Formation of Au/Ag Nanoparticles in a Two Step Micro Flow-Through Process

The optical properties of metal nanoparticles depend strongly on their composition and size. The plasmon absorption band can be shifted between the blue range and the red range of the optical spectrum, if silver and gold are mixed and aggregation of particles is also induced. The formation of different nanoparticles and nanoparticle aggregates was studied in a two-step flow-through process using a glass/Si/glass static micromixer. The educt solution was pumped through the mixer chips by programmable syringe pumps. The process was monitored online by a compact spectrometer connected to the outlet tube by glass fibers. The product solution was collected and investigated by a UV/VIS spectrophotometer. Selected samples were investigated by Scanning Electron Microscopy (SEM) to evaluate the particle morphology. The typical Ag nanoparticle absorption was found, when an excess of Ag ions was introduced into the system. The intensity of the peak decreased when the flow rate of AgNO3 solution was reduced in a reaction system, which included HAuCl4 solution and polyvinyl alcohol, as well as the reducing agent (ascorbic acid). The experiments show that the educt concentration ratios as well as the total flow rates influence the shape and composition of the nanoparticles. Therefore, microreactors with fast mixing and well reproducible mixing conditions are interesting options for the synthesis of nanomaterials.