Nondestructive covalent functionalization of carbon nanotubes by selective oxidation of the original defects with K₂FeO₄

Graphical abstract: - Highlights: • Nondestructive covalent CNT functionalization is realized by defect oxidation. • A green oxidant K₂Fe^VIO₄ is used for CNT oxidation for the first time. • Effective CNT oxidation can be achieved at 60 °C in 3 h. • Oxidized CNTs are obtained in yields of above 100 wt%. • FeO₃, an unusual Fe (VI) specie, is produced when K₂FeO₄ is dissolved in H₂SO₄. - Abstract: Chemical oxidation is still the major approach to the covalent functionalization of carbon nanotubes (CNTs). Theoretically, the defects on CNTs are more reactive than skeletal hexagons and should be preferentially oxidized, but conventional oxidation methods, e.g., HNO₃/H₂SO₄ treatment, have poor reaction selectivity and inevitably consume the C=C bonds in the hexagonal lattices, leading to structural damage, π-electrons loss and weight decrease. In this work, we realized the nondestructive covalent functionalization of CNTs by selective oxidation of the defects. In our method, potassium ferrate K₂Fe^VIO₄ was employed as an oxidant for CNTs in H₂SO₄ medium. The CNT samples, before and after K₂FeO₄/H₂SO₄ treatment, were characterized with colloid dispersibility, IR, Raman spectroscopy, FESEM and XPS. The results indicated that (i) CNTs could be effectively oxidized by Fe (VI) under mild condition (60 °C, 3 h), and hydrophilic CNTs with abundant surface −COOH groups were produced; and (ii) Fe (VI) oxidation of CNTs followed a defect-specific oxidation process, that is, only the sp³-hybridized carbon atoms on CNT surface were oxidized while the C=C bonds remained unaffected. This selective/nondestructive oxidation afforded oxidized CNTs in yields of above 100 wt%. This paper shows that K₂FeO₄/H₂SO₄ is an effective, nondestructive and green oxidation system for oxidative functionalization of CNTs and probably other carbon materials as well