## 3 Results
### Optical Characterization of Metal Titanate Nanorods  
Figure S4 shows the UV-Vis absorption as a function of wavelength for metal titanate nanorods. Table 2 presents the energy gap of metal titanate nanorods. MgTiO\({}_{3}\) and ZnTiO\({}_{3}\) reveal absorption bands at wavelengths lower than 400 nm, explaining their bright white color. However, in ZnTiO\({}_{3}\) a small absorption edge can be spotted in the range 400-550 nm. Moreover, the energy gap of MgTiO\({}_{3}\) was found to be 3.15 eV and for ZnTiO\({}_{3}\) to be 3.5 eV from the Kubelka-Munk formula. CoTiO\({}_{3}\) sample has absorption peaks at 400, 535, 611 nm with the last two bands associated with electron transfer from Co\({}^{2+}\) to Ti\({}^{4+}\)[26]. The sample's energy gap was found to be 2.79 eV. The lack of absorption at the range of 480-550 nm justifies the sample's dark green color. NiTiO\({}_{3}\) sample has a bandgap energy of 2.6 eV and absorption peaks at 448, 505, 744 nm, again justifying the sample's yellow color. Furthermore, the peaks at 448 nm and 505 nm are associated with electron transfer from Ni\({}^{2+}\) to Ti\({}^{4+}\)[69].  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **Energy Gap (eV)** &
\begin{tabular}{c} **Energy Gap (eV)** \\ **Data from Literature** \\ \end{tabular} \\ \hline MgTiO\({}_{3}\) & 3.15 & 3.51 [70] \\ CoTiO\({}_{3}\) & 2.79 & 2.53 [71] \\ NiTiO\({}_{3}\) & 2.6 & 2.33 [72] \\ ZnTiO\({}_{3}\) & 3.5 & 3.16 [73] \\ \hline \hline \end{tabular}
\end{table}
Table 2: Band gap of metal titanate nanorods \(\mathrm{ATIO_{3}}\) (A = Mg, Co, Ni, Zn).  
Figure 6: TEM images of (**a**) MgTiO\({}_{3}\), (**b**) CoTiO\({}_{3}\), (**c**) NiTiO\({}_{3}\), and (**d**) ZnTiO\({}_{3}\) after annealing process.  
The separation efficiency of photo-generated charge carriers is revealed through photoluminescence (PL) spectroscopy. The PL emission spectrum of MTiO\({}_{3}\) (M = Ni, Co, Mg, Zn) is shown in Figure 7. Since A\({}^{2+}\) (A: Metal) itself is non-luminous and the observed luminescence from ATiO\({}_{3}\) nanorods must be due to non-stoichiometry created by the oxygen deficiency in the system, which is expected to arise when ATiO\({}_{3}\) nanorods form in a carbon-rich ambience [65]. The PL emission spectrum of ZnTiO\({}_{3}\) and MgTiO\({}_{3}\) nanorods show the same peaks with different intensity. Although high PL intensity is found for MgTiO\({}_{3}\) and lower PL intensity is observed upon ZnTiO\({}_{3}\) which indicates a lower electron-hole recombination rate. However, NiTiO\({}_{3}\) and CoTiO\({}_{3}\) exhibit the lowest electron-hole recombination. Based on the obtained results, the enhanced photocatalytic activity of NiTiO\({}_{3}\) and CoTiO\({}_{3}\) is ascribed mainly to an increased UV-light harvesting ability and an efficient separation of electron-hole pairs. Especially, it shows that upon 330 nm excitation, a series of emission bands ranging from UV to green region is observed and the bands were centered at 400, 418, 443, 455, 480, 525 nm for ZnTiO\({}_{3}\) and CoTiO\({}_{3}\). In the emission spectra, the broad emission peak at 400 nm is attributed to radiative recombination of photo-generated hole with an electron occupying the oxygen vacancy. Emission band centered at 443 nm is attributed to the recombination of a delocalized electron close to the conduction band with a single charged state of surface oxygen vacancy. The emission band at 455 nm can be attributed to self-trapped excitation luminescence [65]. Two emission peaks centered at 418 and 480 nm are observed also in the PL spectrum of NiTiO\({}_{3}\) and the emission peak at 480 nm is the main peak in the PL spectrum of CoTiO\({}_{3}\)[74; 75]. This might be due to the increase in the crystallites with aspect ratio of the perovskite nanorods [76]. However, the strong emission peak focused at about 418 nm may be also due to radiative recombination process and self-trapped excitations [12].