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

Electrochemical deposition of palladium on an Au(lll) electrode was investi gated using in situ scanning tunneling microscopy (STM) and electrochemical quartz crystal microbalance (EQCM). STM images clearly showed that palladi um deposition was proceeded two-dimensionally even in the relatively large overpotential region up to similar to + 0.3 V (vs. RHE). Many nuclei were c reated, however, in the potential region where hydrogen adsorption took pla ce, i.e. more negative than + 0.3 V. EQCM measurement showed that the surfa ce mass was steadily increased during the potential scan as far as the cath odic current flowed even in the potential region when hydrogen adsorption t ook place. The abrupt surface mass decrease and increase were observed, how ever, when the potential was stepped from + 0.4 V (hydrogen desorbed state) to + 0.1 V (hydrogen adsorbed state) and from + 0.1 to + 0.4 V, respective ly, showing the desorption and adsorption of PdCl42- complex from the elect rode surface upon hydrogen adsorption and desorption, respectively. These r esults support the model that the PdCl42- complex plays an important role t o inhibit the three-dimensional growth and facilitate the two-dimensional g rowth. (C) 1999 Elsevier Science B.V. All rights reserved.