The starting material to produce carbon nanotubes is graphene, a flat network of carbon atoms with a honeycomb geometry. In terms of the structure, a carbon nanotube is a graphene sheet rolled into a seamless hollow cylinder. Single- and multi-wall nanotubes are produced as powders, fibers, and thin films; they differ in length, diameter, and chirality, i.e., the degree of “shift” of the honeycomb pattern. These parameters affect the carbon nanotube properties. For example, electrical conductivity depends on chirality, which is of a particular importance for transparent electronic and optical devices (lasers, LEDs, solar cells).
The main production technology of thin-film single-wall carbon nanotubes is chemical vapor deposition (CVD) which is the process used to obtain high-purity solid materials. One of variations of chemical vapor deposition is aerosol CVD, which allows obtaining a nanotube in one stage: a stream of a gaseous carbon feedstock (hydrocarbons, carbon monoxide, ethanol) and a catalyst precursor, in particular ferrocene, a precursor of iron nanoparticles, is fed into a high-temperature reactor. Under the impact of high temperature, the catalyst precursor disintegrates into catalytic nanoparticles, and as a result, the carbon source is decomposed. Carbon is deposited on the particles’ surface, and then nanotubes begin to form, which after filtration form a two-dimensional grid – a thin film of single-layer carbon nanotubes.
To accelerate the growth of nanotubes, researchers usually introduce carbon dioxide, water, and sulfur compounds into the CVD reactor, which, among other things, increases the catalytic activity. The Skoltech scientists tried to use hydrogen as an accelerator. “Previous studies have shown that introducing hydrogen into a carbon monoxide environment may trigger an additional reaction which would result in carbon formation, in parallel with the Boudoir reaction [disproportionation of carbon monoxide into carbon dioxide: CO + CO → C + CO2 — hydrogenation of CO: CO + H2 → C + H2O]. We have come to the conclusion that this solution could work in our case,” Skoltech quotes Ilya Novikov, a graduate student.
The authors found out that with a 10% hydrogen concentration, productivity of synthesis of single-wall carbon nanotubes increased by 15 times, without any deterioration in their properties as a transparent conductor. “Having studied the technologies of growing nanotubes with an optical spectroscopy and an electron microscopy, as well as the thermodynamics of the process in details, we drew the conclusion that such a remarkable result was achieved thanks to carbon monoxide hydrogenation” Skoltech quotes Albert Nasibulin, head of the Nanomaterials Laboratory.
The scientists also studied various temperature modes of nanotube synthesizing. It turned out that at a relatively low temperature, hydrogen provides a significant increase in catalytic activity, significantly increasing the number of tubes at the output. In turn, at high temperatures, hydrogen accelerates the growth of nanotubes, allowing for production of long nanotubes with a high film conductivity.
