Low-temperature catalyst activator: mechanism of dense carbon nanotube forest growth studied using synchrotron radiation

Abstract

The mechanism of the one-order-of-magnitude increase in the density of vertically aligned carbon nanotubes (CNTs) achieved by a recently developed thermal chemical vapor deposition process was studied using synchrotron radiation spectroscopic techniques. In the developed process, a Ti film is used as the underlayer for an Fe catalyst film. A characteristic point of this process is that C 2 H 2 feeding for the catalyst starts at a low temperature of 450 C, whereas conventional feeding temperatures are $ 800 C. Photoemission spectroscopy using soft and hard X-rays revealed that the Ti underlayer reduced the initially oxidized Fe layer at 450 C. A photoemission intensity analysis also suggested that the oxidized Ti layer at 450 C behaved as a support for nanoparticle formation of the reduced Fe, which is required for dense CNT growth. In fact, a CNT growth experiment, where the catalyst chemical state was monitored in situ by X-ray absorption spectroscopy, showed that the reduced Fe yielded a CNT forest at 450 C. Contrarily, an Fe layer without the Ti underlayer did not yield such a CNT forest at 450 C. Photoemission electron microscopy showed that catalyst annealing at the conventional feeding temperature of 800 C caused excess catalyst agglomeration, which should lead to sparse CNTs. In conclusion, in the developed growth process, the low-temperature catalyst activation by the Ti underlayer before the excess Fe agglomeration realised the CNT densification.

Evaluation of thermal resistance of carbon nanotube film fabricated using an improved slope control of temperature profile growth

“…There is a report showing that Fe catalyst deoxidization starts at 450 °C. 21) At the growth temperature range of 450 to 700 °C, the catalyst activation is low, and over 700 °C, the catalyst activation markedly increases in this temperature rage. There are many parameters; thus, we need to study this result further.…”

Novel growth method of carbon nanotubes using catalyst-support layer developed by alumina grit blasting

“…Because of this, only the group IV metals can act as reducing agents to prevent the iron catalysts from being oxidized in the present CNT growing process. In conventional CNT growth processes with iron sputtering, several researchers [29,30] have reported that a titanium interlayer can behave as a support for the formation of iron nanoparticles, which are susceptible to oxidation but are required for dense CNT growth. Therefore, the group IV metal oxide films on the as-received substrates can support the catalytic activity of iron nanoparticles in the CVD process, as well as the alumina film developed by grit blasting.…”