## 3 Results and Discussion
### Visible-Light Absorption  
Figure 4a,b shows the UV-visible absorption spectra of the sets with C\({}_{2}\)H\({}_{2}\) flow rates of 600 sccm and 800 sccm, respectively, prepared by FFCVC. The anatase-to-rutile ratio (A:R) is also provided from estimation of the areas under the X-ray diffraction peak at 25.3\({}^{\circ}\) (anatase (101)) and under the peak at 27.5\({}^{\circ}\) (rutile (110)) from a previous study. All of these powder samples absorbed visible light in the range of 380-600 nm. In Figure 4a, it can be clearly seen that the visible-light absorption ability for samples decreases in the following order: C\({}_{2}\)H\({}_{2}\) 600 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:4; C\({}_{2}\)H\({}_{2}\) 600 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:5; C\({}_{2}\)H\({}_{2}\) 600 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:6; and C\({}_{2}\)H\({}_{2}\) 600 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:3, as the arrow indicates. The visible-light absorption for sample C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:3 is considerably much lower than that of the other samples. In Figure 4b, all four lines almost overlap one another, but they still follow the decreasing order through the sequence of C\({}_{2}\)H\({}_{2}\) 800 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:6; C\({}_{2}\)H\({}_{2}\) 800 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:5; C\({}_{2}\)H\({}_{2}\) 800 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:5; C\({}_{2}\)H\({}_{2}\) 800 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:3, where the sample at C\({}_{2}\)H\({}_{2}\) 800 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:6 showed slightly stronger absorption than other three samples. The visible light absorption levels for C\({}_{2}\)H\({}_{2}\) 800 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:3; C\({}_{2}\)H\({}_{2}\) 800 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:4; C\({}_{2}\)H\({}_{2}\) 800 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:5; and C\({}_{2}\)H\({}_{2}\) 800 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:6 are comparable with, if not higher than, that for C\({}_{2}\)H\({}_{2}\) 600 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:4, which had the highest visible-light absorption for the acetylene flow rate of 600 sccm. In our previous study [26], the visible-light absorption by TiO\({}_{2}\) nanopowders produced by FFCVC was due to a carbon-doping-related effect. In this study, we wanted to verify whether the anatase/rutile formation resulted in a different conclusion, because the anatase/rutile interface may also create a new energy level between the valence band and conduction band edges of TiO\({}_{2}\) and cause visible light absorption [31; 32; 33]. Figure 5 shows the EPR spectra of all samples under visible-light illumination. The main signal appears at g = 2.003. Comparing Figures 4 and 5, it can be seen that for samples with higher absorption ability in the UV-vis spectra, the signal is generally stronger in the EPR spectra under illumination. Hence, the UV-vis spectral result correlates well with the EPR signal at g = 2.003. The signal under illumination is much stronger than that in the dark (not shown), where the weak signals of the EPR spectra in the dark do not correlate with visible-light absorption. The significant EPR signal increase due to visible-light illumination suggests that the EPR signal at g = 2.003 represents the photonic center for visible-light absorption. Since the illumination response is the same, and the  
Figure 4: UV-visible absorption spectra of TiO\({}_{2}\) powders produced by the FFCVC method with (**a**) C\({}_{2}\)H\({}_{2}\) 600 sccm and with (**b**) C\({}_{2}\)H\({}_{2}\) 800 sccm. The dashed line provided in (**b**) is the spectrum for the sample with C\({}_{2}\)H\({}_{2}\) 600 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1.3, for comparison purposes. The anatase-to-rutile ratio (A:R) is also indicated.