No abstract

Platelets of strontium hexaferrite (SrFe 12 O 19 , SFO), up to several micrometers in width, and tens of nanometers thick have been synthesized by a hydrothermal method. They have been studied by a combination of structural and magnetic techniques, with emphasis on Mössbauer spectroscopy and X-ray absorption based-measurements including spectroscopy and microscopy on the iron-L edges and the oxygen-K edge, allowing us to establish the differences and similarities between our synthesized nanostructures and commercial powders. The Mössbauer spectra reveal a greater contribution of iron tetrahedral sites in platelets in comparison to pure bulk material. For reference, high-resolution absorption and dichroic spectra have also been measured both from the platelets and from pure bulk material. The O-K edge has been reproduced by density functional theory calculations. Out-of-plane domains were observed with 180° domain walls less than 20 nm width, in good agreement with micromagnetic simulations.

We have demonstrated that the annealing process for cleaning pyrite surfaces is a critical parameter in promoting ordering on the surface and driving surface reactivity. Furthermore, we describe a spectroscopic surface characterization of the presence or absence of the surface ordering, as indicated by the Low Energy Electron Diffraction (LEED) pattern, as a function of the surface annealing process. Complementary X-ray photoemission spectroscopy (XPS) results provide evidence that longer annealing processes of over 3 hours repair the sulfur vacancies in the pyrite, making FeS species partially disappear in favor of FeS species. These features play an important role in molecular adsorption. We show that in the case of the cystine amino acid on the (100) pyrite surface, the substrate structure is responsible for the chemical adsorption form. The presence of an ordered structure on the surface, as indicated by the LEED pattern, favors the cystine NH chemical form, whereas the absence of the surface ordering promotes cystine NH adsorption due to the sulfur-deficient surface. The cystine molecule could then act by changing its chemical functionalities to compensate for the iron surface coordination. The chemical molecular adsorption form can be selected by the surface annealing conditions, implying that environmental conditions could drive molecular adsorption on mineral surfaces. These findings are relevant in several surface processes, and they could play a possible role in prebiotic chemistry surface reactions and iron-sulfur scenarios.

A novel thermochemical process based on a single step was optimized to obtain magnetic activated carbons from an industrial biomass waste. Anhydrous iron chloride was used as an activating agent and mixed directly with the chestnut shell waste. The effect of the activation temperature (220–800 °C) on the chemical, morphological, textural, and magnetic properties of the materials was studied. The results demonstrated the presence of different iron compounds depending on the activation temperature set as well as their influence on morphological and textural development of the magnetic activated carbons (BET specific surface area, S BET, up to 568 m2 g–1, total pore volume, V TOT, up to 0.294 cm3 g–1 vs 1 m2 g–1 and 0.007 cm3 g–1, respectively, for the raw biomass waste). The techniques employed, especially Mössbauer spectroscopy, showed relative contributions of the different iron compounds (magnetite, maghemite, metallic iron, and so on) in the materials. The higher activation temperature (800 °C) favored the formation of metal Fe and iron carbide. Additionally, the magnetic properties measured by vibrating sample magnetometry confirmed the coexistence of different ferromagnetic phases with the remanent magnetization, M R, (up to 3.88 emu/g) and coercivity, H c, (up to 140 Oe), being larger as the activation temperature increases. A higher activation temperature favored the development and evolution toward other iron compounds, while at low temperature, 220 °C, the presence of these compounds were null, and their behavior resembled the results obtained for the original biomass waste.