Surface characteristics of Ag-doped Au nanoparticles probed by Raman scattering spectroscopy

We have examined the surface characteristics of Ag-doped Au nanoparticles (below 5 mol% of Ag) by means of the surface-enhanced Raman scattering (SERS) of 2,6-dimethylphenylisocyanide (2,6-DMPI) and 4-nitrobenzenethiol (4-NBT). When Ag was added to Au to form similar to 35-nm-sized alloy nanoparticles, the surface plasmon resonance band was blue-shifted linearly from 523 to 517 nm in proportion to the content of Ag up to 5%. In the SERS spectra of 2,6-DMPI, the N-C stretching peak also shifted almost linearly from 2184 to 2174 cm-1 when the Ag content was 5 mol% or less; the peak then remained the same as that of the pure Ag film. The potential variation of the SERS spectrum of 2,6-DMPI in an electrochemical environment, as well as the effect of organic vapor, also showed a similar tendency. From the SERS of 4-NBT, we confirmed the occurrence of a surface-induced photoreaction converting 4-NBT to 4-aminobenzenethiol, when Ag was added to Au to form alloy nanoparticles. The photoreaction induction ability also increased linearly with the Ag content, reaching a plateau level at 5 mol% of Ag. All these observations suggest that the surface content of Ag should increase almost linearly as a function of the overall mole fraction of Ag and, once the Au/Ag nanoparticles reach 5 mol% of Ag, their surfaces are fully covered with Ag, showing the same surface characteristics of pure Ag nanoparticles. Copyright (C) 2011 John Wiley & Sons, Ltd.