SERS study of Ag-SiO₂ nanoparticles synthesized by photodeposition and stirring

ABSTRACT

This work presents a comparative study of Ag-decorated SiO₂ nanoparticles by photodeposition and stirring methods. SiO₂ NPs were synthesized by the Stöber method. In the photodeposition method, the silver nitrate concentration was set at 2.5 mM, at 532 nm wavelength radiation laser was used and three different exposure times (30, 60 and 90 minutes) were evaluated. In the stirring method, samples synthesized with three different concentrations of silver nitrate (5, 8 and 10 mM) were studied, furthermore, the effect of ammonium hydroxide as a catalyst was also studied. By UV-Vis spectroscopy absorption bands centered between 430 and 435 nm were observed for all samples (photodeposition, stirring and stirring+catalyst), the highest bands were observed in sample where the catalyst was used as evidence of superior concentration of Ag NPs. By Raman spectroscopy the peak associated to Ag-O bond was studied, we found that the SiO₂ NPs decorated by photodeposition exhibited the highest intensity as evidence of better Ag NPs surface distribution that improve plasmonic effects. The final evaluation of the Ag-SiO₂ NPs was carried out by surface enhanced Raman spectroscopy (SERS) measurements, using a solution of rhodamine 6G and using the intensity of mode at 1365 cm^-1 as standard, it is found that the SERS intensity of samples synthesized by photodeposition increases as the exposure time increases and that it is much higher than the intensity observed in the nanoparticles decorated by stirring, even when the AgNO₃ concentration is 4 times lower. With this study is demonstrated that by means of photodeposition is possible to obtain Ag NPs with better surface distribution, in short times and with the use of fewer reagents. Furthermore, Raman spectroscopy (conventional and SERS) is confirmed as cheaper, fast and non-destructive tool for evaluation of colloidal systems whit Ag NPs.

Keywords
Photodeposition; Stirring; SERS; Ag-SiO₂