Production breakthrough creates tallest ever ‘forest’ of carbon nanotubes

Researchers have grown the tallest ever ‘forest’ of carbon nanotubes (CNT), using a new technique they say could make nanomaterials a “ubiquitous reality”.

Describing it as a “novel approach to a conventional technique”, the Japanese team used the process to grow CNT forests roughly 14cm in length – seven-times longer than the previous maximum.

Many industries, including optics, electronics, water purification and drug delivery, are innovating rapidly using CNTs. The nanometre-wide rolls of honeycomb-shaped graphite sheets offer features such as low weight, convenient structure, immense mechanical strength, superior thermal and electrical conductivities and stability. Rising demand means that production needs to scale up, however.

While scientists have grown individual CNTs approximately 50cm in length, when they attempt arrays, or forests, they hit a ceiling at around 2cm. This is because the catalyst, which is key to CNT growth, deactivates or runs out before they can grow any longer. This drives up costs and threatens to cap its industrial use, the Japanese researchers said.

Team leader Hisashi Sugime, assistant professor at Waseda University, said: “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.”

To begin with, the researchers created a catalyst based on their findings in a previous study. They added a gadolinium (Gd) layer to the conventional iron-aluminium oxide (Fe/ Al₂O_x) catalyst coated onto a silicon (Si) substrate. The Gd layer prevented the deterioration of the catalyst to a certain extent, allowing the forest to grow up to around 5cm.

To further prevent catalyst deterioration, the team placed the catalyst in a cold-gas chemical vapour deposition chamber. There, they heated it to 750°C and supplied it with small concentrations (parts per million) of room temperature Fe and Al vapours.

This kept the catalyst working for 26 hours, in which time dense CNT forests could grow to 14cm. Analysis reportedly showed they had high purity and ‘competitive’ strength.

“This achievement not only overcomes hurdles to the widespread industrial application of CNTs, but it opens doors in nanoscience research,” a research announcement said.

Sugime added: “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… the knowledge herein could be pivotal to making nanomaterials a ubiquitous reality."

The study was published in Carbon.

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