Field-Enchanced Photocatalysis at Nanoscale Gaps

Modern industry heavily relies on heterogeneous catalysis that often requires thermal energy. This can be costly and decrease catalyst lifetime due to sintering. Recent works have shown that plasmonic metal nanoparticles can increase reaction rates and product selectivity via the photogeneration of energetic “hot” charge carriers with a characteristic super-linear dependence on light intensity. Because hot charge carrier photogeneration scales as the electric field intensity inside and at the surface of the metal, optimizing the field enhancement is essential. This can be achieved best with metal and high refractive index dielectric (such as Si) nanostructures that are separated with nanoscale gaps. The use of such nanogaps with metal catalysts could break new records in photoconversion efficiency. However, such systems have not yet been investigated in the context of gas phase photocatalysis, mostly due to synthetic issues.
This proposal will investigate CO2 photomethanation at Rh nanocatalysts located within i) plasmonic nanorod dimers with sub-5 nm gaps and ii) Si nanopillars separated with sub-20 nm gaps. State-of-the-art nanostructuring approaches that have been developed the PI Dr. Bourret will be used to prepare these complex architectures, which can provide a field intensity enhancement up to 2500 (2 orders of magnitude larger than at isolated Rh structures). Such large enhancements could potentially make this photocatalytic system industrially relevant and compatible with solar light intensity. CO2 photomethanation at Rh catalysts will be used as a testbed because: It is is industrially and ecologically relevant; recent work reported enhanced reaction rate and selectivity at Rh nanocubes under plasmonic excitation (Nature Communications 2017, 8, 14542); the small molecule products (CH4 and CO) can be detected within our photocatalytic reactor via mass spectrometry; Rh is a lousy plasmonic material, thus providing room for improvement by locating the Rh catalyst within regions of large fields. Hot electron generation will be investigated using three dimensional electromagnetic numerical simulations combined with state of the art X-ray absorption experiments performed under laser irradiation at the synchrotron light source Elettra in Trieste, IT. This will provide a complete picture of the hot electron generation process. This proposal will involve three complementary senior researchers: Dr. Bourret, the PI will provide expertise in metal and Si nanostructure synthesis and electromagnetic simulations. Dr. Diwald will provide expertise in photocatalysis and spectroscopy. Dr. Malvestuto (Elettra) will provide expertise in ultrafast X-ray absorption experiments to quantify light absorption within the different photocatalysts.