Clay-Based Multifunctional Materials: Strategic Design and Applications

PROGRESS REPORT
Multifunctional Materials
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Strategic Design of Clay-Based Multifunctional Materials:
From Natural Minerals to Nanostructured Membranes
Yi Zhou, Anna Marie LaChance, Andrew T. Smith, Hongfei Cheng,* Qinfu Liu,
and Luyi Sun*
Nature not only carefully prepares ingenious raw materials but also
continuously inspires and guides human beings to create a wide variety
of intelligent materials. As the most abundant mineral resource on earth,
clay minerals are no longer synonymous with ceramics and cements. Many
natural clay minerals can be exfoliated into single- or few-layered nanosheets
with exquisite physicochemical properties, which can be reassembled into
functional membranes with a macroscopic controllable size and microscopic
ordered structure. They are thus used in many fields including chemistry,
biology, energy, and environmental science. Strategic design represents one
of the key processes to enhance the value of clay minerals and broaden their
applications. In this work, the three frequently used approaches of exfoliation
are highlighted and the six routes of assembly including casting, dip-coating,
spray coating, vacuum filtration, electrophoretic deposition, and 3D printing
are compared. The corresponding principles and advantages are summarized.
Representative applications of clay-based multifunctional membranes in
protection, separation, responsiveness, flexible electronics, and energy
conversion are presented. The challenges and future perspectives of the claybased multifunctional membranes are discussed.
Y. Zhou, Prof. H. Cheng, Prof. Q. Liu
School of Geoscience and Surveying Engineering
China University of Mining and Technology
Beijing 100083, P. R. China
E-mail: h.cheng@cumtb.edu.cn
Y. Zhou, A. M. LaChance, A. T. Smith, Prof. L. Sun
Department of Chemical and Biomolecular Engineering
and Polymer Program
Institute of Materials Science
University of Connecticut
Storrs, CT 06269, USA
E-mail: luyi.sun@uconn.edu
Prof. H. Cheng
School of Environmental Science and Engineering
Chang’an University
Xi’an 710054, P. R. China
Prof. H. Cheng
Key Laboratory of Subsurface
Hydrology and Ecological Effects in Arid Region
of the Ministry of Education
Chang’an University
Xi’an 710054, P. R. China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201807611.
DOI: 10.1002/adfm.201807611
Adv. Funct. Mater. 2019, 29, 1807611
1. Introduction
Nature produces numerous materials to
fulfill various functions and offers constant
inspiration to the design and preparation
of new materials.[1–4] Inspired by lotus
leaves and water strider’s legs, researchers
synthesized self-cleaning and versatile
super-hydrophobic surfaces.[5,6] Motivated
by mussels and geckos, scientists created
hybrid adhesives that can be used in both
dry and wet environments.[7] Similarly, as
a natural material with ultrahigh strength
and appreciable toughness, nacre has
inspired the design and synthesis of a wide
variety of multifunctional layered composites.[8–10] Combined with the hard inorganic
backbone sheets, soft organic glue layers
act not only as a stress buffer but also as
a natural channel in between the hard
sheets.[10,11] A vast majority of functional
membranes have been synthesized out
of the nacre inspiration. After decades of
development, such nanostructured membranes have been applied to various fields of chemistry, biology,
energy, and environmental science, exhibiting outstanding properties and extraordinary performance including excellent stability,
distinct adaptability, and selective mass transport behavior.[9,12,13]
All of these outstanding properties are inseparable from the
intelligent structure design and proper choice of building blocks.
While graphite is probably the most popular layered material,[14,15] many other inorganic layered materials with different types of layer charges have also attracted high attention, including negatively charged boron nitride,[16] smectite
clays and silicates,[17] and metal phosphates and phosphonates
(e.g., Zr(HPO4)2·H2O),[18–20] positively charged layered double
hydroxides (LDHs),[21,22] and neutral transition metal chalcogenides (e.g., MoS2),[23–26] metal oxides (e.g., TiO2, V2O5, and
MoO3),[27–29] and metal halides (e.g., PbCl4).[30] The common
feature they share is that they are a group of solids with strong
in-plane chemical bonds but weak out-of-plane van der Waals
forces and/or hydrogen bonds.[31,32] And many of them can be
exfoliated into single- or few-layered 2D nanosheets through
physical or chemical processes. Such nanosheets can serve as
ideal building blocks to create various high performance materials, such as functional membranes.
Even though a large number of functional materials reconstructed with 2D nanosheets have already been reported, the
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demand for environmental friendly and cost-effective materials
is ever-increasing. Clays are abundant in the crust and can even
be recovered from mining and mineral-processing waste.[33]
To some extent, unlike other minerals, clays are somewhat
sustainable because weathering and hydrothermal alteration
constantly generate clays at a rate much faster than the formation of other minerals. Clays have shown significant potential
to be exploited as the building blocks for functional materials
because of their special layered structure, high thermal stability,
and remarkable absorption and adsorption capacity.[34–36] Furthermore, clays are easily modified and thus are very versatile
for widespread application.[37] For example, kaolinite possesses
a natural 2D heterostructure and the octahedral sheet exhibits
a high reactivity for further chemical modification because of
the presence of hydrophilic aluminum hydroxyl groups.[10]
Montmorillonite (MMT) can be easily exfoliated into individual
single-layer nanosheets with a thickness of ≈1 nm and a lateral
dimension of 200–500 nm.[38] Palygorskite shows an ideal colloidal property and heat resistance, superb capacity in absorption and adsorption; moreover, their elongated thin particles
are particularly ideal as reinforcers for various materials.[39] The
functional groups, size, and charge density of the intercalants
allow for strategic organizations of layered structure, which
opens new avenues to the formation of functional membranes.
Herein, we review the recent advances in the design, fabrication, and applications of clay-based functional membranes.
2. Design and Fabrication
Clay particles are composed of layers of structural sheets arranged
in two formations: tetrahedral and octahedral, which are held
together by sharing a plane of oxygen atoms. According to the
ratio of tetrahedron and octahedron sheets, the basic structure of
a layered silicate mineral can be categorized into 1:1 or 2:1 type.
For 1:1 type layered clays (kaolinite, nacrite, etc.), the stacking
of unit cells contains one Si-tetrahedral sheet (STS) and one
Al-octahedral sheet (AOS) (sometimes, Al is substituted by Mg)
(Figure 1a). For 2:1 type layered clays (smectites, pyrophyllite,
etc.), the stacking of unit cells contains two STS and one AOS
(Figure 1b).[40] Fibrous clays (sepiolite, palygorskite, etc.) have a
similar structure to 2:1 phyllosilicates; however, discontinuities in
their octahedral sheets lead to nanosized structural tunnels along
the fiber-axis direction, which is unfavorable for the formation of
highly ordered few-layer nanosheets. Figure 1c shows the various
stacking arrangements of different crystal structures. The unit
layers of kaolinite stack to a greater degree than the others; they
are held together by hydrogen bonding accompanied with dipole–
dipole and van der Waals interactions.[41] Kaolinite exhibits a
minimal layer charge and a low exchange capacity because of
the limited substitution in their structural lattice.[42] Kaolinite is
not prone to interlayer expansion in water, but swelling can be
induced in contact with certain compounds (that are able to form
hydrogen bonds with the interlayer surface).[43] The ideal layer
structure of talc and pyrophyllite is electrically neutral; there is
therefore no charge balancing cations in the interlayer space.
