title: Femtosecond X-ray absorption study of electron localization in photoexcited anatase TiO2.
abstract: Transition metal oxides are among the most promising solar materials, whose properties rely on the generation, transport and trapping of charge carriers (electrons and holes). Identifying the latter's dynamics at room temperature requires tools that combine elemental and structural sensitivity, with the atomic scale resolution of time (femtoseconds, fs). Here, we use fs Ti K-edge X-ray absorption spectroscopy (XAS) upon 3.49 eV (355 nm) excitation of aqueous colloidal anatase titanium dioxide nanoparticles to probe the trapping dynamics of photogenerated electrons. We find that their localization at Titanium atoms occurs in <300 fs, forming Ti(3+) centres, in or near the unit cell where the electron is created. We conclude that electron localization is due to its trapping at pentacoordinated sites, mostly present in the surface shell region. The present demonstration of fs hard X-ray absorption capabilities opens the way to a detailed description of the charge carrier dynamics in transition metal oxides.
authors.affiliation: Laboratoire de Spectroscopie Ultrarapide and Lausanne Centre for Ultrafast Science (LACUS), ISIC-FSB, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland.
authors.forename: F G
authors.initials: FG
authors.lastname: Santomauro
authors.affiliation: Laboratoire de Spectroscopie Ultrarapide and Lausanne Centre for Ultrafast Science (LACUS), ISIC-FSB, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland.
authors.forename: A
authors.initials: A
authors.lastname: Lübcke
authors.affiliation: Laboratoire de Spectroscopie Ultrarapide and Lausanne Centre for Ultrafast Science (LACUS), ISIC-FSB, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland.
authors.forename: J
authors.initials: J
authors.lastname: Rittmann
authors.affiliation: Laboratoire de Spectroscopie Ultrarapide and Lausanne Centre for Ultrafast Science (LACUS), ISIC-FSB, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland.
authors.forename: E
authors.initials: E
authors.lastname: Baldini
authors.affiliation: Institut für Quantenelektronik, ETH Zürich, Wolfgang-Pauli-Str. 16, CH-8093 Zürich, Switzerland.|Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen, Switzerland.
authors.forename: A
authors.initials: A
authors.lastname: Ferrer
authors.affiliation: Laboratoire de Spectroscopie Ultrarapide and Lausanne Centre for Ultrafast Science (LACUS), ISIC-FSB, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland.
authors.forename: M
authors.initials: M
authors.lastname: Silatani
authors.affiliation: Laboratoire de Spectroscopie Ultrarapide and Lausanne Centre for Ultrafast Science (LACUS), ISIC-FSB, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland.
authors.forename: P
authors.initials: P
authors.lastname: Zimmermann
authors.affiliation: Institut für Quantenelektronik, ETH Zürich, Wolfgang-Pauli-Str. 16, CH-8093 Zürich, Switzerland.|Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen, Switzerland.
authors.forename: S
authors.initials: S
authors.lastname: Grübel
authors.affiliation: Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen, Switzerland.
authors.forename: J A
authors.initials: JA
authors.lastname: Johnson
authors.affiliation: SwissFEL, Paul Scherrer Institut, CH-5232 Villigen, Switzerland.
authors.forename: S O
authors.initials: SO
authors.lastname: Mariager
authors.affiliation: SwissFEL, Paul Scherrer Institut, CH-5232 Villigen, Switzerland.|Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen, Switzerland.
authors.forename: P
authors.initials: P
authors.lastname: Beaud
authors.affiliation: Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen, Switzerland.
authors.forename: D
authors.initials: D
authors.lastname: Grolimund
authors.affiliation: Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen, Switzerland.
authors.forename: C
authors.initials: C
authors.lastname: Borca
authors.affiliation: SwissFEL, Paul Scherrer Institut, CH-5232 Villigen, Switzerland.|Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen, Switzerland.
authors.forename: G
authors.initials: G
authors.lastname: Ingold
authors.affiliation: Institut für Quantenelektronik, ETH Zürich, Wolfgang-Pauli-Str. 16, CH-8093 Zürich, Switzerland.
authors.forename: S L
authors.initials: SL
authors.lastname: Johnson
authors.affiliation: Laboratoire de Spectroscopie Ultrarapide and Lausanne Centre for Ultrafast Science (LACUS), ISIC-FSB, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland.
authors.forename: M
authors.initials: M
authors.lastname: Chergui