Enhanced catalyst lifetime coupled with high growth rate yields record-length CNTs of high purity, opening doors to scaled up production for industrial applications.The light weight, strength, thermal and electrical conductivity of carbon nanotubes (CNTs) has propelled this unique form of carbon into applications and promises many more. But the ability to produce dense arrays of long CNTs on a large scale has proved challenging. While individual nanotubes up to 50 cm in length have been grown, more useful forests of CNTs reach a limit at around 2 cm when the catalyst essential for growth becomes used up or deactivated.
Now researchers from Waseda and Shizuoka Universities in Japan have devised a novel approach based on chemical vapor deposition (CVD) that yields forests of nanotubes up to 14 cm long, seven times longer than previously reported [Sugime et al., Carbon 172 (2021) 772-780, https://doi.org/10.1016/j.carbon.2020.10.066 ].
“In conventional [CVD], CNTs stop growing due to a gradual structural change in the catalyst, so we focused on developing a new technique that suppresses this change and allows the CNTs to grow for a longer period,” explains Hisashi Sugime of Waseda University, who led the work.
Using the most promising catalysts to date as a starting point, the researchers added gadolinium (Gd) to iron-aluminum oxide (Fe/Al2O3) catalysts on a silicon (Si) substrate. The Gd combats structural changes in the catalyst particles, prolonging the CNT growth time. The introduction of a trace element, Fe, compensates for subsurface diffusion, which depletes the catalyst. These innovations enabled the growth of nanotube bundles 5 cm in length. Another trace element, Al, was then added to suppress further structural changes in the catalyst. The combination allows the catalyst to stay active for 26 hours, with a growth rate of 1.5 µm, producing a forest of 14 cm-long nanotubes. To obtain cleaner CNTs, the team used a growth technique known as cold-gas CVD, where only the substrate is heated (to 750°C) while the carrier gas is kept at room temperature. This allows the trace elements vapors to be introduced efficiently and keeps unnecessary reactions and deposition to a minimum.
“The enhancement of the growth lifetime stems from the suppression of the structural change of the catalyst nanoparticles, which was confirmed by the increased density of the CNT forests,” says Sugime.
Long CNTs make physical and electrical analysis much easier, point out the researchers. Using scanning and transmission electron microscopy, as well as Raman spectroscopy, the researchers recorded comparable values for tensile strength and Young’s modulus. The advance represents a breakthrough in the growth of CNT forests, opening the way for industrial-scale applications in the future.
“This simple but novel method, which drastically prolongs catalyst lifetime by supplying ppm-level vapor sources, is widely applicable in catalyst engineering for other fields such as petrochemistry and nanomaterial crystal growth in general,” adds Sugime.