When a tetrahedral sheet is combined with an octahedral sheet
in a unit layer, the resulting structure exhibits either electrical
neutrality or electronegativity.[33] The negative layer charge arises
Adv. Funct. Mater. 2019, 29, 1807611
Yi Zhou is currently a Ph.D.
candidate under the supervision of Prof. Qinfu Liu and
Prof. Hongfei Cheng at China
University of Mining and
Technology, Beijing. He spent
two years training with Prof.
Wei Guo at the Technical
Institute of Physics and
Chemistry, Chinese Academy
of Sciences. Afterward, he
joined the research group
of Prof. Luyi Sun as a visiting student at the University
of Connecticut from 2017 to 2018. His research interests
include the design and fabrication of functional materials
based on clay minerals for environmental and energy
related applications.
Hongfei Cheng received
his Ph.D. degree in 2011
and worked as an Associate
Professor at China University
of Mining and Technology,
Beijing from 2013 to 2018.
In 2019, he was promoted to
be a Professor in the School
of Environmental Science
and Engineering, Chang’an
University. His research
interest is mainly focused on
the structure, surface reactivity, and applications of clay
minerals for energy and environmental science.
Luyi Sun received his B.S.
from South China University
of Technology in 1998 and
Ph.D. from the University of
Alabama in 2004. After his
postdoctoral training at Texas
A&M University, he worked at
TOTAL Petrochemicals USA,
Inc. from 2006 to 2009. He
was an Assistant Professor at
Texas State University from
2009 to 2013. Dr. Sun joined
the University of Connecticut as an Associate Professor
in 2013 and was promoted to be a Professor in 2018. His
research focuses on the design and synthesis of nanostructured materials for various applications.
from random isomorphous cation substitution. Compared with
illite and vermiculite (VMT), smectites possess a relatively weaker
negative charge which allows the interlayer space to expand from
0.9 nm to complete delamination to form individual layers.[43]
Therefore, smectites can be readily exfoliated into single- or fewlayer nanosheets by proper treatments.
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Figure 1. Different layer structures. Stacking of unit cell of a) 1:1 type clays and b) 2:1 type clays. c) Various stacking arrangements of different crystal
structures. Modified with permission.[43] Copyright 2013, Wiley-VCH.
The exfoliated nanosheets with a high aspect ratio can
be readily assembled into 2D functional membranes with
a uniform microstructure by several assembly approaches
because of the interactions between the basal planes. In the
following sections, three frequently used exfoliation methods
are summarized, and six routes of assembly are highlighted.
Emphasis will be placed on how to achieve the desired function
by a proper selection of assembly technology.
2.1. Exfoliation
Because of the weak out-of-plane interactions, several types
of clays can be exfoliated into 2D nanosheets.[31] The direct
advantage of exfoliation is to remarkably increase the specific
surface area and dramatically enhance surface activity, potentially leading to more significant applications such as fillers
for polymers and inorganic building blocks for free-standing
films.[10,37,44,45] Therefore, various exfoliation methods based
on different stacking microstructures have been developed,
including direct delamination, intercalation-assisted exfoliation,
and ion-exchange-assisted exfoliation (Figure 2). Note that ion
exchange can usually be viewed as a type of intercalation, but
here we would exclude ion exchange from intercalation as a
separate method because of its uniqueness in facilitating exfoliation, which will be discussed below in detail.
2.1.1. Direct Delamination
Stirring and/or ultrasonication are the most facile and efficient ways to exfoliate clays (Figure 2a). For Na+-MMT, which
Adv. Funct. Mater. 2019, 29, 1807611
contains Na+ as the predominant exchangeable cations, the
degree of exfoliation can usually reach virtually 100% by vigorous stirring for one week at pH 5.6 in aqueous dispersions.[9]
Ultrasonication is where a layered material in suspension is
exfoliated into single nanosheets by ultrasonic waves generating
cavitation bubbles that collapse into high-energy jets, breaking
up the layered structure.[31] As such, ultrasonication can usually significantly expedite the exfoliation process. For example,
with the help of ultrasonication, Na+-MMT can be well exfoliated within 30 min (Figure 2b).[38] Exfoliation efficiency is also
related to the selected solvent. In the solvents with appropriate
surface energy, the exfoliated nanosheets can be well dispersed.
Otherwise, reaggregation and sedimentation will occur.[46]
2.1.2. Intercalation-Assisted Exfoliation
Intercalation is a promising strategy to expand the interlayer
distance of clay minerals for their further exfoliation or other
applications.[47] Usually, polar molecules are intercalated into
the layers in a liquid environment, swelling the layered structure and weakening the interlayer attraction. Subsequently,
certain agitation methods such as stir, ultrasonication, and
thermal shock, can help separate the loosened layers, forming
clay nanosheets (Figure 2c).
For example, polymers (epoxy, etc.) can be embedded in the
interlayer space of MMT to prepare a stable intercalation nanocomposite.[48] After adding a suitable curing agent, the curing
reaction in the interlayer space of MMT is greater than the outside, and the migration of the polymer into the interlayer can
continuously increase the spacing, ultimately obtaining exfoliated
nanocomposites.[49] Although intercalation is capable of inducing
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Figure 2. Representative exfoliation methods and exfoliated nanosheets. a) Direct delamination. b) SEM image of exfoliated MMT nanosheets with
intercalation-assisted exfoliation. Reproduced with permission.[38] Copyright 2016, American Association for the Advancement of Science. c) Intercalation-assisted exfoliation. d) Atomic force microscopy (AFM) image of exfoliated kaolinite nanosheets with a combination of multiple methods. Reproduced with permission.[10] Copyright 2017, Wiley-VCH. e) Ion exchange-assisted exfoliation and f) SEM image of exfoliated VMT nanosheets with ion
exchange-assisted exfoliation. Reproduced with permission.[37] Copyright 2015, Springer Nature Limited. a,c,e) Adapted with permission.[31] Copyright
2013, American Association for the Advancement of Science.
the exfoliation of clay minerals, ion exchange-assisted exfoliation
is a more general method for 2:1 type layered clays because of the
existence of exchangeable cations between MMT layers.
Unlike 2:1 type clays, kaolinite does not possess either
exchangeable cations or naturally intercalated water,[50,51] thus
intercalation is a necessary step to exfoliate kaolinite. The most
representative approach is to form kaolinite-dimethyl sulfoxide
(DMSO) intercalation compound, which is typically obtained by
mixing kaolinite with DMSO under agitation at 60 °C for 3 d
in a reactor equipped with a refluxer. The suspension is
centrifuged to obtain kaolinite-DMSO intercalation compound, which is washed several times with acetone and then
dried, ready for exfoliation.[52] But usually clays intercalated by
polar molecules can only be exfoliated into “large particles”
assembled with dozens of monolayers. Note that a repeated
intercalation procedure has been proven to increase the degree
of intercalation, further enhancing the efficiency of exfoliation,
but the disadvantage is that the crystallinity of the kaolinite is
lowered and the structure is partially damaged.[53–55] Although
various reactive guest molecules including urea, formamide,
hydrazine, and alkali salts of short-chain carboxylic acids
have been applied to expand the interlayer distance,[51,56–58]
researchers still fail to effectively exfoliate kaolinite into singleor few-layered nanosheets with a high yield.
