INTRODUCTION
are a diverse class of ultra-low density, highly porous solids with large surface areas, rendering them promising candidates for various advanced applications.1, composition, aerogels can be classified as inorganic,3-6ux5fENREFux5f4ux5fENREFux5f5ux5fENREFux5f6 organic,7-9ux5fENREFux5f8ux5fENREFux5f9 hybrid organic-inorganic carbon-based aerogels.12, Aerogels are commonly fabricated using sol-gel chemistry techniques, including chemical gelation or deposition methods, followed by subsequent template removal to form monoliths.14 The assembled aerogels are notably weak and fragile in monolithic form, consisting of randomly interconnected three-dimensional backbone network skeletons and well-accessible pores, yet, exhibit outstanding performance when integrated in catalyst supports, electrocatalysts, thermal insulators, dust collectors, piezoelectrics and sensors.15 Unlike artificial aerogels, natural light-weight and porous materials such as woods, bones and sea sponges are composed of hierarchical long-range ordered structures, and show enhanced mechanical and optical properties.16 Imprinting similar ordered structures into synthetic aerogels, may significantly improve their properties.
Cellulose nanocrystal (CNC) is a sustainable and renewable nanomaterial that can be produced by controlled sulfuric acid-assisted hydrolysis of lignocellulosic biomass (wood, cotton and sisal, etc. ),17 and holds great promise as an exceptional building block for construction of macroscopic bulk-assembled materials, such as hydrogels, porous monoliths and aerogels.18 CNC contains abundant hydroxyl groups, and is characterized by low density (1.6 g·cm-3), high mechanical strength (2-3 GPa), a large surface area (up to 700 m2·g-1) and high elastic modulus (110-140 GPa), that make it suitable for construction of aerogel skeletons.19 Currently available CNC-based aerogels are mechanically tough and flexible, highly porous and optically transparent.20 Unlike natural cellulose-based materials, artificial CNC-based aerogels are fully amorphous, with randomly connected skeletal microstructures, which fail to demonstrate any significant structure-property relationships. Yet, it is well known that at or above a critical concentration, the dispersed CNCs can self-assemble in aqueous suspension into a chiral nematic or nematic organization, exhibiting anisotropic liquid crystalline structural properties, as well as fluidity and long-range order.21, ordered CNC assembly can be preserved in solid films, yielding a free-standing composite with intense birefringence and brilliant iridescence colours.23-25 This structure can be found in some plants and jewel beetles and is termed a Bouligand-type structure.26,
Directional freeze-casting (also known as ice-segregation-induced self-assembly) is a convenient method used to produce low-density aerogels with ordered structure.28, demonstrated great potential in fabrication of materials with highly sophisticated structures. In this method, the suspensions are transferred into a mold that is frozen in liquid nitrogen, using an unidirectional temperature gradient. In consequence, periodic arrays of ice crystals grow parallel to the freezing direction and exclude the solute from the ice front, forcing them into the inter-crystalline domain boundaries of the ice crystals, leading to an ordered structure of the solute phase.30 Upon selection of an appropriate solute precursor, previous works have shown a wide range of structurally ordered porous materials with aligned nanoparticles in the pore walls, which exhibited desired functionality.31-34
Recently, Munier et al.35 and Chau et al.36 independently reported closely related freeze-casting works involving CNCs. In their studies, they utilized low-concentration CNC suspensions (lacking liquid crystalline order) and polymer as precursors. The resulting freeze-casted products revealed one–dimensional ordering, with lamellar or columnar structure dictated by the ice template matrix. it remains to be determined how liquid crystalline CNCs interact with the growth of the anisotropic ice crystal, without the interference of additives. When there is a large-scale ordered arrangement in the liquid crystalline phase, and the system is rapidly quenched to ultra-low temperatures, such that reorganization can be prevented, the liquid crystalline order can be frozen in solid phase.37
Herein, we demonstrate that hierarchical long-range structured aerogels can be readily fabricated by unidirectional freeze-casting of liquid crystalline CNCs. The resulting CNC-based aerogels exhibited a self-supporting, anisotropic, low-density, porous multi-scale structure. In addition, the directional freezing induced formation of interconnected CNC networks, resulting in strong hydrogels with a distinct macrostructure and birefringence, via a direct thawing process. The growth of ice crystals was accompanied by the formation of fluidic chiral nematic ordering in liquid crystalline CNCs. In particular, we systematically investigated the origin of optical anisotropy in CNCs aerogel, and confirmed that it arises from the alignment of CNCs, rather than from the shape-persistent, rod-like CNCs itself. In summary, this work demonstrates a means of capturing the ordered structure of CNC aerogels through an ice-assisted sol-gel process and solidifying it in silica matrix as a “fossil record”. The presented freeze-casting-induced self-assembly of liquid crystalline CNCs in a confined ice matrix, expands the technique for fabrication of hierarchically structured, free-standing anisotropic aerogels with bio-derived nanoparticle building blocks.