SPR peak give d-metals. Occurrence associated with the formation of surface plasmons (of collective oscillations of conduction electrons ) of the metal near the interface metal / insulator. In Ag2O is covalent and ionic bonds. Look http://pubs.acs.org/doi/pdf/10.1021/acscatal.5b01543.
If researchers found SPR peaks, they represent small islands of metal Ag on Ag2O. It's my opinion.