Generalized 3D Microprinting of Functional Nanocrstal-Based Inorganic Aerogels

Aerogels are porous materials with low densities and exceptionally high surface areas made up of non-fluid colloidal nanoscale networks. Due to their distinctive properties, aerogels are excellent materials for a number of applications, such as thermal and acoustic insulation, energy storage, catalyst, adsorbents, filters, electrodes, and sensors. Numerous materials, including oxides, polymers, carbons, metals, and even semiconductors, have thus far been synthesized as aerogels using sol-gel chemistry techniques or by the gelation of clusters or nanocrystals. However, aerogels' inherent brittleness hinders their processability through subtractive processes like machining. For instance, using conventional subtractive manufacturing techniques, the precision micro-processing of aerogels has never been accomplished.<br/>The development of additive manufacturing enabled the fabrication of a wide variety of materials into unique shapes and dimensions, which is regarded as a next-generation manufacturing technique. Furthermore, micro-stereolithography methods based on multiphoton absorption or using a projection lens, as well as direct ink writing with microneedles, enable the development of microscale 3D architectures that have the potential to revolutionize the production of microscale components for a variety of applications, including microelectronics, micro-electromechanical systems (MEMS), and biomedical systems. Recently, 3D-printed aerogels achieved by an extrusion process using viscoelastic inks containing active particles were reported. Although these reports describe the feasibility of 3D printing for aerogels comprised of some specific materials, such as graphene, silica, and cellulose, the methods described have low resolution and are limited to only a few printable materials. For example, the resolution of reported 3D-printed aerogels is generally in the scale of several hundreds of micrometers, and sub-100-micrometers have rarely been achieved. These reported strategies adhere to the common ink extrusion process that requires a viscoelastic formulation of inks to ensure printability. Particularly, the polymer-based viscosifiers used as additives in aerogel-based inks for inorganic materials deteriorate their intrinsic functionality.<br/>Here, we demonstrate an inorganic ligands-capped nanocrystal ink being directly written in the gelation bath as a generalized approach of high-resolution wet 3D microprinting for inorganic aerogels. Our process was designed to immediately link the thiometallate-based inorganic ligands with the coordinating metal ion linkers in the bath, thereby generating multibranched gel networks of various inorganic materials, such as metals, semiconductors, magnets, and oxides, without any organic additives. This method is designed to produce 3D microarchitectures of purely inorganic aerogels while combining the many advantages of conventional nanocrystal-based aerogels, such as material diversity and integrity of crystalline nanostructures. The microscale aerogel filaments are printed with precisely controlled dimensions from 7 μm to 44 μm. Complex 3D structures are achieved by layer-by-layer deposition of aerogel filaments with highly porous microstructures and high specific surface areas, comparable to those of traditional nanocrystal-based and silica aerogels. In addition, the functionality of primary nanocrystals, including the magnetic, electrical, and luminous properties, were well preserved in the printed aerogels. Additionally, our technology produced multi-material aerogels by sequentially printing each nanocrystal or mixing nanocrystal inks.