Surface modification of TiO₂ nanoparticles with bidentate ligands studied by EPR spectroscopy

The surface of 50 Å TiO₂ nanoparticle colloids was modified in order to improve the kinetic and redox characteristics of this semiconductor. The surface was derivatized with different bidentate ligands (thiolactic, β-mercaptopropionic, mercaptoacetic acids, and alanine) and was investigated by electron paramagnetic resonance (EPR) and IR spectroscopies. Infrared spectroscopy suggests that at pH 4 these compounds bind to Ti(IV) surface atoms through the carboxyl group. However, when a thiol group is in the a position with respect to the carboxyl group, surface Ti(IV) atoms become chelated with both the carboxyl and thiol groups resulting in five-membered ring formation. This results in the formation of a charge transfer complex with an optical absorption threshold at 520 nm. Illumination at 77 K of TiO₂ colloids with surface chelated Ti(IV) atoms in the absence of electron scavengers leads to the formation of the carboxyl cation radical (trapped holes) and three distinct Ti(III) centers (trapped electrons) at 4.2 K. When the temperature is increased, the hole moves to the CH₃ group, which is the farthest from the colloid surface. In the presence of electron accepting species (Pb²⁺, Cd²⁺) the signal for the trapped electron disappears at 250 K indicating electron transfer to the accepting species.