Analysis of oxidation state of multilayered catalyst thin films for carbon nanotube growth using plasma-enhanced chemical vapor deposition

We synthesized vertically aligned carbon nanotubes (CNTs) using multilayered catalyst thin films (Fe/Al2O3 and Al2O3/Fe/Al2O3) by RF (13.56 MHz) CH4/H-2/Ar plasma-enhanced chemical vapor deposition. Pretreatment of the catalyst is crucial for CNT growth. In this paper, we analyzed the effect of catalyst reduction on CNT growth. Catalyst thin films on substrates were reduced by H-2 plasma pretreatment at 550 degrees C to form nanometer-sized catalyst particles. The multilayered thin films were analyzed; the chemical composition and oxidation state by X-ray photoelectron spectroscopy (XPS) and the surface morphology by scanning electron microscopy (SEM). The Fe 2p peak of the XPS spectra showed that FexOy in the as-deposited catalyst was effectively reduced to Fe by a pretreatment of duration 4 min. Using this catalyst, we obtained CNTs with an average diameter of 10.7 nm and an average length of 5.3 mu m. However, pretreatment longer than 4 min resulted in shorter CNTs and the Fe peak was shifted from Fe to Fe3O4. These transitions (Fe2O3 -> Fe3O4 -> Fe -> Fe3O4) can be explained by the enthalpy of the oxides. This result indicates the presence of an optimum ratio between Fe and FexOy to maximize the CNT lengths.