Revealing the role of the Rh valence state, la doping level and Ru cocatalyst in determining the H ₂ evolution efficiency in doped SrTiO ₃ photocatalysts

SrTiO ₃ (STO) has favorable opto-electronic properties for overall water splitting. Nevertheless, realizing a higher efficiency is impeded by its band gap which can only harvest UV light. In order to extend the spectral response towards visible light, STO is (co)doped with lanthanum (La) and rhodium (Rh). However, notwithstanding the amount of visible light absorbed, the H ₂ evolution rates are remarkably governed by the valence state of Rh, La doping level and ruthenium (Ru) cocatalyst loading. Hence, it is essential to unravel the underlying effect of doping on the photophysical processes to gain insight into material design. To this end, charge carrier dynamics was probed over a wide time (sub-picosecond to microsecond) and spectral (visible to IR) region using transient absorption spectroscopy. Depending on the dopant composition, an interplay between the electron trapping and the kinetics of the electron transfer to the Ru cocatalyst was rationalized. For Rh ⁴⁺ :STO, free electrons probed at 3435 nm decayed virtually completely by 20 ps resulting in a kinetic competition between the electron trapping and the electron transfer to Ru cocatalyst. In the case of Rh ³⁺ :STO, free electrons decayed by a factor of three by 100 ps, thus demonstrating the effect of Rh valence state on the electron lifetime. The time constant and quantum yield of electron transfer from Rh ³⁺ :STO to the Ru cocatalyst were found to be 1.6 ps and 14.7%, respectively. In addition to a longer electron lifetime, enhanced electron transfer to the Ru cocatalyst makes Rh ³⁺ :STO one of the promising photocatalysts for H ₂ generation. Engineering the energetic position of the dopant within the band gap to avoid undesirable carrier trapping is crucial to enhance the efficiency of photocatalytic reactions.