Facile synthesis of halogenated multi-walled carbon nanotubes and their unusual photoluminescence

The facile synthesis of halogenated multi-walled carbon nanotubes with Cl, Br and I under high pressure and high temperature was reported, and the unique photoluminescence of the halogenated carbon nanotubes was investigated by means of experimental and theoretical approaches. The one-step and facile method to incorporate these halogens onto nanotubes' surface involves high temperature and high pressure using a sealed vessel. This method is easily handled using SOCl2, Br-2 and I-2 as halogen sources and is accessible for the large-scale production of halogenated carbon nanotubes. The concentration of halogen in the samples by this method reaches up to 3-8% in weight. The UV-Vis photoluminescence of these halogenated carbon nanotubes was observed, and attributed to fluorescence, depending on their lifetimes. The fluorescence of the halogenated carbon nanotubes can be remarkably enhanced by the increase of introduced halogen atoms resulting from prolonging the reaction time. Different species of halogen atoms have little influence on the fluorescence emission behaviour, but affect their fluorescence lifetimes. The fluorescence is more likely to originate from sp(2) carbon clusters isolated from sp(3) carbons formed by the addition of halogen atoms, but a defect mechanism cannot be excluded from the present results. A series of compounds with different aromatic rings were modelled to predict the size of the isolated sp(2) carbon clusters by comparing the calculated excitation and emission maximum wavelengths with their experimental data. Basing on these theoretical results, it is thought that the UV-Vis fluorescence of the halogenated carbon nanotubes may originate from the isolated sp(2) carbon clusters containing 4-8 aromatic rings.