Surface Modification of Small Particle TiO2 Colloids with Cysteine for Enhanced Photochemical Reduction:  An EPR Study

Surface complexation of colloidal titanium dioxide nanoparticles (40−60 Å) with cysteine was investigated by electron paramagnetic resonance (EPR) and infrared (diffuse reflectance infrared Fourier transform−DRIFT) spectroscopies. Cysteine was found to bind strongly to the TiO2 surface, resulting in formation of new trapping sites where photogenerated electrons and holes are localized. Illumination of cysteine-modified TiO2 at 77 K resulted in formation of cysteine radicals with the unpaired electron localized on the carboxyl group. Upon warming to 150 K, these radicals are transformed into sulfur-centered radicals as observed by EPR spectroscopy. We have demonstrated the existence of two surface Ti(III) centers on cysteine-modified TiO2 particles having different extents of tetragonal distortion of the octahedral crystal field. Upon addition of lead ions, a new complex of cysteine that bridges surface titanium atoms and lead ions was detected by IR spectroscopy. Illumination of lead/cysteine-modified TiO2 did not result in the formation of sulfur-centered radicals. Instead, a symmetrical, lattice defect type EPR signal for trapped holes was observed. Addition of methanol to this system resulted in the formation of a ·CH2OH radical at 8.2 K. After the temperature was raised to 120 K, doubling of the signal associated with electrons trapped at the particle surface (Ti(III)surf) was observed. On further increase of the temperature to 200 K, the EPR signal for trapped electrons disappeared due to the reduction of Pb2+ ions, and metallic lead precipitated at room temperature. Conversion of photogenerated holes in the presence of methanol into trapped electrons can lead to the doubled quantum efficiency of metallic lead precipitation.