SURFACE MODIFICATION OF SMALL-PARTICLE TIO2 COLLOIDS WITH CYSTEINE FOR ENHANCED PHOTOCHEMICAL REDUCTION - AN EPR STUDY

Surface complexation of colloidal titanium dioxide nanoparticles (40-6 0 Angstrom) with cysteine was investigated by electron paramagnetic re sonance (EPR) and infrared (diffuse reflectance infrared Fourier trans form-DRIFT) spectroscopies. Cysteine was found to bind strongly to the TiO2 surface, resulting in formation of new trapping sites where phot ogenerated 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 t o 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 havin g different extents of tetragonal distortion of the octahedral crystal field. Upon addition of lead ions, a new complex of cysteine that bri dges surface titanium atoms and lead ions was detected by IR spectrosc opy. Illumination of lead/cysteine-modified TiO2 did not result in the formation of sulfur-centered radicals. Instead, a symmetrical, lattic e defect type EPR signal for trapped holes was observed. Addition of m ethanol 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 s ignal 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 reductio n of Pb2+ ions, and metallic lead precipitated at room temperature. Co nversion of photogenerated holes in the presence of methanol into trap ped electrons can lead to the doubled quantum efficiency of metallic l ead precipitation.