Protecting Carbon Nanotubes from Oxidation for Selective Carbon Impurity Elimination

Abstract

Purity of carbon nanotubes (CNTs) is essential to avoid a dramatic decrease in their performances. In addition to metallic impurities, carbonaceous impurities have been shown to be responsible for pronounced effects. However, they are highly difficult to be selectively removed from CNT samples because of the similar chemical reactivity of these two kinds of carbon species. The existing purification methods often lead to high CNT consumption (>90 wt %). The proposed method consists of a one-pot gas-phase treatment combining chlorine and oxygen. The CNT powder maintained in a chlorine stream is submitted to oxygen at moderate temperature [350 and 500°C for single-walled CNTs (SWCNTs) and double-walled CNTs (DWCNTs), respectively], and the thermal treatment is then pursued at 900−1000°C under chlorine alone. Our work reveals that this approach is able to significantly improve the selectivity of elimination of carbonaceous impurities. Thanks to the proposed purification treatment, only 19 and 11 wt % of carbon species (mainly carbon impurities) are lost for DWCNTs and SWCNTs, respectively. The mechanism proposed involves a protective effect by grafting of chlorine favored to the CNT walls. Because our simple one-pot purification method is also versatile and scalable, it opens new perspectives for CNT applications in high-added value fields.

Multiwalled Carbon Nanotube Purification Probed by Electrochemistry: Low‐Temperature Chlorine Gas Treatment Meets High‐Temperature Annealing

“…The oxygen content is roughly in the 0.5-1 at.% range for the raw and the treated samples. As already reported from our previous works, [25,33] chlorine could be detected in MWCNTs purified by the chlorine-based method. For both pCl1 and pCl2, the Cl 2p feature typically shows two components (each consisting of the 3/2 and 1/2 level with a spin-orbit splitting of 1.6 eV) as shown in Figure S3 (Supporting Information).…”

“…Interestingly, the sublimation temperature of the metal‐based impurities can be significantly reduced in the 800–1200 °C range after their facile reaction with halogens such as bromine [22] or chlorine [23,24] . Moreover, since halogens are advantageously poorly reactive with carbon, the CNTs can hence be preserved or even protected by such treatments [25] . We have already applied such chlorine‐based purification to single walled CNTs (SWCNTs) from several sources and double‐walled CNTs (DWCNTs) with good purification yields [13,23,26–28] .…”

“…[23,24] Moreover, since halogens are advantageously poorly reactive with carbon, the CNTs can hence be preserved or even protected by such treatments. [25] We have already applied such chlorine-based purification to single walled CNTs (SWCNTs) from several sources and doublewalled CNTs (DWCNTs) with good purification yields. [13,23,[26][27][28] Because of well protected metallic impurities within carbon species in CNT samples, further enhancement in metallic removal yield has been only possible by adding O 2 in Cl 2 in a narrow condition window.…”

Critical challenges and advances in the carbon nanotube–metal interface for next-generation electronics

“…Alternatively, it has been shown that the use of a mixture of H 2 O 2 and a non-oxidizing acid such as HCl combined with ultrasonication and high temperature treatments is an effective way of removal of amorphous carbon and dissolving the catalyst residues without changing the surface chemistry of CNTs. 22,23 Gas-phase oxidative processes such as annealing in air and/or O 2 , 24 CO 2 , 25 Cl 2 /O 2 mixture, 26 and steam 27 can also be implemented to selectively remove carbonaceous impurities due to their lower stability than the CNT structure. However, this process should be done with care as defects and inorganic residues can decrease the decomposition temperature of CNTs.…”

A novel purification method of activated carbon-supported carbon nanotubes using a mixture of Ca(OH)2 and KOH as the ablation agent

“…Jeung et al 23 used chloroform (CHCl 3 ) as a gas-phase purification approach to obtain metal-free MWCNTs, while it had no effect on carbon-based impurities. Nawal et al 24 reported a one-pot gas-phase treatment combining chlorine and oxygen. This method can remove both metallic and carbon impurities, however it also resulted in chlorine-containing functional groups present on the wall of CNTs.…”