UV-induced reactions of Mg2+-doped anatase nanocrystals with exposed {0 0 1} facets: An EPR study

The photoinduced processes of pristine and Mg2+-doped anatase nanopowders with the exposed {0 0 1} facets synthesized by a solvothermal method, using HF as the capping agent, were investigated by EPR spectroscopy. The adsorbed fluoride ions on the {0 0 1} facets of the as-prepared anatase samples have been removed by NaOH washing (washed samples) or calcination at 600 °C (calcined samples). EPR spectra monitored at 100 K for the as-prepared and washed anatase nanopowders characterized with high percentage of the exposed {0 0 1} facets, evidenced the presence of paramagnetic signals attributed to the intrinsic Ti3+ centers with narrow-line axially symmetric spectrum (g⊥ = 1.988, g∥ = 1.956) and holes (O−) characterized with g-tensor g⊥ = 2.006, g∥ = 2.003 already before exposure. Upon in situ UV exposure under air, the intensity of EPR signals of Ti3+ and O− increased significantly and two additional signals compatible with the photogenerated O2− and O2H were observed. A series of indirect EPR techniques, e.g., spin trapping using 5,5-dimethyl-1-pyrroline N-oxide (DMPO), hindered amine oxidation, or reduction of radical cation of 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate), were applied to follow the reactions in the irradiated TiO2 dispersions. While in the irradiated aerated aqueous media all anatase samples revealed the generation of DMPO-OH spin-adduct, upon UV exposure of the as-prepared samples in deaerated acetonitrile, the formation of a unique EPR spectrum attributed to DMPO-F was found. The DMPO-F spin-adduct is most probably produced by the inverted spin trapping mechanism via interaction of DMPO+ with the surface fluorides.