The usage of hydrogen gas and carbon monoxide could significantly increase the production efficiency of single-wall carbon nanotubes, the material used for making solar panels, LEDs, and toxic gas detectors. This is the conclusion drawn by the scientists of the Skolkovo Institute of Science and Technology (Skoltech), which is based on the results of the study published by the Chemical Engineering Journal.
Structure of Carbon Nanotubes:
Carbon nanotubes are formed by rolling a graphene sheet (a flat network of carbon atoms with a honeycomb geometry) into a seamless hollow cylinder.
They are produced as powders, fibers, and thin films, and can have single or multiple walls.
Properties such as length, diameter, and chirality (degree of “shift” in the honeycomb pattern) vary and affect their properties, including electrical conductivity.
Importance of Chirality:
Chirality significantly influences the electrical conductivity of carbon nanotubes, which is crucial for applications in transparent electronic and optical devices like lasers, LEDs, and solar cells.
Production Technology – Chemical Vapor Deposition (CVD):
Thin-film single-wall carbon nanotubes are mainly produced using Chemical Vapor Deposition (CVD).
CVD is a method for obtaining high-purity solid materials.
Aerosol CVD, a variation of CVD, allows the production of nanotubes in a single stage by feeding a gaseous carbon source (e.g., hydrocarbons, carbon monoxide, ethanol) and a catalyst precursor (like ferrocene, an iron nanoparticle precursor) into a high-temperature reactor.
Role of Catalyst and High Temperature:
In the CVD process, high temperatures cause the catalyst precursor to disintegrate into catalytic nanoparticles.
The carbon source decomposes, and carbon is deposited on the surface of these particles, leading to the formation of nanotubes.
After filtration, the nanotubes form a two-dimensional grid, resulting in a thin film of single-layer carbon nanotubes.
Enhancing Growth with Additives:
To accelerate nanotube growth, additives such as carbon dioxide, water, and sulfur compounds are introduced into the CVD reactor, which enhance catalytic activity.
Skoltech researchers experimented with using hydrogen as an accelerator, finding that hydrogen introduction into a carbon monoxide environment triggers additional reactions that promote carbon formation through carbon monoxide hydrogenation.
Impact of Hydrogen on Synthesis:
The addition of 10% hydrogen concentration increased the productivity of single-wall carbon nanotube synthesis by 15 times without compromising their properties as transparent conductors.
Studies using optical spectroscopy, electron microscopy, and thermodynamic analysis revealed that the increase was due to the hydrogenation of carbon monoxide.
Temperature Effects on Nanotube Synthesis:
At relatively low temperatures, hydrogen significantly boosts catalytic activity, increasing the number of nanotubes produced.
At higher temperatures, hydrogen accelerates nanotube growth, enabling the production of longer nanotubes with high film conductivity.