The Experts below are selected from a list of 300 Experts worldwide ranked by ideXlab platform
Poul Norby - One of the best experts on this subject based on the ideXlab platform.
-
defect chemistry of a zinc doped Lepidocrocite titanate csxti2 x 2znx 2o4 x 0 7 and its protonic form
Chemistry of Materials, 2009Co-Authors: Helmer Fjellvåg, Poul NorbyAbstract:A zinc-doped layered titanate CsxTi2−x/2Znx/2O4 (x = 0.7) with Lepidocrocite (γ-FeOOH)-type layered structure was prepared via solid-state calcination. A complete extraction of both lattice Zn atoms and interlayer Cs ions was observed upon acid exchange, producing a protonic form H2xTi2−x/2◻x/2O4·H2O that inherited the original Lepidocrocite-type layered structure. This new phase was distinguished from its isomorphous related compounds in terms of high proton content, high charge density, and high defect content as a consequence of extractable Zn atoms located in the host framework. The protonic titanate H2xTi2−x/2◻x/2O4·H2O readily underwent delamination to produce its molecular single sheets Ti1−δ◻δO24δ− (δ = 0.175) with distinctive two-dimensional morphology and small thickness (∼1 nm), suggesting promising applications in the assembly of functional nanostructures.
-
protonic titanate derived from csxti2 x 2mgx 2o4 x 0 7 with Lepidocrocite type layered structure
Journal of Materials Chemistry, 2009Co-Authors: Helmer Fjellvåg, Poul NorbyAbstract:A layered titanate CsxTi2−x/2Mgx/2O4 (x = 0.7) with Lepidocrocite (γ-FeOOH)-type layered structure was prepared via solid-state reaction. Extraction of both Mg2+ ions in the host layers and interlayer Cs+ ions was achieved during an acid-exchange process, producing a new protonic titanate HxTi2−x/2O4−x/2·H2O. This phase was distinguished from isomorphous related compounds in terms of removable lattice Mg and O atoms. The protonic titanate HxTi2−x/2O4−x/2·H2O showed excellent exfoliation/delamination reactivity upon intercalating organic amine ions as well as the ability to produce single two-dimensional titanate nanosheets with small thickness of about 1 nm. These findings offered a new clue for understanding the physicochemical properties of lattice dopants in Lepidocrocite titanates.
-
raman scattering properties of a protonic titanate hxti2 x 4 x 4o4 h2o vacancy x 0 7 with Lepidocrocite type layered structure
Journal of Physical Chemistry B, 2008Co-Authors: Helmer Fjellvåg, Poul NorbyAbstract:Raman scattering spectroscopy is employed to characterize a layered titanate HxTi2-x/4◻x/4O4·H2O (◻: vacancy; x = 0.7) with Lepidocrocite (γ-FeOOH)-type layered structure. Nine Raman lines corresponding to (3Ag + 3B1g + 3B3g) Raman-active modes expected from this orthorhombic structure (space group D2h25-Immm) are recorded at 183, 270, 387, 449, 558, 658, 704, 803, and 908 cm−1, which are assigned to Ti−O lattice vibrations within the two-dimensional (2D) Lepidocrocite-type TiO6 octahedral host layers. These intrinsic Raman bands present a clear signature that can be used for probing the protonic titanate HxTi2-x/4◻x/4O4·H2O and the 2D titanate nanosheets, as well as their corresponding derivates.
-
Raman scattering properties of a protonic titanate HxTi2-x/4[symbol: see text]x/4O4.H2O ([symbol: see text], vacancy; x=0.7) with Lepidocrocite-type layered structure.
