(83m) Effect of Surfactants on the Formation of Au/ag Core/shell Nanoparticles in Micro Segmented Flow Synthesis

Composed metal nanoparticles are of particular interest for different applications due to their specific surface states and optical and electronical as well as chemical properties. The microfluidic conditions influence the formation of AuAg nanoparticles due to the different rates of nucleation and particle growth for the different metals. Here, we investigated the influence of surfactants on the synthesis of core/shell nanoparticles in micro flow-through synthesis using the segmented flow principle.

The synthesis was performed in a three step process. The carrier liquid (tetradecane) and the aqueous reactant solutions (reducing agent, surfactant, silver nitrate, tetrachloroauric acid) were fed into the mixer system by syringe pumps. Mixing was performed by T-type injectors and knot structures. The segmented flow principle supports a narrow distribution of residence times between the T-junctions and contributes to mixing by a shear force-induced segment-internal convection.

The addition of surfactants reduces both the rate of nucleation and the particle growth rate. Obviously, the adsorption of surfactant molecules at the surface of gold particles can reduce the rate of silver deposition at gold cores significantly. So, the reaction behavior shifts the size distribution and the optical properties of the obtained colloidal product solutions. The effect of surfactant is dependent on the order of mixing of reactants and of the volume flow rate. In some cases, a bimodal distribution of colloidal products was observed. An optimization of flow rates leads to an improvement of nanoparticle size distribution.

The obtained colloidal solutions were characterized by centrifugal sedimentation spectroscopy, by UV/VIS spectrophotometry, by SEM and TEM. The size spectra and the ultramicroscopic images confirm the different quality of produced nanoparticles and show, that the optical properties of binary metal nanoparticles prepared by micro reaction technology can be modulated in a large frame in dependence on surfactant addition and micro fluidic conditions.