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

Abstract

Anatase and rutile nanoparticulate samples, prepared via TiCl4 hydrolysis using either ordinary or 17O-enriched water (up to 90%) with subsequent heat-treatment under vacuum, were investigated by EPR spectroscopy upon UV photoexcitation in the temperature range 100–160K. The experimental EPR spectra of photoexcited anatase Ti16O2 revealed the presence of paramagnetic signals attributed to Ti(III) species, two kinds of O− centers produced from lattice O2− ions, along with O2−/HO2 radicals which originated from adsorbed molecular oxygen. The presence of 17O 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 17O nucleus (g⊥=1.962, g||=1.923; A⊥=1.68mT, A||=1.61mT), the signal of a photogenerated hole 17O− (g⊥=2.019, g||=2.004; A⊥=1.76mT, A||=10.61mT) and O2−. 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 17O-enriched rutile sample was simulated as a superposition of Ti(III)–17O 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 DMPO–OH spin-adduct was found (AN=1.492mT, AH=1.476mT; g=2.0059). The dependence of the DMPO–OH concentration on the irradiation time was similar for 16O- and 17O-anatase samples, as well as for both rutile samples. The concentration of DMPO–OH spin-adduct, after 10-min exposure, monitored for rutile was reduced by approximately 40% in comparison with anatase.