La- and Rh-codoped SrTiO3 as a Hydrogen Evolution Photocatalyst in Z-scheme Water Splitting
Kazunari Domen a, Takashi Hisatomi a, Qian Wang a, Su Su Khine Ma a, Yanbo Li a
Oral, Takashi Hisatomi, presentation 005
Publication date: 16th April 2014
Z-scheme overall water splitting is a promising approach to achieving solar energy conversion, because a narrow band gap photocatalyst that is active either for water reduction or oxidation can be applied. Rh-doped SrTiO3 (SrTiO3:Rh) and Ta3N5 are applicable as a hydrogen evolution photocatalyst and an oxygen evolution photocatalyst, respectively, in redox-mediator-free Z-scheme water splitting. However, the solar-to-hydrogen conversion efficiency of the system was limited by the insufficient photocatalytic activity of SrTiO3:Rh.
SrTiO3:Rh generates hydrogen under visible light through the transition from electron donor levels formed by Rh3+ ions to the conduction band. However, coexisting Rh4+ species offer a recombination route. Codoping is a useful strategy for controlling the valence states of dopants and can improve the photocatalytic activity of doped SrTiO3. Recently, we found that La and Rh codoped SrTiO3 (SrTiO3:La/Rh) exhibited higher activity than SrTiO3:Rh for sacrificial hydrogen evolution from an aqueous methanol solution and redox-mediator-free Z-scheme water splitting in combination with Ta3N5. The solar-to-hydrogen efficiency was improved by three times. In this talk, the effect of preparation methods, calcination time, and doping rates of La on crystallinity, visible light absorption, and photocatalytic water-splitting performance of SrTiO3:La/Rh will be discussed. 
SrTiO3:La/Rh was synthesized by a two-step solid state reaction (SSR). SrTiO3 was synthesized by SSR from SrCO3 and rutile-type TiO2. Subsequently, the synthesized SrTiO3, La2O3, and Rh2O3 were calcined to yield SrTiO3:La/Rh. The use of crystalline SrTiO3 as a starting material was essential to prepare single phase SrTiO3:La/Rh. Impurities such as La2O3 and LaRhO3 were produced even at the low doping level of 1% when a sample was prepared directly from SrCO3, TiO2, La2O3, and Rh2O3. It seems that SrTiO3 worked as a perovskite-type template that offered nucleation centers for the perovskite-type SrTiO3 phase and suppressed segregation of impurities. A unique feature of the physical properties underlying the synthesized SrTiO3:La/Rh was found by depth-profiling. The concentration of La and Rh ions decreased with increasing the depth from the surface of SrTiO3:La/Rh particles, because the dopants diffused from the surface into the bulk during the calcination. The photocatalytic activity was sensitive to the calcination times in the second calcination step of the two-step SSR. It is considered that the presence of dopant-rich surface is beneficial in that photoexcited carriers can move to active sites on the surface in shorter times.

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