The journal of physical chemistry. B, 2008Co-Authors: Tao Gao, Helmer Fjellvåg, Poul NorbyAbstract:Raman scattering spectroscopy is employed to characterize a layered titanate HxTi2-x/4[symbol: see text]x/4O4.H2O ([symbol: see text]: vacancy; x=0.7) with Lepidocrocite (gamma-FeOOH)-type layered structure. Nine Raman lines corresponding to (3Ag+3B1g+3B3g) Raman-active modes expected from this orthorhombic structure (space group D2h25-Immm) are recorded at 183, 270, 387, 449, 558, 658, 704, 803, and 908 cm(-1), which are assigned to Ti-O lattice vibrations within the two-dimensional (2D) Lepidocrocite-type TiO6 octahedral host layers. These intrinsic Raman bands present a clear signature that can be used for probing the protonic titanate HxTi2-x/4[symbol: see text]x/4O4.H2O and the 2D titanate nanosheets, as well as their corresponding derivates.
Helmer Fjellvåg - One of the best experts on this subject based on the ideXlab platform.
-
defect chemistry of a zinc doped Lepidocrocite titanate csxti2 x 2znx 2o4 x 0 7 and its protonic form
Chemistry of Materials, 2009Co-Authors: Helmer Fjellvåg, Poul NorbyAbstract:A zinc-doped layered titanate CsxTi2−x/2Znx/2O4 (x = 0.7) with Lepidocrocite (γ-FeOOH)-type layered structure was prepared via solid-state calcination. A complete extraction of both lattice Zn atoms and interlayer Cs ions was observed upon acid exchange, producing a protonic form H2xTi2−x/2◻x/2O4·H2O that inherited the original Lepidocrocite-type layered structure. This new phase was distinguished from its isomorphous related compounds in terms of high proton content, high charge density, and high defect content as a consequence of extractable Zn atoms located in the host framework. The protonic titanate H2xTi2−x/2◻x/2O4·H2O readily underwent delamination to produce its molecular single sheets Ti1−δ◻δO24δ− (δ = 0.175) with distinctive two-dimensional morphology and small thickness (∼1 nm), suggesting promising applications in the assembly of functional nanostructures.
-
protonic titanate derived from csxti2 x 2mgx 2o4 x 0 7 with Lepidocrocite type layered structure
Journal of Materials Chemistry, 2009Co-Authors: Helmer Fjellvåg, Poul NorbyAbstract:A layered titanate CsxTi2−x/2Mgx/2O4 (x = 0.7) with Lepidocrocite (γ-FeOOH)-type layered structure was prepared via solid-state reaction. Extraction of both Mg2+ ions in the host layers and interlayer Cs+ ions was achieved during an acid-exchange process, producing a new protonic titanate HxTi2−x/2O4−x/2·H2O. This phase was distinguished from isomorphous related compounds in terms of removable lattice Mg and O atoms. The protonic titanate HxTi2−x/2O4−x/2·H2O showed excellent exfoliation/delamination reactivity upon intercalating organic amine ions as well as the ability to produce single two-dimensional titanate nanosheets with small thickness of about 1 nm. These findings offered a new clue for understanding the physicochemical properties of lattice dopants in Lepidocrocite titanates.
-
raman scattering properties of a protonic titanate hxti2 x 4 x 4o4 h2o vacancy x 0 7 with Lepidocrocite type layered structure
Journal of Physical Chemistry B, 2008Co-Authors: Helmer Fjellvåg, Poul NorbyAbstract:Raman scattering spectroscopy is employed to characterize a layered titanate HxTi2-x/4◻x/4O4·H2O (◻: vacancy; x = 0.7) with Lepidocrocite (γ-FeOOH)-type layered structure. Nine Raman lines corresponding to (3Ag + 3B1g + 3B3g) Raman-active modes expected from this orthorhombic structure (space group D2h25-Immm) are recorded at 183, 270, 387, 449, 558, 658, 704, 803, and 908 cm−1, which are assigned to Ti−O lattice vibrations within the two-dimensional (2D) Lepidocrocite-type TiO6 octahedral host layers. These intrinsic Raman bands present a clear signature that can be used for probing the protonic titanate HxTi2-x/4◻x/4O4·H2O and the 2D titanate nanosheets, as well as their corresponding derivates.
