Effect of ultrasound pretreatment on bromination of double-walled carbon nanotubes

Bromination of double-walled carbon nanotubes (DWCNTs) was carried out using a saturated vapor of Br-2 at room temperature with or without a pretreatment in bromine water. X-ray photoelectron spectroscopy revealed that ultrasound pretreatment modified the chemical state of bromine in the product. The binding energies of the Br 3d electrons in the pre-sonicated DWCNT sample were between those characteristic of the covalent C-Br bonds and the negatively charged Br-2 molecules, observed when the pretreatment was not performed. Raman spectroscopy, however, clearly evidenced Br-Br vibrations in both brominated samples. Calculations of CNT-Br-2 models within density functional theory were used to propose that the electronic state of a Br-2 molecule depends on the adsorption site. The bromine molecules prefer to be located near edge hydroxyl groups, which acept the electron density from Br-2. This increases the binding energy of Br 3d levels as compared to that for Br-2 molecules in other adsorption sites.