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 N2H3 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.