Au nanoparticles as SERS probes of the silica surface layer structure in the absence and presence of adsorbates

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 An particles (almost spherical, d congruent to 4 nm) are isolated, while at the highest loading clusters of particles are becoming more frequent. The increase of the surface An 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.e., where the aggregates are strongly contributing, allows obtaining a significant enhancement of the Raman spectrum of the external layers of SiO2 support, with special amplification of the modes associated with surface O3Si-OH groups in close contact with the (Au)(n) clusters. Adsorption of pyridine produces a strong modification of the Raman spectrum, with specific intensification of the modes of pyridine adsorbed on gold and simultaneous weakening of the signals from the underlying substrate. These observations allow the following, conclusions: (a) SERS concepts can be extended to the studies of solid surfaces; (b) steadily dispersed "clean" gold particles at a solid surface under controlled conditions behave as surface probes under laser illumination with wavelength near plasmonic absorption; (c) An clusters are responsible for the greatest enhancement factors; (d) the enhancement factor is maximum for the surface SiO2 groups in direct contact with the An clusters; (e) the presence of pyridine in the adsorbed layer causes modifications of the Au/SiO2 interaction, with intensification of the Raman peaks of pyridine adsorbed on An, and weakening of the support modes.