A combination of multiple methods is commonly adopted
to exfoliate natural clay minerals into nanosheets to an
extreme. In a recent report, kaolinite was successfully exfoliated into few-layer nanosheets with combined methods.[10]
The thickness of the exfoliated kaolinite nanosheets ranged
from 2 to 20 nm, more than 75.7% of which were thinner
than 12 nm. Surprisingly, the minimum thickness of the exfoliated nanosheets reached ≈2 nm. In brief, a certain amount
Adv. Funct. Mater. 2019, 29, 1807611
of kaolinite-DMSO intercalation compound was soaked in
water to obtain a kaolinite suspension with the assistance of
stirring. After modification with bis-(γ-triethoxysilylpropyl)tetrasulfide (Si-69) and a subsequent ultrasonication treatment, the few-layered kaolinite nanosheets were prepared
(Figure 2d). Making use of the asymmetric crystal structure,
the Si-69 molecules solely connected with the Al-OH groups
on AOS, forming 2D Janus-like nanobuilding blocks for next
fabrication process.[59]
2.1.3. Ion Exchange-Assisted Exfoliation
For most smectite clays, central metal ions of tetrahedral and
octahedral lattice, typically Si4+ and Al3+, respectively, can
be substituted by lower-valence ions (Al3+, Fe2+, Mg2+, etc.),
resulting in a charge imbalance. Most of the negative charges
are distributed on the surface of each sheet, which is expressed
as a permanent charge and is not affected by the pH of the
surrounding environment. The negative charge imbalance is
usually neutralized by the absorption of hydratable cations,
usually Na+ and Ca2+. Such naturally ion-exchanged clays are
typically still rather stable and thus hard to be directly exfoliated by agitation. Instead, additional ion-exchange treatment
by larger cations to further enhance the interlayer distance is
needed (Figure 2e), during which additional cationic, anionic,
and/or neutral species might be introduced into the interlayer space, further facilitating exfoliation.[31,60,61] For example,
VMT can be typically exfoliated after two consecutive steps of
ion exchange. Typically, VMT is first refluxed with saturated
NaCl aqueous solution for 24 h. Then, NaCl is replaced with
an aqueous solution of LiCl (2.0 m, 100 mL) and refluxed for
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Table 1. Advantages and disadvantages of different assembly methods.
Assemble method
Advantages
Disadvantages
Casting
1) Common instrument
2) High thickness and shape control
3) Conducive to the formation of ordered structures of larger nanosheets
1) Time consuming to prepare thick membranes
2) Coffee ring effect
Dip coating
1) Common instrument
2) Large-scale preparation
3) Time saving
Require additional steps for forming an ordered structure
Spray coating
1) Low dosage, fast drying, and high uniformity
2) Easy to scale up
Hard to prepare well-ordered layered structure
Vacuum filtration
1) Easy preparation of free-standing membranes
2) Simple process
3) Suitable for most clay 2D nanosheets
1) Membrane size is dependent on the filtration instrument
2) Require additional substrate
3) Filtration speed affect structure ordering
Electrophoretic
deposition
1) High speed, high automation, high uniformity, and high coating density
2) Low pollution
1) Limited thickness of the resultant film
2) Higher demands on instrument than the above coating methods
3D printing
Direct printing of ink with desired patterns on multifarious substrates
1) Complicated printing instruments
2) Need to optimize many parameters and settings
an additional 24 h. The ion-exchanged VMT should be washed
with deionized water and ethanol after each step of refluxing.
The two-step ion-exchanged VMT can then be exfoliated by agitation. The thickness and lateral dimensions of the exfoliated
VMT nanosheets are ≈3 nm and tens of micrometers, respectively (Figure 2f).[37] Note that a greater degree of exfoliation
can be achieved when the Li+ exchanged VMT sample was
further refluxed with an aqueous solution of BaCl2 for another
several hours.[1]
2.2. Assembly
Once the natural clays are exfoliated into 2D nanosheets, they
can be readily processed into a range of structures by using
appropriate strategies. In this section, we will discuss various
strategies to assemble 2D clay nanosheets and summarize the
advantages and disadvantages of different assembly methods
(Table 1).[62]
2.2.2. Dip Coating
Dip coating is one of the most widely used coating methods,
performed by immersing a substrate into a dispersion of the
desired building blocks and chemicals (Figure 3b and Table 1)
and subsequently handled and dried.[63] Sun and co-workers
prepared large-scale nanostructured polyvinyl (PVA)/MMT
hybrid nanocoatings via a very simple dip-coating process.[38]
MMT nanoparticles were exfoliated to nanosheets via sonication and then dispersed in a PVA aqueous solution containing
glutaraldehyde (GA) as the crosslinking agent. Various polymer
films including polylactic acid (PLA), polyethylene terephthalate (PET), biaxially oriented polypropylene, high density polyethylene, and low density polyethylene thin films were coated
by dipping them into the as-prepared PVA/MMT aqueous dispersion (1.5 wt% solids) for ≈10 s and hanging them in an oven
2.2.1. Casting
Casting is a facile and controllable method for self-assembly
of nanosheets (Figure 3a and Table 1). The final dispersion
without excessive modifier is usually charged into petri
dishes or other molds and then dried at ambient conditions.
Walther and co-workers fabricated highly oriented, large-area
layered membranes with remarkable mechanical properties
by casting homogeneous dispersions of sodium carboxymethylcellulose and MMT nanosheets.[64] They also compared
film casting via spontaneous water evaporation and water
removal through a filtration membrane. They found that both
approaches yielded self-standing membranes and similar
mesostructures for small nanosheets, but solution casting
exhibited a higher level of ordered mesostructure for larger
nanosheets. Vacuum filtration also resulted in a comparable
mesostructure but led to some microscopic imperfections
that were not desirable.[65]
Adv. Funct. Mater. 2019, 29, 1807611
Figure 3. Assembly methods of nanosheets. a) Casting, b) dip coating,
c) spray coating, d) vacuum filtration, e) electrophoretic deposition, and
f) 3D printing. Adapted with permission.[63] Copyright 2015, American
Association for the Advancement of Science.
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to be dried and crosslinked at 60 °C. Before drying, the PVA/
MMT dispersion flows on the film surface owing to gravity,
generating shear stress along the surface, which helps induce
initial and rough orientation of MMT nanosheets. Meanwhile,
the highly crowded nanosheets force themselves to remain
oriented with each other to accommodate neighboring ones
before the coating is dried. The final drying process further
helps align the nanosheets.
2.2.3. Spray Coating
Spray-coated films are constructed using a spraying apparatus.
