Development of carbon nanotubes (CNTs), including single-wall and multi-wall nanotubes and nanofibers, into useful devices requires fabrication of CNTs that are relatively free from process residues such as amorphous carbon and metallic particles that are not part of a CNT array grown on a (coated) substrate. Conventional techniques for removal of amorphous carbon and other residues often use one of several techniques. One technique uses post-growth thermal or plasma cleaning of the amorphous carbon with Ox (x=2), which non-selectively attacks the CNT material as well as the amorphous carbon. Metal contaminants are removed using an acid bath, which is again a non-selective process and requires disposal of the strong acids and contaminant removal byproducts. A second technique uses certain toxic gases, such as halogens, to remove the amorphous carbon and other residues. This approach requires disposal of one or more hazardous substances and often requires many hours to complete. A third approach requires holding the amorphous carbon and CNTs in a heated Ox atmosphere for several hours, in order to limit the damage to the CNTs.
Each of these techniques is either non-selective, damages the CNTs, requires use and disposal of strong acids and/or other toxic fluids (liquids or gases), and/or requires lengthy time intervals for processing. What is needed is an in-situ approach that allows clean-up of unwanted material such as amorphous carbon after an array of carbon nanotubes has been grown, that does not damage the nanotubes, and that does not use harsh or toxic chemicals. Preferably, the unwanted material should be substantially separated and isolated from the nanotube array after the cleanup. Ideally, the approach should not require more than one hour to complete, and should not require use and/or disposal of any hazardous substances.
In the improved method, purification is accomplished by flowing water vapor through the reaction chamber at elevated temperatures and ambient pressures. The impurities are converted to gaseous waste products by the selective hydrogenation and hydroxylation by the water in a reaction chamber. This process could be performed either immediately after growth or in a post-growth purification process. The water used needs to be substantially free of oxygen and can be obtained by a repeated freeze-pump-thaw process. The presence of oxygen will non-selectively attach the carbon nanotubes in addition to the amorphous carbon.