Abstract
Metal nanoparticles (NPs) are key materials used in a broad range of industries. Among the various synthetic routes of NPs, liquid-phase chemical reactions are promising because of their versatility in reaction conditions as well as their potential productivity. However, because the synthesis of NPs involves not only chemical reactions but also nucleation and growth processes, which are typically higher-order reactions in terms of the concentration, a small degree of nonuniformity in the concentration during mixing of reaction solutions can easily result in a wide size distribution of the resultant particles. A typical solution to this problem is to slow the rate of reactions compared with that of mixing; however, as a result, the synthetic processes often require long reaction periods and complex procedures. In this study, we applied a microreactor with excellent mixing performance to NP synthesis to simplify and intensify the processes. We synthesized nickel and platinum-cobalt alloy NPs as model materials. For the Ni NP synthesis, we demonstrated that the quick mixing provided by the microreactor enabled the precise control of the residence time, and consequently, monodispersed Ni NPs with an average size of 3.8 nm were synthesized. For the Pt-Co bimetallic system, the microreactor successfully produced Pt-Co alloy NPs, while batch-type synthesis with weaker mixing intensity resulted in a bimodal mixture of larger Pt NPs and smaller Co NPs. For both Ni and Pt-Co, monodispersed NPs were synthesized by simply mixing the reaction solutions in the microreactor at room temperature. These results demonstrate that the mixing process plays a key role in NP synthesis, and application of a microreactor enables the establishment of a facile and robust synthetic process.
Highlights
Metal particles exhibit excellent chemical and physical properties, and can be incorporated in various applications, including catalysts, electronics, photonics, sensing, and imaging (Xia et al, 2009)
The X-ray photoelectron spectroscopy (XPS) result of the as-synthesized particles exhibited no specific peaks of binding energies of Ni metal, as shown in Figure 3C, which could be due to the CTAB molecules surrounding the particle surfaces
Our investigation demonstrated that a residence time shorter than ca. 1 s between the microreactors was the key requirement to realize a sharp size distribution so that intermediate nickel–hydrazine complex species were trapped to react with NaOH molecules in the second microreactor, which was enabled by the excellent mixing ability of the microreactor
Summary
Metal particles exhibit excellent chemical and physical properties, and can be incorporated in various applications, including catalysts, electronics, photonics, sensing, and imaging (Xia et al, 2009). We determined the roles of the microreactors in Ni NP synthesis through a comparison with a Y-shaped mixer with weaker mixing intensity and proposed a possible mechanism for the formation of Ni NPs. For the Pt-Co alloy NP synthesis, we used a single microreactor in which a mixed solution of Pt and Co ions was mixed with a reducing agent, and investigated the effects of mixing devices and precursors of Pt and Co on the size and structure of the resultant particles. A premixed aqueous solution of Pt and Co ions and PVP was mixed with an aqueous solution of a strong reducing agent, NaBH4 Both solutions were injected into the microreactor using a syringe pump at the flow rate of 10 ml/min for each syringe. ICP measurements were conducted for supernatants obtained by centrifuging suspensions at 15,000 rpm for 10 min twice
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