Alternating current scanning electrochemical microscopy (AC-SECM) was used for the first time to map key electrochemical attributes of oriented hematite (α-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 from the 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.

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.

This study uncovers bifluoride-type (difluorohydrogenate(i); [HF2](-)) species formed at mineral/water interfaces. Bifluoride forms at [triple bond, length as m-dash]Al-F surface sites resulting from the partial fluoridation of gibbsite (γ-Al(OH3)) and bayerite (α-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 °C) wet mineral pastes revealed coexisting surface F(-) and HF(0) 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 (19)F 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 [triple bond, length as m-dash]Al-F-H-F-Al[triple bond, length as m-dash] and [triple bond, length as m-dash]Al-F-Na-F-Al[triple bond, length as m-dash], 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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Gramsci lecteur des encycliquesL'Église de Pie XI et l'« État intégral » dans le Cahier 6Au XX e siècle, l'Église catholique est considérablement affaiblie. Cependant, Gramsci constate qu'elle peut encore se prévaloir en Italie d'une assise sociale et d'une expérience ancestrale de l'hégémonie dont un parti politique moderne peut tirer enseignement. La lecture attentive qu'il consacre, dans ses Cahiers de prison, aux dernières encycliques, lui permet de ne pas être dupe de l'apolitisme affiché dans les Accords du Latran. L'importance de cette lecture des lettres du pape et de la presse catholique apparaît si on la rapporte au tournant théorique du Cahier 6, lieu d'émergence des principales catégories politiques gramsciennes. Gramsci y élabore en effet sa théorie de l'« État intégral » et de l'« hégémonie » au terme d'une confrontation avec la théologie politique (du pape Pie XI et de l'appareil d'hégémonie qu'elle ordonne).Gramsci, reader of encyclicals.