High-resolution magic angle spinning proton nuclear magnetic resonance of water absorbed in single wall carbon nanotubes
Hochaufgelöste Magisch-Winkel-Rotation Protonen Kernspinresonanz Studie von Wasser absorbiert in einwandigen kohlenstoff-Nanoröhrchen Nano-Physik Studie Summary Single wall carbon nanotubes (SWNTs) have attracted scientific interest because of their fascinating physical and chemical properties and for their application in materials physics and the chemical industry. The main topic of this Nano-Physics study is the investigation of the properties of water in carbon nanotubes, at different temperatures and at different concentrations of water by using the proton Magic Angle Spinning solid state nuclear magnetic resonance technique. Also the influence of metal clusters, which exist near the carbon nanotubes as a result of the synthesis, is investigated. From the Nuclear magnetic Resonance investigations on water in single wall carbon nanotubes (SWCNTs), as described in this thesis, we can draw the following conclusions: • Water is not absorbed inside SWCNTs, synthesized by the HiPCO method with a Fe catalyst, without further purification. The only water that is sorbed by the sample is water adsorbed at the outside of the nanotubes with a chemical shift of 4.6 – 4.8 ppm. • Acid treatment removes the Fe clusters. After this treatment two water signals are detected, at 4.6 – 4.8 ppm and at 1.3 ppm. • Temperature dependent solid state NMR spectra show that the water which is responsible for the 1.3 ppm line freezes at a temperature much lower than the freezing point of bulk water. This water is assigned to water absorbed inside the nanotubes. • The NMR spectra as a function of the amount of water added to the SWCNT sample shows that the first water that is added to the sample, is absorbed in the tubes. In spite of the hydrophobicity of the inside of the carbon nanotubes water is absorbed due to the water surface tension. • The investigated sample contains SWCNTs with diameters ranging from 0.9 to 1.1 nm and different chiralities. Also from theory it is expected that the tubes can be divided in metallic and semi-conducting tubes. Nevertheless the diameter range, the chirality and the conductivity of the tubes seem to have no effect on the chemical shift of absorbed water. • A simple model calculation using the ring current contribution of each benzene ring of the SWCNT to the chemical shift of water located at the centre axis of the tube yields a water chemical shift value very close to the experimental value of 1.3 ppm.
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