## 3 Results and Discussion  
### Characterization of the Hydrogen Evolution Catalyst: Rh-Doped SrTiO\({}_{3}\)  
Figure 2 shows the XRD patterns Rh-doped SrTiO\({}_{3}\) photocatalysts prepared by different methods: conventional solid-state reaction (SS), hydrothermal synthesis (HT) and microwave-assisted hydrothermal method (MW). The XRD patterns of all prepared materials show obvious diffraction peaks at 2\(\theta\) = 32.4\({}^{\circ}\), 40.0\({}^{\circ}\), 46.5\({}^{\circ}\), and 57.8\({}^{\circ}\) which corresponded to the (110), (111), (200) and (221) planes of SrTiO\({}_{3}\) (JCPDS 35-0734). No reflection peak corresponded to rhodium or rhodium oxide species of doping elements suggesting that dopant ions were introduced into structure of SrTiO\({}_{3}\) or were too small to be detected by XRD.  
Figure 1: (**a**) Closed reaction system for photocatalysis water splitting (**b**) Emission spectrum of 150 W Xe lamp.  
Figure 2: XRD patterns of Rh-doped SrTiO\({}_{3}\) catalysts (**a**) SS-STO2 Rh (**b**) HT-STO2 Rh (**c**) MW-STO2 Rh-A (**d**) MW-STO2 Rh-T.  
The SS-STO2 Rh showed a much weaker diffraction peaks than samples prepared via hydrothermal route. As compared with conventional solid-state reaction method, a better crystalline structure could be formed for Rh-doped SrTiO\({}_{3}\) prepared by hydrothermal synthesis. The small diffraction peaks at 2\(\theta\) of 31.5\({}^{\circ}\) were considered as the presence of Sr\({}_{2}\)TiO\({}_{4}\) (CPDS-39-1471).  
On the other hand, the XRD results also shows that samples prepared via hydrothermal route contained trace amount of SrCO\({}_{3}\) (JCPDS 84-0418, diffraction peaks located in 2\(\theta\) of 20.5\({}^{\circ}\) and 25.4\({}^{\circ}\) as the (110) and (111) planes SrCO\({}_{3}\)) for which was due to the reaction of environment CO\({}_{2}\) with the precursors such as Sr(OH)\({}_{2}\) and NaOH. The sharp diffraction peaks indicated that the samples were fully crystallized SrTiO\({}_{3}\) with cubic space group Pm3m (221). The average crystallite sizes of Rh-doped SrTiO\({}_{3}\) samples (HT-STO2 Rh, MW-STO2 Rh-A and MW-STO2 Rh-T) prepared by hydrothermal method were about 3.6 nm, determined by Scherrer equation. It should be noted that while conventional hydrothermal synthesis of Rh-doped SrTiO\({}_{3}\) required a long synthesis time (\(>\)12 h), high crystallinity Rh-doped SrTiO\({}_{3}\) was formed in a very short time (about 1 h) during microwave-assisted hydrothermal synthesis [21]. These results indicated that the lattice deformation of Rh-doped SrTiO\({}_{3}\) are associated with the synthesis route.  
To obtain the effects of synthesis route on their size and morphology, the samples were subjected to TEM analysis. Figure 3 depicts the TEM images of the Rh-doped SrTiO\({}_{3}\) prepared from different methods. The TEM results show that their morphology was significant different. The high-temperature solid-state synthesis caused the formation of large SS-STO2 Rh particles with a diameter of 1-5 \(\upmu\)m while the HT-STO2 Rh catalyst showed smaller particles ranged from 50-500 nm. The TEM images also revealed that the MW-STO2 Rh-A and MW-STO2 Rh-T samples prepared by microwave-assisted hydrothermal method have a smallest particle sizes of about 100-200 nm. The TEM images also illustrated that the morphology Rh-doped SrTiO\({}_{3}\) by microwave-assisted hydrothermal method was changed sensitively by the titanium precursor. The MW-STO2 Rh-A showed uniform cubic nano particles whereas the MW-STO2 Rh-T showed a porous nanocrystal structure.  
Figure 3: TEM images of Rh-doped SrTiO\({}_{3}\) catalysts (**a**) SS-STO2 Rh (**b**) HT-STO2 Rh (**c**) MW-STO2 Rh-A (**d**) MW-STO2 Rh-T.  
SrTiO\({}_{3}\) prepared by microwave-assisted hydrothermal method consisted less Rh\({}^{4+}\) species than SS-STO2 Rh prepared by solid-state method [23]. These results indicated donor and acceptor levels were introduced in the band gap of SrTiO\({}_{3}\) by the doping Rh.