| dc.description.abstract | This work reports 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 solution. With a 1 mV/s scan rate cyclic voltammograms of Pt(111) reveal two well-defined UPD peaks at 0.65 and 0.61 V,while only one UPD peak at 0.44 V is observed for Rh(111). Real-time STM imaging along the UPD processes of Cu on Pt(111) reveal that these features correspond to the formation of a (√3 ×√3)R30°structure and a disorder phase. The entire surface was covered with an (√3 × √7)oblique structure at the onset of bulk deposition. These STM results are tentatively assigned to the (bi)sulfate anions coadsorbed with Cu adatoms sandwiched between the Pt electrode and sulfate anions. In contrast, a monolayer of Cu is plated in a single step on Rh(111), which was always accompanied by (bi)sulfate anions arranging in a well-ordered (
√3 × √7)oblique structure. A series time-dependent in situ STM images were acquired to unveil the deposition processes of Cu, which clearly show the initiation of deposition at defect sites, particularly upper step ledges, and the subsequently rapid lateral growth of Cu islands 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 observed. All the STM results indicate that sulfate anions actively participated in the UPD processes of Cu at these two electrodes. The contrast between these two electrodes illustrates the important roles of anions and interfacial forces existing among the electrodes, adatoms and anions.
The cyclic voltammograms for well-ordered Pt(111) electrodes in 0.05 M HCl + 1 mM Cu(ClO4)2 show a marked effect of chloride on UPD of Cu, as two CV feature are total resolved .In situ STM imaging at 0.7 V(1st UPD peak),indeed indicate the formation of an incommensurate (4 × 4) structure, presumably due to the upper chloride layer, whereas the lower Cu layer remained hidden. At 0.65V,in situ STM revealed (√7 × √7)R19.1°. Because this structure appeared at the end of the 1st peak, the structural transformation from (4 × 4) to (√7 ×√7)R19.1°was induced by deposition of more Cu atoms. Changing imaging condition led to revelation of a hexagonal array with a lattice constant of 0.28 nm, attributable to the lower Cu layer.In situ STM imaging at 0.45 V still revealed a well-ordered (√7 × √7)R19.1°-Cl adlayer, but a hexagonal array with a lattice constant of 0.256 nm.The 2nd peak could be continuous deposition of Cu, resulting in from a pseudomorphoric to a close-pack(1 × 1) structure. | en_US |