Kevin M. Rosso

Ferromagnetism in oxide semiconductors

Over the past five years, considerable work has been carried out in the exploration of candidate diluted oxide magnetic semiconductors with high Curie temperatures. Fueled by early experimental results and theoretical predictions, claims of ferromagnetism at and above room temperature in doped oxides have abounded. In general, neither the true nature of these materials nor the physical causes of the magnetism have been adequately determined. It is now apparent that these dilute magnetic systems are deceptively complex. We consider two well-characterizedn-type magnetically doped oxide semiconductors and explore the relationship between donor electrons and ferromagnetism.

Linked Reactivity at Mineral-Water Interfaces Through Bulk Crystal Conduction

The semiconducting properties of a wide range of minerals are often ignored in the study of their interfacial geochemical behavior. We show that surface-specific charge density accumulation reactions combined with bulk charge carrier diffusivity create conditions under which interfacial electron transfer reactions at one surface couple with those at another via current flow through the crystal bulk. Specifically, we observed that a chemically induced surface potential gradient across hematite (α-Fe2O3) crystals is sufficiently high and the bulk electrical resistivity sufficiently low that dissolution of edge surfaces is linked to simultaneous growth of the crystallographically distinct (001) basal plane. The apparent importance of bulk crystal conduction is likely to be generalizable to a host of naturally abundant semiconducting minerals playing varied key roles in soils, sediments, and the atmosphere.

Charge transport in metal oxides: A theoretical study of hematite α-Fe2O3

Transport of conduction electrons and holes through the lattice of α-Fe2O3 (hematite) is modeled as a valence alternation of iron cations using ab initio electronic structure calculations and electron transfer theory. Experimental studies have shown that the conductivity along the (001) basal plane is four orders of magnitude larger than the conductivity along the [001] direction. In the context of the small polaron model, a cluster approach was used to compute quantities controlling the mobility of localized electrons and holes, i.e., the reorganization energy and the electronic coupling matrix element that enter Marcus’ theory. The calculation of the electronic coupling followed the generalized Mulliken–Hush approach using the complete active space self-consistent field method. Our findings demonstrate an approximately three orders of magnitude anisotropy in both electron and hole mobility between directions perpendicular and parallel to the c axis, in good accord with experimental data. The anisotropy ...

The roles of outer membrane cytochromes of Shewanella and Geobacter in extracellular electron transfer

As key components of the electron transfer (ET) pathways used for dissimilatory reduction of solid iron [Fe(III)] (hydr)oxides, outer membrane multihaem c-type cytochromes MtrC and OmcA of Shewanella oneidensis MR-1 and OmcE and OmcS of Geobacter sulfurreducens mediate ET reactions extracellularly. Both MtrC and OmcA are at least partially exposed to the extracellular side of the outer membrane and their translocation across the outer membrane is mediated by bacterial type II secretion system. Purified MtrC and OmcA can bind Fe(III) oxides, such as haematite (α-Fe2 O3 ), and directly transfer electrons to the haematite surface. Bindings of MtrC and OmcA to haematite are probably facilitated by their putative haematite-binding motifs whose conserved sequence is Thr-Pro-Ser/Thr. Purified MtrC and OmcA also exhibit broad operating potential ranges that make it thermodynamically feasible to transfer electrons directly not only to Fe(III) oxides but also to other extracellular substrates with different redox potentials. OmcE and OmcS are proposed to be located on the Geobacter cell surface where they are believed to function as intermediates to relay electrons to type IV pili, which are hypothesized to transfer electrons directly to the metal oxides. Cell surface-localized cytochromes thus are key components mediating extracellular ET reactions in both Shewanella and Geobacter for extracellular reduction of Fe(III) oxides.

Role of extracellular polymeric substances in bioflocculation of activated sludge microorganisms under glucose-controlled conditions

Extracellular polymeric substances (EPS) secreted by suspended cultures of microorganisms from an activated sludge plant in the presence of glucose were characterized in detail using colorimetry, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy. EPS produced by the multi-species community were similar to literature reports of pure cultures in terms of functionalities with respect to C and O but differed subtly in terms of N and P. Hence, it appears that EPS produced by different microorganisms maybe homologous in major chemical constituents but may differ in minor components such as lipids and phosphodiesters. The role of specific EPS constituents on microbial aggregation was also determined. The weak tendency of microorganisms to bioflocculate during the exponential growth phase was attributed to electrostatic repulsion when EPS concentration was low and acidic in nature (higher fraction of uronic acids to total EPS) as well as reduced polymer bridging. However, during the stationary phase, polymeric interactions overwhelmed electrostatic interactions (lower fraction of uronic acids to total EPS) resulting in improved bioflocculation. More specifically, microorganisms appeared to aggregate in the presence of protein secondary structures including aggregated strands, beta-sheets, alpha- and 3-turn helical structures. Bioflocculation was also favored by increasing O-acetylated carbohydrates and overall C-(O,N) and O=C-OH+O=C-OR functionalities.

