Study of paramagnetic defect centers in as-grown and annealed TiO anatase and rutile nanoparticles by a variable-temperature X-band and high-frequency (236 GHz) EPR.

Detailed EPR investigations on as-grown and annealed TiO nanoparticles in the anatase and rutile phases were carried out at X-band (9.6 GHz) at 77, 120-300 K and at 236 GHz at 292 K. The analysis of EPR data for as-grown and annealed anatase and rutile samples revealed the presence of several paramagnetic centers: Ti, O, adsorbed oxygen (O) and oxygen vacancies. On the other hand, in as-grown rutile samples, there were observed EPR lines due to adsorbed oxygen (O) and the Fe ions in both Ti substitutional positions, with and without coupling to an oxygen vacancy in the near neighborhood. Anatase nanoparticles were completely converted to rutile phase when annealed at 1000° C, exhibiting EPR spectra similar to those exhibited by the as-grown rutile nanoparticles. The high-frequency (236 GHz) EPR data on anatase and rutile samples, recorded in the region about g = 2.0 exhibit resolved EPR lines, due to O and O ions enabling determination of their g-values with higher precision, as well as observation of hyperfine sextets due to Mn and Mn ions in anatase.