Plasma-induced brightening and coarsening of tarnished Ag nanoparticles

The recovery of the optical properties of tarnished silver nanoparticles (Ag NPs) has been demonstrated using exposure to plasma. As-prepared Ag NPs on a SiO2 substrate exhibited an intense localized surface plasmon resonance (LSPR) in the UV-vis optical extinction spectrum. The LSPR band weakened gradually when an Ag NP sample was stored in ambient air forming tarnished Ag. X-ray photoelectron spectroscopy (XPS) and ion-beam analysis revealed the presence of impurity elements, such as N and S. In addition, the shape of the valence-band XPS spectra changed significantly with exposure to ambient air. Ar plasma treatments recovered the intense LSPR and the XPS spectral shape. We conclude that both the reduction of Ag2S to Ag and the morphological change toward isolated Ag NPs most likely lead to their recovery. A simulation based on the Mie theory, as well as direct observation with an atomic force microscope, supports our findings.