The interaction of pyrite {100} surfaces with O 2 and H 2 O; fundamental oxidation mechanisms

Abstract The interaction of gaseous O2, H2O, and their mixtures with clean {100} surfaces of pyrite (FeS2) were investigated in ultra-high vacuum using scanning tunneling microscopy and spectroscopy (STM-STS), ultraviolet photoelectron spectroscopy (UPS) and ab initio calculations. He I UPS spectra of O2 exposed surfaces show that the density of states decreases at the top of the valence band but increases deeper in the valence band. These changes indicate oxidative consumption of low binding energy electrons occupying dangling bond surface states localized on surface Fe atoms, and the formation of Fe-O bonds. No such changes in the valence band spectra are observed for pyrite surfaces exposed to H2O. However, UPS spectra of surfaces exposed to mixtures of O2 and H2O demonstrate that the combined gases more aggressively oxidize the surface compared to equivalent exposures of pure O2. Atomically resolved STM images of O2 and O2-H2O exposed surfaces show discrete oxidation “patches” where reacted surface Fe sites have lost surface state density to the sorbed species. STS spectra show the removal of highest occupied and lowest unoccupied surface state density associated with dangling bond states consistent with the interaction of sorbates with surface Fe sites. Ab initio cluster calculations of adsorption energies and the interaction of O2 and water species with the surface show that O2 dissociatively sorbs and H2O molecularly sorbs to surface Fe. For the mixtures, the calculations indicate that H2O dissociatively sorbs when O2 is present on the surface. Charge population analyses also show that the surface S sites become more electropositive in this environment which should allow for easier formation of S-O surface bonds, thus promoting the production of sulfate during oxidation.

Microthermometric and Raman spectroscopic detection limits of C02 in fluid inclusions and the Raman spectroscopic characterization of C02

In many geologic environments, dominantly aqueous solutions contain low concentrations of CO2. At ambient temperature, the typical phase assemblage in fluid inclusions which trap these solutions, consists of a CO2-rich vapor (where PrmCO2 ≈ Pinternal and an aqueous phase containing dissolved salts and CO2. In this study, the CO2 detection limits (DLs) using microthermometry and Raman spectroscopy are established in terms of PCO2 using synthetic H2OCO2 inclusions of known composition. The purpose of the microthermomeri experiments was to identify the diagnostic CO2 phase changes and determine the quantity of CO2 necessary to result in observable solid CO2 melting. The results of these experiments show that an observable solid CO2 melting event in liquid-rich aqueous inclusions requires PCO2 ≥ 45 bars at 25°C. The Raman spectroscopic detection limits were investigated using a multichannel Raman spectrometer. The CO2 DLs were obtained by determining signal-to-noise ratios for both the upper and lower ν1-2ν2 bands as a function of CO2 pressure (5–60 bars) over a range of integration times and incident laser power. The resulting CO2 DLs are on the order of 1 bar for the instrument used. The band splitting of the ν1-2ν2 diad as a function of CO2 pressure, converted to CO2 density, was measured up to 500 bars at ambient temperature. The results are given in terms of the frequency separation between the upper and lower bands and are compared to results of previous studies. An analysis of the estimated errors indicates that the technique can be used to determine CO2 densities in fluid inclusions containing a homogeneous, free CO2 phase to a precision of approximately ± 0.02 g/cm3. The temperature dependence of the intensity ratio of the hot bands to the ν1-2ν2 diad was measured from 270–315 K. The close agreement between the calculated and observed resultindicates that laser induced sample heating is not significant. The intensity ratio can be used to estimate the CO2 temperature and, combined with the Raman density determination, allows calculation of the CO2 pressure.

