Study On The SFG Activity Induced By Polar Solvents For Multi-walled Carbon Nanotubes

In this thesis,the surface phonon vibration mechanism of multi-walled carbon nanotubes(MWCNTs)wetted by different polar solvents was systematically investigated at the molecular scale by the second order nonlinear optic technology,sum frequency generation vibrational spectroscopy(SFG-VS)using a total internal reflection experimental setup(TIR)complemented by transmission electron microscopy(TEM),and Raman spectroscopy etc.The results of experiments MWCNTs of different diameters wetted by different polar solvents and exploratory validation experiments such as a mixture of sodium dodecyl sulphate(SDS)and water wetting MWCNTs and MWCNTs with increased defects such as carboxyl groups and terminal openings being wetted by water showed that SFG-VS can detect surface optical phonon vibrational signals induced by polar solvents changing the interfacial optical properties of MWCNTs.The intensity of polar solvent polarity and the number of surface defects in MWCNTs affected the intensity of phonon vibration peaks.Specifically,firstly,when MWCNTs were wetted by polar solvents,such as water and dimethylformamide(DMF),the vibration modes of interfacial optical phonons were detected.Whereas,when the solvents were non-polar(i.e.,decahydronaphthalene and air),SFG signal would be silent,indicating that polar solvents can induce SFG activity of MWCNTs.Secondly,when the wetting solvent was replaced with a mixture of water and SDS,the interfacial optical phonon vibrations were undetectable,indicating that the appearance of vibrations required a direct contact between the polar solvent and MWCNTs.Finally,the SFG vibration peak was blue-shifted when the pristine carbon nanotubes with fewer defects were treated to generate more defects and then wetted by water.This showed that the vibration peak frequency would be blue-shifted when the wetting solvent was water and the number of defects on the surface of carbon nanotubes was abundant and proved that the interfacial photoacoustic phonon vibration frequency had a significant dependence on the number of surface defects of MWCNTs.Combining theoretical analysis with experimental verification,we proposed a reasonable conjecture about the phonon vibration activity on the surface of MWCNTs.The polar solvent can wet the sample asymmetrically at the interface formed by MWCNTs and prisms,while MWCNTs have effective Raman vibration modes,and the hydrogen bonding in the polar solvent can interact with the carbon ring lattice,the constituent unit of carbon nanotubes,to induce the interfacial phonon vibration,namely,the SFG activity of MWCNTs.This phenomenon of SFG activity for MWCNTs induced by polar solvents has potential applications in optical detection and environmental sensing in the near future.