes. Compared to the quasiharmonicity for the line shift, the explicit anharmonicity is found to be relatively smaller for the B 1g mode with the cubic anharmonicity leading the quartic term. The E g mode has a smaller line shift which mostly follows a quartic phonon decay mechanism while the cubic contribution is canceled by thermal expansion effects. The cubic anharmonicity dominates mostly the line broadening with more prominent quartic effects responsible for the larger broadening of the E g mode. Multiple potential asymmetric cubic and quartic anharmonic decay channels including down-conversion and up-conversion are identified with their characteristics discussed and correlated to the positive and negative anhamonicities found for the two phonon modes studied. ■ INTRODUCTIONPdO is a catalytically active material and has found extensive use in heterogeneous and homogeneous catalysis for chemical conversions, energy production, and pollution mitigation based on the reversible and energetic creation of Pd 2+ or oxygen vacancies in the oxide. 1−6 The material is also an appreciable source of oxygen because of its reversible equilibrium transformation with Pd at elevated temperatures within a sealed cavity. This characteristic has been employed as an oxygen reference material in the development of harsh environment oxygen sensors. 7 Owing to the technological interest in the oxide as a critical component in a variety of applications, the material has been actively studied. Crystalline PdO has a tetragonal cooperite structure (space group D 4h 9 -P4 2 /mmc) and is composed of edge-sharing chains of PdO 4 planes. 8,9 Strong resonance Raman scattering has been observed from PdO at room temperature, with the resonance enhancement possibly correlated to an exciton model of the electronic structure. 10 The temperature dependence of Raman scattering carries a great deal of information on the interactions of the vibrational phonon modes at the Brillouin zone center. The phonon line shift and broadening with temperature can be understood through a determination of the lattice anharmonicity properties. It has been shown in the literature that for a variety of materials studied the temperature-dependent phonon properties can be modeled by the effects of thermal expansion, phonon−phonon coupling, and perhaps electron−phonon coupling, depending on the specific material and electronic structures. 11−16 Anharmonic effects in crystals strongly affect a number of processes including energy transport, 17,18 structural phase transformation, 19 optical properties, 20 and electronic transport in devices, 21,22 which contrib...

Au-YSZ nanocomposite films exhibited a surface plasmon resonance absorption band around 600 nm that underwent a reversible blue shift and narrowed upon exposure to CO in air at 500 degrees C. A linear dependence of the sensing signal was observed for CO concentrations ranging between 0.1 and 1 vol % in an air carrier gas. This behavior of the SPR band, upon exposure to CO, was not observed when using nitrogen as the carrier gas, indicating an oxygen-dependent reaction mechanism. Additionally, the SPR band showed no measurable signal change upon exposure to CO at temperatures below approximately 400 degrees C. The oxygen and temperature-dependent characteristics, coupled with the oxygen ion formation and conduction properties of the YSZ matrix, are indicative of charge-transfer reactions occurring at the three-phase boundary region between oxygen, Au, and YSZ, which result in charge transfer into the Au nanoparticles. These reactions are associated with the oxidation of CO and a corresponding reduction of the YSZ matrix. The chemical-reaction-induced charge injection into the Au nanoparticles results in the observed blue shift and narrowing of the SPR band.

The surface plasmon resonance (SPR) band of Au nanoparticles embedded in an YSZ matrix was monitored at 500 °C under varying gas exposure concentrations of H 2 and O 2 in N 2 . Because the peak position of the SPR band relies closely on the number of driven oscillating free electrons per gold nanoparticle, we were able to monitor electrochemical charge transfer from Au nanoparticles to diffusing oxygen ions by monitoring the optical properties of the Au-YSZ nanocomposite thin film, specifically the peak position of the SPR band. A direct relation was observed for the change in the equilibrium ratio, p H 2 1/4 /p O 2 1/8 , contributing to oxygen ion diffusion into and out of the YSZ matrix, and the change in the square of the SPR band peak position. Free electron theory states that this change in the square of the SPR band peak position is directly proportional to the change in conduction electrons available per Au nanoparticle; thus, our observations agree with the expected trend for charge transfer vs the redox gas mixture.