Quantification and description of photothermal heating effects in plasmon-assisted electrochemistry.

A growing number of reports have demonstrated plasmon-assisted electrochemical reactions, though debate exists around the mechanisms underlying the enhanced activity. Here we address the impact of plasmonic photothermal heating with cyclic voltammetry measurements and finite-element simulations. We find that plasmonic photothermal heating causes a reduction in the hysteresis of the anodic and cathodic waves of the voltammograms along with an increase in mass-transport limiting current density due to convection induced by a temperature gradient. At slow scan rates, a temperature difference as low as 1 K between the electrode surface and bulk electrolytic solution enhances the current density greater than 100%. Direct interband excitation of Au exclusively enhances current density by photothermal heating, while plasmon excitation leads to photothermal and nonthermal enhancements. Our study reveals the role of temperature gradients in plasmon-assisted electrochemistry and details a simple control experiment to account for photothermal heating.

Potential-Dependent Temporal Dynamics of CO Surface Concentration in Electrocatalytic CO2 Reduction.

Beyond the identity and structure of an intermediate, changes in its concentration on and near the electrode surface with time are a critical component to understand and improve selectivity and reactivity in electrochemical transformations. We apply pulsed-potential electrochemical Raman scattering microscopy to measure the potential-dependent temporal evolution of CO formed during electrocatalytic CO2 reduction in acetonitrile on Ag electrodes. At driving potentials positive of the onset potential as determined by cyclic voltammetry, CO accumulates on the electrode surface at time scales longer than 1 s. Near the ensemble onset potential, CO resides on the electrode surface for approximately 100 ms. At potentials known to evolve CO from the electrode surface, CO remains adsorbed on the electrode for less than 10 ms. The time scales accessible in our strategy are nearly 3 orders of magnitude faster than transient Raman or infrared measurements, allowing direct measurement of the temporal evolution of intermediates.

Tracking Electrochemistry on Single Nanoparticles with Surface-Enhanced Raman Scattering Spectroscopy and Microscopy.

Studying electrochemical reactions on single nanoparticles is important to understand the heterogeneous performance of individual nanoparticles. This nanoscale heterogeneity remains hidden during the ensemble-averaged characterization of nanoparticles. Electrochemical techniques have been developed to measure currents from single nanoparticles but do not provide information about the structure and identity of the molecules that undergo reactions at the electrode surface. Optical techniques such as surface-enhanced Raman scattering (SERS) microscopy and spectroscopy can detect electrochemical events on individual nanoparticles while simultaneously providing information on the vibrational modes of electrode surface species. In this paper, a protocol to track the electrochemical oxidation-reduction of Nile Blue (NB) on single Ag nanoparticles using SERS microscopy and spectroscopy is demonstrated. First, a detailed protocol for fabricating Ag nanoparticles on a smooth and semi-transparent Ag film is described. A dipolar plasmon mode aligned along the optical axis is formed between a single Ag nanoparticle and Ag film. The SERS emission from NB fixed between the nanoparticle and the film is coupled into the plasmon mode, and the high-angle emission is collected by a microscope objective to form a donut-shaped emission pattern. These donut-shaped SERS emission patterns allow for the unambiguous identification of single nanoparticles on the substrate, from which the SERS spectra can be collected. In this work, a method for employing the SERS substrate as a working electrode in an electrochemical cell compatible with an inverted optical microscope is provided. Finally, tracking the electrochemical oxidation-reduction of NB molecules on an individual Ag nanoparticle is shown. The setup and the protocol described here can be modified to study various electrochemical reactions on individual nanoparticles.

Plasmonically enhanced electrochemistry boosted by nonaqueous solvent.

Plasmon excitation of metal electrodes is known to enhance important energy related electrochemical transformations in aqueous media. However, the low solubility of nonpolar gases and molecular reagents involved in many energy conversion reactions limits the number of products formed per unit time in aqueous media. In this Communication, we use linear sweep voltammetry to measure how electrochemical H2O reduction in a nonaqueous solvent, acetonitrile, is enhanced by excitation of a plasmonic electrode. Plasmonically excited electrochemically roughened Au electrodes are found to produce photopotentials as large as 175 mV, which can be harnessed to lower the applied electrical bias required to drive the formation of H2. As the solvent polarity increases, by an increase in the concentration of H2O, the measured photopotential rapidly drops off to ∼50 mV. We propose a mechanism by which an increase in the H2O concentration increasingly stabilizes the photocharged plasmonic electrode, lowering the photopotential available to assist in the electrochemical reaction. Our study demonstrates that solvent polarity is an essential experimental parameter to optimize plasmonic enhancement in electrochemistry.

Super-Resolution Imaging and Plasmonics.

This review describes the growing partnership between super-resolution imaging and plasmonics, by describing the various ways in which the two topics mutually benefit one another to enhance our understanding of the nanoscale world. First, localization-based super-resolution imaging strategies, where molecules are modulated between emissive and nonemissive states and their emission localized, are applied to plasmonic nanoparticle substrates, revealing the hidden shape of the nanoparticles while also mapping local electromagnetic field enhancements and reactivity patterns on their surface. However, these results must be interpreted carefully due to localization errors induced by the interaction between metallic substrates and single fluorophores. Second, plasmonic nanoparticles are explored as image contrast agents for both superlocalization and super-resolution imaging, offering benefits such as high photostability, large signal-to-noise, and distance-dependent spectral features but presenting challenges for localizing individual nanoparticles within a diffraction-limited spot. Finally, the use of plasmon-tailored excitation fields to achieve subdiffraction-limited spatial resolution is discussed, using localized surface plasmons and surface plasmon polaritons to create confined excitation volumes or image magnification to enhance spatial resolution.