Preparation, Testing and Characterization of Doped TiO2 Active in the Peroxidation of Biomolecules under Visible Light

Abstract

Doped TiO₂ samples using different preparative procedures were synthesized using either urea or thiourea leading to N- or S-doped TiO₂. Photocatalytic peroxidation and oxidation (mineralization) of phosphatidylethanolamine (PE) lipid with doped TiO₂ were carried out under light irradiation λ > 410 nm. The formation of conjugated double bonds in PE molecules was followed to detect the formation of peroxy radicals (peroxidation index) under light excitation (λ > 410 nm) when doped TiO₂ was used. The kinetics of CO₂ production was monitored during the mineralization of PE. Colored TiO₂ powders were studied in detail by different and complementary physicochemical techniques. The band gap energies of colored TiO₂ were determined by diffuse reflectance spectroscopy (DRS). The visible absorption shoulder of TiO₂ was observed to follow Urbach's law. The variation of the transient decay after 354 nm laser pulse excitation does not correlate with the different N− and S−TiO₂ doping levels introduced by the addition of urea or thiourea. This suggests that the states (recombination centers or traps) introduced by the doping are not effective in varying the decay kinetics within the nanosecond and microsecond time scale. Elemental analysis shows comparable amounts of S- and N-doping of TiO₂ when thiourea is used as dopant. X-ray diffraction reveals no rutile in S−TiO₂ samples heated to 600 °C, suggesting that the addition of sulfur precludes rutilization during sample crystallization. X-ray photoelectron spectroscopy (XPS) of the S−TiO₂ samples confirms the preferential localization of S on the 20 topmost layers of S−TiO₂ upon calcination at 500 °C for 2 h.