Surface heterogeneity in Au-Ag nanoparticles probed by hyper-Rayleigh scattering

Aqueous solutions of alloyed gold-silver nanoparticles, the diameter of which ranged between 15 and 50 nm, were prepared with varying molar fractions. The UV-Visible photoabsorption spectra for all solutions were dominated by a single surface-plasmon resonance, the wavelength of which was found to change linearly with the molar fraction. A model, in agreement with these findings, was then derived to compute a dielectric constant for the different alloyed gold-silver nanoparticles based on a weighted function of the pure gold and pure silver dielectric constants. Hyper-Rayleigh scattering measurements were performed for these alloyed metallic nanoparticles solution to determine their quadratic hyperpolarizability. Theses values, once corrected for their size and local-field factors dependencies, are found to be larger than the corresponding values for the pure gold and pure silver nanoparticles. This is attributed to the atomic surface heterogeneity of these alloyed gold-silver particles, underlining the potential of the method to investigate the surface of nanometer scale metallic particles, especially in catalysis. This atomic surface heterogeneity is assigned to the differences between the Au and Ag atoms in their interband transitions since their intraband parameters are very similar.