Ultrafast Nanoscale Raman Thermometry Proves Heating Is Not a Primary Mechanism for Plasmon-Driven Photocatalysis.

Abstract

Plasmonic materials efficiently convert light to various forms of energies for many applications, including photocatalysis, photovoltaics, and photothermal therapies. In particular, plasmonic photocatalysts hold incredible promise for highly selective sunlight-driven catalysis through the generation of highly energetic holes and electrons used to drive chemical reactions. However, plasmons are also known to generate heat, and the partitioning of photoexcitation energy into hot carriers and heat on molecularly relevant time scales is not well understood, yet plays a crucial role in designing and understanding these photocatalysts. Using an ultrafast surface-enhanced Raman thermometry technique, we probe the effective temperature, equivalent to the mode-specific increase of vibrational kinetic energy, of molecules adsorbed to gold nanoparticle aggregates in the most active hot spots on the picosecond time scale of chemical reactivity. This represents the first measurement of vibrational energy deposition for coupled molecular-plasmonic systems on the picosecond time scale of molecular motion. We find that upon plasmon excitation, the adsorbates in the hot spots undergo an initial energy transfer within several picoseconds that changes the effective temperature of the system by less than 100 K, even at peak flux values 108 times stronger than focused sunlight. The energy quickly dissipates from the adsorbates into the surroundings in less than 5 ps, even at the highest values of photoexcitation. This surprisingly modest energy transfer of the most active regions of the plasmonic materials on the ultrafast time scale decisively proves that most plasmonic photocatalysis is not primarily thermally driven.