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

• Published in: Journal of physical chemistry. C Vol. 112; no. 15; pp. 5756 - 5761
• Main Authors: de Resende, Valdirene G, De Grave, Eddy, Peigney, Alain, Laurent, Christophe
• Format: Journal Article
• Language: English
• Published: American Chemical Society 17-04-2008
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp711679w

The surface state of carbon nanotubes-Fe-alumina nanocomposite powders was studied by transmission and integral 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 the maximum temperature (800−1070 °C) were investigated, demonstrating that integral low-energy electron Mössbauer spectroscopy is a promising tool complementing transmission Mössbauer 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 Mössbauer spectra are markedly different. Indeed, α-Fe and Fe3C were not observed as surface species, while γ-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 γ-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 °C and 1070 °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 Mössbauer spectroscopic study of a powder oxidized in air at 600 °C suggests that all Fe3C particles oxidize to α-Fe2O3, while the α-Fe and/or γ-Fe−C are partly transformed to Fe1- x O and α-Fe2O3, the latter phase forming a protecting layer that prevents total oxidation.