Dark-Field Microscopy of Au@Ag Core Shell Nanoparticles during Electrochemical Conversion Reactions

Abstract

Nanoparticles (NPs), which can nowadays be synthesized in various shapes and compositions, provide special physical properties that can strongly differ from their respective macroscopic counterparts. In particular due to the highly increased surface-to-volume ratios, NPs are frequently used as catalyst material.(1) Furthermore, metal NPs exhibit adjustable spectral properties arising from the interaction of the conduction band electrons with electromagnetic radiation, called localized surface plasmon resonance (LSPR).(2) The NPs appear colorful, where spectra of single NPs contain fundamental information about the size, shape and composition of the NP. Using Dark-Field Microscopy (DFM), the LSPR spectra of individual NPs can be precisely detected by hyperspectral imaging and CCD imaging. Combining this method with electrochemistry allows the monitoring of electrochemical processes on a single NP level.(3) In our work we focus on Au@Ag Core Shell NPs which are a promising candidate for catalytic applications. The LSPR spectra of the core shell NPs were herein tracked during the electrochemically induced dissolution of the Ag shell in nitrate solution exhibiting a “naked” Au core with possibly increased catalytic activities. With this spectro-electrochemical approach, an intensity decrease accompanied by a spectral red-shift was found during the dissolution of the Ag, which correlated with the simultaneously obtained current responses.Figure 1: Left: DFM CCD images of core shell NPs before and after linear sweep voltammetry (LSV) are displayed (-0.2 to 0.6 V vs Pt, 5 mV/s), center: LSPR intensity gradient of > 600 core shell NPs which were individually tracked during the LSV (black) including the current response (red), right: LSPR spectra of a representative core shell NP before and after the LSV.References(1) Ealia S. A. M.; Saravanakumar M. P. IOP Conf. Ser.: Mater. Sci. Eng. 2017, 263, 032019.(2) Sambur J. B.; Chen P. Annu Rev Phys Chem 65, 395–422.(3) K. Wonner et al. J. Am. Chem. Soc. 2018, 140, 12658–12661. Figure 1