Singlet Oxygen Photocatalytic Generation by Silanized TiO₂ Nanoparticles

A commercial TiO₂ sample, used as received or hydrothermally treated to increase surface hydroxylation, has been functionalized by surface modification with hexadecyltrimethoxysilane. The anchoring of the silane has been characterized by means of FTIR and solid-state NMR spectroscopies, and the grafting density was determined by thermogravimetric and N₂ physisorption analyses. The silane moieties induce a partial decrease of the shielding of the valence electrons of the Ti ions at the surface, and a local modification of their crystal field, as demonstrated by XPS and UV/Vis spectroscopy, respectively. The changes in coordination and the produced oxygen vacancies result in the formation of Ti³⁺ defects localized in the sub-surface region, as revealed by EPR spectroscopy. These paramagnetic centers are stabilized in the silanized samples, as the electron transfer to O₂ is efficiently inhibited even under UV irradiation. However, the amount of Ti³⁺ centers appears to be correlated with the singlet oxygen (¹O₂) formation rate. Accordingly, epoxidation of limonene under UV light, chosen as a model photocatalytic reaction triggered by ¹O₂, occurred with higher selectivity when TiO₂ was silanized and upon simultaneous NIR irradiation. These evidences suggest that in the silanized sample ¹O₂ may be generated through Förster-type energy transfer from excited sub-surface Ti³⁺ centers.