Polar Solvents Induce Sum Frequency Generation Activity for Multiwalled Carbon Nanotubes

Regarding methods of process and use of carbon nanotubes (CNTs), solvents are generally employed to disperse or dissolve CNTs as a pretreatment or intermediate process step. This naturally imposes an essential issue on how CNTs and solvents interact with each other, which seems trivial, comparatively inconsequential, and might often be overlooked from the perspective of engineering scenarios. However, as a matter of fact, it is indeed a fascinating and significant topic. In this article, to investigate the interfacial properties of multiwalled CNTs (MWCNTs) exposed to widely utilized solvents, we applied sum frequency generation vibrational spectroscopy (SFG-VS) to probe solvent-wetted MWCNTs and proved that polar solvents can substantially alter the interfacial optical property of MWCNTs. First, the interfacial optical phonon vibrational modes were detected when MWCNTs were wetted by polar solvents, i.e., water and dimethylformamide (DMF), while such modes were inactive when the solvents were nonpolar, i.e., decalin and air. Second, the interfacial optical phonon vibration frequency displayed distinct dependence on surface defects of MWCNTs. Combining theoretical analysis with experimental verification, a valid conjecture with respect to surface phonon vibration activity for MWCNTs was proposed. This phenomenon of polar solvent-induced SFG activity may have the potential to find applications in optical detection and environmental sensing in the near future.