Electrodeposition of Gold Ultrathin Films on Indium Tin Oxide (ITO) Promoted by <i>In Situ</i> Formation of an Indium Layer

Continuous and smooth gold ultrathin films (UTFs) on transparent substrates are key for spectroelectrochemical techniques, (photo)electrocatalysis, and optical imaging of electrochemical interfaces. In this work, we study the electrodeposition of gold on ITO from a mildly acidic HAuCl4 electrolyte (pH ≈ 3) over a wide range of potentials and thicknesses. The morphology, structure, and optical properties of the deposits are characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and UV–vis spectroscopy. Complementary characterizations are also conducted to characterize the chemical stability of the ITO surface in the gold-free electrolyte, the surface roughness factor of the deposits, and the gold deposition rate. Results demonstrate that a potential of −1.05 VAg/AgCl must be applied to grow smooth, continuous, and adherent gold UTFs on ITO. The critical thickness above which the gold layer becomes continuous lies between 4.1 and 6 nm. At potentials more positive than −1.05 V, the deposits are not mechanically stable, and they do not close the ITO surface, even for a thickness of 26 nm. A detailed discussion is provided to rationalize this finding. In particular, it is demonstrated that the in situ formation of metallic In0 on the ITO surface during gold nucleation plays a decisive role in growing gold UTFs. The strategy used in this work opens new perspectives to grow gold UTFs on other transparent conductive oxide electrodes.