## 3 Results and Discussion
### Photocatalytic Water-Splitting Activity  
Figure 9 shows the H\({}_{2}\) and O\({}_{2}\) evolution time course on Ru/Rh:SrTiO\({}_{3}\)-BiVO\({}_{4}\) photocatalysts taken from the 0.5 mM Co(bpy)\({}_{3}\)\({}^{2+}\) solution. The gas production stopped when the light was turned off and similar hydrogen and oxygen production rates were observed in 8 h. This indicates that the Z-scheme overall water splitting on these catalysts was photocatalytic and that the photocatalysts can be used without deactivation. Furthermore, the H\({}_{2}\) and O\({}_{2}\) ratio of products were constant during the reaction and close to the theoretical stoichiometric ratio (H\({}_{2}\)/O\({}_{2}\) = 2). The related reactions of the Z-scheme photocatalytic water splitting are written as following [13]: On H\({}_{2}\) evolution photocatalyst:  
Co(bpy)\({}_{3}\)\({}^{2+}\) + h\({}^{+}\)\(\rightarrow\) Co(bpy)\({}_{3}\)\({}^{3+}\)  
2H\({}^{+}\) + 2e\({}^{-}\)\(\rightarrow\) H\({}_{2}\)  
On H\({}_{2}\) evolution photocatalyst:  
2 H\({}_{2}\)O + 4h\({}^{+}\)\(\rightarrow\) O\({}_{2}\) + 4H\({}^{+}\)  
Co(bpy)\({}_{3}\)\({}^{3+}\) + e\({}^{-}\)\(\rightarrow\) Co(bpy)\({}_{3}\)\({}^{2+}\)  
The average production rates of hydrogen and oxygen from water splitting over this period are listed in Table 1 which increased as the following order: Ru/HT-STO2 Rh < Ru/SS-STO2 Rh < Ru/MW-STO2 Rh-A < Ru/SS-STO2 Rh-T. These results show that the Rh-doped SrTiO\({}_{3}\) photocatalyst prepared by microwave-assisted hydrothermal method exhibited a very high hydrogen evolution rate of water-splitting reaction under visible light irradiation. The activity of Ru/SS-STO2 Rh-T drastically increased more than 5 times as compared to Ru/SS-STO\({}_{2}\)Rh, and 10 times than Ru/HT-STO prepared by convention hydrothermal method. \begin{table}
\begin{tabular}{c c c} \hline
**Catalyst** & **H\({}_{2}\) Evolution Rate (umol eg\({}^{-1}\) h\({}^{-1}\))** & **O\({}_{2}\) Evolution Rate (umol eg\({}^{-1}\) h\({}^{-1}\))** \\ \hline Ru/SS-STO2 Rh & 56 & 35 \\ Ru/HT-STO2 Rh & 28 & 16 \\ Ru/MW-STO2 Rh-A & 141 & 75 \\ Ru/MW-STO2 Rh-T & 317 & 168 \\ \hline \end{tabular}
\end{table} 
Table 1: Photocatalytic water-splitting activity of Rh-doped SrTiO\({}_{3}\)-BiVO\({}_{4}\) in 0.5 mM CoBpy\({}^{2+/3+}\) solution. Figure 9: Time course of H\({}_{2}\) evolution and O\({}_{2}\) evolution for the Z-scheme photocatalysts water splitting on 1 wt % Ru/Rh:SrTiO\({}_{3}\)–BiVO\({}_{4}\) where (**a**) Ru/SS-STO2 Rh (**b**) Ru/HT-STO2 Rh (**c**) Ru/MW-STO2 Rh-A (**d**) Ru/MW-STO2 Rh-T was used as H\({}_{2}\) evolution catalyst. Conventional solid-state reaction is widely used to synthesize semiconductor photocatalyst. However, the high-temperature synthesis process leads to large particles and low surface area. It is known that preparation of photocatalyst nanoparticles via hydrothermal method. However, crystallinity is one of the most important factors of the photocatalytic activity of photocatalyst. Although HT-STO2 Rh sample consisted smaller particles than SSR-STO2 Rh sample, it showed a lower activity as compared to the SSR-STO2 Rh sample. [PERSON] et al. [16] reported that the SrO defects in the Rh-doped SrTiO\({}_{3}\) photocatalyst prepared by hydrothermal method are the recombination centers for the photogenerated electron and holes which is the main reason of low activity. Their results indicated that a post calcination at 1273 K with excess Sr was necessary to decrease SrO defects of in the Rh-doped SrTiO\({}_{3}\) photocatalyst prepared by hydrothermal method. The low activity of Ru/HT-STO2 Rh was attributed to the SrO defects formed in hydrothermal reaction. The high activity of Ru/MW-STO2 Rh-A and Ru/SS-STO2 Rh-T samples indicated that the microwave-assisted hydrothermal method has distinct advantages in the synthesis of high crystallinity Rh-doped SrTiO\({}_{3}\) nanoparticles with low SrO defects.