Mapping of the Photoinduced Electron Traps in TiO₂ by Picosecond X‐ray Absorption Spectroscopy

Abstract

Titanium dioxide (TiO2) is the most popular material for applications in solar‐energy conversion and photocatalysis, both of which rely on the creation, transport, and trapping of charges (holes and electrons). The nature and lifetime of electron traps at room temperature have so far not been elucidated. Herein, we use picosecond X‐ray absorption spectroscopy at the Ti K‐edge and the Ru L3‐edge to address this issue for photoexcited bare and N719‐dye‐sensitized anatase and amorphous TiO2 nanoparticles. Our results show that 100 ps after photoexcitation, the electrons are trapped deep in the defect‐rich surface shell in the case of anatase TiO2, whereas they are inside the bulk in the case of amorphous TiO2. In the case of dye‐sensitized anatase or amorphous TiO2, the electrons are trapped at the outer surface. Only two traps were identified in all cases, with lifetimes in the range of nanoseconds to tens of nanoseconds.

Photoinduced Structural Dynamics of Molecular Systems Mapped by Time-Resolved X-ray Methods

“…The first study of the electron injection with element-selective probing of both the sensitizer and the substrate was carried out by the Chergui group on Ruthenium dyes adsorbed on anatase TiO2 nanoparticles (NPs) and compared with the results of band gap excitation of bare NPs. 271,272 These experiments found that a full electron charge is trapped in both cases, but that the traps due to charge injection are different from those resulting from band gap excitation as can be seen from Figure 18. Figure 18: Titanium K-edge spectra of anatase and amorphous TiO2 nanoparticles (pre-edge peaks A1-3 and B and X-ray absorption near-edge structure): a) steady state Ti K-edge absorption spectra of bare anatase and amorphous TiO2; b) Transient X-ray absorption spectra of bare anatase TiO2 nanoparticles at a time delay of 100 ps (black), after excitation at 355 nm above the band gap; c) Transient XAS spectrum of ruthenium N719-dyesensitized anatase TiO2 NPs at a time delay of 100 ps (blue) after excitation at 532 nm.…”

“…The subtle differences between b) and c) have been discussed in ref. 271,272 and are due to the trapping of electrons deep inside the shell (band gap excitation) of the nanoparticles or on its surface (injection case). Reprinted with permission from ref.…”

“…Reprinted with permission from ref. 271,272 . Copyright 2014 John Wiley and Sons; d) Femtosecond pump-probe transient recorded at 4.981 keV of bare nanoparticles excited at 355 nm.…”

Atomic-Scale Perspective of Ultrafast Charge Transfer at a Dye–Semiconductor Interface

“…We note that a recent picosecond time-resolved X-ray absorption study lent support to the existence of intermediate interfacial charge-transfer complexes in solvated dye-sensitized TiO 2 nanocrystals based on the detection of surface trap states. 22 The combined experimental and theoretical study presented here highlights the potential of TRXPS to monitor transient interfacial electron configurations with femtosecond time resolution and on an atomic level of detail. The novel method complements existing techniques such as time-resolved X-ray absorption and provides, in particular, exquisite surface sensitivity to monitor interfacial charge dynamics on application-like electrode configurations.…”

Royal Society of Chemistry Prizes 2019

“…Nach Postdoktoraten an der UniversitØ de Paris-Sud (bei Odile Eisenstein) und an der Australian National University begann er 1997 als Lecturer an der Heriot-Watt University.Seit 2009 ist er Full Professor. [10] Gregory Voth ( [11] In diesem Jahr wurde ihm bereits der Joel Henry Hildebrand Award der American Chemical Society zuerkannt. In einer Angewandte-Chemie-Zuschrift beschrieb er eine katalytische Hydrodefluorierung über einen konzertierten nukleophilen Ru-H-Angriff.…”