Hyperpolarized 129Xe NMR Study of TiO2 Nanotubes

Abstract

TiO2 nanotubes (Ti-NTs) have uniform tubular structure with quasi-one dimensional hydrophilic nano-channels, and show many remarkable physical characteristics. In order to investigate nondestructively the internal characteristics of nanotubes, N2 sorption isotherms or mercury porosimetry could be applied. However, the former is based on a number of theoretical assumptions and is time consuming

the latter often requires high pressures due to its low diffusion which may affect the structure of pore wall. In this thesis, an indirect probing method for Ti-NTs using hyperpolarized (HP) xenon is presented. Most of work mainly consists of HP 129Xe NMR investigation for Ti-NTs channel with the following two parts:Part I discusses how to discriminate inside of channel from outside. A continuous flow (CF) HP 129Xe NMR spectroscopy was employed to investigate Ti-NTs with different reaction times of hydrothermal synthesis. A single peak attributing to channels for Ti-NTs was observed for variable temperature HP 129Xe NMR spectra. It was also noted that there was alteration in the value for heat of adsorption, and variation in chemical shift of the xenon adsorbed in channels, which were closely correlated to channel length and it was shown that P25-24 Ti-NTs with longest channel is most favorite Ti-NTs for xenon adsorption.Part II deals with water and cyclohexane adsorption in Ti-NTs channels to investigate adsorption behavior of water and cyclohexane in nanotube channels

for this, two different series of HP 129Xe NMR spectra for Ti-NTs were observed with increasing exposure time to them. The results show that xenon can simultaneously probe the channels with and without cyclohexane, whereas this phenomenon was rarely observed in channels with water. It could be thought that water was sticky enough to interfere with xenon adsorption in channels entrance. This was also confirmed by integral and chemical shift analysis of xenon peaks which could be applied to quantitative adsorption analysis for a specific adsorption site like channel as well as overall nanotubes.