Targeted clay dispersions are aerosolized and sprayed onto
selected substrates (Figure 3c and Table 1).[66] The obvious
advantages of spray coating are low dosage, fast drying, and
easy to scale up. More importantly, compared with conventional casting method, spray coating can minimize the coffee
ring effect,[67] so that the sample can be deposited with high
uniformity during the preparation process, ensuring a uniform color distribution and light transmission. For example,
by precisely controlling the spraying time and concentration
of the dispersion, Cheng and co-workers sprayed a modified
MMT nanosheets dispersion with some patterned templates on
different substrates and successfully prepared nanostructured
films with thermochromic properties.[68]
2.2.4. Vacuum Filtration
Vacuum filtration is one of the most popular approaches
for assembling 2D clay nanosheets (Figure 3d and
Table 1).[1,10,37,61,68] It is facile, cost-effective, and scalable. In
a typical process, clay nanosheets are dispersed in water and
filtered under a negative pressure. The nanosheets in the dispersion are reconstructed into a highly uniform and ordered
structure under the external pressure. The thickness of the
resultant membrane can be adjusted by controlling the amount
of clay dispersion and/or clay concentration and usually
ranges from a few micrometers to tens of micrometers.[10,68]
Compared with graphene oxide (GO), the filtration process of
clay dispersion is usually faster because of the higher thickness, lower aspect ratio, and higher stiffness of individual clay
nanosheets.[69] In addition, multistep filtration is also a highly
efficient means of preparing layered membranes with multiple components. In a recent report, Raidongia and co-workers
fabricated a series of monolayer, bilayer and trilayer membranes
via vacuum filtration.[1] The monolayer was prepared from the
well-dispersed VMT nanosheets through vacuum filtration, and
once the VMT membrane was dried, an MMT dispersion was
subsequently filtered through it to form the bilayer membrane.
The trilayer membrane was prepared by sequential vacuum
filtration of VMT, MMT, and reduced GO dispersion.
2.2.5. Electrophoretic Deposition
Electrophoretic deposition is a low cost process capable of
producing materials with complicated geometry. It typically
Adv. Funct. Mater. 2019, 29, 1807611
consists of two steps: first, particles dispersed in liquid are
driven toward an electrode by an external electric field; then,
the particles at the electrode form a coherent deposit on it
(Figure 3e and Table 1).[70] The electrophoretic deposition of
coatings has been widely accepted in many different fields
and adopted to various applications.[71] Recently, Wang and
co-workers fabricated polyacrylamide/MMT films by following
a potentiostatic procedure with a working voltage of 5 V. The
coated electrode was immediately withdrawn from the emulsion after 5 min of treatment to obtain a uniform off-white
coating.[70] This technique shows huge advantages in higher
speed, higher automation, higher uniformity, and lower pollution. Compared with dip coating and spray coating, the
improved adherence of electrophoretically deposited structures
also endows the coatings with higher density.
2.2.6. 3D Printing
3D printing has already been developed for nearly four decades and widely applied to the fabrication of various physical
prototypes.[72–76] Direct-ink writing (DW), a powerful and facile
strategy of extrusion-based 3D printing, has been explored to
fabricate various types of functional membranes with printable inks (Figure 3f and Table 1).[77,78] High viscosity and shearthinning behavior are essential features of an enabled ink.[72]
Clay nanosheets can interact with a variety of polymers to form
a dispersion with high viscosity. As such, clay/polymer nanocomposites are considered one of the most popular types of
inks for 3D printing. Recently, Compton and co-workers fabricated garamite/epoxy nanocomposites by using a custom DW
3D printing platform comprised of a three-axis positioning
stage.[79] The inks were prepared by mixing epoxy resin with
proper quantities of garamite nanosheets. Sufficient mixing is
necessary to make the ink free of bubbles and uniformly dispersed. The flexural strengths of the printed composites tested
along both transverse and parallel to the print directions are
comparable to those of the cast samples.[80]
2.2.7. Summary
The commonality of casting, dip coating, and spray coating is
evaporation-induced assembly. Some nonvolatile solutes such
as polymers and dispersed particles such as clay nanosheets
can lead to inconsistencies in an ordered structure during
evaporation.[81] Thus, in order to take the extreme simplicity of
the top-down technique to its full potential and obtain a regular
structure, the choice of solvent and solution concentration, the
control of evaporation flux and interfacial interaction are the
most important considerations.[82,83] Although evaporation is
widely considered as a slow and gentle assembly process, it in
fact can help generate a highly ordered structure.[10] Furthermore, if proper process control is applied, one can even obtain
complicated structures by design. For example, layer-by-layer
(LbL) assembly is basically a unique combination of various
evaporation-induced assembly steps (sometimes other assembly
methods as well) based upon the successive adsorption of different components exhibiting attractive forces between them,
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including electrostatics interactions, hydrogen bonds, and van
der Waals forces.[84] Electrophoretic deposition can be used
to rapidly assemble polymers, dyes, ceramics, and metals, as
well as nanosheets.[63] Depending on specific applications, the
thickness of the coatings can be controlled from the order of
nanometers to micrometers.[85,86] Vacuum filtration is versatile
and thus a better choice to fabricate macroscopic multilayered
membranes with asymmetric structures.[1,87] 3D printing shows
the potential to create a wide range of multifunctional materials
with almost any geometrical shape or unique features.[88]
3. Applications
The applications of clay has existed since prehistoric times,
which have influenced civilization significantly.[10,31,37] In the
past few decades, clay minerals have been considered as important industrial components or precursors in geology, agriculture, and construction. In addition to their conventional uses
in ceramics, paper coatings, plastics and rubbers, and catalysis,
clay nanosheets have found many novel applications when they
are properly handled.[8,42,43,89–93] In this section, we summarize
some unique nanotechnology-driven phenomena and features
of clay-based functional materials, compare the exfoliation
methods, the size of nanosheets, the assembly methods, and
the related properties and applications (Table 2). Some representative applications are discussed in more detail.
3.1. Clay-Based Membranes for Protection
Due to their rigid layered structure with rich surface chemistry, clay nanosheets are ideal building blocks to assemble
hybrid functional materials. Inspired by nacre, Sun and coworkers prepared large-scale nanostructured PVA/MMT
nanocoatings with exceptional mechanical, barrier, and flameretardant properties.[38] MMT was chosen as the nanobuilding
blocks because of its low cost and high performance. After a
vertical dip-coating process, highly ordered clay nanosheets
aligned by gravity-induced shear were created together with
PVA (Figure 4a). In some cases, a GA crosslinking agent was
added to co-crosslink PVA and MMT nanosheets to improve
the stability as well as the mechanical properties of the hybrid
nanocoatings (Figures 4b and 4c). The assembled PVA/MMT
nanocoatings exhibited remarkably improved tensile strength
and Young’s modulus, particularly for the crosslinked ones
(Figures 4d and 4e). For a crosslinked nanocoating with 50 wt%
MMT nanosheets, the tensile strength of the freestanding film
reached 315.7 MP, which is ≈171% of that of aerospace grade
aluminum alloy 2014, and thanks to the low density of both
PVA and MMT, the specific strength and modulus of the freestanding hybrid film are higher than those of stainless steel
304, respectively. The highly ordered nanosheets also served as
an exceptional barrier to gas molecules, showing a reduction
in oxygen permeability of a 20 µm thick PLA substrate from
275.29 to 0.05 [10−16 cm3 standard temperature and pressure
(STP)·cm/cm2·s·Pa] with only a 620 nm coating layer. In
addition, such nanocoatings possess excellent flame retardant
properties. A highly flammable polyurethane (PU) exhibited
significantly reduced flammability after coating a thin layer of
PVA/MMT hybrid. After 10 s of horizontal combustibility test
at 1300 °C, the neat PU foam was almost consumed but the
coated PU foam was briefly burned only in the frame-contact
surface (Figure 4f). As such, the PVA/MMT thin films possess
excellent protection performance against mechanical forces,
gas, and heat.
