Underlying Mechanisms of Hot Carrier-Driven Reactivity on Bimetallic Nanostructures

Abstract

Bimetallic nanostructures exhibit unique catalytic activity and selectivity that are not evident for their monometallic analogues. Such nanostructures contain plasmonic metals, such as gold or silver, which afford highly efficient harvesting of electromagnetic radiation and its conversion into hot carriers. These highly energetic species are transferred to the catalytic metal subcomponent of the bimetallic nanostructure, where a large spectrum of chemical reactions may be catalyzed. The strength of the electric field and the interplay between catalytic and plasmonic metals at the nanoscale are thus critically important for the catalytic activity of bimetallic nanostructures. In this study, we investigate the relationship between the catalytic activity and local electric fields sustained on the surface of gold–palladium (Au@PdNPs) and gold–platinum (Au@PtNPs) nanoplates using tip-enhanced Raman spectroscopy. We image the spatially varying magnitudes of rectified (DC) local electric fields on the surface of these nanostructures and compare them to the fields sustained on the surface of monometallic nanoplates. We find substantially larger electric field magnitudes on Au@PdNPs and Au@PtNPs as compared to their monometallic analogues. These findings suggest that catalytic efficiency of bimetallic nanostructures may be mediated and potentially tuned through precise control of electric fields sustained on their surfaces.

Probing the Redox Selectivity on Au@Pd and Au@Pt Bimetallic Nanoplates by Tip-Enhanced Raman Spectroscopy

“…Specifically, at 30 μW (λ = 633 nm), AuNPs possess ∼12 MW·cm –1 whereas Au@PdNPs and Au@PtNPs show ∼50 MW·cm –1 and ∼70 MW·cm –1 , respectively. Our own results, as well as experimental findings reported by other groups, show that the intensity of the rectified electric field depends on the intensity of the incident laser light. , …”

“…Our group showed that AuNPs, Au@PdNPs, and Au@PtNPs exhibit drastically different intensities of the rectified electric field on their surfaces . Optical rectification is a second-order nonlinear effect that originates from damping of LSPRs on the nanostructures. , Such damping transforms the alternating current (AC) into a constant direct current (DC) field that can be quantified using molecular reporters. Using TERS and 4-nitrobenzenethiol as a molecular reporter, we found that Au@PtNPs and Au@PdNPs exhibited 4–6 times larger magnitudes of the rectified DC field compared to the intensity of electric field observed on the surface of AuNPs.…”

“…Our own results, as well as experimental findings reported by other groups, show that the intensity of the rectified electric field depends on the intensity of the incident laser light. 36,37 These findings suggest that reduction of 4-NBT to DMAB requires only 12 MW•cm −1 , which can be reached on AuNPs upon nanostructure illumination by electromagnetic radiation. At the same time, higher rectified electric field intensity that can be produced on Au@PdNPs allow for stronger reduction of 4-NBT to 4-ATP.…”

“…29,40 Our group showed that AuNPs, Au@PdNPs, and Au@ PtNPs exhibit drastically different intensities of the rectified electric field on their surfaces. 36 Optical rectification is a second-order nonlinear effect that originates from damping of LSPRs on the nanostructures. 36,37 Such damping transforms the alternating current (AC) into a constant direct current (DC) field that can be quantified using molecular reporters.…”

“…36 Optical rectification is a second-order nonlinear effect that originates from damping of LSPRs on the nanostructures. 36,37 Such damping transforms the alternating current (AC) into a constant direct current (DC) field that can be quantified using molecular reporters.…”