-
Raman scattering properties of a protonic titanate HxTi2-x/4[symbol: see text]x/4O4.H2O ([symbol: see text], vacancy; x=0.7) with Lepidocrocite-type layered structure.
The journal of physical chemistry. B, 2008Co-Authors: Tao Gao, Helmer Fjellvåg, Poul NorbyAbstract:Raman scattering spectroscopy is employed to characterize a layered titanate HxTi2-x/4[symbol: see text]x/4O4.H2O ([symbol: see text]: vacancy; x=0.7) with Lepidocrocite (gamma-FeOOH)-type layered structure. Nine Raman lines corresponding to (3Ag+3B1g+3B3g) Raman-active modes expected from this orthorhombic structure (space group D2h25-Immm) are recorded at 183, 270, 387, 449, 558, 658, 704, 803, and 908 cm(-1), which are assigned to Ti-O lattice vibrations within the two-dimensional (2D) Lepidocrocite-type TiO6 octahedral host layers. These intrinsic Raman bands present a clear signature that can be used for probing the protonic titanate HxTi2-x/4[symbol: see text]x/4O4.H2O and the 2D titanate nanosheets, as well as their corresponding derivates.
Georges Onanguema - One of the best experts on this subject based on the ideXlab platform.
-
constraining the origins of the magnetism of Lepidocrocite γ feooh a mossbauer and magnetization study
Frontiers in Earth Science, 2016Co-Authors: Yohan Guyodo, P Bonville, J L Till, Georges Onanguema, Nicolas MenguyAbstract:Lepidocrocite, a widespread environmentally relevant iron oxyhydroxide, has been investigated for decades using 57Fe Mossbauer spectroscopy and magnetic measurements. However, a coherent and comprehensive interpretation of all the data is still lacking due to seemingly contradictory interpretations. On one hand, temperature dependence of magnetic susceptibility and Mossbauer spectra resemble those of superparamagnetic nanoparticles with diameters less than 10 nm even though physically particles are lath-shaped with lengths on the order of 100-300 nm. On the other hand, in-field Mossbauer spectra show that Lepidocrocite is an antiferromagnet and becomes paramagnetic above 50-70 K, a temperature close to the blocking temperature deduced from susceptibility data. The present study investigates a well-characterized synthetic sample of Lepidocrocite, includes modelling of Mossbauer spectra and dc and ac magnetization data, and proposes a solution to this paradox. The new data are coherent with the presence of two entities in Lepidocrocite: a bulk antiferromagnetic matrix and sparse ferrimagnetic nanosized inclusions (d = 3.4 nm), akin to maghemite, embedded within. The presence of nanosized ferrimagnetic inclusions is confirmed for the first time by Mossbauer spectroscopy.
-
arsenic iii and arsenic v speciation during transformation of Lepidocrocite to magnetite
Environmental Science & Technology, 2014Co-Authors: Yuheng Wang, Georges Onanguema, Guillaume Morin, Gordon E BrownAbstract:Bioreduction of As(V) and As-bearing iron oxides is considered to be one of the key processes leading to arsenic pollution in groundwaters in South and Southeast Asia. Recent laboratory studies with simple aqueous media showed that secondary Fe(II)-bearing phases (e.g., magnetite and green rust), which commonly precipitate during bioreduction of iron oxides, captured arsenic species. The aim of the present study was to follow arsenic speciation during the abiotic Fe(II)-induced transformation of As(III)- and As(V)-doped Lepidocrocite to magnetite, and to evaluate the influence of arsenic on the transformation kinetics and pathway. We found green rust formation is an intermediate phase in the transformation. Both As(III) and As(V) slowed the transformation, with the effect being greater for As(III) than for As(V). Prior to the formation of magnetite, As(III) adsorbed on both Lepidocrocite and green rust, whereas As(V) associated exclusively with green rust, When magnetite precipitated, As(III) formed surfa...
