From Optical to Chemical Hot Spots in Plasmonics

Abstract

In recent years, the possibility to induce chemical transformations by using tunable plasmonic modes has opened the question of whether we can control or create chemical hot spots in these systems. This can be rationalized as the reactive analogue of the well-established concept of optical hot spots, which have drawn a great deal of attention to plasmonic nanostructures for their ability to circumvent the far-field diffraction limit of conventional optical elements. The parameters that determine the reactivity of plasmonic systems are deeply influenced by the dynamics and interplay of photons, plasmon-polaritons, carriers, phonons and molecular states. Although optical hot spots can be mainly defined by the geometry and permittivity of the nanostructures, the degrees of freedom influencing their photocatalytic properties appear to be much more numerous. These degrees of freedom can affect the reaction rates, the product selectivity or the spatial localization of a chemical reaction. In this Account, we show how the control of chemical hot spots can be achieved by carefully tuning the parameters that influence the cascade of events following the optical excitation of plasmonic modes in nanostructures. We discuss here a series of single photocatalyst techniques and ideas of how plasmonic nanoscale reactivity can be spatially mapped and imaged, and how the lifetime of hot and thermalized carriers can be altered by trap states on semiconductors and by metal-semiconductor interfaces. In addition, the tailored generation of non-thermal phonons in metallic nanostructures and their dissipation is shown as a promise to understand and exploit thermal photocatalysis at the nanoscale. Surpassing or enhancing each of these energy channels should enable to engineer solar nanometric photocatalysts. Nevertheless, the ultimate capability of a plasmonic photocatalyst to trigger a chemical reaction is correlated to its ability to navigate through, or even modify, the potential energy surface of a given chemical reaction. Here we reunite both worlds, the plasmonic photocatalysts and the molecular ones, identifying different energy transfer pathways and their influence on selectivity and efficiency of chemical reactions. We foresee that the migration from optical to chemical hot spots will greatly assist the understanding of ongoing plasmonic chemistry.

Prospects of Coupled Organic–Inorganic Nanostructures for Charge and Energy Transfer Applications

“…[146] In order to achieve an absorption dominated by Landau damping, it is also recommended to use nanostructures with dimensions below the electrons mean free path length. [146,151] For gold and silver, this criterion applies to particles with a diameter smaller than 30 nm. In this size range little scattering is to be expected and interactions are preferentially located at the surface.…”

“…In the classical indirect transfer mechanism, hot electrons are first generated in the metal and then transferred to the LUMO of the adsorbed molecule. [151] Although this process is generally regarded as the dominant mechanism, recent studies suggest a second mechanism via a direct electron transfer pathway resulting from chemical interfacial damping (CID). [163] Experimental observations have shown that a material that is chemically bound to a plasmonic nanoparticle can lead to the acceleration of the plasmon dephasing and thus CID.…”

Anisotropic Au-ZnO photocatalyst for the visible-light expanded oxidation of n-hexane

“…Likewise, these metallic nanostructures can greatly enhance the photocatalytic activity of regular semiconductors given their plasmonic properties that facilitate the effective transport of hot carriers (i.e. hot electrons or hot holes) [26][27][28][29][39][40][41][42][43][44][45][46][47][48][49]. Another important topic of interest is currently devoted to the search for valid alternatives to TiO2 as semiconductor photocatalytic supports.…”

Addressing the plasmonic hotspot region by site-specific functionalization of nanostructures

“…These volumes are usually referred to as optical hot-spots, as the electromagnetic eld can reach values magnitudes higher than the incoming light. [1][2][3][4] Such hotspots can be useful to enhance properties arising from the interaction of radiation with the nanostructure and the surrounding environment and are therefore applied, for example, to increase the local temperature, 5 catalyze chemical reactions, 4,[6][7][8] increase photovoltaic efficiency, 9 or increase spectroscopic signals. 10,11 The latter has been particularly useful in surface-enhanced Raman spectroscopy (SERS), 4 where optimized design of such hotspots has shown to be capable of the detection on the single-molecule level.…”

“…12,13 Two very recent reviews highlighted the use of plasmonic hot spots in chemical transformations and SERS, while underlining the demand for reliable methods for substrate fabrication. 4,8 An ideal nanostructure to take advantage of signal, energetic, or chemical enhancement should have the following properties. First, its structure should provide a high near-eld enhancement.…”