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 tetrachloropallada te complex, PdCl42-, on on Au(lll) electrode in H2SO4 solution contain ing PdCl42- was investigated using an electrochemical quartz crystal m icrobalance (EQCM) and in situ electrochemical scanning tunneling micr oscopy (STM). The adlayer of PdCl42- with at (root 7 x root 7)R19.1 de grees structure on the Au(lll) substrate was observed by in situ STM m easurement, and the amount of adsorbed PdCl42- determined by the EQCM measurement was in good agreement with that estimated from this adlaye r structure. In situ STM observation showed also that the electrochemi cal deposition of palladium proceeded with an epitaxial layer-by-layer growth mode and the PdCl42- complex adsorbed on the deposited palladi um layer with the same adlayer structure. The adsorption of the PdCl42 - complex seemed to play an important role for the layer-by-layer grow th of palladiurn in a large area, since it inhibits the vertical but f actors the lateral growth of the palladium layer. The formation of the Pd(111) bulk phase and the surface structure of Pd(lll) was confirmed by X-ray diffraction (XRD) and the underpotential deposition of coppe r, respectively.