Porous silica beads produced by nanofluid emulsion freezing

TL;DR

This paper demonstrates a novel method to produce porous silica beads using nanofluid emulsion freezing, revealing how microstructure varies with emulsion parameters and uncovering unexpected macroporosity linked to dendrite formation.

Contribution

It introduces an emulsion freezing technique to synthesize porous silica beads with controllable microstructure and reveals the formation of macroporosity due to dendrite growth during freezing.

Findings

Porous silica beads with mesopores and macropores were produced.

Microstructure depends on emulsion volume fraction and droplet size.

Unexpected macroporosity linked to dendrite formation during freezing.

Abstract

It is shown that porous spherical particles can be obtained via the freezing of silica nanoparticle aqueous suspensions emulsified in a continuous oil phase. After two freeze-thaw cycles, nanoparticles turn aggregated into flocculated objects with microstructure that depends upon emulsion volume fraction and droplet size. For low volume fractions, regular microspheres are produced while for large ones, irregular beads with several tens of micrometer radius are formed. Electronic microscopy, mercury porosimetry and nitrogen adsorption are used to get insights into these porous particles typical radius, pore size distribution, surface area and pore network structure. All exhibit mesopores that result from inter-nanoparticle spacing after flocculation. An unexpected macroporous domain appears which is not observed when drying non-emulsified suspensions. This macroporosity is interpreted as the signature of dendrite formation during the undercooled period, right before freezing occurs. Beside this additional macroporosity, the protocols presented in this article constitute also promising emulsion-based routes for porous material synthesis with original geometry, chemical composition and porosity.