Visualizing Plasmon Mediated Nanochemistry During Liquid Phase Transmission Electron Microscopy

Visible light illumination of coinage metal nanoparticles, such as gold nanorods (AuNRs), excites localized surface plasmon modes that decay via phonon scattering, radiative emission, and generation of hot carriers. The latter decay pathway generates hot electrons and holes capable of driving chemistry at the nanoparticle surface, such as catalytic and redox reactions. The prevailing approaches to investigating hot carrier generation are numerical first principles simulations and optical scattering, but these methods cannot probe the nanoscale spatial distribution of hot carrier injection efficiency on nanoparticle surfaces. A detailed understanding of how nanoparticle characteristics, such as shape, size, faceting, and surface ligands, impact hot carrier generation rate and injection across the nanoparticle/liquid interface is required for rational design of nanoparticle photocatalysts.

Prior work has utilized liquid phase transmission electron microscopy (LPTEM) to visualize nanoparticle shape changes driven by electron beam excitation of plasmons. However, LPTEM also generates highly reactive radicals in the solvent that can overshadow reactions driven by less reactive plasmonic hot carriers. In this talk, we utilize silver metal deposition to demonstrate that radical chemistry dominates over plasmonic hot carriers for AuNRs in water and show that a radiolysis resistant solvent enables isolating and quantifying plasmonic hot electron driven metal deposition and AuNR reshaping. In the first part of the talk, we investigate the deposition mechanism of silver shells onto AuNRs suspended in water. At relatively high surface ligand coverage the silver deposited as bipyramidal faceted shells and self-nucleated in regions adjacent to the AuNR. The silver deposited preferentially at the tips of the AuNR when the ligand was removed by repeated rinsing. Ex situ synthesis experiments showed that chemical reduction of silver onto AuNRs generated core-shell particles with shapes that were similar to the LPTEM experiments. On the other hand, photoillumination deposited thin (~5 nm) conformal silver shells with no significant faceting. Taken together, these experiments indicate that in water silver deposition onto AuNRs during LPTEM is primarily driven by chemical reduction of silver ions by radicals instead of plasmonic hot electrons.

The second part of the talk will demonstrate a radiation resistant solvent consisting of toluene and isopropanol to diminish radical generation during LPTEM and isolate plasmon hot carrier mediated chemistry. LPTEM observations showed that silver deposition on AuNRs in this solvent occurred preferentially at plasmonic hot spots predicted by finite difference time domain (FDTD) simulations. Self-nucleation of silver nanoparticles was minimized in this solvent. Systematic experiments demonstrated shell growth rates increased linearly with electron flux, consistent with simulated hot electron generate rates. Quantitative analysis of the silver deposition rate on the AuNR surface provided experimental estimates of the hot electron injection efficiency. FDTD simulations demonstrated that the transverse and higher order plasmon modes were responsible for hot electron generation. The higher order dark modes generated by the point electron source generated orientation dependent silver deposition kinetics due to the raster pattern of the scanning TEM (STEM) beam. Overall, this work demonstrates that plasmonic hot carrier driven reactions are convoluted by chemical radicals in water during LPTEM, but can be isolated if radiolysis is minimized with judicious solvent choice. This work demonstrates a powerful LPTEM method for characterizing how plasmonic nanoparticle characteristics impact hot carrier generation and reactivity that has the potential to further characterize hot carrier driven catalytic reactions that generate gaseous or solid products.