-
magnetic comparison of abiogenic and biogenic alteration products of Lepidocrocite
Earth and Planetary Science Letters, 2014Co-Authors: Yohan Guyodo, J L Till, Georges Onanguema, F Lagroix, J BrestAbstract:Abstract Lepidocrocite is a potentially important Fe-bearing precursor phase for the production of nanoscale Fe-oxide particles in the environment. We present a detailed magnetic characterization of various alteration products of Lepidocrocite resulting from thermal dehydroxylation reactions and bacterially induced bioreduction and remineralization, accompanied by characterization with x-ray diffraction (XRD) and transmission electron microscopy. Dehydroxylation during annealing at moderate temperatures produces a topotactic transformation from Lepidocrocite to maghemite when heated in an oxidizing atmosphere, or to magnetite when heated in a reducing atmosphere. The abiotic Fe-oxide products form an oriented framework of strongly interacting superparamagnetic crystallites and are characterized by a distinctive porous nanostructure observed by electron microscopy. Lepidocrocite bioreduction by the iron-reducing bacterium Shewanella putrefaciens ATCC 8071 produces nanoscale particles of a strongly magnetic phase. This Fe(II)-bearing mineral produced by bioreduction is highly crystalline and euhedral in shape, with a broad grain size distribution and is indicated by magnetic and XRD measurements to be a cation-excess magnetite. We highlight the distinguishing microscopic characteristics of magnetite from both abiotic and bacterially induced mineralization that should allow them to be identified in natural settings. Moreover, both mechanisms of alteration represent potential pathways for the direct formation of strongly magnetic fine-grained Fe-oxide particles in sedimentary environments.
-
arsenite sequestration at the surface of nano fe oh 2 ferrous carbonate hydroxide and green rust after bioreduction of arsenic sorbed Lepidocrocite by shewanella putrefaciens
Geochimica et Cosmochimica Acta, 2009Co-Authors: Georges Onanguema, Nicolas Menguy, Guillaume Morin, Yuheng Wang, Farid Juillot, Luca Olivi, Giuliana Aquilanti, Mustapha Abdelmoula, Christian RubyAbstract:X-ray Absorption Fine Structure (XAFS) spectroscopy was used in combination with high resolution transmission elec- tron microscopy (HRTEM), electron energy loss spectroscopy (EELS), X-ray energy dispersive spectroscopy (XEDS), X- ray powder diffraction, and Mo ssbauer spectroscopy to obtain detailed information on arsenic and iron speciation in the products of anaerobic reduction of pure and As(V)- or As(III)-adsorbed Lepidocrocite (c-FeOOH) by Shewanella putrefaciens ATCC 12099. We found that this strain of S. putrefaciens is capable of using Fe(III) in Lepidocrocite and As(V) in solution or adsorbed on Lepidocrocite surfaces as electron acceptors. Bioreduction of Lepidocrocite in the absence of arsenic resulted in the formation of hydroxycarbonate green rust 1 [FeII4FeIII2(OH)12CO3: GR1(CO3)], which completely converted into ferrous-car- bonate hydroxide (FeII2(OH)2CO3: FCH) over nine months. This study thus provides the first evidence of bacterial reduction of stoichiometric GR1(CO3) into FCH. Bioreduction of As(III)-adsorbed Lepidocrocite also led to the formation of GR1(CO3) prior to formation of FCH, but the presence of As(III) slows down this transformation, leading to the co-occur- rence of both phases after 22-month of aging. At the end of this experiment, As(III) was found to be adsorbed on the surfaces of GR1(CO3) and FCH. After five months, bioreduction of As(V)-bearing Lepidocrocite led directly to the formation of FCH in association with nanometer-sized particles of a minor As-rich Fe(OH)2 phase, with no evidence for green rust formation. In this five-month experiment, As(V) was fully converted to As(III), which was dominantly sorbed at the surface of the Fe(OH)2 nanoparticles as oligomers binding to the edges of Fe(OH)6 octahedra at the edges of the octahedral layers of Fe(OH)2. These multinuclear As(III) surface complexes are characterized by As–As pairs at a distance of 3.32 ± 0.02 A and by As–Fe pairs at a distance of 3.50 ± 0.02 A and represent a new type of As(III) surface complex. Chemical analyses show that the majority of As(III) produced in the experiments with As present is associated with iron-bearing hydroxycarbonate or hydroxide solids,
