Vibrationally Hot Reactants in a Plasmon-Assisted Chemical Reaction

Abstract

Recent studies on plasmon-assisted chemical reactions postulate that the hot electrons of plasmon-excited nanostructures may induce a non-thermal vibrational activation of metal-bound reactants. However, the postulate has not been fully validated at the level of molecular quantum states. We directly and quantitatively prove that such activation occurs on plasmon-excited nanostructures: The anti-Stokes Raman spectra of reactants undergoing a plasmon-assisted reaction reveal that a particular vibrational mode of the reactant is selectively excited, such that the reactants possess >10 times more energy in the mode than is expected from the fully thermalized molecules at the given local temperature. Furthermore, a significant portion (∼20%) of the excited reactant is in vibrational overtone states with energies exceeding 0.5 eV. Such mode-selective multi-quantum excitation could be fully modeled by the resonant electron-molecule scattering theory. Such observations suggest that the vibrationally hot reactants are created by non-thermal hot electrons, not by thermally heated electrons or phonons of metals. The result validates the mechanism of plasmon-assisted chemical reactions and further offers a new method to explore the vibrational reaction control on metal surfaces.

Vibrational Energy Flow in Molecules Attached to Plasmonic Nanoparticles

“…We note that the NO stretch can be selectively excited by plasmonic excitation of the attached metal, which can be exploited to facilitate a NBT decomposition reaction . The extent to which the population of the NO stretch can be enhanced and the reaction accelerated depends sensitively on the lifetime of this mode.…”

“…The extent to which the population of the NO stretch can be enhanced and the reaction accelerated depends sensitively on the lifetime of this mode. The population of the mode, n NO , exceeds the equilibrium population, ⟨ n NO ⟩ by the ratio of the rate of excitation, w ex , to the rate, w rel , of relaxation rate due to IVR, n NO – ⟨ n NO ⟩ = w ex / w rel . It would thus be of interest to explore ways to enhance the lifetime, perhaps by introducing chemical groups that might shift the frequency of the 655 cm –1 mode out of 2:1 resonance with the NO stretch, a possibility we are presently investigating.…”

“…The ability to facilitate chemical reactions via interactions between light and molecules attached to or near metal nanoparticles has motivated many studies of photochemical and photophysical processes in these systems. − Plasmonic nanoparticles catalyze, e.g., dimerization, dissociation, and reduction of chemisorbed 4-nitrobenzenethiol (NBT), − accelerate enzyme catalysis, and can trigger reactions with medical applications. − While detailed mechanisms for the catalysis of these reactions remain unclear, there is evidence that vibrational energy flow plays a central role in the kinetics of many of them. The NO stretch of NBT has been the subject of particular interest, as selective excitation of this mode has been recently observed to drive dissociation, where the extent to which the reaction can be accelerated by selective excitation depends on vibrational energy flow . Picosecond time-resolved surface-enhanced Raman spectroscopy (SERS) carried out on NBT and other substituted benzenethiols attached to metal nanoparticles has provided information about vibrational dynamics and energy flow in these molecules. , Still, intriguing trends in the lifetimes of some of the vibrations that have been measured for different substituted benzenethiols remain unexplained, as do the relative contributions of intra- and intermolecular energy flow to the observed energy dynamics.…”

Bond Selective Photochemistry at Metal Nanoparticle Surfaces: CO Desorption from Pt and Pd

“…Finally, there are recent reports of nonthermal energy distributions within comparatively larger molecules bound to metal surfaces upon photoexcitation. , These reports highlight that photon fluxes can induce nonthermal energy exchange within adsorbates on metal surfaces based on differences in the vibrational “temperature” of bonds within the same molecule. This spectroscopic picture is consistent with our observations based on induced chemistry–photon fluxes enable precision in energy deposition into adsorbed species on metal surfaces that equilibrium heating cannot.…”

Key Factors for Controlling Plasmon-Induced Chemical Reactions on Metal Surfaces

“…However, the role of plasmonic heating in these chemical reactions remains controversial. Some studies reported that plasmonic heating was dominant for plasmon-induced chemical reactions. , In addition, other studies reported that localized heat increased the reaction rate. , By contrast, some reports concluded that plasmonic heating was not a principal mechanism and that nonthermal effects were dominant. , The details of the effects of plasmonic heating on chemical reactions are still under debate, and further study is required to elucidate the critical factors governing chemical reactions caused by localized heat.…”