Electrochemical layer-by-layer growth of palladium on an Au(111) electrode surface: Evidence for important role of adsorbed Pd complex

The adsorption and electrochemical reduction of the tetrachloropalladate complex, PdCl₄^2-, on an Au(111) electrode in H₂SO₄ solution containing PdCl₄^2- was investigated using an electrochemical quartz crystal microbalance (EQCM) and in situ electrochemical scanning tunneling microscopy (STM). The adlayer of PdCl₄^2- with a (√7 × √7)R19.1° structure on the Au(111) substrate was observed by in situ STM measurement, and the amount of adsorbed PdCl₄^2- determined by the EQCM measurement was in good agreement with that estimated from this adlayer structure. In situ STM observation showed also that the electrochemical deposition of palladium proceeded with an epitaxial layer-by-layer growth mode and the PdCl₄^2- complex adsorbed on the deposited palladium layer with the same adlayer structure. The adsorption of the PdCl₄^2- complex seemed to play an important role for the layer-by-layer growth of palladium in a large area, since it inhibits the vertical but favors the lateral growth of the palladium layer. The formation of the Pd(111) bulk phase and the surface structure of Pd(111) was confirmed by X-ray diffraction (XRD) and the underpotential deposition of copper, respectively.