Willes H. Weber

Raman and x‐ray studies of Ce1−xRExO2−y, where RE=La, Pr, Nd, Eu, Gd, and Tb

Powdered samples of the type Ce1−xRExO2−y, where RE=La, Pr, Nd, Eu, Gd, and Tb, are synthesized over the range 0≤x≤0.5 starting from nitrate solutions of the rare earths. X‐ray diffraction and Raman scattering are used to analyze the samples. These compounds, at least in the low doping regime and for strictly trivalent dopants, form solid solutions that maintain the fluorite structure of CeO2 with a change in lattice constant that is approximately proportional to the dopant ionic radius. The single allowed Raman mode, which occurs at 465 cm−1 in pure CeO2, is observed to shift to lower frequency with increasing doping level for all the rare earths. However, after correcting for the Gruneisen shift from the lattice expansion, the frequency shift is actually positive for all the strictly trivalent ions. In addition, the Raman line broadens and becomes asymmetric with a low frequency tail, and a new broad feature appears in the spectrum at ∼570 cm−1. These changes in the Raman spectrum are attributed to O va...

Empirical method for determining CeO2-particle size in catalysts by raman spectroscopy

The addition of ceria, CeO{sub 2}, to automotive exhaust-gas catalysts has been linked to greater thermal stability of the alumina support, higher precious-metal dispersion, and wider range of parameters for three-way operation. Some, if not all, of these beneficial effects are undoubtedly influenced by the dispersion of the ceria, which is typically added at the 5-10 wt% (including the monolith) level. Methods of measuring its dispersion, or particle size, are thus important tools for studying how ceria functions. Traditionally, either transmission electron microscopy (TEM) or X-ray diffraction (XRD) have been applied to the problem of particle size determination in catalysts. In this report, the authors show how Raman spectroscopy may be used to determine the size of CeO{sub 2} particles in a way that is easier than TEM and faster than XRD.

Dispersion studies on the system La2O3γ-Al2O3

Abstract According to the literature and earlier studies in this laboratory, lanthanum oxide appears to be the best of the known additives for inhibiting the sintering of high-surface-area aluminas. The substrates of the present studies were largely γ-alumina, with minor components of transition aluminas, σ-alumina, and possibly θ-alumina. Alumina samples with different lanthanum concentrations, produced by impregnation with aqueous lanthanum nitrate, followed by calcination at various temperatures, were studied by chemisorption of carbon dioxide, Auger electron spectroscopy, Raman spectroscopy, and by X-ray powder diffraction (XRD). All results are consistent with the following description: Up to a concentration of 8.5 μmol La m 2 , the lanthana is in the form of a two-dimensional overlayer, invisible by XRD or Raman spectroscopy. For greater lanthana concentrations, the excess lanthana forms crystalline oxides, detectable by XRD. In samples calcined to 650 °C, this crystalline phase is cubic lanthanum oxide. After calcination at 800 °C, the lanthana reacts to form the lanthanum aluminate LaAlO3.

Growth and characterization of reactively sputtered thin‐film platinum oxides

Thin‐film oxides of platinum have been prepared by reactive sputtering and characterized primarily by x‐ray diffraction (XRD) and Raman scattering. Different phases of the Pt‐O system were obtained by adjusting sputtering parameters such as gas composition, deposition rate, and substrate temperature. Optimum conditions for producing crystalline α‐PtO2 and PtO were identified. Raman spectra of α‐PtO2 films show two sharp lines at 514 and 560 cm−1, in agreement with those from commercially available powders. XRD and high‐resolution scanning electron microscopy (HRSEM) analyses indicate that these samples have a poorly crystallized structure with lack of order along the c axis. The Raman spectra from PtO show two broad peaks at 438 and 657 cm−1, close to the lines seen in PdO, which has the same structure as PtO. Estimates from XRD line broadening and HRSEM photographs indicate mean crystallite sizes on the order of 300 A. Infrared reflectivity spectra yield two of the three ir‐active phonons, and along with...

Materials for luminescent greenhouse solar collectors

Luminescent greenhouse solar collectors are potentially useful for concentrating sunlight onto photovoltaic power cells. Measurements of the performance of small-scale collectors made of two commercially available materials (Owens-Illinois ED2 neodymium-doped laser glass and rhodamine 6G-doped plastic) are presented. The results are encouraging, but they indicate a need for further spectral sensitization and for reduced matrix loss coefficient. The measurements with monochromatic illumination agree with the predictions of a mathematical model developed to take account of reemission following the absorption of luminescence. Under solar illumination, the model predicts photon flux concentrations of about 15 for optimized full-scale collectors made of the materials studied and concentrations of 110 for reasonably improved glass.

