| Abstract
| - A simple synthesis method of gold nanoparticles dispersed at the silica surface which allows a progressiveand controlled increase of Au particle population is described. An important step of the preparation is treatmentin H2 at 573 K, which allows cleaning the surface of Au from chlorine, a frequently present impurity whichcan 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. Theincrease of the surface Au particle concentration is accompanied by a gradual change of the absorption spectrumin the visible−NIR, with progressive appearance of an intense tail in the NIR region. This tail is ascribed tothe formation of a growing amount of particle aggregates. This assignment is fully confirmed by an HRTEMstudy, which clearly shows the presence of aggregates at the highest loading. The Raman spectra obtained byirradiation with a laser at 785 nm, i.e., where the aggregates are strongly contributing, allows obtaining asignificant enhancement of the Raman spectrum of the external layers of SiO2 support, with special amplificationof the modes associated with surface ⋮O3Si−OH groups in close contact with the (Au)n clusters. Adsorptionof pyridine produces a strong modification of the Raman spectrum, with specific intensification of the modesof pyridine adsorbed on gold and simultaneous weakening of the signals from the underlying substrate. Theseobservations allow the following conclusions: (a) SERS concepts can be extended to the studies of solidsurfaces; (b) steadily dispersed “clean” gold particles at a solid surface under controlled conditions behave assurface probes under laser illumination with wavelength near plasmonic absorption; (c) Au clusters areresponsible for the greatest enhancement factors; (d) the enhancement factor is maximum for the surfaceSiO2 groups in direct contact with the Au clusters; (e) the presence of pyridine in the adsorbed layer causesmodifications of the Au/SiO2 interaction, with intensification of the Raman peaks of pyridine adsorbed onAu, and weakening of the support modes.
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