Charge dynamics in photocatalytic systems investigated by ultrafast X-ray and electron techniques

• Research unit: COST
• Project number: C13.0062

Key words (English)

photocatalysis; electrons; holes; TiO2; ultrafast; X-ray absorption; X-ray emission; EELS

Research programs (English)

COST-Action CM1202 - Supramolecular photocatalytic water splitting (PERSPECT-H2O)

Short description (English)

We aim at probing at probing the charge carrier (electrons and holes) dynamics in supramolecular-TiO2 assemblies upon photoexcitation, using novel and cutting-edge methodologies. For solution systems, femtosecond X-ray absorption spectroscopy will address the issue of electron transport, while picosecond X-ray emission spectroscopy will identify the trapping site of holes. For films, we will use femtosecond EELS (Electron energy loss spectroscopy), which allows a direct and simultaneous visualization of both the electron and hole dynamics, so that they can be correlated. The various tools proposed here are universal and will allow probing the charge carrier dynamics from sub-ps to hundreds of ns.

Further information (English)

Full name of research-institution/enterprise: EPF Lausanne Laboratoire de Spectroscopie Ultrarapide (LSU), Faculté des Sciences de Base Institut des Sciences et Ingénierie Chimiques (ISIC)

Partners and International Organizations (English)

AT; BE; CY; CZ; DK; FI; FR; DE; EL; IE; IL; IT; NL; PL; PT; RO: ES; SE; TR; UK

Abstract (English)

Lead-halide perovskites are an emerging promising class of materials in the field of photovoltaics due to a solar energy conversion efficiency reaching ca. 20%. Several time-resolved spectroscopic studies attempted to find correlation of such performances with the charge carrier dynamics in these materials, but the lack of elemental and structural sensitivity of optical spectroscopic techniques did not allow to find clear answers in this respect. Picosecond X-ray absorption spectroscopy (ps-XAS) combines elemental and structural sensitivity together with a picosecond time resolution and therefore is the ideal tool to study charge carrier localization, meaning electrons and holes, at operational conditions (room temperature and ambient pressure). We performed ps-XAS at Swiss Light Source in Villigen (Switzerland) probing the Br K-edge and Pb L-edge, namely the valence band (VB) and the conduction band (CB) of the semiconductor, upon photoexcitation at 355 nm. These band-selective measurements show that hole localization happens within 100 ps at Br atoms and that the change in oxidation state persists for hundreds of nanosecond. A hint of transient signal was found at the Pb L-edge at 100 ps, but the limited signal/noise ratio doesn't allow to unambiguously assign this feature to electron localization within the gap states. In order to correlate these results with the optical ones, time-resolved luminescence studies were very recently carried out and the data is still being analysed.

References in databases (English)

Swiss Database: COST-DB of the State Secretariat for Education and Research Hallwylstrasse 4 CH-3003 Berne, Switzerland Tel. [phone] Swiss Project-Number: C13.0062