Structural characterization of self-organized TiO2 nanoclusters studied bysmall angle neutron scattering

Thiolactate-modified 45 Angstrom TiO2 particles were observed to self-assem ble into rodlike structures. The rod formation was observed using small-ang le neutron scattering (SANS). The diameter of the rods was determined to be 43.2 +/- 1.6 Angstrom, the same diameter as unassembled TiO2 particles. Th e rod diameter was found to be independent of TiO2 concentration or pH, whi le rod length was sensitive to both. An independent determination of the mo lecular weight per unit length of a rod using SANS confirms the beadlike st ructure of the rods with a bead diameter of 40 +/- 3 Angstrom. Further conf irmation of self-assembly was obtained from transmission electron microscop y where long rods with a diameter of 42.8 +/- 3.5 Angstrom were observed. T he 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 fund to be a consequence of the special conforma tions of surface modifiers upon bidentate: binding of mercapto-carboxylic a cids. These special bidentate bindings introduce conformations of surface m odifiers that allow a significant increase of a dipole moment at the surfac e. Single-point ZINDO calculations suggest a change of the dipole moment fr om 1.5 D for unbound thiolactic acid to 11.2 D for bidentate bound thiolact ate surface Ti atoms.