Drawings and maps detail the localized surface plasmon resonances in Au/Pd octopod nanoparticles. The light-driven oscillations on the surface, especially at the tips, help define and enhance the ability of the Pd catalysts. Scale bar is 50 nm.Eight-armed nanoparticles—or ‘octopods’—of Au and Pd, which combine the attractive catalytic and plasmonic capabilities of each element in a single nanostructure, could both speed up chemical reactions and monitor them [Ringe et al., Sci. Rep. (2015) 5:17431].
Metallic nanoparticles like Au, Ag, Cu, and Al demonstrate localized surface plasmon resonances—oscillations of conduction electrons in response to light—which can be useful in biological sensing. Other nanoparticles, like Pd and Pt, however, are poor plasmonic metals but good catalysts.
‘‘Gold is plasmonic, palladium is not. Palladium is catalytic, gold is not. When mixed together simply, palladium damps the plasmonic behavior of gold, and gold damps the catalytic activity of palladium, such that one gets less than the sum of the parts,’’ explains lead author Emilie Ringe of Rice University.
So Ringe and colleagues from Indiana University, the University of Cambridge, and Forschungszentrum Jülich, used a solution-based process to synthesize octopod nanoparticles with Au cores and Pd arms. When the team investigated the nanostructures with a unique combination of electron energy loss spectroscopy, cathodoluminescence, and energy dispersive X-ray spectroscopy, they found that the octopods retain the plasmonic properties of their Au cores while their surfaces are catalytic.
‘‘By scanning an electron beam around the octopods, we were able to map the position of Au and Pd atoms, confirming that most of the Pd was on the outside of the particle,’’ explains Ringe. ‘‘Using the same electron beam, we then looked at the electric field distribution around the particles—which gives us information about the local plasmon enhancement.’’
The electric field maps reveal that strong fields are found at the tips of the octopods, where non-plasmonic Pd is concentrated. The observations confirm that strong localized plasmons can be sustained on catalytically active metals such as Pd, which were previously thought not to be strongly plasmonic.
‘‘[The octopods] combine plasmonics and catalysis, in a unique and advantageous way,’’ says Ringe. ‘‘They could be useful in light-enhanced catalysis, in both industrial settings or rural areas, where concentrated sunlight could drive sanitization reactions, for example.’’
Richard P. Van Duyne of Northwestern University believes the findings are very compelling. ‘‘This is truly first-rate work,’’ he says. ‘‘The case made by the authors for cleverly combining plasmonic and catalytic activity will advance the field.’’ The researchers are now looking at different alloy structures, says Ringe, with other catalytically active metals like iron oxide. Other plasmonic metals such as Al will also be investigated, which could bring down the costs of such multifunctional systems.
This article was originally published in Nano Today (2016), doi:10.1016/j.nantod.2016.01.003