-
exafs analysis of arsenite adsorption onto two line ferrihydrite hematite goethite and Lepidocrocite
Environmental Science & Technology, 2005Co-Authors: Georges Onanguema, Guillaume Morin, Farid Juillot, Georges Calas, Gordon E BrownAbstract:The modes of As(III) sorption onto two-line ferrihydrite (Fh), hematite (Hm), goethite (Gt), and Lepidocrocite (Lp) have been investigated under anoxic condition using X-ray absorption spectroscopy (XAS). X-ray absorption near-edge structure spectroscopy (XANES) indicates that the absence of oxygen minimized As(III) oxidation due to Fenton reactions. Extended X-ray absorption fine structure spectroscopy (EXAFS) indicates that As(III) forms similar inner-sphere surface complexes on two-line ferrihydrite and hematite that differ from those formed on goethite and Lepidocrocite. At high surface coverage, the dominant complex types on Fh and Hm are bidentate mononuclear edge-sharing (2E) and bidentate binuclear corner-sharing (2C), with As−Fe distances of 2.90 ± 0.05 and 3.35 ± 0.05 A, respectively. The same surface complexes are observed for ferrihydrite at low surface coverage. In contrast, As(III) forms dominantly bidentate binuclear corner-sharing (2C) sorption complexes on Gt and Lp [d(As−Fe) = 3.3−3.4 A]...
Andreas U Gehring - One of the best experts on this subject based on the ideXlab platform.
-
low temperature magnetic properties of Lepidocrocite
Journal of Geophysical Research, 2002Co-Authors: Ann M Hirt, L Lanci, J Dobson, Peter G Weidler, Andreas U GehringAbstract:[1] The magnetic properties of synthetic Lepidocrocite (γ-FeOOH) were investigated between 5 and 300 K. The purity of the samples was verified by X-ray diffraction and Mossbauer spectra at room temperature. The Lepidocrocite is paramagnetic at room temperature with a low field susceptibility of 57.8 × 10−8 m3 kg−1. It undergoes magnetic ordering at low temperature and a peak in in-phase susceptibility occurs around 52 K, where it is antiferromagnetic. Thermal demagnetization of low-temperature remanence after cooling in zero field and in a 2.5-T field display a large loss in remanence from 5 to 30 K. Hysteresis loops are open and symmetric about the origin below 50 K; the remanent magnetization probably arises from a defect magnetic moment. The field-cooled hysteresis loop at 5 K is shifted along the field axis, which implies an exchange bias between antiferromagnetic and ferrimagnetic alignment in the Lepidocrocite.
-
Low‐temperature magnetic properties of Lepidocrocite
Journal of Geophysical Research, 2002Co-Authors: Ann M Hirt, L Lanci, J Dobson, Peter G Weidler, Andreas U GehringAbstract:[1] The magnetic properties of synthetic Lepidocrocite (γ-FeOOH) were investigated between 5 and 300 K. The purity of the samples was verified by X-ray diffraction and Mossbauer spectra at room temperature. The Lepidocrocite is paramagnetic at room temperature with a low field susceptibility of 57.8 × 10−8 m3 kg−1. It undergoes magnetic ordering at low temperature and a peak in in-phase susceptibility occurs around 52 K, where it is antiferromagnetic. Thermal demagnetization of low-temperature remanence after cooling in zero field and in a 2.5-T field display a large loss in remanence from 5 to 30 K. Hysteresis loops are open and symmetric about the origin below 50 K; the remanent magnetization probably arises from a defect magnetic moment. The field-cooled hysteresis loop at 5 K is shifted along the field axis, which implies an exchange bias between antiferromagnetic and ferrimagnetic alignment in the Lepidocrocite.
