Surface composition of carbon nanotubes-Fe-alumina nanocomposite powders: An integral low-energy electron Mossbauer 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 alpha-Al-1.8-Fe0.2O3 precursor in a H-2-CH4 atmosphere applying the same heating and cooling rate and changing only the maximum temperature (800-1070 degrees 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, alpha-Fe, gamma-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 degrees C), for which the transmission and integral low-energy electron Mossbauer spectra are markedly different. Indeed, alpha-Fe and Fe3C were not observed as surface species, while gamma-Fe-C is present at the surface and in the bulk in the same proportion independent of the temperature of preparation. This could show that most of the nanoparticles (detected as Fe3C and/or gamma-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 T-r of 990 degrees C and 1070 degrees C, all Fe and Fe-carbide particles formed during the reduction are distributed evenly in the bulk and the surface of the matrix grains. The integral low-energy electron Mossbauer spectroscopic study of a powder oxidized in air at 600 degrees C suggests that all Fe3C particles oxidize to alpha-Fe2O3, while the alpha-Fe and/or gamma-Fe-C are partly transformed to Fe1-xO and alpha-Fe2O3, the latter phase forming a protecting layer that prevents total oxidation.