Integral Low-energy Electron Mossbauer Spectroscopy Research Articles

Surface effects in α-Fe2O3 nanoparticles studied by ILEEMS and TMS

To understand the particular properties of thin α-Fe2O3 films, hematite nanoparticles with sizes between 20 and 140 nm have been examined by integral low-energy electron Mossbauer spectroscopy (ILEEMS) and transmission Mossbauer spectroscopy (TMS) at room temperature and 80 K. By comparing the results of both Mossbauer variants, surface effects of the Morin transition are studied. The results clearly indicate that the Morin transition takes place at higher temperature in the interior of the particles as compared to the outer layers. It is found that the lowering of the Morin transition is much stronger at the surface of smaller particles.

Integral low-energy electron Mössbauer spectroscopic studies of the surfaces of carbon nanotube-nanocomposite powders

The surface state of CNTs-Fe-Al2O3 and CNTs-Fe-MgAl2O4 nanocomposite powders was studied by integral low-energy electron Mossbauer spectroscopy (ILEEMS). Several samples, prepared by reduction of α-(Al,Fe)2O3 or (Mg,Fe)Al2O4 precursors in a H2-CH4 atmosphere, were investigated, demonstrating that ILEEMS is a promising tool completing transmission Mossbauer spectroscopy for the investigation of the metal Fe and iron-carbide particles in the different carbon nanotube systems.

Presence of Metallic Fe Nanoclusters in α-(Al,Fe)2O3 Solid Solutions

Powders of R-(Al1-xFex)2O3 solid solutions prepared by the calcination in air of the corresponding γ-(Al1- xFex)2O3 powders were studied by several techniques including X-ray diffraction, field-emission-gun scanning electron microscopy, transmission Mossbauer spectroscopy, integral low-energy electron Mossbauer spectroscopy (ILEEMS), and Fe K-edge X-ray absorption near-edge structure (XANES) measurements. The asymmetry of the characteristic Mo¨ssbauer doublet representing Fe3+ ions substituting for Al3+ ions in the corundum lattice of R-(Al1-xFex)2O3 solid solutions was resolved and explained for the first time by using two additional subspectra, i.e., a broad second doublet characteristic of a very distorted octahedral site for Fe3+ and a singlet attributable to R-Fe, suggesting the presence of metallic iron nanoclusters consisting of only a few number of atoms within the solid solution grains. ILEEMS studies showed that the Fe nanoclusters are evenly distributed among the surface layers and the cores of the grains. Fe K-edge XANES measurements further confirmed the occurrence of metallic iron. The proportion of Fe nanoclusters increases when the total iron content is decreased, as does the proportion of distorted octahedral site, suggesting that they are located around the iron nanoclusters. The formation of the metallic Fe nanoclusters in the R-(Al1-xFex)2O3 grains is thought to be a consequence of the γ f R phase transition which implies structural rearrangement on both the cationic and anionic sublattices.

Surface Composition of Carbon Nanotubes-Fe-Alumina Nanocomposite Powders: An Integral Low-Energy Electron Mössbauer Spectroscopic Study

The surface state of carbon nanotubes-Fe-alumina nanocomposite powders was studied by transmission and integral low-energy electron Mossbauer spectroscopy. Several samples, prepared under reduction of the α-Al1.8Fe0.2O3 precursor in a H2−CH4 atmosphere applying the same heating and cooling rate and changing only the maximum temperature (800−1070 °C) were investigated, demonstrating that integral low-energy electron Mossbauer spectroscopy is a promising tool complementing transmission Mossbauer spectroscopy for the investigation of the location of the metal Fe and iron-carbide particles in the different carbon nanotube−nanocomposite systems containing iron. The nature of the iron species (Fe3+, Fe3C, α-Fe, γ-Fe−C) is correlated to their location in the material. In particular, much information was derived for the powders prepared by using a moderate reduction temperature (800, 850, and 910 °C), for which the transmission and integral low-energy electron Mossbauer spectra are markedly different. Indeed, α-F...

ILEEMS: Methodology and Applications to Iron Oxides

ILEEMS is the acronym for Integral Low-energy Electron Mossbauer Spectroscopy. In this variant of Mossbauer spectroscopy the low-energy electrons, E < ∼15 eV, emitted by the probe nuclei in the absorber are counted as a function of source velocity. As a consequence of their low energy, the detected electrons’ origin lies within a very thin surface layer with thickness of a few nanometers and consequently, ILEEMS is a useful technique to examine surfaces of Fecontaining substances. In a first part of this paper the authors briefly describe the design of a homemade ILEEMS equipment allowing the temperature of the investigated sample to be varied between 77 K and room temperature. The second part of this contribution deals with a selection of results obtained from ILEEMS spectra for various Fe oxides. In particular, the following items are covered: (1) surface versus bulk Morin transition in small-particle and near-bulk hematite, α-Fe2O3; (2) bulk and thin-film magnetite, Fe3O4; (3) ferrihydrite, 5Fe2O3 · 9H20, goethite, α-FeOOH, and lepidocrocite, γ-FeOOH, all to some extent in relation to their morphology. Interesting and intriguing findings concerning the surface properties of these oxides were obtained and it is argued that the results encourage more systematic research in this and related fields using the surface-sensitive ILEEMS technique.