In situ ellipsometric study of a palladium catalyst during the oxidation of methane

Ellipsometry is used to follow the growth of a PdO layer on the surface of a thick Pd-film catalyst during methane oxidation at ∼ 500°C. The oxide layer that develops under rich conditions (excess CH4) is quite porous and roughens with time. Little CO is formed during this period, but the CO2 formation rate increases until spontaneous oscillations develop, which correlate with changes in the ellipsometric data. These changes indicate that the porous oxide rapidly converts to a metal-rich state, which has decreased catalytic activity, and then slowly reoxidizes.

Raman spectroscopy of palladium oxide on γ-alumina applicable to automotive catalysts: Nondestructive, quantitative analysis; oxidation kinetics; fluorescence quenching

Abstract Raman scattering is used to analyze a series of model catalysts consisting of γ-alumina powder with up to 20 wt.-% Pd, and two monolithic catalyst samples with palladium concentrations typically employed in automotive emission control. By using the 514.5 nm Ar laser line to take advantage of a resonant Raman effect, the 651-cm −1 Raman line in PdO can be detected in the oxidized catalysts with palladium loadings as small as 0.2 wt.-%. A strong fluorescence signal is observed in reduced pure γ-Al 2 O 3 . This signal is quenched by the addition of palladium, admixed either from aqueous solution or from Pd black, provided the palladium was allowed to interact with the γ-Al 2 O 3 during at least one oxidation treatment at 600°C, followed by reduction in hydrogen at 300°C. Raman spectroscopy is found to be applicable for a quantitative, nondestructive analysis of PdO on γ-alumina, including automotive formulations, within certain limitations. At low concentrations, some palladium may interact with the support and thus may not contribute to the signal associated with crystalline PdO. The analysis is also suited for studying sintering and reduction/oxidation kinetics in situ.

Analysis of Doppler-limited spectra of the CO stretch fundamental of CD3OH

Abstract Doppler-limited linewidth diode laser spectra of the low- J P and R branches of the ν 4 band (CO stretch) of CD 3 OH were recorded and analyzed. Hindered internal rotation and molecular asymmetry are handled by exact matrix diagonalization. Fermi resonance between ν 4 and the τ = 1, n = 3 and n = 4 torsional levels of the ground vibration state was found to be responsible for several anomalous spectral features. Direct LMS fitting of the data to a set of 11 excited-state parameters lead to the assignment of 160 lines with a rms error of ±0.00088 cm −1 .

Optical study of silicon-containing amorphous hydrogenated carbon

Silicon-containing amorphous hydrogenated carbon films deposited by a plasma-enhanced chemical vapor deposition process were studied using both Raman and ellipsometry spectroscopies. Analyses of the experimental data from both these techniques yielded valuable information about the microstructure of the films. The silicon incorporation in amorphous hydrogenated carbon breaks down large size sp2 carbon clusters and enhances sp3 bonding. The reduction of large sp2 graphitic defects, the enhancement of sp3 bonding, and the associated microstructure changes are responsible for the desired properties of silicon-containing amorphous hydrogenated carbon.

Double injection in semiconductors heavily doped with deep two-level traps

Abstract Double injection is discussed for semiconductors heavily doped with deep recombination centers which act both as acceptors and donors. The current-voltage characteristics are derived using a generalization of the method due to Lampert for long structures in which diffusion can be neglected. Thermal emission from the traps and trapped space-charge effects are included. The resulting j - V curve shows in order of increasing current the following general features: an Ohmic regime, a low injection square law related to thermal emission, a negative resistance regime, a high injection square law, and finally a space-charge-limited cube law. A complete computer solution for the case of gold-doped silicon is given. Simple power laws for the various regimes, derived using the quasineutrality approximation, are found to agree well with the numerical solution. An expression is obtained for the voltage at the threshold of the negative resistance, and the effects of field dependent mobility are discussed quantitatively. Experimental results are presented for long (∼100 μm) gold-doped (∼10 16 cm −3 ) silicon p - i - n structures. The devices are fabricated from high purity n -type silicon, and large area alloy junctions are used. Pulsed I – V measurements at room temperature show an Ohmic regime, a square-law regime, and a negative resistance regime. These data are found to agree quantitatively with the theoretical calculations up to and including the onset of the negative resistance. In particular a strong temperature dependence is observed in the low injection square-law regime, which is different from that found in the Ohmic regime. This difference is explained in terms of the level structure of the gold impurity. The current for voltages above the onset of the negative resistance is approximately two orders of magnitude below the theoretical predictions. This indicates that filamentary conduction is probably occurring.