EPR, spectroscopic and photocatalytic properties of N-modified TiO2 prepared by different annealing and water-rinsing processes

Samples of TiO2, calcined at 350 °C, 400 °C, 450 °C and 500 °C, have been prepared without and with water rinsing. The crystallite sizes were determined to be below 32 nm for all types of samples. Temperature dependence of magnetic resonance spectra of the obtained samples has been investigated. The electron paramagnetic resonance (EPR) spectra of free radicals were recorded for all samples, non-rinsed and water-rinsed, centered between geff = 2.0027(3) and geff = 2.0032(3) with linewidths ΔHpp = 5(1) G and ΔHpp = 8(1) G. The temperature dependence of the EPR integrated intensities has shown the presence of different relaxation processes operating in high temperature range, which could have arisen due to different concentrations of conducting electrons. For the water-rinsed sample annealed at 400 °C the ferromagnetic resonance (FMR) spectrum of magnetic agglomerates was recorded. Very intense reorientation processes of a correlated spin system have been observed in different temperature ranges. The EPR spectra of trivalent titanium ions were registered in all samples. The number of oxygen vacancies could have increased, possibly involving complexes and agglomerates containing trivalent titanium ions.

► Photocatalytic activity of TiO2 nanopowders increased after rinsing samples with water.
► Ferromagnetic resonance spectrum of TiO2 nanopowder has appeared after rinsing with water.
► An increase of number of oxygen vacancies could account for both phenomena.