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

Abstract

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 R-Al 1.8-Fe 0.2 O 3 precursor in a H 2-CH 4 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 nanotubenanocomposite systems containing iron. The nature of the iron species (Fe 3+ , Fe 3 C , R-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, R-Fe and Fe 3 C 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 Fe 3 C 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 Fe 3 C particles oxidize to R-Fe 2 O 3 , while the R-Fe and/or γ-Fe-C are partly transformed to Fe 1-x O and R-Fe 2 O 3 , the latter phase forming a protecting layer that prevents total oxidation.

Influence of iron contaminations on local and bulk magnetic properties of nonfunctionalized and functionalized multi‐wall carbon nanotubes

“…As one may expect, the values of hyperfine parameters of the subsequent Fe‐components increase with decreasing temperature. In contrast to the other studies we identify more iron components (not only Fe 3 C and α‐Fe) in our samples and we do not detect γ‐Fe .…”

In situ high-temperature Mössbauer spectroscopic study of carbon nanotube–Fe–Al2O3 nanocomposite powder

“…This finding suggests that the initial weight gain (250-300 • C) observed in the TGA curve is due to oxidation of the topmost (5 nm) surface ␣-Fe particles. More details on ILEEMS studies on similar powders can be found elsewhere [14]. The differences between the present results and those of the earlier ILEEMS study arise because of the different oxidation conditions applied: quenching the powder in air immediately after reaching the maximum desired temperature (present study) or oxidation in air at 600 • C during 2 h [14].…”

Iron carbide nanoparticles produced by laser ablation in organic solvent

“…Whereas iron-based materials such as ferrite, austenite, martensite, and cementite (Fe 3 C) are well understood, Hägg (Fe 5 C 2 ) and EckstromAdcock (Fe 7 C 3 ) carbides, which are metastable iron carbides, are less well understood. Fe-C compounds have recently attracted renewed interest because Fe was found to catalyze the synthesis of carbon nanotubes [1,2]. Fe-C core-shell structured nanoparticles were also investigated [3,4].…”