Table 2. Comparison of typical clay-based nanostructured membranes.
Membrane
MMT
Exfoliation method
Thickness/lateral dimensions
Assembly methods
Properties and applications
Reference
Stirring, centrifugation
≈1 nm/up to ≈500 nm
Vacuum filtration
Reversible thermochromism
[68]
Ion exchange
≈0.96 nm/from one to several
hundred nanometer
Vacuum filtration
Surface-charge-governed ion transport;
ionic current rectification
[61,94]
Stirring
–
Layer-by-layer assembly
Li–S batteries anode protector
[95]
Stirring
≈1 nm in thickness
Casting and evaporationintroduced method
Bulk scale, toughness and fracture
properties
[96]
Stirring
≈1 nm in thickness
Film-casting
Flexible electronic substrate
[97]
VMT
Thermal shock, ion exchange Thinnest 3 nm/from a few to tens
of micrometers
(Na+ and Li+)
Vacuum filtration
Nanofluidic proton transport,
high thermal stability
[37]
VMT-MMT
Ion exchanged (Na+ and Li+)
≈4.5 nm/in the range of 6–24 µm
Vacuum filtration
Multiple responsive materials
[1]
Ion exchange (Na+ followed
by Li+, followed by Ba2+)
≈3 nm/≈500 nm
75% are thinner than 12 nm/over Evaporation and vacuum filtration
70% in the range of 800–1000 nm
Energy conversion based on
nanofluidic ion transport
[10]
Kaolinite
Intercalation, stirring and
ultrasonication
Halloysite
Natural nanotubes
Mostly ≈30–45 nm/≈375–750 nm
Evaporation
Separation of dye/salt solution
[81]
Attapulgite
Nanofibers
≈a few micrometers in length
Papermaking technology, sintered
in nitrogen
Oil/water emulsion separation
[98]
Bentonite
Ultrasonication
≈0.7 nm in thickness
A process of interfacial polymerization between two monomers
Removal of metal ions and humic acids,
permeability and selectivity
[99,100]
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Figure 4. Properties of dip-coated PVA/MMT nanocoatings. a) Schematic of one-step coassembly of MMT and PVA in an aqueous solution. b,c) Schematic of GA crosslinking with MMT nanosheets and PVA matrix. d) Stress–strain curves of the freestanding films. In the labels “PVA/MMT-##-C,” “##”
represents MMT mass percentage in the dried PVA.MMT hybrid and “C” denotes a crosslinked system. e) Stress–strain curves of non-crosslinked (“N”)
and crosslinked (“C”) freestanding hybrid films at a 50/50 PVA/MMT mass ratio. f) Digital images of the neat PU (two on the left) and coated PU foam
(two on the right) after horizontal combustibility test. Adapted with permission.[38] Copyright 2016, American Association for the Advancement of Science.
Inorganic clay based coatings not only protect macroscopic
combustibles from burning but also act as a protector for battery anodes. Compared with Li battery, Li–S battery has a higher
theoretical energy density thus promising for various applications but suffers from limited deep cycle life due to the leakage
of soluble polysulfide, which results in capacity decay and selfdischarge.[101,102] Recently, Zhang and co-workers reported a
unique separator which was fabricated with laponite nanosheets
(LNS) and a carbon black (CB) coated Celgard (CB-Celgard)
membrane.[103] By using the LNS/CB-Celgard membrane as the
separator, it not only inhibits the leakage of polysulfides but also
enhances the Li+ conductivity. Typically, a certain amount of LNS
was mixed with CB and then dispersed in 20 mL ethanol to form
a homogeneous suspension after vigorous stirring and ultrasonication treatments. The LNS/CB-Celgard membrane was
fabricated by depositing the suspension on a Celgard2400 commercial polypropylene separator via vacuum filtration. Together
with a control Celgard membrane (Figure 5a), the prepared
CB-Celgard membrane (Figure 5b) and LNS/CB-Celgard membrane (Figure 5c) were assembled into a Li–S battery to test the
Adv. Funct. Mater. 2019, 29, 1807611
electrochemical performance. Figure 5d shows the discharge/
charge profile at 0.1 C, compare with Celgard and CB-Celgard
separators, the battery with LNS/CB-Celgard separator exhibited the lowest voltage hysteresis (ΔV), which indicates a low
resistance and rapid redox reaction. The capacity of the battery
with Celgard and CB-Celgard separators decreased to 141 and
397 mA h g−1 after 500 cycles, but for the LNS/CB-Celgard separator, it became stable after 100 cycles (Figure 5e). Thanks to the
presence of Li+ in LNS and the high conductivity of CB, the battery with the LNS/CB-Celgard separator exhibited a superior performance.[104,105] The effect of the separators on preventing the
diffusion of polysulfides can be drawn by studying the discharge
behavior of the battery. Figures 5f and 5g show the discharge
curves of the 26th cycle (continuous discharge) and the 27th
cycle (7 d of rest at 2.05 V during discharge) of the batteries with
the Celgard and LNS/CB-Celgard separators at 0.2 C. During
the rest period, the amount of polysulfides leakage reached the
maximum. The polysulfides diffused to the Li anode and were
reduced to Li2S/Li2S2, causing self-discharge.[106] Compare irreversible capacity decay due to self-discharge, the battery with
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Figure 5. Protection performance of polymer/clay nanocomposite films. Li–S batteries with a) Celgard, b) CB-Celgard, and c) LNS/CB-Celgard
separators. d) Discharge/charge profiles at 0.1 C and e) cycling performance. f) Celgard and g) LNS/CB-Celgard separators at 0.2 C. The insets
show cycling performance of the batteries. h) UV–vis spectra of the polysulfides solution before and after being adsorbed by LNS. The inset shows
color change of the solution in the adsorption process. Reproduced with permission.[103] Copyright 2018, Wiley-VCH. Comparison of sulfur core
particle wrapped in i) polymer/polymer and j) polymer/clay membranes (scale bar: 100 nm). Reproduced with permission.[95] Copyright 2017, Royal
Society of Chemistry.
