Nanoscale chemical imaging of zinc oxide nanowire corrosion

The ability to monitor corrosion on a nanometer scale is a powerful tool for a fundamental understanding of surface chemical processes. Nanoscale chemical images of individual bare and alumina-coated zinc oxide nanowires (NWs) were recorded using tip-enhanced second harmonic generation (SHG) spectroscopy before and after exposure to carbon dioxide and water vapor. Images were collected for the same bare nanowire after each successive two-day exposure period. Corrosion of the bare ZnO NW to zinc carbonate was evident from far-field and near-field SHG images and simultaneously recorded atomic force microscopy (AFM) data. The expected zinc carbonate corrosion product is SHG inactive. The AFM profile of the NW showed vertical and lateral expansion in different regions of the nanowire. The lower resolution far-field SHG signal decreased gradually and uniformly. The near-field SHG signal provided a profile of the evolving NW with a spatial resolution approaching 100 nm. In contrast, exposed alumina-coated ZnO NWs showed reduced, but still observable, degradation. The 3 nm thick alumina protective layer may have been insufficient to fully protect the NW, or the coating may have been incomplete. Thicker coatings preclude the tip-enhanced method. Further nanometer-scale imaging should lead to the discovery of protective layers to prevent or delay ZnO degradation.