Abstract:
Generation of hot carriers in transition metal catalysts through photoexcitation has been demonstrated to be a promising approach capable of significantly lowering activation temperature of the catalysts, which could have a widespread impact on substantially reducing the current energy demands and improving the selectivity of heterogeneous catalysis.  Plasmonic nanoparticles made of Au, Ag, Cu, and Al are recently focused because they can actively absorb light at the corresponding surface plasmon resonance (SPR) frequencies, which are usually in the visible spectral region.  The high optical absorptions lead to the generation of hot carriers in plasmonic nanoparticles, on which the hot carriers can directly drive chemical transformations.  Despite the promise, plasmonic metal nanoparticles are not useful catalysts for a wide range of important reactions.  In contrast, platinum-group metals (PGMs) such as Pt, Pd, Ru or Rh are excellent catalytic materials but exhibit SPR in the ultraviolet (UV) spectral region, which represents a significant disadvantage for photocatalysis due to the poor overlap with the solar spectrum.  Although increasing size of PGM nanoparticles shifts SPR absorption to the red, it increases cost and reduces surface area and thus catalytic activity.  Moreover, increasing the size of metal nanocrystals significantly reduces the yield of hot electron generation, lowering the efficiency of photochemical energy conversion.  In this presentation, a new light absorption model will be discussed to demonstrate a transformative way to enhance optical absorption in small PGM nanoparticles in the visible spectral region by adjusting their dielectric environment instead of changing their size.  In this model, the “quantum-sized” metal nanocrystals are attached to surfaces of transparent silica spheres, which can support a variety of dielectric scattering resonances (e.g., Fabry-Perot or Whispering Gallery modes depending on the size of silica spheres) capable of creating strong electric fields near the silica surface.  The intensified nearfields can dramatically enhance the absorption cross-section of the metal nanocrystals, which are on the silica surface, thus improving the yield of “hot electrons” in the metal nanocrystals.  This new model provides a unique opportunity to efficiently generate hot carriers in the PMG metal nanoparticles upon excitation of solar energy.