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

Abstract

We report a unique alloy–dealloy phenomenon of Au–Ag nanoparticles inside SiO₂ films with clear plasmonic modifications between the absorptions of Ag (∼415 nm) and Au (∼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 °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 H₂ 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 H₂ 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% H₂ (balance Ar) and air in the temperature range from 50 to 400 °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.