| Abstract
| - The surface state of carbon nanotubes-Fe-alumina nanocomposite powders was studied by transmission andintegral low-energy electron Mössbauer 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 themaximum temperature (800−1070 °C) were investigated, demonstrating that integral low-energy electronMössbauer spectroscopy is a promising tool complementing transmission Mössbauer spectroscopy for theinvestigation 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 correlatedto their location in the material. In particular, much information was derived for the powders prepared byusing a moderate reduction temperature (800, 850, and 910 °C), for which the transmission and integrallow-energy electron Mössbauer spectra are markedly different. Indeed, α-Fe and Fe3C were not observed assurface species, while γ-Fe−C is present at the surface and in the bulk in the same proportion independentof the temperature of preparation. This could show that most of the nanoparticles (detected as Fe3C and/orγ-Fe−C) that contribute to the formation of carbon nanotubes are located in the outer porosity of the material,as opposed to the topmost (ca. 5 nm) surface. For the higher reduction temperatures Tr of 990 °C and1070 °C, all Fe and Fe-carbide particles formed during the reduction are distributed evenly in the bulk andthe surface of the matrix grains. The integral low-energy electron Mössbauer spectroscopic study of a powderoxidized in air at 600 °C suggests that all Fe3C particles oxidize to α-Fe2O3, while the α-Fe and/or γ-Fe−Care partly transformed to Fe1-xO and α-Fe2O3, the latter phase forming a protecting layer that prevents totaloxidation.
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