(713b) Advanced Emulsion-Templating of Microreactors for the Scalable Production of Semiconductor Nanowires

We demonstrate how particle stabilized double emulsion droplets prepared by simple batch emulsification can be used to generate microreactors for the scalable production of semiconductor nanowires. The vapor-liquid-solid (VLS) mechanism enables the synthesis of semiconductor nanowires with different functional blocks encoded along their length. However, current substrate-based syntheses cannot offer the economically competitive scale-up needed for applications such as large-area electronics and solar energy harvesting. Nanowire growth inside porous hollow silica microspheres, by contrast, offers the advantage of 3D scaling and compatibility with standard fluidized bed reactors. The requisite hollow microspheres have a target size of 30 to 100 micrometers and gold catalyst nanoparticles exposed on their inner surface. We discuss in detail the steps and rationale of their preparation procedure, which involves (a) the emulsification of a moderately salty aqueous gold nanoparticle dispersion in an extractable organic solvent using partially hydrophobized silica particles as the emulsion stabilizer; (b) the addition of strongly hydrophobic silica particles to the continuous solvent phase; (c) re-emulsification in an aqueous solution of low ionic strength, yielding a w/o/w emulsion with multiple water droplets inside a typical solvent droplet; (d) osmotic swelling and coalescence of the inner water droplets resulting in double emulsion droplets with a single aqueous core, (e) solvent extraction to produce aqueous-core, solid-shell microsphere capsules dispersed in an aqueous medium, and (f) drying. Si nanowires grown inside the resulting hollow microspheres from SiH₄ and Au catalyst nanoparticles are single crystalline and exhibit excellent morphologies.