Chemical Requirement for Extracting Energetic Charge Carriers from Plasmonic Metal Nanoparticles to Perform Electron-Transfer Reactions

Performing electron-transfer reactions on metal nano particles requires separation of charge carriers at the nanoparticle and their transfer to the reacting molecules. Inducing these reactions using light is challenging due to the exceedingly short lifetimes of energetic charge carriers formed in metal nanoparticles under light illumination. The results described here show that certain conditions must be met to drive these electron-transfer reactions on plasmonic nanoparticles. One critical requirement is that the process of electronic excitation takes place at the nanoparticle/molecule interface. This is accomplished by high plasmonic electric fields at the surface of plasmonic nanoparticles. Furthermore, it is also evident from our study that the electron (or hole)-donating capacity of the hole (or electron) scavengers needs to be high enough to allow for the extraction of holes (or electrons) from the nanoparticle/molecule complex, therefore completing the catalytic cycle. We discuss these findings through a case study of the conversion of methylene blue (MB) into a reduced MB ion radical on the surface of plasmonic Ag and Ag-Pt core shell nanoparticles. To directly monitor the reduction reaction of MB on the nanoparticle surfaces, we have used time-dependent in situ surface-enhanced Raman scattering measurement, which also informs us about the underlying mechanistic details of plasmon-driven charge transfer.