-
the transformation of Lepidocrocite during heating a magnetic and spectroscopic study
Clays and Clay Minerals, 1994Co-Authors: Andreas U Gehring, A M HofmeisterAbstract:Infrared (IR) spectroscopy, in combination with magnetic methods, was used to study the thermally induced transformation of synthetic Lepidocrocite (3,-FeOOH) to maghemite (~-Fe203). Mag- netic analyses showed that the thermal conversion began at about 175"C with the formation of super- paramagnetic maghemite clusters. The overall structural transformation to ferrimagnetic'y-Fe203 occurred at 200"C and was complete around 300"C. At higher temperatures, the maghemite converted into hematite (a-Fe203). Observation of the transformation from 3,-FeOOH to "y-Fe203 using variable-temperature IR spectroscopy indicated that dehydroxilation on a molecular level was initiated between 145"C and 155*C. The lag time between the onset of the breaking of OH bonds and the release of H20 from Lepidocrocite around 1750C can be explained by diffusive processes. Overall dehydroxilation and the subsequent break- down of the Lepidocrocite structure was complete below 219*(3. The comparison of the magnetic and IR data provides evidence that the dehydroxilation may precede the structural conversion to maghemite.
Paul Fenter - One of the best experts on this subject based on the ideXlab platform.
-
templating growth of a pseudomorphic Lepidocrocite microshell at the calcite water interface
Chemistry of Materials, 2018Co-Authors: Ke Yuan, Jun Wang, Neil C Sturchio, Paul FenterAbstract:The growth of Lepidocrocite (γ-FeOOH) has been observed through oxidation of Fe(II) on calcite (CaCO3). Here, we seek to understand the structural relation between Lepidocrocite and the calcite substrate and its growth mechanism. The formation of iron oxyhydroxide layers having distinct morphologies was observed during the dissolution of calcite in acidic Fe(II)-rich solutions. A pseudomorphic Lepidocrocite shell together with multiple iron oxyhydroxide layers encapsulated within the shell was imaged by optical and transmission X-ray microscopies. The presence of a several-nanometer-thick ordered Lepidocrocite film was observed by X-ray reflectivity, with the Lepidocrocite (100) plane oriented parallel to the calcite (104) surface. Lath-shaped Lepidocrocite aggregates formed during the initial precipitation, which eventually grew into clusters of parallel platy crystals. The formation of a nanometer-thick well-ordered Lepidocrocite film on a pristine calcite surface appears critical for the subsequent pse...
-
Templating Growth of a Pseudomorphic Lepidocrocite Microshell at the Calcite–Water Interface
Chemistry of Materials, 2018Co-Authors: Ke Yuan, Jun Wang, Neil C Sturchio, Paul FenterAbstract:The growth of Lepidocrocite (γ-FeOOH) has been observed through oxidation of Fe(II) on calcite (CaCO3). Here, we seek to understand the structural relation between Lepidocrocite and the calcite substrate and its growth mechanism. The formation of iron oxyhydroxide layers having distinct morphologies was observed during the dissolution of calcite in acidic Fe(II)-rich solutions. A pseudomorphic Lepidocrocite shell together with multiple iron oxyhydroxide layers encapsulated within the shell was imaged by optical and transmission X-ray microscopies. The presence of a several-nanometer-thick ordered Lepidocrocite film was observed by X-ray reflectivity, with the Lepidocrocite (100) plane oriented parallel to the calcite (104) surface. Lath-shaped Lepidocrocite aggregates formed during the initial precipitation, which eventually grew into clusters of parallel platy crystals. The formation of a nanometer-thick well-ordered Lepidocrocite film on a pristine calcite surface appears critical for the subsequent pse...
