## 3 Results and Discussion
### Photonic Center for Visible-Light Absorption  
The interesting phenomenon that the rutile formation is at the expense of visible-light absorption is worth noticing. In the set with acetylene flow of 600 sccm, as the oxygen flow rate reduces to approach C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:3, rutile forms and is accompanied by a significant reduction of light absorption. This experiment was repeated many times and confirmed that the onset of rutile formation always coincides with a substantial decrease in light absorption for acetylene flow of 600 sccm. Thus, we claim that the visible-light-absorbing photonic center acts as the nucleation site for rutile formation for acetylene flow of 600 sccm, such that the rutile formation accompanies the elimination of the visible-light-absorbing photonic center. Furthermore, since rutile formation reduces the visible-light absorption, the visible-light-incorporating photonic center should locate neither at the anatase/rutile interface nor in rutile grains, but in anatase grains. This agrees well with our previous study that single-phase anatase TiO\({}_{2}\) photocatalyst prepared by FFCVC can absorb visible light while showing poor photocatalytic ability. For the samples prepared with an acetylene flow rate of 800 sccm, rutile formation is not at the expense of the photonic center. For example, C\({}_{2}\)H\({}_{2}\) 800 sccm, C\({}_{2}\)H\({}_{2}\)/O\({}_{2}\) = 1:6 forms rutile (Figure 1b) without compromised visible-light absorption (Figure 4b); the rutile formation under elevated temperature and sufficient oxygen availability is through phase transformation from anatase aggregates and not through nucleation from the photonic center. Thus, the remaining anatase may still possess the photonic center for visible light absorption for samples prepared under acetylene flow of 800 sccm. As discussed before, the formation of rutile by nucleation is feasible in an oxygen-deficient environment even when the temperature is not high enough for rutile formation by phase transformation from large anatase aggregates (\(>\)15 nm). Oxygen vacancy should play a vital role in the unlocking of the edge-sharing octahedra towards corner-sharing octahedra, which leads to rutile nucleation. In light of the finding that the visible-light-absorbing photonic center acts as the nucleation site for rutile formation for acetylene flow of 600 sccm, the visible-light-incorporating photonic center should be closely related to the defect of oxygen vacancy on the surface of anatase grains. The literature suggests that the defect level of oxygen vacancy in anatase lies 0.75 eV-1.18 eV below the conduction band edge of anatase. The defect level described above agrees quite well with our visible-light absorption spectrum starting from the lowest photonic energy with an equivalent wavelength around 600 nm. Thus, the visible-light absorption by our samples should yield electron excitation from the valance band edge to the defect level of oxygen vacancy in anatase. A schematic energy band diagram is required, shown in Figure 7, for the anatase/rutile photocatalyst prepared by the FFCVC process to illustrate the possible electron flow direction and to highlight the role of the anatase/rutile dual-phase structure in enhancing photocatalytic activity under visible light illumination. The anatase and rutile were aligned following the consensus suggested in the literature that the conduction band edge has about 0.2 eV offset. In contrast, the valance band edges of anatase and rutile are close to each other. Recall that the defect level of the photonic center is located on the anatase side. When visible light illuminates the nanopowder prepared with C2H2 600 sccm, C2H2/O2 = 1:3, there is little visible-light absorption, and the electron-hole pair cannot be formed by visible-light illumination, as shown in Figure 7a. When visible light illuminates the nanopowder with significant visible-light absorption, the electrons are excited to the defect energy level in anatase, as shown in Figure 7b.