Carbon nanotubes from human-crafted nanopores?

[norrrbit]

Would it be feasible to synthesize carbon nanotubes from a carbon gas source that had passed through a man crafted nanopores (nanoholes, nanochannels), the outflowing end of which is covered with a catalyst? This artificial membrane would separate gas source chamber from a product chamber. Intuitively (for a layman like me) such method could allow to obtain a uniform, pure, and high-quality product.
But after some web search I couldn't find anyone to employ such a method. The closest research I found were carbon nanotubes grown on a nanoporous alumina (i.e. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4147107/).
What is the main obstacle? Maybe producing such uniform and very deep nanopores, so the membrane would hold its weight and gas pressure?
Thanks in advance!

 

[TeethWhitener]

If you could localize the catalyst exactly at the mouth of the pores, then maybe. But it’s a lot easier just to grow them off nanoparticles (covering the entire surface with catalyst would give a 2D carbon film like graphene, rather than nanotubes).

 

[norrrbit]

Thanks for your suggestions.

Nevertheless, I can imagine that first the catalyst would be deposited on the entire surface of the membrane, and next deposition with an inert layer of some kind would cover it, to inhibit carbon film formation. Nanoholes could be cleared by passing a reactive gas. Eventually one would obtain nanoholes, whose endings would have a catalyst wall of a short length.
Why all the fuss?
According to the intuition of a layman like me such nanoholes:
1. could be more uniform then nanoparticles (nanohole diameter designed precisely, concernig catalyst thickness - PVD is very accurate), thus producing more uniform, better quality product
2. nanoparticles would not conatminate end product
3. gas source would be abundant at the catalyst site at high pressure even when dense nanotube forest would be grown, perhaps allowing for faster growth.

Just wondering...

 

[TeethWhitener]

There’s a lot going on here. You’ve put a decent amount of thought into this, which is good, but there’s a lot of specialized knowledge you’re still missing. Below is some food for thought.

Nanotube diameters tend to be on the order of 1-10 nm, whereas nanoporous alumina, for example, can’t be made with any kind of control over pore diameter less than about 25 nm or so. What can you do to span that order of magnitude? If you can get down to ~1 nm pores, how are you going to keep them from getting clogged? Typically at that level, you’re looking at zeolites, which are really only used for adsorption, not for passing reactive gases through. Diffusion works differently when the pore diameter is less than the mean free path of the gas molecule (google Knudsen diffusion). This will affect growth kinetics in unforeseen ways. In addition, there’s the issue of getting the right combination of materials to make the structure you’re talking about. Everything has to survive temperatures of at least 700 C or so, so your choices are limited to ceramics and metals. How will your inert layer prevent carbon film growth? Nickel and iron dissolve carbon, so that film growth occurs when the metal is cooled and the carbon crashes out of the solid solution. So your scheme requires a catalyst with a negligible solubility of carbon, which limits you to copper and a few other more expensive metals. What reactive gas will you use to etch only the inert film and not the metal or the nanoporous support? Might there be better ways of creating a nanopore with a catalyst around its mouth (or possibly as a ring within the pore)?

From a practical standpoint, let’s say it works. The next question is who cares? Nanoparticles can be made pretty monodisperse now, and separation techniques for nanotubes have gotten quite good. Density ultracentrifugation is quite cheap and effective. What makes you think your method will be better/cheaper/higher quality?

Keep working on it. It’s an interesting idea, albeit potentially riddled with landmines. Using nanopores to pattern other materials (as a shadow mask, for instance) is not unheard of, so maybe look into that literature for ideas. Best of luck.

 

[norrrbit]

I realize that going from tens of nm to a few nm is a huge leap. As I understand the biggest hurdle in my concept would be to transfer enough gaseous substrate through such a tiny pore. As I heard about ballistic, extremely efficient transport of water molecules in carbon nanotubes of certain diameter, I hoped that similar phenomenon might be possible for example with methane or other carbon sources. If true than high pressure would literally spill out nanotubes from the pores and clogging wouldn't be an issue. They'd be prefectly alligned and uniform. Maybe even with identical electrical properties, if pore charactersitics could be better controlled than these of the nanoparticles. They'd be continously produced with practically endless length, as no problems with carbon source would occur (as far as I know there is a problem with nanoparticle catalysis when nanotube forest reaches certain length and density).

This was my motivation, but wishful thinking too. Of course I knew from the beginning that my idea is too obvious not to be considered by professionals long time ago and there must have been some serious obstacles to realizing it, but it was annoying not to know what these were.

I really appreciate your effort to clarify me the matter.
Thank you again.