(98c) 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 spectra, if silver and gold are mixed and aggregation of particles is induced, in addition.

The formation of different nano particles and nano particle aggregates was studied in a two step flow-through process using a glass/Si/glass static micro mixer. The educt solution was pumped through the mixer chip by programmable syringe pumps. The process was monitored online by a compact spectrometer connected with the outlet tube by glass fibers. The product solution was collected and investigated in addition by an UV/VIS spectraphotometer (Specord, Analytic Jena). Selected samples were investigated by Scanning Electron Microscopy (SEM) in order to evaluate the particle morphology.

The typical Ag nanoparticle absorption at 460 nmwas found, if an excess of Ag ions is introduced into the system (Fig. 1). The intensity of the peak is decreased with the reduction of the flow rate of AgNO3 solution in a reaction system including HAuCl4 solution and polyvinylalcohol beside the reducing agent (ascorbic acid). For a flow rate ratio of 5 :l/min Ag NO3 (0.1 M) and 50 :l/min HAuCl4 (1 mM), a broad absorption covering te whole range between about 370 nm and 520 nm was observed.

The change of the flow rate of only one component changes the total of the flow rate as well as the ratio of educt concentrations. But the properties of the product are also dependend on the flow rate in case of constant concentration ratios. So, a silver dominated absorption peak arised at higher flow rate, a broad absorption band indicating the importance of gold-dominated nano particles was found at a lower flow rate (Fig. 2). This effect is obviously due to the importance of nucleation rate and the growth of the nuclei in comparison with the liquid transport and the mixing processes.

Heterogenities in the particle composition can not be distingushed by SEM in most cases. But, in some experiments aggregat particles showing small cores of the heavy material (Au) onto larger particles of the lighter component (Ag) were found (Fig. 3).

The experiments show that 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 for the synthesis of complex nanomaterials.