Dayne F. Swearer

Heterometallic antenna−reactor complexes for photocatalysis

Significance Plasmon-enhanced photocatalysis holds significant promise for controlling chemical reaction rates and outcomes. Unfortunately, traditional plasmonic metals have limited surface chemistry, while conventional catalysts are poor optical absorbers. By placing a catalytic reactor particle adjacent to a plasmonic antenna, the highly efficient and tunable light-harvesting capacities of plasmonic nanoparticles can be exploited to drastically increase absorption and hot-carrier generation in the reactor nanoparticles. We demonstrate this antenna−reactor concept by showing that plasmonic aluminum nanocrystal antennas decorated with small catalytic palladium reactor particles exhibit dramatically increased photocatalytic activity over their individual components. The modularity of this approach provides for independent control of chemical and light-harvesting properties and paves the way for the rational, predictive design of efficient plasmonic photocatalysts. Metallic nanoparticles with strong optically resonant properties behave as nanoscale optical antennas, and have recently shown extraordinary promise as light-driven catalysts. Traditionally, however, heterogeneous catalysis has relied upon weakly light-absorbing metals such as Pd, Pt, Ru, or Rh to lower the activation energy for chemical reactions. Here we show that coupling a plasmonic nanoantenna directly to catalytic nanoparticles enables the light-induced generation of hot carriers within the catalyst nanoparticles, transforming the entire complex into an efficient light-controlled reactive catalyst. In Pd-decorated Al nanocrystals, photocatalytic hydrogen desorption closely follows the antenna-induced local absorption cross-section of the Pd islands, and a supralinear power dependence strongly suggests that hot-carrier-induced desorption occurs at the Pd island surface. When acetylene is present along with hydrogen, the selectivity for photocatalytic ethylene production relative to ethane is strongly enhanced, approaching 40:1. These observations indicate that antenna−reactor complexes may greatly expand possibilities for developing designer photocatalytic substrates.

From tunable core-shell nanoparticles to plasmonic drawbridges: Active control of nanoparticle optical properties

Redox electrochemistry was used to reversibly tune the optical properties of plasmonic core-shell nanoparticles and dimers. The optical properties of metallic nanoparticles are highly sensitive to interparticle distance, giving rise to dramatic but frequently irreversible color changes. By electrochemical modification of individual nanoparticles and nanoparticle pairs, we induced equally dramatic, yet reversible, changes in their optical properties. We achieved plasmon tuning by oxidation-reduction chemistry of Ag-AgCl shells on the surfaces of both individual and strongly coupled Au nanoparticle pairs, resulting in extreme but reversible changes in scattering line shape. We demonstrated reversible formation of the charge transfer plasmon mode by switching between capacitive and conductive electronic coupling mechanisms. Dynamic single-particle spectroelectrochemistry also gave an insight into the reaction kinetics and evolution of the charge transfer plasmon mode in an electrochemically tunable structure. Our study represents a highly useful approach to the precise tuning of the morphology of narrow interparticle gaps and will be of value for controlling and activating a range of properties such as extreme plasmon modulation, nanoscopic plasmon switching, and subnanometer tunable gap applications.

Plasmon-induced selective carbon dioxide conversion on earth-abundant aluminum-cuprous oxide antenna-reactor nanoparticles

The rational combination of plasmonic nanoantennas with active transition metal-based catalysts, known as ‘antenna-reactor’ nanostructures, holds promise to expand the scope of chemical reactions possible with plasmonic photocatalysis. Here, we report earth-abundant embedded aluminum in cuprous oxide antenna-reactor heterostructures that operate more effectively and selectively for the reverse water-gas shift reaction under milder illumination than in conventional thermal conditions. Through rigorous comparison of the spatial temperature profile, optical absorption, and integrated electric field enhancement of the catalyst, we have been able to distinguish between competing photothermal and hot-carrier driven mechanistic pathways. The antenna-reactor geometry efficiently harnesses the plasmon resonance of aluminum to supply energetic hot-carriers and increases optical absorption in cuprous oxide for selective carbon dioxide conversion to carbon monoxide with visible light. The transition from noble metals to aluminum based antenna-reactor heterostructures in plasmonic photocatalysis provides a sustainable route to high-value chemicals and reaffirms the practical potential of plasmon-mediated chemical transformations.Plasmon-enhanced photocatalysis holds promise for the control of chemical reactions. Here the authors report an Al@Cu2O heterostructure based on earth abundant materials to transform CO2 into CO at significantly milder conditions.

Quantifying hot carrier and thermal contributions in plasmonic photocatalysis

Hot carriers reducing thermal barriers Plasmonic catalysts can generate hot charge carriers that can activate reactants and, in turn, reduce the overall barrier to a reaction. Zhou et al. studied the decomposition of ammonia to hydrogen on a copper alloy nanostructure that absorbed light and generated electrons that activated nitrogen atoms on ruthenium surface atoms (see the Perspective by Cortés). By measuring reaction rates at different wavelengths, light intensities, and catalyst surface temperatures, the light-induced reduction of the apparent activation barrier was quantified. Science, this issue p. 69; see also p. 28 A Ru-Cu alloy plasmonic photocatalyst substantially reduced the thermal activation barrier for ammonia decomposition. Photocatalysis based on optically active, “plasmonic” metal nanoparticles has emerged as a promising approach to facilitate light-driven chemical conversions under far milder conditions than thermal catalysis. However, an understanding of the relation between thermal and electronic excitations has been lacking. We report the substantial light-induced reduction of the thermal activation barrier for ammonia decomposition on a plasmonic photocatalyst. We introduce the concept of a light-dependent activation barrier to account for the effect of light illumination on electronic and thermal excitations in a single unified picture. This framework provides insight into the specific role of hot carriers in plasmon-mediated photochemistry, which is critically important for designing energy-efficient plasmonic photocatalysts.

Mapping near-field environments of plasmonic and 2D materials with photo-induced force imaging(Conference Presentation)

We demonstrate the ability to map photo-induced gradient forces in materials, using a setup akin to atomic force microscopy. This technique allows for the simultaneous characterization of topographical features and optical near-fields in materials, with a high spatio-temporal resolution. We show that the near-field gradient forces can be translated onto electric fields, enabling the mapping of plasmonic hot-spots in gold nanostructures, and the resolution of sub-10 nm features in photocatalytic materials. We further show that the dispersion-sensitive nature of near-field gradient forces can be used to image and distinguish atomically thin layers of 2-D materials, with high contrast.

Tuning the Resonance Frequency of Surface Plasmons Localized in Au-Ag Bimetallic Hollow Nanorods In-situ in a Transmission Electron Microscope

Metallic nanostructures can couple with electromagnetic radiation exciting collective oscillations of conduction electrons in resonance with the incident radiation. By tailoring the shape, size, composition, surrounding environment and interior structure of metallic nanostructures, it is possible to tune the resonance frequency of such localized surface plasmon resonances (LSPRs). Taking the advantage of these unique optical properties in metallic nanostructures, researchers have demonstrated a wide range of novel applications from photocatalysis, nanophotonics, and solar cells to biomedicine [1]. This application range can be extended significantly further if the frequency of LSPR can be tuned reversibly in individual metallic particles using external stimuli such as electric field, electric current, light or magnetic field. Here, we report on shifting reversibly the frequency of LSPR in individual Au-Ag hollow nanorods by using an electron beam in-situ in a transmission electron microscope (TEM).