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

Abstract

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.

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

“…There is very little difference between the TAS signal obtained in water and that obtained in the presence of 1.8 × 10 −5 mM of CAN (figures 4(A) and (B)). In previous work [31], we obtained a similar trend in the gas phase for a fresh TiO 2 (110) Figure 2. TAS decay of the signal at 500 nm of TiO 2 (110) rutile single crystal in the presence of n-heptane, water, and CAN (1.08 × 10 −4 mM) (A).…”

“…It is however composed of patterns that are reproducible on a TiO 2 (110) single crystal. These are presented by arrows in the figure (a similar shape is obtained in air [31,39]). The TAS decay of three wavelengths (720 nm, 820 nm and 870 nm) for the series was plotted and fitted with a bi-exponential function from which the average time constants are obtained (table S1).…”

“…The system is based on a regeneratively amplified Ti: sapphire laser system that produces 800 nm laser pulses of 90 fs pulse width at 1 kHz repetition rate and in conjunction with Excipro pump-probe spectrometers (Moscow). More details on the system can be found in [31]. Briefly, the pump pulses at 340 nm were generated after passing through 90% of the 800 nm beam into a spectrally tuneable (240 − 2600 nm) optical parametric amplifier (TOPAS Prime, Spectra-Physics) and a frequency mixer (NIR vis UV, Light conversion).…”

“…While many pump-probe femtosecond TAS studies have been conducted on powder oxide semiconductors, one of the main issues in photocatalysis is the effect of the state of the material. That is because the catalytic performance changes with the crystallite and particle sizes, with the degree of crystallinity [30], and with the extent of bulk defects (largely viewed as oxygen vacancies and interstitial metal atoms [31], such as the Magneli phase [32]) among other factors). While all will share largely the same main characteristics (similar band gap energy for example) changes in the lifetime of the excited charge carriers, from one system to the other, prevent extracting fundamental information that may be used as a reference.…”

Hydrogen-Induced Reduction Improves the Photoelectrocatalytic Performance of Titania

“…Our results indicate that the samples that exhibit the best photoelectrochemical performance are those that have undergone a superficial reduction localized at the topmost layers, while heavy reduction reaching the bulk is detrimental to their performance, in good agreement with recent results. 19 , 20 Furthermore, the combination of mesoscopic and nanoscopic measurements allows rationalizing the hydrogen-induced etching mechanism, demonstrating that model studies performed on single crystalline substrates using surface science characterization techniques under highly controlled UHV conditions constitute a privileged framework to access the atomic-scale properties of the treated materials and achieve a valuable structure-performance correlation. This work will contribute to the comprehension and control of the hydrogenation process as a clean, economical, and versatile method for the development of broadband absorbers from the UV to the IR regions, efficient photocatalysts, and photothermal energy conversion devices.…”