Selective Removal of Metallic Single-Walled Carbon Nanotubes with Small Diameters by Using Nitric and Sulfuric Acids

Abstract

Coexistence of metallic and semiconducting carbon nanotubes has often been a bottleneck in many applications and much fundamental research. Single-walled carbon nanotubes (SWCNTs) were dissolved in HNO₃/H₂SO₄ mixture to confirm differing reactivity between metallic (m) and semiconducting (s) SWCNTs. With HNO₃/H₂SO₄ treatment, s-SWCNTs remained intact, while m-SWCNTs were completely removed for SWCNTs with small diameters less than 1.1 nm, which was confirmed by resonant Raman and optical absorption spectra. We also showed that nitronium ions (NO₂⁺) dissolved in the HNO₃/H₂SO₄ solution could preferably attack the m-SWCNTs, which was supported by our theoretical calculation. This clear selectivity can be explained by the preferential adsorption of positively charged NO₂⁺ on m-SWCNTs due to more available electron densities at the Fermi level in the m-SWCNTs. We report for the first time a selective removal of small-diameter m-SWCNTs by using HNO₃/H₂SO₄ solution, which presented a striking contrast to the diameter-selective removal of SWCNTs by oxidative etching reported previously.