Recent progress in solution-based synthetic techniques has allowed the synthesis of Ag nano-structures with well-controlled, highly uniform sizes and particle geometries. These Ag particles exhibit a strong localized surface plasmon resonance (LSPR) due to the nanometer scale spatial confinement and the metal's inherent electronic structure. For nanometer-sized Ag structures, the resonance frequency falls in the ultraviolet to visible light range, and it can be tuned by changing the geometry and size of the Ag particles.
The excitation and subsequent decay of surface plasmon states is manifested through three resonant processes: (a) non-radiative decay: absorption of photons which excite phonon modes, lattice vibrations, that locally heat the particle, (b) radiative decay: Rayleigh scattering process where photons are concentrated at the surface of the particle and scattered in high intensity in the vicinity of the particle without being degraded into heat, and (c) direct transfer of electrons from plasmon states to surrounding, electron-acceptor medium. In this work we will show examples of how the shape and size of Ag nanostructures can be tuned to allow different decay mechanisms to drive photocatalytic reactions. In the first case large Ag nanostructures (>75 nm) are utilized to take advantage of the efficient scattering cross sections (mechanism b) to enhance photochemistry on neighboring TiO2 nanoparticle photocatalysts. In another example we will discuss the utility of mechanism c, and coupling this effect with a thermal heat bath to drive important industrial oxidation reactions directly on Ag nanoparticles at temperatures far below what is necessary in pure thermocatalytic reactions.
This work shows how the rich photo-physical characteristics of surface plasmons can be used to drive chemical processes, by utilizing a thorough understanding of the different plasmon decay mechanisms.
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