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Farewell: the NCCR MUST ended 
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NCCR MUST at Scientifica 2021Pump - probe X-ray absorption on metal nano-particles
 | Professor Dr. Jeroen van Bokhoven ETH Zürich Heterogeneous Catalysis / ICB / D-CHAB HCI E-127 CH-8093 Zürich http://www.vanbokhoven.ethz.ch/ +41 44 632 55 42 | ||||
| Project start | January 2013 | ||||
| Project end | 2015 | ||||
| Abstract | Metal nanoparticles (NP) offer the intriguing possibility to extend the absorp- tion spectrum of a wide class of materials into the visible range, opening up the way to directly use solar light to drive catalytic processes (visible-light- driven water splitting, for example). However, the process involved behind the absorption of photons via the so-called plasmon resonance is very fast. Consequently, the study of the resulting redirection of the collected energy into the electronic system and into the reactants adsorbed on the active sites of the catalysts requires a high temporal resolution. The femtosecond optical pump – pico-second X-ray probe setup is the right tool to face such problems and it offers the exciting opportunity to deepen our knowledge about the physics and chemistry behind photocatalysis. Time-resolved X-ray absorption will allow the observation of the electronic and structural changes caused by the optical excitation and the study of the involved excited states together with the relaxation paths. | ||||
| Goals | For the short term, a femtosecond optical pump – picosecond X-ray probe setup at the SuperXAS beamline at the Swiss Light Source (Paul Scherrer Institute, Villigen) is being built up and characterized. The laser is a high- repetition rate (50-150 kHz) femtosecond Ti:Sapphire laser (40 fs pulse duration) which will be phase-locked with the radio frequency signal of the synchrotron source. First optical pump/X-ray probe experiments with a temporal resolution of about 100 ps will be performed on liquid-jet targets containing suspensions of metal or combined semiconductor/metal nanoparti- cles. On a longer term perspective, the project aims at a deeper understanding of the relationship between light and catalytic properties of the matter. | ||||
| Results | A first time-resolved X-ray absorption spectroscopy experiment on the Au-L3 edge has been performed, employing a picosecond laser to optically pump gold nanoparticles. The analysis of the differential data showed the occurance of thermal effects (expansion and premelting, followed by resolidification) on a nanosecond time scale, but no indication of an electron transfer to the sur- rounding medium. This is promising since it suggests that the energy of the hot electrons within the NP is not directly dissipated to the environment. These electrons may therefore be transferred to an adjacent semiconductor, where they trigger catalytic reactions for the ultimate goal of energy storage in chemi- cal bonds. | ||||
| Outlook | Two follow-up beam times have been accepted, where the role of charge trans- fer as the energy transport mechanism from a metal to a semiconductor nano- particle after optical excitation of the metal NP will be investigated in depth using again an ultrafast pump/probe scheme. As the samples, gold NP supported on Fe2O3 and ZnO will be used. The simultaneous monitoring of the Au-L3 and the Fe/Zn-K edges will give the possibility to observe the electron behavior both while leaving the gold NP and while reaching the semiconductor NP. This will possibly give additional support to the hypothesis of an electron-transfer-based mechanism of the energy transport between the two NP species. | ||||
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