(17e) Continuous Flow Synthesis of Core-Shell Nanoparticles Using Droplet-Based Microreactors

Core-shell bimetallic nanoparticles have large advantages over their monometallic counterparts in many applications. Colloidal synthesis offers a good control over the size, shape, and morphology of capped, non-agglomerating nanocrystals. However, two main disadvantages of this conventional approach are: only small quantities (sub- milligram) are produced, and irreproducible results are obtained while scaling-up. The solution to this problem is to use scalable droplet-based microreactors.

In this work, we demonstrate the continuous production of core-shell nanoparticles of defined morphology and translated a batch method to a microfluidic setup. The aim of this work is to address the following two key questions: (i) how to synthesize monodisperse nanoparticles inside microdroplets, and (ii) how to coat these nanoparticles with a uniform shell.

To address the first question, we quantified the effect of mixing and residence time distribution on the polydispersity index of gold nanoparticles. Comparing the results obtained in a microreactor with results obtained in a conventional batch process, we found that the polydispersity index of particles produced in the microreactor is two times smaller. Additionally, we found that particles much smaller than the average particle size observed in samples taken from the batch process, do not form in the microreactor.

To address the second question, we coated gold nanoparticles with silver layer as a model system [1]. We developed a droplet-based microreactor, which allows the controlled injection of silver precursor and mild reducing agent into droplets containing freshly formed gold nanoparticles. To inject precise amounts of reagents in the moving droplets, we adapted the so-called picoinjector approach [2]. Our results show that we obtain core-shell nanoparticles that have a monodisperse core (σ<10%) with a diameter of typically 7 nm and a uniform 4 nm thick silver coating. We also show that the thickness of coating can be easily tuned by changing the temperature, and the concentrations of the reagents. Our results are reproducible over multiple runs and show a superior control over particle morphology in comparison to conventional batch reactors.

We believe that this integrated microfluidic device will be particularly useful for the controlled synthesis of core-shell structures in the fields of optics, photo-voltaics, and catalysis.

References

[1] Ma Y, Li W, Cho EC, Li Z, Yu T, Zeng J, Xia Y. Au@Ag Core-Shell Nanocubes with Finely Tuned and Well-Controlled Sizes, Shell Thicknesses, and Optical Properties. Acs Nano. 2010;4:6725-34.

[2] Abate AR, Hung T, Mary P, Agresti JJ, Weitz DA. High-throughput injection with microfluidics using picoinjectors. Proc Natl Acad Sci U S A. 2010;107:19163-6.