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

Abstract

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 TiO 2 surface, resulting in formation of new trapping sites where photogenerated electrons and holes are localized. Illumination of cysteine-modified TiO 2 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 TiO 2 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 TiO 2 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 ·CH 2 OH 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 Pb 2+ 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.