In-Situ Electrochemical Scanning Tunneling Microscopy (ECSTM) Study of Cyanide-Induced Corrosion of Naked and Hexadecyl Mercaptan-Passivated Au (111)

摘要

In-Situ electochemical scanning tunneling microscopy (ECSTM) was used to studythe corrosion of naked and n-alkanethiol-modified Au (111) surfaces in basic CN(-) solutions. In these studies, the potential of a naked Au electrode is poised at a sufficiently negative value that no electrochemical etching takes place. Small positive potential excursions from the initial potential permit observation of the initial stages of corrosion. The data indicates that initial corrosion of naked Au (111) occurs only at high energy defect sites such as pits and step edges. At slightly higher overpotentials, pitting occurs in the middle of terraces while continuing at step edges. At even higher overpotentials, etching occurs rapidly and uniformly and the surface becomes very rough. Results from this experiment are compared to those obtained after coating the Au surface with a hexadecyl mercaptan self-assembled monolayer (SAM) to determine the extent to which the organomercaptan SAM inhibits Au corrosion. On the SAM-modified Au (111) surface the onset potential for a significant level of etching is shifted several hundred millivolts more positive than on the naked surface. Additionally, the rate of etching is significantly slower and the corrosion process is very different: etching initially occurs on terraces at defect sites within the monolayer instead of on step edges. We determined that potential, rather than time, is the primary factor that controls the rate of corrosion.