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|Catechol-Functionalized TiO2 Nanoparticles: From Fundamental Surface Chemistry to Promising Applications
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|Titanium dioxide (TiO2) is an abundant, non-toxic material that has found numer-ous applications in diverse research fields over the last few decades. To enhance its sur-face functionality, TiO2 nanoparticles (NPs) are frequently modified with catechol an-chor molecules, which incorporate the benefits of a high binding strength and an ex-tended spectral response. However, this strategy is often pursued with minimal knowledge of the underlying surface chemistry, thus making accurate assessments very difficult. Therefore, the goal of this thesis is to investigate the fundamental surface process-es taking place when hydrophobic TiO2 NPs are reacted with catecholic molecules and to utilize thus-functionalized TiO2 NPs in applications of photocatalysis and nanocom-posites. The first part of the thesis lays a solid foundation for the surface chemistry during catechol ligand adsorption on hydrophobic, oleate-stabilized TiO2 NPs. Using a broad set of NMR techniques (1D and 2D, 1H and 13C, solution and solid state), the surface coverage of the incoming catechol and the displacement of the native oleate were pre-cisely monitored. Contrary to the prevailing view, catechol was not able to substitute the entirety of the native ligand shell. Instead, a mixed ligand shell was formed after dis-placement of only a minor fraction (~20%) of the native ligand shell. Starting at a value of 2.3 nm−2 for the native oleate ligand shell, the total ligand density more than doubled to 4.8 nm−2 for the mixed ligand shell at maximum catechol loading. This surprising behavior can be rationalized by the assumption that the incoming catechol ligand pref-erably adsorbs to unoccupied Ti surface sites rather than displacing native oleate ligands. The second part of the thesis shows a promising application of a specially de-signed catecholic anchor as a visible-light sensitizer for TiO2 NPs in a photocatalytic oxidative cyanation reaction. The sensitizer molecule, 6,7-dihydroxy-2-methylisoquinolinium chloride (DHMIQ), was designed in such a way that it incorpo-rates the structural features of a number of powerful homogeneous organic photocata-lysts such as the 2,7-diazapyrenium dication. When attached to TiO2 NPs, DHMIQ al-lows for a panchromatic sensitization over the whole visible range and even extending into the near-infrared (NIR) region. The extended spectral response of the TiO2−DHMIQ photocatalyst was exploited in aerobic photocyanation reactions of ter-tiary amines with visible and NIR light. When irradiated with light of different wave-lengths, the photocatalyst achieved remarkable catalytic performance not only for ener-getic blue light of 462 nm, but also for low-energy NIR light of 730 nm. The third part of the thesis deals with 4-allylcatechol-functionalized TiO2 NPs, which were covalently embedded into thiol−ene (TE) polymer matrices through their terminal double bonds. Thus, hybrid nanocomposites with varying inorganic filler frac-tions were fabricated in order to study the effect of the inorganic filler on the material’s mechanical and thermal properties. To access these properties, the sound velocity (and thus elastic modulus) and the thermal conductivity of the TE/TiO2 nanocomposites were measured by Brillouin light scattering spectroscopy (BLS) and by the 3ω method, re-spectively. Both properties increased with increasing TiO2 NP filler fraction: The effec-tive elastic modulus increased from 6.2 GPa at 0 wt% of TiO2 NPs to 37.5 GPa at 90 wt% of TiO2 NPs (by a factor of 6), while the effective thermal conductivity in-creased even more significantly, from 0.04 to 0.76 W/m∙K (by a factor of 18). The in-crease of the effective elastic modulus was attributed to the covalent cross-linking of the nanocomposite constituents. The pronounced enhancement of the effective thermal conductivity was ascribed to the addition of a high-conductivity filler in the form of nanoparticulate TiO2 and the associated formation of conductive channels at high TiO2 NP fractions.
540 Chemistry and allied sciences
|Johannes Gutenberg-Universität Mainz
|FB 09 Chemie, Pharmazie u. Geowissensch.
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|xiv, 203 Seiten, Illustrationen, Diagramme
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