3D‐ TEM microstructure analyses of anisotropic and isotropic aerogels of TiO2 nanoparticles

Abstract

Aerogels are fascinating materials with low density and high surface area with great application potential in battery materials, fuel‐ and solar cells and many more.(1‐3) These porous structures are conventionally prepared by sol‐gel chemistry and subsequent supercritical drying.(4) In the case of TiO 2 nanoparticles, the ability of the crystalline nanoparticles to undergo oriented attachment is fundamentally important for the self‐assembly into three‐dimensional interconnected networks. Such structures (Fig.1) are prepared by destabilizing highly concentrated dispersions of well‐defined TiO 2 nanocrystals. The gelation results in a percolating network of ultrafine structures which is preserved by supercritical drying. This processing technique allows for the preparation of crystalline, isotropic and translucent aerogels with broad pore size distributions and disordered pore arrangements (Fig.1).(5) The aim of our study is to produce structures from destabilized dispersions and subsequent unidirectional freeze‐drying. Compared to the aerogel route, this process results in a different macro‐ and microstructure consisting of about 3 µm large units of compacted aerogel structures. Here, the pores are of a more ordered nature due to the freeze drying procedure. In this work, aerogels prepared by supercritical drying and structures made via freeze‐drying were analyzed microstructurally. Using TEM tomography, we study the morphology of the 3D networks of the nanocrystals within both structures. Critical performance parameters like pore size, connectivity and tortuosity of those structures are analyzed.