EPR study of 17O-enriched titania nanopowders under UV irradiation

Anatase and rutile nanoparticulate samples, prepared via TiCl4 hydrolysis using either ordinary or (17)Oenriched water (up to 90%) with subsequent heat-treatment under vacuum, were investigated by EPR spectroscopy upon UV photoexcitation in the temperature range 100-160 K. The experimental EPR spectra of photoexcited anatase (TiO2)-O-16 revealed the presence of paramagnetic signals attributed to Ti(III) species, two kinds of O- centers produced from lattice O2- ions, along with (O2 center dot-/HO2 center dot) radicals which originated from adsorbed molecular oxygen. The presence of O-17 atoms in the anatase crystal lattice was reflected in substantial EPR line broadening and the experimental spectrum was interpreted as a superposition of signals attributed to Ti(III) with axial symmetry arising from an interaction with an O-17 nucleus (g(perpendicular to) = 1.962, g(parallel to) = 1.923; A(perpendicular to) = 1.68 mT, A(parallel to) = 1.61 mT), the signal of a photogenerated hole O-17(-) (g(perpendicular to) = 2.019, g(parallel to) = 2.004; A(perpendicular to) = 1.76 mT, A(parallel to) = 10.61 mT) and O-2(center dot-). Due to the thermal treatment at higher temperature (1000 C) both rutile samples showed a variety of paramagnetic signals attributed to Ti(III) sites already present before photoexcitation. The EPR spectrum of (17)O(-)enriched rutile sample was simulated as a superposition of Ti(III)-O-17 sites and photogenerated ozonide in the rutile matrix. The ability of synthesized anatase and rutile nanopowders to produce hydroxyl radicals upon UV photoexcitation in the aerated aqueous suspensions was monitored via an EPR spin trapping technique using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the spin trapping agent. Upon continuous exposure of titania suspensions a typical four-line EPR signal of the center dot DMPO-OH spin-adduct was found (A(N) = 1.492 mT,A(H) = 1.476 mT; g = 2.0059). The dependence of the (DMPO)-D-center dot-OH concentration on the irradiation time was similar for O-16- and O-17-anatase samples, as well as for both rutile samples. The concentration of center dot DMPO-OH spin-adduct, after 10-min exposure, monitored for rutile was reduced by approximately 40% in comparison with anatase. (c) 2013 Elsevier B.V. All rights reserved.