Celgard separator was 201 mA h g−1, which is much higher than
that of 24 mA h g−1 with LNS/CB-Celgard. It shows that LNS/
CB-Celgard separator can effectively inhibit the diffusion of poly­
sulfides, reduce the self-discharge of the battery, and enhance
battery life. The intercalation of LNS by polysulfides was studied
by UV–vis spectroscopy. When mixing the polysulfides solution
with LNS, the peak of polysulfides became weak after vibrating
for 5 min, the color changed from red orange to transparent
after 6 h, and the concentration sharply decreased from 206.2 to
5.6 mg L−1 (Figure 5h), indicating a high adsorption capacity and
adsorption rate for polysulfides.
Another approach to protect Li anode is to inhibit the leakage
of soluble polysulfide from S cathode. Muldoon and co-workers
designed a selective membrane that consists of branched poly­
ethylenimine (or polydiallyldimethylammonium chloride) and
MMT nanosheets via LbL assembly,[95] which was able to both
impede polysulfides dissolution and improve the utilization
of the Li anode. Governed by the difference in hydrophobicity
Adv. Funct. Mater. 2019, 29, 1807611
of the varied polymer backbones, pockets often form in the
membrane wrapping, which allows for polysulfide dissolution. (Figure 5i). However, the bPEI/MMT hybrid eliminated
the formation of pockets, which prevents the undesired loss of
polysulfides during battery operation (Figure 5j). Also because
of the low flammability of MMT, the hybrid coatings are
expected to lead to safer batteries by providing flame-retardant
properties.[38]
3.2. Clay-Based Membranes for Separation
Membranes are very effective and low-cost options for separation and purification.[107–111] The separation process can
be driven by pressure, concentration gradient, and electrical
potential, with pressure driven process as the top choice
because of its facile and low cost operation.[112] Various
membranes have been developed for separation, including
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microporous membranes, asymmetric membranes, composite
membranes, electrically charged membranes, inorganic membranes, and their combinations.[112] Compared with other
types, inorganic membranes are stable at temperatures from
500 to over 1000 °C and are also more resistant to chemical
attack.[113–117] Clay minerals can be used to make separation
membranes by virtue of their numerous varieties and relatively
reactive chemical properties.
Lvov and co-workers fabricated a halloysite-based membrane
with a well-arranged nanotube layer that exhibited much better
selectivity for dye/salt solution, and it also had high water permeability. What is more, the micro/nano organic–inorganic
composite membranes showed excellent antifouling behavior
against various organic dyes and bovine serum albumin
(BSA).[81] Natural halloysite nanotubes (HNTs) were modified
with poly(sodium-p-styrenesulfonate) (PSS) (m-HNTs) to
become more strongly negatively charged. A homogeneous
suspension was obtained after modification according to the
digital and transmission electron microscopy (TEM) images.
The difference between inside AOS and outside STS surface of
HNTs facilitates selective functionalization (Figure 6a). Then it
was mixed with a PVA solution (0.2 wt%) and coated on the surface of polyacrylonitrile (PAN) membranes and polysulfone (PS)
membranes by crosslinking with GA. The nanotubes were well
aligned on the PAN substrate because of its hydrophilic surface
but disordered on PS because of its hydrophobic nature. Both
m-HNTs/PAN and m-HNTs/PS composite membranes exhibited a much higher selectivity for reactive black 5 (RB 5), but the
well-aligned m-HNTs/PAN membrane had a higher dye selectivity than that of the irregular m-HNTs/PS membrane, indicating that a better-ordered structure was more efficient in solute
selectivity (Figure 6b). The permeability of the m-HNTs/PAN
membrane was almost constant and the rejection of RB 5 could
be maintained above 95.0%, indicating a good operational
stability (Figure 6c). The antifouling ability of the PAN, m-HNTs/
PS, and m-HNTs/PAN membranes was also investigated, which
were fouled by 0.1 g L−1 BSA solution (Figure 6d). Both types of
the coated membranes showed an improved water flux recovery
Figure 6. Fabrication and properties of halloysite-based membranes. a) Fabrication of m-HNTs/PAN composite membrane, dispersion of HNTs and
m-HNTs in water, structure of HNTs, and TEM images of HNTs and m-HNTs. b) Permeability and dye/salt selectivity of PAN, m-HNTs/PS (loading
10 mg m-HNTs on a 28.3 cm2 PS substrate), m-HNTs-10, m-HNTs-20, m-HNTs-30, and m-HNTs-40 membrane (loading 10, 20, 30, and 40 mg m-HNTs
on a 28.3 cm2 PAN substrate). c) Rejection and permeability of m-HNTs-40 membrane at pH 7 under a pressure of 0.8 MPa. d) Normalized flux, water
flux recovery ratio, irreversible fouling ratio, and reversible fouling ratio of PAN (black), m-HNTs/PS (red), and m-HNTs-10 (blue) membrane under
0.8 MPa. Adapted with permission.[81] Copyright 2016, American Chemical Society.
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ratio and anti-fouling performance, but the m-HNTs/PAN membrane reached a water flux recovery ratio of up to 100% with
almost zero reversible fouling, even after three cycles. The excellent antifouling properties are due to the well-aligned halloysite
surface film, which functioned as a selective nanopore layer to
enhance the membrane hydrophilicity and reduce the membrane roughness. The reversible fouling value for the m-HNTs/
PAN membrane was maintained at about 12.0%. Therefore, the
decrease of flux was mainly due to the reversible pollution of the
ordered m-HNTs/PAN membrane by BSA. For the disordered
m-HNTs/PS membrane, the blocking of large membrane pores
by BSA molecules and the reversible absorption of BSA are the
main reasons for the membrane fouling.
3.3. Clay-Based Membranes for Responsiveness
As technology continues to progress, what is becoming
apparent is the need to develop automated solutions to improve
the efficiency of processes. Various stimuli responsive materials have been developed, including alloys, hydrogel hybrids,
and organic/inorganic nanocomposites.[1,87,118–121] Multifarious stimuli such as force, heat, light, voltage, humidity, and
pH have been used to change the physical properties of smart
materials,[61,118,122–127] including size, shape, color, and conductivity.[123,124,126,128] Color and shape, which belong to visual
perception, is the most important sense for humans to collect
information from their surrounding environment.
Color changes are the most obvious and easily perceptible
responses. It can be induced by many external stimuli such
as electronic current, light, solvent polarity, mechanical force,
temperature, organic solvent vapor, and pH.[129–133] Among all
chromic materials, thermochromic materials have attracted a
great deal of attention not only due to their traditional applications in paints and textiles but also thanks to their new applications, especially in biosensors and chemosensors.[134–137] For
example, Cheng and co-workers designed a thermochromic
synthetic nacre based on MMT.[68] 10,12-Pentacosadiynoic
acid (PCDA) monomers were coated on MMT nanosheets via
hydrogen bonding and self-polymerized into polyPCDA in
solution under 254 nm UV and crosslinked by (3-aminopropyl)
triethoxysilane (APTES). MMT/polyPCDA/APTES hybrid membranes were finally obtained after vacuum-assistant filtration.
