Conversion of holes into reducing species on surface modified small-particle TiO{sub 2}

Complexation of colloidal titanium dioxide nanoparticles (40 {angstrom}) by cysteine as a surface derivative was investigated by electron paramagnetic resonance (EPR) and infra-red (diffusion reflectance infra-red Fourier Transform DRIFT) spectroscopies. It was found that cysteine strongly binds to the colloid surface. The authors have demonstrated with EPR spectroscopy that cysteine modifies the TiO{sub 2} surface with formation of new trapping sites where photogenerated electrons and holes are localized. Illumination of cysteine modified TiO{sub 2} at 77K resulted in formation of a sulfur centered radical observed by EPR spectroscopy at 200 K. 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{sub 2} did not result in the formation of sulfur centered radical, but symmetrical, lattice defect type EPR signal for trapped holes was observed. However, addition of methanol to this system resulted in the formation of {center_dot}CH{sub 2}OH radical following illumination at 8.2 K. After the temperature was raised to 120 K, doubling of the signal associated with electrons trapped at particle surface (Ti(3){sub surf}) was observed. On further increase of the temperature to 200 K the EPR signal for trapped electrons disappeared as a result of the reduction of Pb{sup 2+} ions, and metallic lead was observed to precipitate. Conversion of photogenerated holes into trapped electrons due to the presence of methanol doubles the yield of trapped electrons that can reduce Pb{sup 2+}. Direct reduction of Pb{sup 2+} ions by {center_dot}CH{sub 2}OH radical on TiO{sub 2} was not detected