Mossbauer spectroscopy analysis of iron compounds in carboxylated multiwall carbon nanotubes and their ammonium salt

Different forms of iron compounds embedded in non-functionalized (as prepared) and functionalized multiwall carbon nanotubes (MWCNTs) were studied. Iron used to catalyze MWCNT synthesis can be found inside nanotubes and built-in within their walls. Mossbauer spectroscopy was applied to study valence and spin states as well as magnetic ordering of the iron compounds. In the Mossbauer spectra of as prepared MWCNTs the Fe3C carbide provides the main contribution. The content of Fe3C decreases by about 20 and 40% in carboxylated carbon nanotubes (MWCNTs-COOH) and ammonium salt of carboxylated carbon nanotubes (MWCNTs-COONH4), respectively. A small amount of alpha-Fe and FexCx iron forms are always observed in all studied carbon nanotubes. In MWCNTs-COOH, additionally, ferrihydrates and/or FexCy with oxygen in the second coordination sphere of iron are present (at a level of about 17%). In MWCNTs-COONH4 their content increases by a factor of 3. The experimental data shows that the purification and functionalization of as prepared MWCNTs result in removal of about 90% of iron contaminations. Additionally, we observe the modification of iron compounds inside MWCNTs. All these results are important in optimizing the as prepared MWCNT purification process and in finding a new way to produce different Fe phases inside MWCNTs. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim