SURFACE MODIFICATION OF TIO2 NANOPARTICLES WITH BIDENTATE LIGANDS STUDIED BY EPR SPECTROSCOPY

The surface of 50 Angstrom TiO2 nanoparticle colloids was modified in order to improve the kinetic and redox characteristics of this semicon ductor. The surface was derivatized with different bidentate ligands ( thiolactic, beta-mercaptopropionic, mercaptoacetic acids, and alanine) and was investigated by electron paramagnetic resonance (EPR) and IR spectroscopies. Infrared spectroscopy suggests that at pH 4 these comp ounds bind to Ti(IV) surface atoms through the carboxyl group. However , when a thiol group is in the alpha position with respect to the carb oxyl group, surface Ti(IV) atoms become chelated with both the carboxy l and thiol groups resulting in five-membered ring formation. This res ults in the formation of a charge transfer complex with an optical abs orption threshold at 520 nm. Illumination at 77 K of TiO2 colloids wit h 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 i s the farthest from the colloid surface. In the presence of electron a ccepting species (Pb2+, Cd2+) the signal for the trapped electron disa ppears at 250 K indicating electron transfer to the accepting species.