Structural Characterization of Self-Organized TiO2 Nanoclusters Studied by Small Angle Neutron Scattering

Abstract

Thiolactate-modified 45 Å TiO2 particles were observed to self-assemble into rodlike structures. The rod formation was observed using small-angle neutron scattering (SANS). The diameter of the rods was determined to be 43.2 ± 1.6 Å, the same diameter as unassembled TiO2 particles. The rod diameter was found to be independent of TiO2 concentration or pH, while rod length was sensitive to both. An independent determination of the molecular weight per unit length of a rod using SANS confirms the beadlike structure of the rods with a bead diameter of 40 ± 3 Å. Further confirmation of self-assembly was obtained from transmission electron microscopy where long rods with a diameter of 42.8 ± 3.5 Å were observed. The formation of rodlike structures was not detected when the mercapto group was either blocked by a methyl group or separated from the carboxyl group with an additional methylene group. The self-organization of TiO2 particles in rodlike structures was found to be a consequence of the special conformations of surface modifiers upon bidentate binding of mercapto−carboxylic acids. These special bidentate bindings introduce conformations of surface modifiers that allow a significant increase of a dipole moment at the surface. Single-point ZINDO calculations suggest a change of the dipole moment from 1.5 D for unbound thiolactic acid to 11.2 D for bidentate bound thiolactate surface Ti atoms.