In the early days of scientists’ trysts with carbon nanotubes, arc discharge was used to produce multi-walled carbon nanotubes. There was no control on the number of layers, length or diameter of the nanotubes. It was in the late 90’s, that CNR Rao’s group succeeded in creating single-walled nanotubes. They used chemical a vapour deposition technique, sublimating ferrocene powder.
Ferrocene – iron sandwiched between two C5H5 groups – is a catalyst found in many chemistry labs. Thus, it is a source of both carbon and iron. This provides iron nanoparticles around which single-walled carbon nanotubes can grow.
Ben Mills via Wikimedia Commons
Researchers in different parts of the world were soon using a two-zone furnace – the first for sublimating ferrocene and the second for pyrolysing the vapour.
Vapour is transported by a carrier gas, such as argon. During pyrolysis, the hydrogen in the aromatic group separates and the carbon atoms wrap themselves around the iron. But this process produced nanotubes that are not completely filled with iron. There was a deposition of metal particles outside the nanotubes, as well.
To improve the filling efficiency of iron within the core cavity of the carbon nanotubes and to reduce the deposition of metal nanoparticles outside, researchers pumped in another carbon source – typically, acetylene.
Soon researchers made attempts to synthesize carbon nanotubes filled with nickel and cobalt by replacing ferrocene with nicklocene or cobalotocene. However, they could not get well formed carbon nanotubes. The filling efficiencies of these metals within carbon nanotubes were negligibly small. Scientists had no control over the diameter and length of individual carbon nanotubes.
Even in the case of iron filled carbon nanotubes synthesised by the pyrolysis of ferrocene, the output was a mess of entangled tubes.
This month, Ashna Bajpai and team from the IISER Pune reported solving that problem.
“Most other labs use furnaces with two zones . But we use a single zone furnace. We monitor the temperature profile of the tubular furnace and introduce the metallocene at the appropriate place where the temperatures are conducive for sublimation, using a compression seal arrangement”, says Aakanksha Kapoor, Ph D scholar.
” Instead of using acetylene to improve filling efficiency, we use camphor. Camphor is non-toxic, and environment friendly. It has been used earlier to make carbon nanotubes. And it sublimates, like the metallocenes”, explains Nitesh Singh, Project Assistant.
So he puts the metallocene powder along with camphor powder in the compression seal and inserts it into the appropriate position that sublimates both the powders.
“We use argon, as carrier gas for transporting the vapour to the middle of the furnace, to the pyrolysis zone”, he says.
“Initially we tested camphor from the market, routinely used in Indian households. And we got good results. So we got hold of research grade camphor for our experiments with metallocenes. Using camphor also enabled a new control parameter to tune degree of entanglement”, says Aakanksha.
“Our key result is well formed carbon nanotubes with high filling efficiency for all three transition metals. It is the co-sublimation of camphor with metallocene that enabled this”, adds Ashna Bajpai, IISER Pune.
“These are self-organised structures. Such highly aligned forests of metal nanowires with a narrow distribution in the length and diameter of carbon nanotubes can be collected from the region where the pyrolysis temperature is carefully regulated”, says Ashna.
“In the variable temperature region, we still get entangled carbon nanotubes”, adds Nitesh.
“While aligned forests are important for spintronic applications, entangled carbon nanotubes are suitable for battery related applications. Now we have better control to form both types”, explains Ashna.
“Now we have a pyrolytic region that is about sixty centimetres. With a longer pyrolysis zone, we can provide a larger yield”, says Aakanksha Kapoor.
“Now our challenge is to convert the metal filling to various types of metal oxides. Metal oxides are functional materials. They exhibit a wide variety of magnetic states. They can be inherently metallic, insulating or semiconducting. When trapped inside carbon nanotubes, these tiny nano magnets can be tuned by magnetic fields, electric fields or even by stress.
The constraint, so far, was getting consistent yield and morphology of nanotubes filled with metals. This delayed anticipated uses – in spintronics, batteries. Now that the hurdle has been overcome, technological innovations have become feasible.
Forget miniaturisation, we are at a tipping point for technology micro-aturation.
The study, funded by the DST, India through a Ramanujan Grant and the Nano mission Thematic Unit Grant, was published in Carbon, 12928 (2018)
Udham P K