## 3 Results and Discussion
### Visible-Light Photocatalytic Reactivity  
Figure 6 shows the degradation of methylene blue as a function of reaction time by the samples under visible light illumination. The dashed lines represent the degradation of methylene without the presence of TiO\({}_{2}\) powders, suggesting that photolysis of methylene blue can be neglected for the 5 h period of time examined. As can be seen from Figure 6a, samples with a single anatase phase such as 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; and C\({}_{2}\)H\({}_{2}\) 600 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:6 appear to have poor photocatalytic activity for methylene blue degradation. C\({}_{2}\)H\({}_{2}\) 600 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:3, the only sample with mixed phase prepared under a C\({}_{2}\)H\({}_{2}\) flow of 600 sccm, shows the highest photocatalytic activity among the samples. Figure 6b shows that for the samples with an acetylene flow rate of 800 sccm, the photocatalytic methylene blue degradation ability is significantly higher than for those with an acetylene flow rate of 600 sccm, and this ability increases with the oxygen flow rate.  
Figure 5: EPR spectra of visible-light-illuminated 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.  
Figure 6: The degradation of methylene blue by TiO\({}_{2}\) powders under visible light for powder produced with (a) C\({}_{2}\)H\({}_{2}\) 600 sccm and with (b) C\({}_{2}\)H\({}_{2}\) 800 sccm. The dashed lines represent the degradation of methylene without the presence of TiO\({}_{2}\) powders.  
As discussed earlier, the EPR signals relate to the UV-visible absorption spectra. The higher the intensity of the signal in EPR, the stronger the absorption of visible light, such as that of C\({}_{2}\)H\({}_{2}\) 600 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:4 with the highest light absorption and EPR signal in Figures 4a and 5a and that of C\({}_{2}\)H\({}_{2}\) 800 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:6 with the highest light absorption and EPR signal in Figures 4b and 5b. However, good absorption of visible light does not necessarily ensure good photocatalytic ability, as can be seen from Figures 4a and 6a, in which the sample with C\({}_{2}\)H\({}_{2}\) 600 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:4 has the strongest absorption of visible light among the four samples prepared under an acetylene flow rate of 600 sccm, but shows a poor result of methylene blue degradation under visible light. On the other hand, the sample with C\({}_{2}\)H\({}_{2}\) 600 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:3 has the weakest light absorption ability but shows the best degradation activity against methylene blue among the samples in the set for acetylene flow of 600 sccm. Recall that the sample with C\({}_{2}\)H\({}_{2}\) 600 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:3 is the only sample which has rutile formation coexisting with anatase in the set with C\({}_{2}\)H\({}_{2}\) flow of 600 sccm. Apparently, the anatase/rutile interface in C\({}_{2}\)H\({}_{2}\) 600 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:3 does play an important role in the enhancement of the photocatalytic reaction. Note that the rutile formation is at the expense of visible-light absorption. Hence, the significant role of the anatase/rutile interface in this study is electron-hole separation instead of photonic center creation for visible-light absorption.