Mapping of the Photoinduced Electron Traps in TiO sub(2) by Picosecond X-ray Absorption Spectroscopy

• Published in: Angewandte Chemie International Edition Vol. 53; no. 23; pp. 5858 - 5862
• Main Authors: Rittmann-Frank, MHannelore, Milne, Chris J, Rittmann, Jochen, Reinhard, Marco, Penfold, Thomas J, Chergui, Majed
• Format: Journal Article
• Language: English
• Published: 01.06.2014
• Subjects: Absorption spectroscopy; Anatase; Dyes; Nanostructure; Surface chemistry; Titanium; Titanium dioxide; Trapping; X-rays
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201310522

Titanium dioxide (TiO sub(2)) 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 L sub(3)-edge to address this issue for photoexcited bare and N719-dye-sensitized anatase and amorphous TiO sub(2) nanoparticles. Our results show that 100ps after photoexcitation, the electrons are trapped deep in the defect-rich surface shell in the case of anatase TiO sub(2), whereas they are inside the bulk in the case of amorphous TiO sub(2). In the case of dye-sensitized anatase or amorphous TiO sub(2), 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. Electrons in the trap: Picosecond Ti K-edge and Ru L sub(3)-edge X-ray absorption spectra of photoexcited bare and dye-sensitized TiO sub(2) nanoparticles (NPs) showed that electrons are trapped deep in the defect-rich surface shell of bare anatase TiO sub(2), whereas injection from the dye on both anatase and amorphous NPs leads to surface trapping (see picture). In bare amorphous TiO sub(2), trapping occurs at preexisting defects within the NP.