SPATIALLY-RESOLVED ELECTROCHEMISTRY OF THE LEAD SULFIDE (GALENA) (001) SURFACE BY ELECTROCHEMICAL SCANNING-TUNNELING-MICROSCOPY

Electrochemical scanning tunneling microscopy (STM) has been used to investigate the chemical and electrochemical reactions of naturally-occurring lead sulfide (galena) (001) surfaces. Both oxidation and reduction reactions are shown to be spatially anisotropic, involving almost exclusively atoms located at step edges. STM images show that the oxidation reaction PbS --> Pb2+ + S + 2e(-) occurs by removal of atoms from step edges, and that impurity-related defects on the surface lead to etch pit formation under oxidizing conditions. STM images at ambient lead concentrations reveal that the electrochemical reduction PbS + 2H(+) 2e(-) --> Pb-0 + H2S occurs exclusively by removal of material from step edges, with simultaneous 2 formation of Pb(111) islands on the PbS terraces. Re-oxidation of the Pb islands causes regrowth of PbS at the step edges and the simultaneous disappearance of Pb islands from the terraces, indicating that this reaction is partially reversible at the atomic level. Using atomic-resolution images to establish the crystallographic orientation, it is shown that the rate of removal of material from step edges is directionally anisotropic and is fastest along the [100] directions, leaving behind steps with outward normal directed along the equivalent [110] directions.