Pump - probe X-ray absorption on metal nano-particles

Jvbok    Professor Dr. Jeroen van Bokhoven

ETH Zürich
Heterogeneous Catalysis / ICB / D-CHAB
HCI E-127
CH-8093 Zürich

+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-
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.
Dr. Arno Schneider
+41 56 310 4075
Swiss Light Source (SLS)
5232 Villigen PSI
Institute of Chemical and Bioengineering

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