Effect of Water Filling on the Electronic and Vibrational Resonances of Carbon Nanotubes: Characterizing Tube Opening by Raman Spectroscopy

Insertion of various compounds into single-walled carbon nanotubes (SWCNTs) is a challenging approach for the design of one-dimensional nanostructures with novel functionalities. It has already been shown that fullerenes as well as other organic and inorganic compounds can be introduced in SWCNTs. [1] Molecular dynamics simulations have suggested that water can also enter the SWCNTs despite the hydrophobic nature of the CNT wall, and that even very fast transport can take place. [2] Very recently, remarkably efficient water transport through nanotube membranes was observed, [3] and also the use of water-filled CNTs as nanovalves to control gas flow inside CNTs was demonstrated. [4] Monitoring and optimization of the accessibility of the nanotubes are of prime importance in these different applications. In spite of the many reports concerning the opening/filling of CNTs, a quantitative evaluation of the tube opening for the different processes is still not available. SWCNT opening has been shown to occur as a side effect in various acid-based purification methods and has been intensively studied with electron microscopy. [5] A series of experimental studies including nuclear magnetic resonance (NMR), [6] X-ray diffraction (XRD), [7] IR spectroscopy, [8] and neutron scattering, [9] have demonstrated that water can enter SWCNTs, by probing the encapsulation induced changes in the water. Raman scattering of H2O vibrations and the tangential modes of CNTs indicate adsorption/desorption of water on CNTs, however no quantitative information on the CNTs could be obtained. [10] Very recently, Longhurst and Quirke [11] calculated that filling of CNTs with water would lead to an upshift of the radial breathing mode (RBM) of the CNTs by 2–6 cm –1 . Here, we show that the RBM resonant Raman features of empty and water-filled SWCNTs can be very well resolved, even in measurements on bulk solutions, provided the nanotubes are solubilized using bile salt surfactants. [12] Furthermore, the vibrational and electronic shifts and damping upon filling are accurately determined for a series of SWCNTs. With this knowledge we show that resonant Raman spectroscopy can be used as a quantitative ratiometric technique to monitor the opening and closing of CNTs under different treatments. The electronic properties of CNTs depend critically on their structure, [13] which is uniquely defined by the chiral indices (n,m). Resonance Raman scattering (RRS) is a powerful technique to study CNTs, because the RBM frequency (xvib )o f the CNTs is directly related to the diameter of the tube: xvib= c1/D+c2, where D is the nanotube diameter. Different values for the constants have been reported, among which c1=223.5 nmcm –1 and c2=12.5 cm –1 are the most commonly used. [14] RRS was applied to bundles or individual nanotubes