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  • 1.
    Lucas, Marie
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Boily, Jean-Francois
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Mapping Electrochemical Heterogeneity at Iron Oxide Surfaces: A Local Electrochemical Impedance Study2015In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 31, no 50, p. 13618-13624Article in journal (Refereed)
    Abstract [en]

    Alternating current scanning electrochemical microscopy (AC-SECM) was used for the first time to map key electrochemical attributes of oriented hematite (alpha-Fe2O3) single crystal surfaces at the micron-scale. Localized electrochemical impedance spectra (LEIS) of the (001) and (012) faces provided insight into the spatial variations of local double layer capacitance (C-dl) and charge transfer resistance (R-ad). These parameters were extracted by LEIS measurements in the 0.4-8000 Hz range to probe the impedance response generated by the redistribution of water molecules and charge carriers (ions) under an applied AC. These were attributed to local variations in the local conductivity of the sample surfaces. Comparison with global EIS measurements on the same Samples uncovered highly comparable frequency-resolved processes, that were broken: down into contributions fromthe bulk hematite, the interface as well as the microelectrode/tip :assembly. This work paves the way for new studies aimed at mapping electrochemical processes at the mesoscale on this environmentally and technologically important material.

  • 2.
    Lucas, Marie
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Boily, Jean-François
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Electrochemical Response of Bound Electrolyte Ions at Oriented Hematite Surfaces: A Local Electrochemical Impedance Spectroscopy Study2017In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 121, no 50, p. 27976-27982Article in journal (Refereed)
    Abstract [en]

    The electrochemical response of millimeter-sized hematite (α-Fe2O3) electrode surfaces to bound ions of NaCl, NH4Cl, and NaHCO3 salts was monitored by alternating current scanning electrochemical microscopy (AC-SECM). Local electrochemical impedance spectroscopy (LEIS) measurements along 100 μm lines on the (001) and (012) faces of hematite were used to extract capacitance and resistance parameters affected by bound inorganic ions. Equivalent circuit modeling was used to suggest that (1) double layer capacitances are affected by the spatial distribution of ions, and that (2) compact plane capacitance and resistance are affected by the closeness of association of ions to surface hydroxo groups. This study confirms the sensitivity of the technique to electrolyte ion binding, and provides new and key insight into the micrometer-scale electrochemical properties of iron oxides exposed to environmentally relevant conditions.

  • 3.
    Lucas, Marie
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Yeşilbaş, Merve
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Shchukarev, Andrey
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Boily, Jean-Francois
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    X-ray photoelectron spectroscopy of fast-Frozen hematite colloids in aqueous solutions. 6. Sodium halide (F–, Cl–, Br–, I–) ion binding on microparticles2018In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 34, no 45, p. 13497-13504Article in journal (Refereed)
    Abstract [en]

    Electrolyte ion binding at mineral surfaces is central to the generation of surface charge and key to electric double-layer formation. X-ray photoelectron spectroscopy of fast-frozen (−170 °C) mineral wet pastes provides a means to study weakly bound electrolyte ions at the mineral/water interface. In this study, we build upon a series of articles devoted to ion binding at hematite (α-Fe2O3) particle surfaces to resolve the nature of sodium halide ion binding. Measurements on micron-sized hematite particles terminated by the charged and amphoteric (012) and the relatively uncharged (001) faces point to the formation of salt loadings of similar composition to those of cryosalts of NaCl, NaBr, NaI, and NaF. These coatings could be likened to those of the better-known hydrohalite (NaCl·2H2O) phase, one that typically forms under concentrated (≫0.1 M) aqueous solutions of NaCl under freezing conditions. As we have previously shown that these reaction products do not occur in nanosized hematite particles, our work points to the involvement of the basal (001) face and/or the juxtaposition of these faces in packed tabular microparticles of hematite (1–3 μm in width) in stabilizing these cryosalts. One possible formation pathway involves first-layer Na+ and Cl– ions serving as an anchoring layer for a topotactic-like growth of amorphous to low-crystalline salt hydrates at the (001) face. Thus, by contrasting reaction products of four sodium halides at surfaces of tabular microparticles of hematite, this work revealed the formation of cryosalt-like solids. The formation of such solids may have especially important ramifications to ice nucleation mechanisms in the atmosphere, as well as in saline permafrosts on Earth and on planet Mars where salt-laden mineral particles prevail.

  • 4.
    Shimizu, Kenichi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry. Physical and Theoretical Chemistry Laboratory, Department of Chemistry, University of Oxford, UK.
    Driver, Gordon W.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Lucas, Marie
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Sparrman, Tobias
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Shchukarev, Andrey
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Boily, Jean-Francois
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Bifluoride ([HF2](-)) formation at the fluoridated aluminium hydroxide/water interface2016In: Dalton Transactions, ISSN 1477-9226, E-ISSN 1477-9234, Vol. 45, no 22, p. 9045-9050Article in journal (Refereed)
    Abstract [en]

    This study uncovers bifluoride-type (difluorohydrogenate(I); [HF2](-)) species formed at mineral/water interfaces. Bifluoride forms at equivalent to Al-F surface sites resulting from the partial fluoridation of gibbsite (gamma-Al(OH3)) and bayerite (alpha-Al(OH3)) particles exposed to aqueous solutions of 50 mM NaF. Fluoride removal from these solutions is proton-promoted and results in a strongly self-buffered suspensions at circumneutral pH, proceeds at a F : H consumption ratio of 2 : 1, and with recorded losses of up to 17 mM fluoride (58 F nm(-2)). These loadings exceed crystallographic site densities by a factor of 3-4, yet the reactions have no resolvable impact on particle size, shape and mineralogy. X-ray photoelectron spectroscopy (XPS) of frozen (-155 degrees C) wet mineral pastes revealed coexisting surface F- and HF0 species. Electron energy loss features pointed to multilayer distribution of these species at the mineral/water interface. XPS also uncovered a distinct form of Na+ involved in binding fluoride-bearing species. XPS and solid state magic angle spinning F-19 nuclear magnetic resonance measurements showed that these fluoride species were highly comparable to a sodium-bifluoride (NaHF2) reference. First layer surface species are represented as =Al-F-H-F-Al= and =Al-F-Na-F-Al=, and may form multi-layered species into the mineral/water interface. These results consequently point to a potentially overlooked inorganic fluorine species in a technologically relevant mineral/water interfacial systems.

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