Extremely fast Au–Ag alloy–dealloy associated reversible plasmonic modifications in SiO2 films

We report a unique alloy-dealloy phenomenon of Au-Ag nanoparticles inside SiO2 films with clear plasmonic modifications between the absorptions of Ag (similar to 415 nm) and Au (similar to 524 nm). An Au-Ag (1 : 1) alloy nanoparticles (average size: 4.5 nm)-incorporated transparent SiO2 film is prepared on a glass substrate using mercaptosuccinic acid capped Au nanoparticles and Ag+ co-doped hybrid sol. The Au-Ag (1 : 1) alloy-originated plasmon band (465 +/- 1 nm) is gradually red-shifted with increasing temperature (50 to 400 degrees C) due to the partial oxidation of Ag, causing a systematic modification of the alloy composition. The 1 : 1 alloy, however, reverted very quickly, showing its original plasmon band in the presence of a small amount of H-2 due to re-reduction of the oxidized Ag and instantaneous re-alloying. During the Ag oxidation, the Si-OH groups associated with the embedded SiO2 matrix exchange Ag+ to form Ag-O-Si linkages; they subsequently release Ag very quickly in H-2 and dissolve again into the parent alloy. As a result, the films exhibit reversible and rapid optical changes while cycling in 0.1% to 1% H-2 (balance Ar) and air in the temperature range from 50 to 400 degrees C. This unique reversible alloy-dealloy phenomenon clearly demonstrates the mechanism of plasmonic modification associated with Au-Ag nanoparticles embedded in the sol-gel SiO2 film matrix.