The artificial nacre shows reversible thermochromic properties
and great robustness. Different patterns of artificial nacre can
be prepared by spraying the MMT-polyPCDA solution with a
spray gun through a template (Figure 7a). The shapes of (b)
rose and leaf, (c) flower bud, and (d) rose were patterned on
(b) paper, (c) glass slide, and (d) steel plate, respectively, as
shown in Figure 7. All of the samples show reversible thermochromism in 20–50 °C range. A flower bud-shaped image
sprayed onto a curved plastic cup was able to quickly change
color (from purple to orange) upon pouring hot water into
the cup (Figure 7e). Superior mechanical properties and reversible thermochromic properties extend the potential use of this
artificial nacre to armor and even aerospace materials.
Shape morphing is another readily achieved response
that can be induced by external stimuli. The diverse physicochemical properties of clay minerals not only allow clay
nanosheets to easily interact with guest substances to form
new nanobuilding blocks but also greatly enhance the responsiveness to various stimuli. In a recent report, Raidongia
and co-workers fabricated responsive membranes by assembling MMT and VMT nanosheets.[1] The clay–clay bilayer
Figure 7. Thermochromic artificial nacre with different patterns. a) Illustration of the spray coating via a spray gun. Different patterns of artificial nacre
can be obtained by spraying the thermochromic solution with the assistance of a template. The artificial nacres with shapes of b) rose and leaf, c) flower
bud, and d) rose patterned on b) paper, c) glass slide, and d) steel plate. e) Color transforms on a curved plastic cup. Scale bar: 1 cm. Reproduced
with permission.[68] Copyright 2017, American Chemical Society.
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Figure 8. Vapor-induced shape transformation of CCBMs. a) Snapshots showing the bending movement of a CCBM strip after 0, 5, and 10 s in
methanol vapor. b) Bending speeds of CCBM in different vapors. c) Forces generated by a CCBM strip (dimensions 30 mm × 3 mm × 0.026 mm and
weight 2 mg) in different vapors. d) Moisture driven bending of a CCBM strip at 10%, 50%, and 100% relative humidity (RH). e) Temperature responsive
properties of a CCBM strip at 23, 25.4, and 34 °C. f) Responses of a CCBM strip upon human touch. g) Morphing of a U-shaped trilayer strip induced
by a voltage of 80 V. Reproduced with permission.[1] Copyright 2017, Wiley-VCH.
membranes (CCBMs) were prepared by vacuum filtrating
a dispersion of MMT nanosheets and a dispersion of VMT
nanosheets in successive steps. The thickness of CCBMs could
be easily controlled by varying the concentration and volume
of the dispersions.
The shape and mechanical property of the as-prepared
CCBMs were found to change after exposure to various vapors
including methanol, acetone, tetrahydrofuran, ethanol, 2-propanol, and ethyl acetate (Figure 8a). Among all of the above
vapor-driven responsiveness, the response speed reached up
to 85° s−1 when driven by methanol (Figure 8b). The force
from the bending of the strip was also characterized by a
balance. After exposure to methanol, the CCBM strip could
generate a force equivalent to 6.4 times of its own weight
(Figure 8c), which can be further improved after heating.
The output force from morphing was positively correlated
with the sensitivity. Moreover, atmospheric moisture content (Figure 8d) and environmental temperature fluctuations
(Figure 8e) were found to affect the shape of the CCBMs.
Interestingly, the bending of a CCBM strip could also be
induced by the warmth of human skin (Figure 8f). In order
to test the response to electrical voltage, a U-shaped trilayer
device made from VMT/MMT/rGO was measured with the
help of a source meter. When a 80 V voltage was applied to
both ends of the U-shaped device, it responded by bending
its shape (Figure 8g). Therefore, the CCBM can be potentially
exploited for energy harvesting from day/night temperature
variations through coating a thin piezoelectric material. What
is more, the CCBM did not lose its responsiveness even in
extreme temperatures and hazardous chemical environments,
which indicates that it could possibly provide a method for
developing self-operating devices used in areas where human
reach is not desired.
Adv. Funct. Mater. 2019, 29, 1807611
3.4. Clay-Based Membranes for Flexible Electronics
Flexible electronic devices have attracted extensive attention
owing to their widespread application. One of the key areas
under rapid development is wearable electronics.[138–141] However, when it comes to applications with unconventional interfaces, traditional substrates result in a problem of mechanical
mismatch.[142] The substrate of flexible electronics is a very
critical part, requiring a unique combination of various properties typically including high thermal durability, low thermal
expansion, and high gas barrier. Recently, Ryo and co-workers
designed a flexible electronics substrate film by using natural
clay and wood, which partially addressed the above requirements.[97] In order to overcome the poor resistance to moisture
of Li+-MMT, they first migrated partial Li+ ions into the silicate
sheets by thermal treatment; the corresponding benefit is to
convert the MMT from hydrophilic to hydrophobic (Figure 9a).
For the organic section, an acrylic polymer crosslinker was used
to improve the functionality of the glycol-modified lignin (GL,
as shown in Figure 9b). The final resultant gel was cast on a
PET sheet by using a film-casting knife. The paper-like film was
obtained after being dried under ambient conditions for one
week, peeled off from the PET, and annealed at 60 °C for 15 h.
The cross-sectional TEM image shows that the Li+-MMT, GL,
and crosslinker (MLC) nanosheets were able to form a multi­
layered parallel stacked structure thanks to the ordered selfassembly of MMT nanosheets during preparation (Figure 9c).
After printing on an electrode with Ag ink, bending the film
showed no peeling or breaking of the electrodes, demonstrating
their potential for use as a substrate for flexible electronics
(Figure 9d). The integrated circuits and rigid elements provide
the potential for other flexible electronics as well as wearable
devices.
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Figure 9. Fabrication of flexible electronics substrate. a) Li+-MMT was preannealed for partial Li+ migration. b) Crosslinking chemical reaction occurred
between phenolic-OH (and/or carboxyl groups) of GL and the oxazoline groups on the side chains of the crosslinking polymer. c) Cross-sectional TEM
image of the film. d) Photograph of a rolled film with printed electrode. Reproduced with permission.[97] Copyright 2017, Wiley-VCH.
3.5. Nanofluidic Channels and Energy Conversion
Compared with other 2D materials, clays have significant
advantages for fabricating lamellar membranes for molecule
and ion transport because of their ease of exfoliation, sufficient
surface charge, and structural diversity.[37,94] Recently, Huang
and co-workers reported self-assembled VMT-based 2D nanofluidic channels with surface charge-governed proton conductivity.[37] Thermally expanded VMT could be swollen by several
steps of ionic exchange to replace the interlayer cations with
Li+, which has a larger hydration diameter. A hydrogen peroxide
treatment was applied to further exfoliate the VMT layers. The
exfoliated VMT nanosheets perfectly dispersed in water and
could be readily reassembled by vacuum filtration to form
a flexible thin film (Figure 10a inset) with 2D layered microstructure according to the scanning electron microscope (SEM)
characterization (Figure 10a). The proton conductivity through
the nanofluidic ionic channels in the reconstructed VMT membranes (RVMs) was measured in a homemade electrochemical
setup (Figure 10b) wherein a rectangular RVM was embedded
in a polydimethylsiloxane (PDMS) elastomer and its two
opposing ends were exposed to an acid solution (Figure 10c).
