Coalescence of carbon nanotubes while preserving the chiral angles

Atomically precise coalescence of graphitic nanocarbon molecules is one of the most challenging reactions in sp² carbon chemistry. Here, we demonstrate that two carbon nanotubes with the same chiral indices (n, m) are efficiently coalesced into a single (2n, 2 m) nanotube with preserved chiral angles via heat treatment at less than 1000 °C. The (2n, 2 m) nanotubes constitute up to ≈ 20%-40% of the final sample in the most efficient case. Additional optical absorption peaks of the (2n, 2 m) nanotubes emerge, indicating that the reaction occurs over the entire sample. The reaction efficiency strongly depends on the chiral angle, implying that C-C bond cleavage and recombination occurs sequentially. Furthermore, the reaction occurs efficiently even at 600 °C in an atmosphere containing trace amounts of oxygen. These findings offer routes for the structure-selective synthesis of large-diameter nanotubes and modification of the properties of nanotube assemblies via postprocessing.