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.