Scientific Publications 2005
2005. "Near-edge X-ray Absorption Fine Structure Study of Disordering in Gd₂(Ti1-yZry)₂O₇ Pyrochlores." Journal of Physical Chemistry B 109(4):1337-1339. Abstract Disorder in Gd₂(Ti1-yZry)₂O₇ pyrochlores, for y=0.0-1.0, is investigated by Ti 2p and O 1s near-edge x-ray absorption fine structure spectroscopy. Ti⁴⁺ ions are found to occupy octahedral sites in Gd₂Ti₂O₇ with a tetragonal distortion induced by vacant oxygen sites. As Zr substitutes for Ti, the tetragonal distortion decreases, and Zr coordination increases from 6 to 8. The migration of oxygen ions from 48f or 8b sites to vacant 8a sites compensate for the increased Zr coordination, thereby reducing the number of vacant 8a sites, which further reduces the tetragonal distortion and introduces more disorder around Ti. This is evidence for simultaneous cation disorder with anion migration.
2005. "Near-Edge X-ray Absorption Fine-Structure Study of Ion-Beam-Induced Phase Transformation in Gd2(Ti1-yZry)2O7." Journal of Applied Physics 97(3):Article 033518. Abstract The structural and electronic properties of Gd2(Ti1-yZry)2O7 (y=0-1) pyrochlores following 2.0 MeV Au2+ ion-beam irradiation (~5.0x1014 Au2+/cm2) have been investigated by Ti 2p and O 1s near-edge x-ray absorption fine structure (NEXAFS). The irradiation of Gd2(Ti1-yZry)2O7 leads to the phase transformation from the ordered pyrochlore structure (Fd3m) to the defect fluorite structure (Fm3m) regardless of Zr concentration. Irradiated Gd2(Ti1-yZry)2O7 with y≤0.5 are amorphous although significant short-range order is present. Contrasting to this behavior, compositions with y≥0.75 retain crystallinity in the defect fluorite structure following irradiation. The local structures of Zr4+ in the irradiated Gd2(Ti1-yZry)2O7 with y≥0.75 determined by NEXAFS are the same as in cubic fluorite-structured yttria-stabilized zirconia (Y-ZrO2), thereby providing conclusive evidence for the phase transformation. The TiO6 octahedra present in Gd2(Ti1-yZry)2O7 are completely modified by ion-beam irradiation to TiOx polyhedra, and the Ti coordination is increased to eight with longer Ti-O bond distances. The similarity between cation sites and the degree of disorder in Gd2Zr2O7 facilitate the rearrangement and relaxation of Gd, Zr, and O ions/defects. This inhibits amorphization during the ion-beam induced phase transition to radiation-resistant defect fluorite structure contrasting to the ordered Gd2Ti2O7. Thus, during the ion-beam induced phase transition, the Gd2(Ti1-yZry)2O7 with y≤0.5 becomes amorphous whereas compositions with y≥0.75 retain crystalline structure.
2005. "Cell Adhesion of Shewanella Oneidensis to Iron Oxide Minerals: Effect of Different Single Crystal Faces." Geochemical Transactions 6(4):77-84. Abstract The results of experiments designed to test the hypothesis that near-surface molecular structure of iron oxide minerals influences adhesion of dissimilatory iron reducing bacteria are presented. These experiments involved the measurement, using atomic force microscopy, of interaction forces generated between Shewanella oneidensis MR-1 cells and single crystal growth faces of iron oxide minerals. Significantly different adhesive force was measured between cells and the (001) face of hematite, and the (100) and (111) faces of magnetite. A role for electrostatic interactions is apparent. The trend in relative forces of adhesion generated at the mineral surfaces is in agreement with predicted ferric site densities published previously. These results suggest that near-surface structure does indeed influence initial cell attachment to iron oxide surfaces; whether this is mediated via specific cell surface-mineral surface interactions or by more general interfacial phenomena remains untested.
2005. "Low-frequency Electrical Response to Microbial Induced Sulfide Precipitation." Journal of Geophysical Research. Solid Earth 110:Article NO. G02009. doi:10.1029/2005JG000024 Abstract We investigated the sensitivity of low-frequency electrical measurements to microbeinduced metal sulfide precipitation. Three identical sand-packed monitoring columns were used; a geochemical column, an electrical column and a control column. In the first experiment, continuous upward flow of nutrients and metals in solution was established in each column. Cells of Desulfovibrio vulgaris (D. vulgaris) were injected into the center of the geochemical and electrical columns. Geochemical sampling and post-experiment destructive analysis showed that microbial induced sulfate reduction led to metal precipitation on bacteria cells, forming motile biominerals. Precipitation initially occurred in the injection zone, followed by chemotactic migration of D. vulgaris and ultimate accumulation around the nutrient source at the column base.
