Study of paramagnetic defect centers in as-grown and annealed TiO₂ anatase and rutile nanoparticles by a...

Detailed EPR investigations on as-grown and annealed TiO₂ 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: Ti³⁺, O^-, adsorbed oxygen (O₂^-) and oxygen vacancies. On the other hand, in as-grown rutile samples, there were observed EPR lines due to adsorbed oxygen (O₂^-) and the Fe³⁺ ions in both Ti⁴⁺ 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 O₂^- ions enabling determination of their g-values with higher precision, as well as observation of hyperfine sextets due to Mn²⁺ and Mn⁴⁺ ions in anatase.