Electrodeposition of Copper at Well-Defined Pt(111) and Rh(111) Electrodes in Sulfuric Acid Solutions: Studying with In Situ Scanning Tunneling Microscopy

We report in situ scanning tunneling microscopy (STM) results of underpotential deposition (UPD) of copper at well-ordered Pt(111) and Rh(111) electrodes in sulfuric acid solutions. Cyclic voltammograms of Pt(111) at 1 mV/s in 0.05 M H 2 SO 4 and 5 mM CuSO 4 reveal two well-defined UPD peaks at 0.65 and 0.61 V, whereas one doublet UPD peak at 0.44 V is observed for Rh(111). Real-time STM imaging revealed that the two sharp UPD features for Pt(111) correspond to the formation of a (√3 × √3) R 30° structure and a disorder phase, respectively. A long-range ordered (√3 × √7)oblique structure was imaged after a full monolayer of Cu was deposited, tentatively assigned to the (bi)sulfate anions lying atop the Cu adlayer. In contrast, a monolayer of Cu was deposited in a single step on Rh(111), where (bi)sulfate anions also actively participated in the process. In situ STM revealed a well-ordered (√3 × √7)oblique structure throughout the deposition process, likely because of the coadsorbed (bi)sulfate anions. A series of time-dependent in situ STM images were acquired to unveil the deposition processes of Cu. Deposition of Cu preferentially began at defect sites, particularly upper step ledges. Lateral growth and coalescence of Cu islands followed to cover nearly the whole surface. Decreasing potential to the bulk deposition region led to the formation of local Cu islands with a thickness of 4−5 layers of Cu, on which a well-ordered (√3 × √7)oblique structure was still observed. All the STM results indicate that sulfate anions were heavily involved in the UPD processes of Cu at these two electrodes. The different adsorption energies of Cu adatoms on Pt and Rh electrodes also affect the deposition processes.