Measurement of the Charge Density during the Reduction of Gold (Hydr)Oxides Formed at Different Experimental Conditions

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The distinctive characteristics of gold have found diverse applications in advanced technologies, including but not limited to organic photovoltaics, sensory probes, therapeutic agents, drug delivery systems in biomedical contexts, and catalytic processes. Many of these applications hinge on surface interactions occurring at liquid-solid, gas-solid, or electrode-electrolyte interfaces, where knowing the active surface area of the gold substrate is pivotal. One method employed to quantify this surface area leverages the inherent electrochemical activity of gold. This technique entails the formation of an oxide layer on the surface of a gold sample during an anodic scan of a cyclic voltammetry experiment. Subsequently, during the cathodic scan, this oxide layer undergoes reduction, resulting in a distinct reduction peak. The surface area of the gold sample, termed as the electrochemical surface area (ECSA) in this context, can then be calculated by dividing the reduction charge, denoted as Q, by a predetermined charge density value, represented by q: ECSA=Q/qWhile the electrochemical properties of gold have been extensively studied for decades, the value of q is still in debate. Reported estimates of charge densities for an AuO monolayer vary widely; 386 μC/cm2 [1], 400 μC/cm2 [2],[3], 484 μC/cm2 [4] and others. The first two values are the most common and are based on the somewhat ambiguous assumptions regarding the formation of AuO monolayer. Furthermore, these values are contingent upon experimental conditions, primarily the upper potential limit (UPL).This study aims to accurately determine the charge density necessary for the reduction of Au-(hydr)oxides formed under various upper potential limits and scan rates. To achieve this objective, polycrystalline gold rotating disk electrodes (RDEs, AFE3T050AU, Pine Research) were utilized. The real geometric surface area of these electrodes was determined by assessing their roughness factors employing the iodine adsorption technique [5],[6]. Subsequently, the electrodes underwent potential cycling across different upper potential limits (ranging from 1.5V to 1.75V vs RHE, Figure 1a) at varying scan rates (20, 50, and 100 mV/sec). The charge densities required for the reduction of the Au-(hydr)oxides were then calculated by integrating the reduction charge within the potential range of 0.9–1.3V vs RHE.It was found that, as expected, the charge density is significantly influenced by the UPL, exhibiting a roughly linear growth up to a potential ~1.6V vs RHE and a subsequent change of the slope towards lower values afterwards (Figure 1b). Additionally, a noticeable dependance of the charge density on the scan rate was observed, with lower scan rates exhibiting higher charge densities. The results obtained in this study allow for an accurate determination of the (electrochemical) surface area of polycrystalline gold macro, micro and nanostructures by using potential cycling up to different UPLs and at different scan rates.