EPR spectra of all samples did not change during months being conserved at room temperature in darkness. EPR signals of surface carbon-doped TiO2
-3) are practically isotropic and are characterized by rather high intensity. Their parameters are equal to the following: g
0.0005; the line width ΔH2
G and ΔH3
G (Figure ). Samples with higher carbon concentration (C-TiO2
-2) have higher content of paramagnetic centers: N2
spin/g. Similar EPR signals were reported in [13
] for the carbon dangling bonds in amorphous carbon particles. Another possible explanation of the nature of the such-type EPR signal can be found in [8
]. The authors of [8
] ascribed a symmetric single line with g
2.0030 to the conduction electrons trapped by oxygen vacancies. Unfortunately, a mechanism of such process is not clear from both papers.
Figure 1 EPR spectra of surface-doped samples at 5K. 1, C-TiO2-2; 2, C-TiO2-3. Inset shows the same samples but at 300K. Arrow shows the position of g-values.
It should be mentioned that the shape of EPR spectrum and the main parameters were unchanged for both samples at different temperatures: 300 and 5
K (Figure , inset). This fact reflects the negligible role of spin–lattice relaxation in these samples. The volume-doped samples (C-TiO2
-1) had completely different EPR signals (Figure ). The asymmetric shape of the signal is known for the 17e−
three atomic π-radical with g
-factor values: g1
0.0005, and g3
0.0005. This signal can be assigned to CO2−
radicals, which were previously detected in MgO, NaHCO2
, and KHCO2
]. Seems, this anion-radical has been observed in C-TiO2
samples firstly. The EPR signal of CO2−
radicals was also detected at room temperature but with lower intensity (Figure , inset). We assume that CO2−
radicals are located in the interstitial sites of TiO2
lattice. Taking into account a shoulder of the EPR signal in a magnetic field within g
2.0043-2.034 (Figure ) and the absence of EPR signals from Ti3+
centers, one can propose the following mechanisms of CO2−
formation at the stage of C-TiO2
-1 synthesis: CO2
(lattice). The g
-values of O−
radicals are the following for various matrixes: g1
2.009-2.019, and g3
]. Therefore, we assume that the shoulder of the EPR line mentioned above can be assigned to EPR signal of O−
radicals. The content of paramagnetic centers in C-TiO2
-1 samples was equal to N1
Figure 2 EPR spectrum of volume-doped samples C-TiO2-1 at 5K. Inset shows the same sample but at 300K. Arrows show the position of g-values.
Under illumination of all doped-TiO2 samples, a growth of EPR signal intensity was registered. As an example, the effect of illumination of C-TiO2-2 sample on the EPR spectrum is shown in Figure . Partial reduction of the EPR signal intensity has been observed after illumination (Figure ). Such changes of the EPR signal intensity under and after illumination can be explained due to a light absorbance by negatively or positively charged carbon dangling bonds, which are located inside the energy gap of TiO2. During illumination, the dangling bonds are changing to a neutral paramagnetic state; therefore, the spin density of paramagnetic centers increases. After illumination, density of paramagnetic centers decreases due to capture of electrons and holes by neutral paramagnetic centers.
Figure 3 EPR spectra of surface-doped sample C-TiO2-2 at 5K. 1, before illumination; 2, under illumination; 3, after illumination.