Au Nanoparticles as SERS Probes of the Silica Surface Layer Structure in the Absence and Presence of Adsorbates

Abstract

A simple synthesis method of gold nanoparticles dispersed at the silica surface which allows a progressive and controlled increase of Au particle population is described. An important step of the preparation is treatment in H2 at 573 K, which allows cleaning the surface of Au from chlorine, a frequently present impurity which can interfere with the Au/silica interaction. At the lowest concentration, the Au particles (almost spherical, d ≅ 4 nm) are isolated, while at the highest loading clusters of particles are becoming more frequent. The increase of the surface Au particle concentration is accompanied by a gradual change of the absorption spectrum in the visible−NIR, with progressive appearance of an intense tail in the NIR region. This tail is ascribed to the formation of a growing amount of particle aggregates. This assignment is fully confirmed by an HRTEM study, which clearly shows the presence of aggregates at the highest loading. The Raman spectra obtained by irradiation with a laser at 785 nm, i....