Dark-Field Scattering Spectroelectrochemistry Analysis of Hydrazine Oxidation at Au Nanoparticle-Modified Transparent Electrodes

Au nanoparticles (NPs) have interesting optical properties, such as local field enhancement for improving light absorption and Raman scattering cross-section of an organic chromophore, and catalytic properties of improving the kinetics of redox reactions involved in clean energy transformations. Real-time electrochemical measurements of catalytic Au NPs would help resolve their local structure–function relationship, which can further provide insights into developing an optimal catalytic condition. It is extremely challenging to resolve the electrochemical events of electrocatalytic Au NPs at a single-particle level using conventional ensemble averaging methods. Here, we present a light-scattering-based spectroelectrochemistry analysis of single catalytic Au NPs at a transparent planar electrode and ultramicroelectrode (UME) with combined methods of electrochemistry and dark-field light scattering (DFS). Hydrazine oxidation reaction is used as a model system to characterize the catalytic characteristics of single Au NPs. Real-time light-scattering responses of Au NPs to surface adsorbates, Au oxide formation, double-layer charging, and nitrogen bubble formation upon hydrazine oxidation are investigated for both ensemble and single Au NPs. Such a light-scattering response to catalytic hydrazine oxidation at single Au NPs is highly sensitive to Au NP sizes. The DFS study of single Au NPs shows a minor decrease in the light-scattering signal in the low overpotential region because of the double-layer charging in the absence of hydrazine and the surface adsorbates N<sub>2</sub>H<sub>3</sub> in the presence of hydrazine. A significant decrease in the DFS signal of Au NPs upon Au oxidation in the high-overpotential region can be obtained in the absence of hydrazine. Such an oxide-induced light-scattering signal loss effect can be weakened in the presence of hydrazine and completely eliminated in the presence of >50 mM hydrazine. Strong light scattering can be obtained because of nitrogen bubble formation on the Au NP surface. Theoretical modeling with COMSOL Multiphysics is applied to support the abovementioned conclusions.