Hongliang Zhang, Yunqi Liu,* Lingchao Cao, Dacheng Wei, Yu Wang, Hisashi Kajiura,* Yongming Li, Kazuhiro Noda, Guangfu Luo, Lu Wang, Jing Zhou, Jing Lu,* and Zhengxiang Gao

Single-walled carbon nanotubes (SWNTs) are one of the most promising materials for future electronics. However, they are generally grown as a mixture of metallic and semiconducting tubes, which is a major obstacle to their widespread application. Recently, several approaches for the enrichment of SWNTs with one electronic type have been developed, such as selective elimination of metallic SWNTs (M-SWNTs) by electrical breakdown or methane plasma etching, selective adsorption, electrophoresis, and density-gradient centrifugation. In these reported methods, however, either the amount of the separated samples is quite small, or long-time or ultrahigh-speed centrifugation are needed in order to achieve good results.

        Here, we report a facile and scalable method of selective etching of semiconducting SWNTs (S-SWNTs) with an amount larger than 5 mg within 2 h. This method is based on a gas-phase reaction at high temperature, in a furnace filled with inert gas and SO3 as the etchant. Different from other selective chemical reactions in solution, this high-temperature gas-etching strategy has some advantages. First, since reacted carbon nanotubes and/or amorphous carbon will turn to gas through high-temperature SO3 etching, no elaborate post-treatment is necessary to separate the reacted SWNTs from their unreacted counterparts, as previously reported. Second, during the etching reaction at high temperature, there is a chemical equilibrium between functionalization and defunctionalization: M-SWNT-enriched samples can be obtained after the etching process without an increase of the disorder band in the Raman spectra. Third, M-SWNTs, which have always been considered more active in chemical reactions due to their more-abundant electron-charge density at the Fermi level than their semiconducting counterparts, are preserved in our method. The enrichment of M-SWNTs has been characterized by Raman spectroscopy, near-IR absorption spectroscopy, and conductivity measurements.

Advanced Materials , 2009, 21, 813-816.

Experimental setup for selective etching of single-walled carbon nanotube.