Study of rutile TiO2(110) single crystal by transient absorption spectroscopy in the presence of Ce4+cations in aqueous environment. Implication on water splitting.

Ce4+cations are commonly used as electron acceptors during the water oxidation to O2reaction over Ir- and Ru-based catalysts. They can also be reduced to Ce3+cations 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 TiO2(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 Ce4+resulted in a clear ground-state bleaching (GSB) signal at the band gap energy of TiO2(ca. 400 nm) with a time constantt= 4-5 ps. This indicated that the Ce4+cations 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 Ce4+. We attribute the decrease in the lifetime of this signal to electrostatic repulsion between Ce4+at the surface of TiO2(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 Ce4+cations. Results point out that the presence of Ce4+cations 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 O2in the presence of Ce4+cations.

Monitoring the Lifetime of Photoexcited Electrons in a Fresh and Bulk Reduced Rutile TiO2 Single Crystal. Possible Anisotropic Propagation.

Defects (oxygen vacancies and interstitial cations) in oxide semiconductors have recently been invoked as a key property behind increased photocatalytic reaction rates. In this work, we have monitored by transient absorption spectroscopy (TAS) excited electrons in the conduction band decaying into the invoked traps to extract their lifetime using a rutile single crystal instead of the more conveniently used powder homologue. This is preferred in order to rule out grain boundary, degree of crystallinity, and size effects among other parameters that would obscure the results. It was found, in the energy region investigated (1.3-1.8 eV), that the lifetime of excited electrons is about four times shorter for the bulk defect crystal when compared to the fresh one. This indicates that the created defects (mostly oxygen defects and interstitial Ti cations) are unlikely to contribute to reaction rate enhancement.

Up-conversion luminescence coupled to plasmonic gold nanorods for light harvesting and hydrogen production.

The conversion of infrared light to visible-light which allows a larger fraction of sun-light to be used is needed to improve light-harvesting. In this work a tri-functional material composed of an up-converter (NaYF4-Yb-Tm), plasmonic gold nanorods and CdS was made photocatalytically active using 980 nm wavelength light for the reduction of H+ to H2.