Scientists grow a carbon nanotube forest of record length

Japanese scientists have used a novel technique to grow a ‘forest’ of carbon nanotubes (CNTs) with greater length than has ever before been recorded, potentially revolutionising the future of industries including optics, electronics, water purification and drug delivery.

Today many industries innovate at an unprecedented scale thanks to nanometre-wide rolls of honeycomb-shaped graphite sheets called carbon nanotubes (CNTs). Features such as light weight, convenient structure, immense mechanical strength, superior thermal and electrical conductivity, and stability put CNTs a notch above other material alternatives; however, to supply their rising industrial demand, their production must be constantly scaled up.

While scientists have been able to grow individual CNTs approximately 50 cm in length, when they attempt arrays, or forests, they hit a ceiling at around 2 cm. This is because the catalyst, which is key to CNT growth occurring, deactivates and/or runs out before CNTs in a forest can grow any longer, driving up monetary and raw-material costs of CNT production and threatening to cap its industrial use.

Now scientists from Waseda University and Shizuoka University have presented a novel approach that yields CNT forests of ~14 cm — seven times greater than the previous maximum. Their breakthrough has been published in the journal Carbon.

“In the conventional technique, the CNTs stop growing due to a gradual structural change in the catalyst, so we focused on developing a new technique that suppresses this structural change and allows the CNTs to grow for a longer period,” said Hisashi Sugime, an assistant professor at Waseda University, who led the team.

To begin with, the scientists created a catalyst based on their findings in a previous study, adding a gadolinium (Gd) layer to the conventional iron-aluminium oxide (Fe/Al₂Ox) catalyst coated onto a silicon (Si) substrate. This Gd layer prevented the deterioration of the catalyst to a certain extent, allowing the forest to grow up to around 5 cm in length.

To further prevent catalyst deterioration, the team placed the catalyst in what they call a cold-gas chemical vapour deposition (CVD) chamber. There, they heated it to 750°C and supplied it with small concentrations of room-temperature Fe and Al vapours. This kept the catalyst going strong for 26 hours, in which time a dense CNT forest could grow to 14 cm. Analysis to characterise the grown CNTs showed that they were of high purity and competitive strength.

“This simple but novel method that drastically prolongs catalyst lifetime by supplying ppm-level vapour sources is insightful for catalyst engineering in other fields such as petrochemistry and nanomaterial crystal growth,” Sugime said. “The knowledge herein could be pivotal to making nanomaterials a ubiquitous reality.”

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