Through two-terminal DC measurements, three representative
current–voltage (I–V) curves were recorded at different HCl
concentrations (Figure 10d). The pH-dependent ionic conductance through the nanochannels was calculated based on the
slope of the I–V curves (Figure 10e). At high acid concentrations, the trans-membrane proton conductivity was determined
by the concentration, due to the very thin electrical double layer
(EDL). While at lower concentrations below 0.01 m, the proton
conductance converges to a saturated value because of the
overlap of EDL inside the nanochannels,[143] suggesting strong
Adv. Funct. Mater. 2019, 29, 1807611
surface charge-governed proton transport behavior.[144] Therefore, cations are the dominant charge carriers, so the surface
charge density has a greater impact on the concentration of the
cations in the nanochannels than the bulk electrolyte concentration. Shao and co-workers fabricated a reconstructed MMT
membrane (RMM) with a similar process (Figure 10f).[94] Surface charge-governed ion transport behavior was also detected
through the nanochannels of the RMM with KCl electrolyte.
Furthermore, by conducting a drift-diffusion experiment (top
scheme in Figure 10g), a negative zero-bias current and a positive zero-current potential of 46.43 mV were observed for the
nanofluidic device (Figure 10h), which shows a cation-selective
property of the nanochannels.[145,146] Ionic current rectification
in the nanofluidic device was also investigated by using an
asymmetric strip of RMM (bottom scheme in Figure 10g). The
observed ionic current is higher at positive potentials than that
at negative potentials with a highest rectification ratio of ≈2.6
for KCl electrolyte (Figure 10i), indicating ionic rectification
function of a nanofluidic diode.[147]
Energy conversion in nanofluidic channels is a significant
application in reconstructed 2D nanomaterials. The inspiration comes from electric eel, which generates bioelectricity
by passing ions through a series of nanoscale conductors
in the form of ion pumps and ion channels on its cell membranes.[11,148] It usually operates transient current to defend
itself with peak values up to 600 V with currents of 1 A.[149–151]
By using natural 2D heterostructures of kaolinite nanosheets,
Guo and co-workers successfully modified few-layered kaolinite nanosheets into Janus-like nanobuilding blocks with
Si-69 and then assembled them into a paper-like, free-standing,
and hydrophobic reconstructed kaolinite membrane (RKM)
through a two-step evaporation and vacuum filtration process
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Figure 10. 2D nanofluidic channels in reconstructed membranes. a) Cross-sectional SEM image of the RVM showing a lamellar structure (scale bar,
1 µm). Inset: Photograph of the RVM (scale bar, 1 cm). b) Schematic illustration and c) photograph of the experimental setup for electrochemical
test. d) Representative I–V responses through the nanofluidic channels. e) Ionic conductance of 2D channels deviating from the bulk solution value at
0.1 m, showing a characteristic surface charge-governed property. Reproduced with permission.[37] Copyright 2015, Springer Nature Limited. f) Crosssectional SEM image of the RMM (scale bar, 5 µm). Inset: flexible MMT membrane with a diameter of 4 cm. g) Schemes of nanofluidic channels with
asymmetric bulk electrolyte concentrations (top) and asymmetric geometry shape (down) and h,i) their corresponding I−V responses. Reproduced
with permission.[94] Copyright 2017, American Chemical Society.
(Figure 11a).[10] The RKM possessed both sub-nanometer
(6.8 Å) and nanometer (13.8 Å) channels depending on the
possibility of restacking STS and AOS, confirmed by the X-ray
diffraction (XRD) characterization (Figure 11b). The 2D nanochannels showed prominent surface charge-governed ion transport behaviors and nearly perfect cation-selectivity. Two types
of electrokinetic energy conversion through the network of 2D
nanochannels were demonstrated. Driven by a trans-membrane
hydraulic flow, applying an electrolyte flow led to a synchronized
streaming ionic current with a magnitude of typically several
Adv. Funct. Mater. 2019, 29, 1807611
hundred nA. This process can be repeated for many cycles
(Figure 11c). The streaming current exceeded that of the GO
membranes (GOM) by nearly 300% at neutral pH (Figure 11d).
This is because the stiffness of the kaolinite nanosheets is
higher than that of GO laminas.[69] Moreover, the fluid flow
rate was increased due to the hydrophobicity of the many 2D
nanochannels in the RKM, creating many slip boundary conditions.[152,153] Under a trans-membrane concentration gradient, asymmetric ion diffusion without externally applied
voltage through the lamellar nanochannels converted osmotic
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Figure 11. Electrokinetic energy conversion in kaolinite-based 2D nanofluidic channels. a) Kaolinite crystal is composed of a STS and an AOS. After
modification with Si-69 molecules, the kaolinite nanosheets has three ways to reconstruct: i) with Si-69 pillar-like structure in between, AOS-to-AOS
(A); ii) STS-to-STS (S); iii) similar with bulk phase, AOS-to-STS (B). b) XRD patterns of bulk kaolinite and RKM. The A, S, and B ways of restack can
be clearly observed. c) Streaming ionic current observed with the applied electrolyte flow. d) Streaming current generated from the RKM remarkably
larger than that from the GOM in pH 3, 6, and 10. e) Output power density of the osmotic currents from the RKM comparable to that from the GOM.
Adapted with permission.[10] Copyright 2017, Wiley-VCH.
energy into electric power. Under a 100-fold concentration difference, the output power density from the RKM approached
0.18 W m−2, a value comparable to the systems using GOMs
(Figure 11e). This work provides a research platform for the
fundamental ion transport in nano- and sub-nanoscale confinements and contributes a further step toward high-performance,
chemically stable, and cost-competitive membrane materials for
energy, environmental, and healthcare applications.
4. Conclusions and Outlook
In this article, we briefly summarized the exfoliation and
assembly methods of natural clays and reviewed the strategies
to fabricate nanostructured membranes that possess unique
functions. Having a prominent role in the 2D materials family,
clay minerals, which possess unique physical and chemical
Adv. Funct. Mater. 2019, 29, 1807611
properties, continue to flourish. In the past few decades, claybased membranes have successfully participated in cutting-edge
research because of their novel properties including mechanical performance, electrokinetic behavior, corrosion resistance,
and thermal and optical properties.[96,154,155] However, there are
still quite a few challenges to solve and much room to further
improve the quality and function of clay-based membranes.
For example, some clay minerals cannot be exfoliated into
single-layer nanosheets on a large scale, which severely limits
their applications. Researchers have already shown fabrication
of various reconstructed membranes with classic assembly
approaches, going forward assembly methods need to follow
the development of modern technology, 3D printing or even
4D printing.[73,156,157] In addition, proper selection of organic
components or synthesize specific organic components should
facilitate the further development of clay/organic functional
membranes.
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Acknowledgements
The authors acknowledge the support by the National Science
Foundation (CMMI-1562907). Q.L. thanks the support by the Science
and Technology Major Projects of Shanxi Province of China (Project
Number: 20181101003). Y.Z. thanks the support from the China
Scholarship Council (CSC).
Conflict of Interest
The authors declare no conflict of interest.
Keywords
application, clay, membranes, minerals, multifunctional materials
Received: October 26, 2018
Revised: December 22, 2018
Published online: January 30, 2019
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