Study of rutile TiO(110) single crystal by transient absorption spectroscopy in the presence of Cecations in aqueous environment. Implication on water splitting.

Cecations are commonly used as electron acceptors during the water oxidation to Oreaction over Ir- and Ru-based catalysts. They can also be reduced to Cecations by excited electrons from the conduction band of an oxide semiconductor with a suitable energy level. In this work, we have studied their interaction with a rutile TiO(110) single crystal upon band gap excitation by femtosecond transient absorption spectroscopy (TAS) in solution in the 350-900 nm range and up to 3.5 ns. Unlike excitation in the presence of water alone the addition of Ceresulted in a clear ground-state bleaching (GSB) signal at the band gap energy of TiO(ca. 400 nm) with a time constant= 4-5 ps. This indicated that the Cecations presence has quenched the e-h recombination rate when compared to water alone. In addition to GSB, two positive signals are observed and are attributed to trapped holes (in the visible region, 450-550 nm) and trapped electrons in the IR region (>700 nm). Contrary to expectation, the lifetime of the positive signal between 450 and 550 nm decreased with increasing concentrations of Ce. We attribute the decrease in the lifetime of this signal to electrostatic repulsion between Ceat the surface of TiO(110) and positively charged trapped holes. It was also found that at the very short time scale (<2-3 ps) the fast decaying TAS signal of excited electrons in the conduction band is suppressed because of the presence of Cecations. Results point out that the presence of Cecations increases the residence time (mobility) of excited electrons and holes at the conduction band and valence band energy levels (instead of being trapped). This might provide further explanations for the enhanced reaction rate of water oxidation to Oin the presence of Cecations.