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

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 C2H2 feeding for the catalyst starts at a low temperature of 450 degrees C, whereas conventional feeding temperatures are similar to 800 degrees C. Photoemission spectroscopy using soft and hard X-rays revealed that the Ti underlayer reduced the initially oxidized Fe layer at 450 degrees C. A photoemission intensity analysis also suggested that the oxidized Ti layer at 450 degrees 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 degrees C. Contrarily, an Fe layer without the Ti underlayer did not yield such a CNT forest at 450 degrees C. Photoemission electron microscopy showed that catalyst annealing at the conventional feeding temperature of 800 degrees 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.