New insight into singlet oxygen generation at surface modified nanocrystalline TiO2 – the effect of near-infrared irradiation

Abstract

The generation of singlet oxygen in aqueous colloids of nanocrystalline TiO₂ (anatase) modified by organic chelating ligands forming surface Ti^IV complexes was studied. Detailed studies revealed a plausible and to date unappreciated influence of near-infrared irradiation on singlet oxygen generation at the surface of TiO₂. To detect ¹O₂, direct and indirect methods have been applied: a photon counting technique enabling time-resolved measurements of ¹O₂ phosphorescence, and fluorescence measurements of a product of singlet oxygen interaction with Singlet Oxygen Sensor Green (SOSG). Both methods proved the generation of ¹O₂. Nanocrystalline TiO₂ modified with salicylic acid appeared to be the most efficient photosensitizer among the tested materials. The measured quantum yield reached the value of 0.012 upon irradiation at 355 nm, while unmodified TiO₂ colloids appeared to be substantially less efficient generators of singlet oxygen with the corresponding quantum yield of ca. 0.003. A photocatalytic degradation of 4-chlorophenol, proceeding through oxidation by OH˙, was also monitored. The influence of irradiation conditions (UV, vis, NIR or any combination of these spectral ranges) on the generation of both singlet oxygen and hydroxyl radicals has been tested and discussed. Simultaneous irradiation with visible and NIR light did not accelerate OH˙ formation; however, for TiO₂ modified with catechol it influenced ¹O₂ generation. Singlet oxygen is presumably formed according to Nosaka's mechanism comprising O₂˙⁻ oxidation with a strong oxidant (hole, an oxidized ligand); however, the energy transfer from NIR-excited titanium(III) centers (trapped electrons) plays also a plausible role.