Intrinsic and impurity defects in chromium-doped SrTiO3 nanopowders: EPR and NMR study



Electron Paramagnetic Resonance (EPR) and Nuclear Magnetic Resonance (NMR) spectra were studied in solvothermal- and citrate sol–gel-grown (CSGG) SrTiO3:Cr(0.1%) crystalline nanopowders. Various growth conditions and thermal treatments were used to determine defects and to tailor nanoparticles structure and crystallization processes. Solvothermal-grown nanopowders with 5 nm particle size revealed the dominant presence of the single perovskite-type phase. Annealing of the CSGG nanoparticles in air up to 800 °C leads to an increase of the particles size and the amount of the titanium dioxide (rutile and/or anatase) phase, which is initially present in small amounts in CSGG powders. It was found that paramagnetic Cr3+, Cr5+, equation image, O, O2− ions are present in solvothermal-grown nanopowders and in all CSGG powders. In the annealed particles the majority of paramagnetic centers (PC) are located in the particle surface and in the TiO2 phase. The nature of the PC in nanopowders exhibiting different degrees of catalytic activity in nanopowders was studied too. A correlation between O and O2− content, synthesis conditions and particle size was found. The NMR spectra of 47Ti (I = 5/2) and 49Ti (I = 7/2) reveal a pronounced dependence of the spectral line intensity on synthesis conditions, evidencing coexistence of Ti3+ and Ti4+ ions in SrTiO3:Cr nanoparticles.