Electrochemical deposition of palladium on an Au(111) electrode: Effects of adsorbed hydrogen for a growth mode

Electrochemical deposition of palladium on an Au(111) electrode was investigated using in situ scanning tunneling microscopy (STM) and electrochemical quartz crystal microbalance (EQCM). STM images clearly showed that palladium deposition was proceeded two-dimensionally even in the relatively large overpotential region up to approx. +0.3 V (vs. RHE). Many nuclei were created, however, in the potential region where hydrogen adsorption took place, i.e. more negative than +0.3 V. EQCM measurement showed that the surface mass was steadily increased during the potential scan as far as the cathodic current flowed even in the potential region where hydrogen adsorption took place. The abrupt surface mass decrease and increase were observed, however, when the potential was stepped from +0.4 V (hydrogen adsorbed state) to +0.1 (hydrogen adsorbed state) and from +0.1 to +0.4 V, respectively, showing the desorption and adsorption of PdCl₄^2- complex from the electrode surface upon hydrogen adsorption and desorption, respectively. These results support the model that the PdCl₄^2- complex plays an important role to inhibit the three-dimensional growth and facilitate the two-dimensional growth.