Exploiting Plasmonic and Plexcitonic Nanomaterials in Industrial Catalysis

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Could energy intensive chemical reactions such as CO2 reduction and steam reforming be performed close to room temperature using light as the energy source? Could the hydrogen economy be realized within the next 5-10 years through efficient sunlight-driven water-splitting ? How do quantum quasiparticles such as excitons, plasmons and phonons interact in the context of heterogeneous catalysis? Could we reduce civilization's extraordinary reliance on precious metals such as platinum, palladium, gold and silver for a range of catalytic reactions? These are the sorts of scientific questions and technological possibilities that the research program described in this proposal seeks to examine and answer. At the heart of this research are plasmonic nanoparticles, and their heterojunctions with semiconductors and pi-conjugated dye molecules. Plasmons are collective and coherent oscillations of the free electron gas in metals. In metal nanoparticles, the spatial confinement of collective excitations of electrons at the metal-dielectric interface result in strong localized surface plasmon resonances (LSPR). LSPR resonances at visible & near-infrared wavelengths are desired for photocatalytic applications. Nanoparticles of a handful of materials such as Ag, Au, Cu, Al, TiN, ZrN and Cu2S have been shown to have LSPR peaks in the visible and near-infrared. Surface plasmons decay in femtoseconds through either Landau damping (LD) or chemical interface damping (CID) to form hot electron-hole pairs with energies much higher than what would be expected from a Boltzmann distribution. Therefore, hot carriers are particularly attractive as agents of chemical transformation in order to drive chemical reactions such as water-splitting to generate H2, reduction of CO2 into value-added products, synthesis of azo dyes, ammonia synthesis, hydrocarbon reforming, photooxidation of organic compounds, etc. However, hot electrons experience electron-electron scattering over ~ 100 fs timescales and collisions with phonons over ~ 1 ps timescales resulting in very fast relaxation to a purely thermal carrier distribution. The key technological challenge that this proposal seeks to address is to how to shuttle the hot electrons away from the metal and get them to drive a chemical reaction before their excess energies are lost to a variety of dissipation processes. Heterojunctions of plasmonic metal nanoparticles (nanoprisms, nanocubes, nanoshells, etc) with inorganic semiconductors and pi-conjugated organic dyes, offer the most promising routes for efficient separation and exploitation of plasmonic hot carriers. The underlying physical processes at these heterojunctions are not completely understood. We also seek to advance our fundamental scientific understanding of plasmonic metal-semiconductor heterojunctions. The research in this proposal has the potential to impact the $5 billion semiconductor photocatalysis industry and the $30 billion global catalyst industry (2018).