Ab Initio Determination of Edge Surface Structures for Dioctahedral 2 : 1 Phyllosilicates: Implications for Acid-Base Reactivity

The atomic structure of dioctahedral 2:1 phyllosilicate edge surfaces was calculated using pseudopotential planewave density functional theory. Bulk structures of pyrophyllite and ferripyrophyllite were optimized using periodic boundary conditions, after which crystal chemical methods were used to obtain initial terminations for ideal (110)- and (010)-type edge surfaces. The edge surfaces were protonated using various schemes to neutralize the surface charge, and total minimized energies were compared to identify which schemes are the most energetically favorable. The calculations show that significant surface relaxation should occur on the (110)-type faces, as well as in response to different protonation schemes on both surface types. This result is consistent with atomic force microscopy observations of phyllosilicate dissolution behavior. Bond-valence methods incorporating bond lengths from calculated structures can be used to predict intrinsic acidity constants for surface functional groups on (110)- and (010)-type edge surfaces. However, the occurrence of surface relaxation poses problems for applying current bond-valence methods. An alternative method is proposed that considers bond relaxation, and accounts for the energetics of various protonation schemes on phyllosilicate edges.

The structure of hematite (α-Fe2O3) (001) surfaces in aqueous media: scanning tunneling microscopy and resonant tunneling calculations of coexisting O and Fe terminations

The iron oxide–water interface is of interest not only in geochemical and environmental processes, but also in fields ranging from corrosion to photocatalysis. The structure of α-Fe2O3 (001) surfaces is not fully understood, and questions have arisen recently concerning different terminations of (001) terraces; a so-called Fe-termination is expected, but under some conditions an O-termination may also be possible. Ultra-high vacuum (UHV) scanning tunneling microscope (STM) studies report evidence for an O-termination in coexistence with an Fe-termination, but other studies find no evidence for an O-termination. Molecular mechanics studies suggest that an O-termination should be possible in an aqueous environment. An O-termination could result from dissolution; if Fe atoms were to dissolve from an Fe-termination, an O-termination would presumably result (and vice-versa). We imaged hematite (001) surfaces in air and aqueous solution using STM. To aid interpretation of the images, we use a resonant tunneling model (RTM) parameterized using ab initio calculations. Our STM and RTM results are consistent with mixed O- and Fe-terminated (001) surfaces. For acid-etched surfaces we find evidence for a periodic (with wavelength of 2.2 ± 0.2 nm) arrangement of nominal O- and Fe-terminated domains. Two different borders between domains should occur, one in which the Fe-termination is high relative to the O-termination, and the reverse. The different domain borders have significantly different heights, consistent with RTM calculations. This agreement allows us to conclude that the Fe-termination is topographically high on most terraces. Surface domains are observed in aqueous solutions at the atomic scale, and appear to be very unreactive on tens-of-seconds time scales at pH 1.

Molecular Underpinnings of Fe(III) Oxide Reduction by Shewanella Oneidensis MR-1.

In the absence of O2 and other electron acceptors, the Gram-negative bacterium Shewanella oneidensis MR-1 can use ferric [Fe(III)] (oxy)(hydr)oxide minerals as the terminal electron acceptors for anaerobic respiration. At circumneutral pH and in the absence of strong complexing ligands, Fe(III) oxides are relatively insoluble and thus are external to the bacterial cells. S. oneidensis MR-1 and related strains of metal-reducing Shewanella have evolved machinery (i.e., metal-reducing or Mtr pathway) for transferring electrons from the inner-membrane, through the periplasm and across the outer-membrane to the surface of extracellular Fe(III) oxides. The protein components identified to date for the Mtr pathway include CymA, MtrA, MtrB, MtrC, and OmcA. CymA is an inner-membrane tetraheme c-type cytochrome (c-Cyt) that belongs to the NapC/NrfH family of quinol dehydrogenases. It is proposed that CymA oxidizes the quinol in the inner-membrane and transfers the released electrons to MtrA either directly or indirectly through other periplasmic proteins. A decaheme c-Cyt, MtrA is thought to be embedded in the trans outer-membrane and porin-like protein MtrB. Together, MtrAB deliver the electrons through the outer-membrane to the MtrC and OmcA on the outmost bacterial surface. MtrC and OmcA are the outer-membrane decaheme c-Cyts that are translocated across the outer-membrane by the bacterial type II secretion system. Functioning as terminal reductases, MtrC and OmcA can bind the surface of Fe(III) oxides and transfer electrons directly to these minerals via their solvent-exposed hemes. To increase their reaction rates, MtrC and OmcA can use the flavins secreted by S. oneidensis MR-1 cells as diffusible co-factors for reduction of Fe(III) oxides. Because of their extracellular location and broad redox potentials, MtrC and OmcA can also serve as the terminal reductases for soluble forms of Fe(III). In addition to Fe(III) oxides, Mtr pathway is also involved in reduction of manganese oxides and other metals. Although our understanding of the Mtr pathway is still far from complete, it is the best characterized microbial pathway used for extracellular electron exchange. Characterizations of the Mtr pathway have made significant contributions to the molecular understanding of microbial reduction of Fe(III) oxides.