Correlating the plasmonic and structural evolutions during the sulfidation of silver nanocubes.

Ag/Ag2S hybrid nanostructures have recently received much attention, because of their synthetically tunable plasmonic properties and enhanced chemical stability. Sulfidation of pregrown Ag nanocrystals is a facile process for making Ag/Ag2S nanostructures. Understanding the sulfidation process can help in finely controlling the compositional and structural parameters and in turn tailoring the plasmonic properties. Herein we report on our study of the structural and plasmonic evolutions during the sulfidation process of Ag nanocubes, which is carried out at both the ensemble and single-particle levels. Ensemble extinction measurements show that sulfidation first causes the disappearance of the high-order triakontadipolar plasmon modes, which have electric charges located on the sharp vertices and edges of Ag nanocubes, suggesting that sulfidation starts at the vertices of Ag nanocubes. As sulfidation goes on, the dipolar plasmon peak gradually red-shifts, with its intensity first decreasing and then increasing. Electron microscopy characterizations reveal that sulfidation progresses from the outer region to the center of Ag nanocubes. The cubic shape is maintained throughout the sulfidation process, with the edge length being increased gradually. Single-particle scattering measurements show that the dipolar plasmon peak red-shifts and decreases in intensity during sulfidation. An additional scattering peak appears at a shorter wavelength at the late stage of sulfidation. The difference in the sulfidation behavior between ensemble and single-particle measurements is understood with electrodynamic simulations. During ensemble measurements, the Ag core is increasingly truncated, and it becomes a nanosphere eventually. Sulfidation stops at an intermediate stage. During single-particle measurements, Ag nanocubes are completely transformed into Ag2S, leading to the observation of the shorter-wavelength scattering peak.