Photogreying of TiO2 nanoparticles

Photogreying, the change in brightness on UV irradiation in the absence of oxygen, of TiO2 nanoparticulate dispersions is shown to depend on the nature of the liquid, consistent with a surface reaction. Measurements on a series of TiO2 particles (mainly 75×10 nm) dispersed in, e.g., alkyl benzoate correlate well with those on the same TiO2’s dispersed in a second liquid (e.g. propan-2-ol). Photogreying in propan-2-ol is paralleled by photocatalytic-oxidation activity, indicating a common origin – UV-generation of charge carriers. Further, photogreying parallels Ti3+ formation. Hence, although appearance and the visible spectra of photogreyed particles both differ from those of Ti3+ in ≤10 nm colloidal TiO2, we suggest that photogreying is caused by capture of UV excited electrons to form Ti3+. Surface treatment reduces photogreying, and we speculate that differences between uncoated samples reflect differences in the number of potentially reducible Ti’s.