2005. "Characterization and Properties of Metallic Iron Nanoparticles: Spectroscopy, Electrochemistry, and Kinetics." Environmental Science and Technology 39(5):1221-1230. Abstract There are reports that nano-sized zero-valent iron (Fe⁰) exhibits greater reactivity than micro-sized particles of Fe⁰, which may impart advantages for groundwater remediation or other environmental applications. However, most of these reports are preliminary in that they leave a host of potentially significant (and often challenging) material or process variables either uncontrolled or unresolved. To better understand the reactivity of iron nanoparticles, we have used a variety of complementary techniques to characterize two widely studied nano Fe⁰ preparations: one synthesized by heat-reduction of goethite under H₂ (FeH₂) and the other by reductive precipitation with borohydride (FeBH). X-ray diffraction (XRD), transmission electron microscopy (STXM) showed particles of similar size (40-80 nm), but surface area measurements varied widely with method of measurement (4-60 m² g-¹). FeH₂ is a two-phase material consisting of ⍺-Fe⁰ and Fe₃O₄, doped with reduced sulfur, whereas FeBH is mostly metallic Fe with an oxide shell that is high in boron. Both materials exhibit corrosion potentials that are more negative than nano-sized Fe₂O₃, Fe₃O₄, micro-sized Fe⁰, or a solid Fe⁰ disk, consistent with their rapid reduction of oxygen, benzoquinone, and carbon tetrachloride. Benzoquinone-which presumably probes inner-sphere surface reactions-reacts more rapidly with FeBH than with FeH₂, whereas with carbon tetrachloride, FeBH and FeH₂ react at similar rates, presumably by outer-sphere electron transfer. Whether either material reacts more rapidly with the probes than micro-sized Fe⁰ is unclear due to uncertainties in the appropriate specific surface areas. The distribution of products from reduction of carbon tetrachloride is more favorable with FeH₂, which produces less chloroform than reaction with FeBH.
2005. "Interaction of Wide-Band-Gap Single Crystals with 248-nm Excimer Laser Irradiation: X. Laser-Induced Near-Surface Absorption in Single-Crystal NaC1." Journal of Applied Physics 97(4):Article no. 043501. Abstract Ultraviolet laser-induced desorption of neutral atoms and molecules from nominally transparent, ionic materials can yield particle velocities consistent with surface temperatures of a few thousand Kelvin, even in the absence of visible surface damage. The origin of the laser required for this surface heating has been often overlooked. In this work, we report simultaneous neutral emission and laser transmission measurements on single crystal NaCI exposed to 248-nm excimer laser radiation. As much as 20% of the incident radiation at 248 nm must be absorbed in the near surface region to account for the observed particle velocities. We show that the laser absorption grows from low values over several pulses and saturates at values sufficient to account for the surface temperatures required to explain the observed particle velocity distributions. The growth of absorption in these early pulses is accompanied by a corresponding increase in the emission intensities. Diffuse reflectance spectra acquired after exposure suggest that near surface V-type centers are responsible for most of the absorption at 248 nm in single crystal NaCI.
2005. "Interaction of Wide-Band-Gap Single Crystals With 248-nm Excimer Laser Radiation: XI. The Effect of Water Vapor and Temperature on Laser Desorption of Neutral Atoms From Sodium Chloride." Journal of Applied Physics 97:043502-1-10. Abstract We investigate the effect of water vapor and temperature on the desorption of neutral Na and C1 from cleaved, single crystal NaC1 during pulsed laser irradiation at 248-nm (KrF excimer). Neutral emissions in the presence of ~10-5 Pa of water vapor are much more intense than in ultra high vacuum (total pressure <10-7 Pa). Emission intensities are also increased by raising the substrate temperature or the laser fluence. The neutral time-of-flight signals are well described by Maxwell Boltzmann velocity distributions for effusing particles, which we use to estimate the peak surface temperatures during the laser pulse. The neutral emission intensities display Arrhenius behavior when plotted against both the background substrate temperature and the peak surface temperature. The resulting activation energies correspond to different, rate limiting processes, one of which is enhanced in the presence of water vapor. We propose a mechanism for the effect of water on these neutral emissions and discuss the implications.