ИСТИНА

Experimental and calculated EPR spectra of the Nb(3%)-N-TiO2 sample (as example) are shown in Fig. 1. They show the existence of superposition of sev- eral EPR signals, which may be related to different spin centers. The simulation of EPR spectra allowed us to determine spin-Hamiltonian parameters (ĝ and hyperfine splitting, Â tensor values). These data are necessary for the correct interpretation of the chemical nature of spin centers contributing to the EPR spectra. According to the simulation results, only a low-intensity rhombic-type EPR signal corresponding to Ti3+ centers [2] with g1 = g2 = 1.9897, g3 = 1.9592 can be clearly distinguished in the Nb(3%)-N-TiO2 EPR spectrum recorded in the dark. Also, the spectrum shows a weak line in the region of ~338 mT, the nature of which is not quite clear if we do not consider the spectra of illuminated samples. Under illumination we observed not only a sharp increase in the amplitude of the Ti3+ center signal, but also we see the appearance of a new signal with peaks corresponding to the superfine splitting in nitrogen radicals (g1 = 2.0055, g2 =2.0053,g3 =2.0035,A1 =0.2mT,A2 =0.34mT,A3 =3.2mT)[2].The relatively low concentration of these spin centers corresponds to the selected low level of nitrogen doping. Now we can say that the weak signal in the dark spectrum also corresponds to nitrogen centers. We have detected the appearance of an additional signal near H = 340 mT corresponding to O2− radicals accord- ing to our simulation and literature data (g1 = 2.025, g2 = 2.011, g3 = 2.0043) [2]. We assume the following reactions of O2− radical formation: (a) ħω → h + e− (interband light absorption), and O2 + e− → O2– (b). Nb(6%)-N-TiO2 samples have similar EPR signals but O2– signal has lower intensity. Nb(3%)-TiO2 samples are characterized only by Ti3+ centers, no O2– radicals! Therefore, the presence of nitrogen in the samples promotes the formation of O2– radicals. The results obtained open up new possibilities for the development of gas sensors with improved selectivity based on nanocrystalline Nb-N-doped titania.