Plasma-induced formation of flower-like Ag₂O nanostructures

Graphical abstract: Flower-like Ag₂O nanostructures. - Highlights: • Flower-like Ag₂O nanostructures were synthesized from Ag colloids using plasma. • XPS was used to monitor plasma treatment effect on Ag colloids. • SERS of methyl orange was used to monitor the plasma oxidation–reduction processes. • Photocatalytic degradation of methylene blue was performed using Ag₂O. • Ag₂O is a more efficient visible light photocatalyst than Ag colloids. - Abstract: Plasma treatment effect on Ag colloids was investigated using X-ray photoelectron spectroscopy (XPS) and surface-enhanced Raman scattering (SERS) techniques. XPS showed that O₂ plasma was critical in removing organic residues in Ag colloids synthesized using citric acid as a reducing agent. With O₂ 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 O₂ plasma treatment. Followed by H₂ plasma treatment, the SERS intensity of MO on Ag regained, which indicated that Ag₂O has been reduced to Ag. Nonetheless, the reduction by H₂ 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