Analysis of Fe Catalyst during Carbon Nanotube Synthesis by Mossbauer Spectroscopy

The behavior of Fe catalyst, from its deposition on a SiO2/Si substrate with or without an Al buffered layer to the end of carbon nanotube (CNT) synthesis under hydrogen- and water vapor-added conditions, was investigated by Mossbauer spectroscopy and scanning electron microscopy. The analyses revealed that the chemical states of the catalyst, not only the morphology, change drastically in the presence of the Al buffered layer. In the case of Fe/SiO2/Si without Al, a quarter of the deposited alpha-Fe changed to Fe2SiO4 at the heat-up stage, and then not only alpha-Fe but also Fe2SiO4 appeared to act as CNT synthesis catalysts. In the case of Fe/Al/SiO2/Si the amount of alpha-Fe once decreased down to 8 at. % at the heat-up stage and then increased up to 76 at. % during CNT synthesis. The phenomenon in the latter case can be explained by the two following steps: (i) formation of nonmagnetic and magnetic Fe-Al alloys and (ii) segregation of Fe on the surface (where it changes to Fe3C) and at the Al2O3/SiO2 interface (where it stays as alpha-Fe) accompanied by selective water vapor oxidation of Al in the Fe-Al alloys. The segregated Fe on the surface is thought to act as a CNT synthesis catalyst.