| Eight samples of nanocrystalline TiO2, subjected to three different modifications (various calcination temperatures, N doping levels and water rinsing) have been prepared. The crystallite sizes of all studied samples were determined to be smaller than 15 nm. Electron paramagnetic resonance (EPR)measurements at temperatures 4-290K have been conducted. The EPR spectra displayed a signal, in form of a resonance line, attributed to the trivalent titanium complex. In the low temperature range (T<45K), the temperature dependence of the EPR integrated intensities showed a similar paramagnetic behavior in all cases. At high temperatures (T>50K) a different behavior prevailed and ferromagnetic interactions between the paramagnetic centers were revealed. These interactions are attributed to the presence of free electrons, whose concentration strongly depends on the used modification process. |
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