Microwave-Assisted Bromination of Double-Walled Carbon Nanotubes

The ability to functionalize carbon nanotubes (CNTs) through controlled structural modifications is an essential prerequisite for application purposes, as it may lead to increased solubility and processability(1-3) as well as opportunities for fine-tuning the physical and chemical properties,(4, 5) thereby enabling the exploitation of the mechanical, thermal, and optical properties of CNTs(6) in a wide range of applications in both material(7) and biomedical sciences.(5, 8, 9) A major challenge in this field, therefore, is to develop a range of reliable and effective functionalization methodologies that allow the construction of CNTs-based macromolecular species of highly versatility and usefulness, to be further modified by coupling them to a variety of organic functionalities and in the mean time do not extensively disrupt the graphitic π-conjugated surface (consequently altering their electronic(10) and thus optical properties(11-13)). In this respect, halogenated CNTs-based materials can represent a useful class of intermediates as they could act as precursors to a number of organometallic species and suitable substrates for metal-catalyzed cross coupling reactions. Halogenation of CNTs has mainly been performed with highly oxidizing F2 gas,(14-16) with electrochemically generated Cl2 and Br2(17) and with I2 through a modified Hunsdiecker-type reaction.(18) The low-susceptibility of CNTs to weak oxidizing species, such as Br2, was instead utilized as a means of purification for MWCNTs containing carbon nanoparticles(19) and to the best of our knowledge, no definitive reports about the C−Br bond formation on single- or double-walled CNTs have been reported at time. We thus report the synthesis and characterization of double-walled carbon nanotubes (DWCNTs) functionalized with Br2 (Br-DWCNTs) under microwave-assisted (MiW) conditions,(20-22) to link covalently the Br atoms to DWCNTs with a percentage of functionalization of ca. 5−8 % in weight.