Additive‐Free, Gelled Nanoinks as a 3D Printing Toolbox for Hierarchically Structured Bulk Aerogels

Abstract Aerogels are highly porous solids that maintain the properties of individual nanomaterials at a macroscopic scale. However, the inability to fabricate hierarchical architectures limits technological implementation in energy storage, gas‐sorption, or catalysis. A 3D‐printing methodology for additive‐free TiO₂ nanoparticle‐based aerogels is presented with full control of the nano‐, micro‐, and macroscopic length‐scales. To compensate for ink's low solid loading of 4.0 vol% and to enable subsequent processing into aerogels via supercritical drying, the printing is done in a liquid bath of alkaline pH. The 3D‐printing protocol retains a high specific surface area of 539 m² g^–1 and a mesopore diameter of 20 nm of conventionally casted aerogels while offering an unparalleled designability on the micrometer scale. To illustrate the new geometric freedom of 3D‐printed aerogels, the microstructure of a strongly light‐absorbing, photothermal Au‐nanorod/TiO₂ aerogel is defined. To date, photothermal nanomaterials are mainly applied in the form of unstructured films where scalability is limited by light attenuation. Microstructures in 3D enhance light penetration by a factor of four and facilitate spatially defined heating on a macroscopic scale. The process can be generalized for a broad material library and allows to design inks with specific functionality, thus making aerogels adaptable for their target application.