Simple and green synthesis of monodisperse silver nanoparticles and surface-enhanced Raman scattering activity

We describe a simple and green method for preparing silver nanoparticles with an average diameter in the range from 4.0 nm to 5.0 nm and with the standard deviation as low as 0.3 nm (polydispersity: 5.5%). The pre-synthesized silver oleate precursor in oleic acid was thermally reduced to obtain the monodisperse silver nanoparticles without a size selection procedure. This environmentally friendly chemistry approach requires only three reagents (silver nitrate, sodium oleate and oleic acid) which all come from renewable resources. The average diameter of these nanoparticles can be greatly adjusted by varying reaction time and finely regulated by varying reaction temperature at the same precursor concentration. The nucleation/diffusional growth model can be used to interpret the size and morphology difference of silver nanoparticles synthesized under different conditions. Raman spectra of graphene with and without an active layer of silver nanoparticles were measured to investigate the surface-enhanced Raman scattering properties of the silver nanoparticles. The Raman signal of graphene with an active layer increased by fifteen-fold in comparison to that without an active layer. The surface-enhanced Raman scattering activity can be explained using interparticle coupling induced Raman enhancement.