Electrochemical Deterioration of a Gold Thin Film in Bicarbonate Solution: Correlative Optical, Nano-Mechanical, and Chemical Considerations

The electrochemical dissolution of metals in liquid electrolytes is of great concern for various electrochemical technologies. However, it is also the focus for boosting metal recovery processes, e.g., from electronic wastes, one of which is the extraction of gold (Au) using eco-friendly electrolytes. To shed more light on the possibility and improvement of such metal leaching, this work presents the investigation of electrochemical deterioration of a Au nanofilm coated on a silicon (Si) support─ubiquitous materials in electronic components─in aqueous potassium bicarbonate (KHCO₃) electrolyte, a solution widely used in food products. In addition to the time-dependent in situ Raman spectra revealing the reduced reflectivity of Au associated with its molecular degradation, the significant role of the electric double layer (EDL) at the metal/electrolyte interface is also indicated. Analyzing surface adhesion maps and force-distance spectra acquired using atomic force microscopy and spectroscopy (AFM/AFS), metal degradation is related to nanoscale worm-shaped reconstructed surface features that act as local sites of reduced refractive index. Complemented by the non-negligible fraction of K observed on the treated Au surfaces using scanning electron microscopy and energy dispersive spectroscopy (SEM/EDS), an electrochemical framework of metal degradation is proposed, depicting the electric-field-induced transport of K⁺ and H⁺ ions toward the Au surface regardless of bias polarity and implying the formation of ternary gold hydrides. The consideration of faradic current in the EDL circuit also suggests the occurrence of ternary gold oxides. Such determination is reinforced by the attributability of the polar and electrostatic characteristics of the worm-like features to residual negatively charged anionic clusters of the hydrides and the oxides. The analysis process and the new understanding of metal degradation provide a potential route for inspecting other metal/solution systems.