Understanding Activation of Small Molecules with Light:
Photothermal Vs. Charge Carrier Effects in Photocatalysis
Jennifer Strunk
Technical University of Munich
When photocatalysts are irradiated with high intensity light sources, it is expected that
the samples will also heat up, at least locally. Then, in addition to direct light-driven
reactions with excited charge carriers, more conventional thermal reactions are also
possible, similar to classical catalysis. This is especially the case when nanoparticles
of the coinage metals Cu, Ag and Au are involved, which show intense plasmon
absorption bands.
Here, two photocatalyst systems are presented in which we were able to clearly identify
photothermal effects as main reason for improved (photo)catalytic performance. For
the case of Au/TiO2, Anti-Stokes Raman spectroscopy was used to track temperature
changes in the anatase phase of TiO2, when the plasmon excitation of gold was
triggered with a green laser. Under these conditions, the titania reached temperatures
significantly exceeding 1000 K, which over time caused severe rutilization of titania.
Even under irradiation with common UV-Vis research light sources, rutilization of titania
was identified in Au/TiO2, so local temperatures above the rutilization threshold
(~850 K) can be reached in this case, too. At these temperatures, many catalytic
reactions of relevance reach thermodynamic equilibrium, so it is expected that thermal
effects are dominant for Au/TiO2, with little or no influence of photogenerated charge
carriers in either Au or TiO2.
In the second case, Cu/ZnO-based catalysts were considered for light-assisted
thermocatalytic CO2 valorization near ambient pressure. Both methanol synthesis and
CO formation by the reverse water gas shift were detected to be influenced by light
irradiation. The endothermic reverse water gas shift was always favored over
exothermic methanol synthesis, regardless of whether UV or visible light or both was
used. The behavior of forward and reverse reaction rate, together with detailed in situ
characterization of the material and the reaction mechanism, suggested that the light
simply caused the active sites, predominantly the copper, to be locally at much higher
temperature. The larger influence of visible light suggests a dominant influence of the
Cu plasmon absorption rather than band-gap excitation in ZnO.