Abstract
The charge-carrier dynamics of anatase TiO2 nanoparticles in an aqueous solution were studied by femtosecond time-resolved X-ray absorption spectroscopy using an X-ray free electron laser in combination with a synchronized ultraviolet femtosecond laser (268 nm). Using an arrival time monitor for the X-ray pulses, we obtained a temporal resolution of 170 fs. The transient X-ray absorption spectra revealed an ultrafast Ti K-edge shift and a subsequent growth of a pre-edge structure. The edge shift occurred in ca. 100 fs and is ascribed to reduction of Ti by localization of generated conduction band electrons into shallow traps of self-trapped polarons or deep traps at penta-coordinate Ti sites. Growth of the pre-edge feature and reduction of the above-edge peak intensity occur with similar time constants of 300–400 fs, which we assign to the structural distortion dynamics near the surface.
Highlights
Photoexcitation of TiO2 promotes an electron from the valence band to the conduction band to create an exciton, which is followed by charge separation and formation of self-trapped
The almost instantaneous K-edge shift within about 90 fs indicates that electrons in the conduction band are localized very rapidly by reducing a Ti4þ lattice site to form Ti3þ
The edge shift in this study suggests that almost one full electron charge is localized around Ti to form Ti3þ,9,10 which is consistent with theoretical prediction that $80% of the charge is localized at a single self-trapped Ti site
Summary
Tamai have studied anatase TiO2 nanoparticles in an aqueous solution using TAS, and they found that 360 nm photoexcitation creates an immediate rise of the photoabsorption signal at 520 nm; the estimated time constant was shorter than 50 fs.. Tamai have studied anatase TiO2 nanoparticles in an aqueous solution using TAS, and they found that 360 nm photoexcitation creates an immediate rise of the photoabsorption signal at 520 nm; the estimated time constant was shorter than 50 fs.15 Since this signal disappeared by addition of SCNÀ, a well-known scavenger of holes, into the solution, Yang and Tamai concluded that the absorption at 520 nm must be due to trapped holes at the surface and that these holes were transferred to SCNÀ on an ultrashort timescale. Monitor of X-ray pulses to make full use of the SACLA’s ultrashort pulses, and we determine the time constants for the K-edge shift and the growth of the pre-edge peak accurately
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