From colloidal dispersions to aerogels: How to master nanoparticle gelation

Nanoparticle-based aerogels are one of the few examples of 3-dimensional nanoparticle assemblies that can achieve macroscopic sizes while fully retaining the size-specific properties of the initial nanobuilding blocks. A typical synthesis involves three main steps: i) Preparation of a stable dispersion containing all the building blocks with the desired composition, size, shape and crystallinity, ii) controlled destabilization of the dispersion into a gel, and iii) drying of the gels under preservation of the 3-dimensional nanoparticle architecture. In this review, we focus on the first two steps. We discuss the stability of colloidal dispersions, offering an overview of the different interparticle forces and how they can experimentally be modulated. In the next section, we discuss the gelation process in its entirety, including gelation mechanisms, and, most importantly, how it can rationally be controlled under consideration of the interparticle forces in the dispersion. Finally, the review concludes with an overview of the synthesis pathways to nanoparticle-based aerogels, divided into the gelation of sterically and charge-stabilized particles.