Gram-Scale Continuous Flow Synthesis of Silver-on-Silica Patchy Nanoparticles with Widely Tunable Resonances for Plasmonics Applications

Abstract

Extensive research has been conducted on colloids that incorporate nanostructured noble metals, motivated in particular by their highly tunable optical properties. However, the lack of approaches that offer reproducibility, tunability, and scalable production is limiting their exploitation in many application areas. Here, we present an innovative process for the controlled formation of silver patches on silica nanospheres using gold nanocrystal seeds. The synthesis of the seeds and their attachment in very small numbers to the silica spheres is achieved without employing any surfactants, stabilizers, or linker molecules. Using advanced transmission electron microscopy techniques, we show that electroless deposition of nanostructured silver occurs at the introduced seeds and leads to a central protrusion from the metal patches not observed in earlier, seed-free work. We demonstrate that both seeding and growth processes can be achieved in millimixer flow reactors. With this simple and scalable approach, we provide compelling evidence for the use of seeds in producing targeted morphology and function. Specifically, patch size and optical properties can be tailored in a predictive manner by adjusting the density of seeds on the silica nanospheres as well as reagent concentrations during growth. As a result, our process can be used to obtain colloidal dispersions with localized surface plasmon resonance peaks anywhere in the range 400 to nearly 1300 nm. We confirm the scalability of our method by operating the growth process for around 70 min. Apart from changes in the first few minutes, the product properties remained rather stable and over a gram of high quality silver patch-coated silica could be recovered. Our approach opens up exciting new avenues for plasmonics applications, which typically require large amounts of nanoparticles with specifically designed optical properties. These include color and IR pigments, photocatalysts, and as light management components in optoelectronic devices like photovoltaic cells.