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.
There are many potential sensing applications for Au nanorods due to a tunable localized surface plasmon resonance (LSPR) frequency that changes with aspect ratio. However, their application at high temperatures is limited due to a shape change that can take place well below the melting point of bulk Au, driven by a reduction in surface energy. A method of stabilizing Au nanorods is provided here by encapsulating them with a 15 nm capping layer of yttria stabilized zirconia (YSZ). After annealing rods with nominal dimensions of 100 × 44 nm to a temperature of 600 °C, small reductions in length were observed, but the rods remained stable for all subsequent sensing tests at 500 °C, which amounted to 80 h. It was shown with a separate sample that the rod geometry can be preserved even up to 800 °C over a 12 h annealing period, although a significant shortening of the rod length occurred, leaving a void space in the YSZ. The sensing response of both the transverse and the longitudinal LSPR peaks was monitored for H2, CO, and NO2 exposures in an air background at 500 °C. In all cases, the longitudinal LSPR peak shows a larger shift upon gas exposure than does the transverse peak.
An optical plasmonic-based sensing array has been developed and tested for the selective and sensitive detection of H(2), CO, and NO(2) at a temperature of 500 °C in an oxygen-containing background. The three-element sensing array used Au nanoparticles embedded in separate thin films of yttria-stabilized zirconia (YSZ), CeO(2), and TiO(2). A peak in the absorbance spectrum due to a localized surface plasmon resonance (LSPR) on the Au nanoparticles was monitored for each film during gas exposures and showed a blue shift in the peak positions for the reducing gases, H(2) and CO, and a red shift for the oxidizing gas, NO(2). A more in-depth look at the sensing response was performed using the multivariate methods of principal component analysis (PCA) and linear discriminant analysis (LDA) on data from across the entire absorbance spectrum range. Qualitative results from both methods showed good separation between the three analytes for both the full array and the Au-TiO(2) sample. Quantification of LDA cluster separation using the Mahalanobis distance showed better cluster separation for the array, but there were some instances with the lowest concentrations where the single Au-TiO(2) film had separation better than that of the array. A second method to quantify cluster separation in LDA space was developed using multidimensional volume analysis of the individual cluster volume, overlapped cluster volume, and empty volume between clusters. Compared to the individual sensing elements, the array showed less cluster overlap, smaller cluster volumes, and more space between clusters, all of which were expected for improved separability between the analytes.
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