Plasma treatment effect on Ag colloids was investigated using X-ray photoelectron spectroscopy (XPS) and surface-enhanced Raman scattering (SERS) techniques. XPS showed that O-2 plasma was critical in removing organic residues in Ag colloids synthesized using citric acid as a reducing agent. With O-2 plasma treatment, Ag colloids were also oxidized to form flower-like Ag2O nanostructures. The formation mechanism is proposed. The SERS spectral intensity of methyl orange (MO) adsorbed on Ag surface became deteriorated with O-2 plasma treatment. Followed by H-2 plasma treatment, the SERS intensity of MO on Ag regained, which indicated that Ag2O has been reduced to Ag. Nonetheless, the reduction by H-2 plasma could not bring Ag back to the original as-synthesized nanoparticle morphology. The flower-like nanostructure morphology still remained. The photocatalytic degradation reactions of methylene blue (MB) aqueous solutions were carried out using Ag colloids and Ag2O nanostructures. The results show that Ag2O is more efficient than Ag colloids and many other metal oxides for the photocatalytic degradation of MB in solution when utilizing visible light. (C) 2015 Elsevier B.V. All rights reserved.