ARTICLE
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a.
Polymer Program, Institute of Materials Science, University of Connecticut, Storrs,
CT 06269, USA.
E
-mail: luyi.sun@uconn.edu
b
.
Department
of Chemical and Biomolecular Engineering, University of Connecticut,
Storrs, CT 06269, USA
c.
CAS Key Laboratory of Mineralogy and Metallogeny/Guangdong Provincial Key
Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry,
Chines
e Academy of Sciences, Guangzhou 510640, China.
E
-mail: zhurl@gig.ac.cn
d
.
Institutions of Earth Science, Chinese Academy of Sciences
e
.
Chemistry Research Laboratory, Department of Chemistry, University of Oxford,
12 Mansfield Road, Oxford, OX1 3TA, UK.
E
-mail: dermot.ohare@chem.ox.ac.uk
f
.
Department of Biomedical Engineering, University of Connecticut, Storrs, CT
06269, USA.
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Functionalized Layered Double Hydroxides for Innovative
Applications
Minwang Laipan,
a,b,c,d
Jingfang Yu,
e
Runliang Zhu,
c,d,
* Jianxi Zhu,
c,d
Andrew T. Smith,
a,b
Hongping
He,
c,d
Dermot O’Hare,
e,
* Luyi Sun
a,b,f,
*
Two-dimensional layered double hydroxides (LDHs) are currently a topic of significant interest due to their extraordinary
physiochemical properties. LDHs are potentially useful in a wide range of applications, particularly in environmental,
energy, catalysis, and biomaterials related fields. Despite the unique intrinsic properties of LDHs, various functionalization
strategies have been applied to LDHs that yield even more exciting performance opportunities, offering guides to design
novel functional nanomaterials. In this review, we address how these strategies can improve the various properties of
LDHs. We provide an overview of the functionalizing strategies of intercalation, surface modification, hybridization,
layered compositions regulation, size and morphology control, and defect creation. These strategies contribute
significantly to the enhancement of the performance of LDHs, across a diverse range of areas such as adsorptive, catalytic,
electronic, electrochemical, and optical. As a result, functionalized LDHs exhibit great potential in a wide range of
applications in the environmental and energy domains. We have comprehensively highlighted their emerging potential in
the environmental, energy, catalysis, and biomaterials related fields, including heavy metal removal, radionuclide capture,
organic contaminants purification, oil pollution elimination, hydrogen generation, supercapacitors, batteries, solar cells,
catalysis, and biomaterial fabrication.
1. Introduction
Two-dimensional (2D) layered materials are currently a topic of
significant interest due to their extraordinary physiochemical
properties.
1
Their unique anisotropic structural characteristics make
them potentially useful in a wide range of applications, such as
optoelectronics, photonics, catalysis, piezoelectric devices,
environmental pollution control, energy storage and conversion,
and biomaterials.
1-3
Layered double hydroxides (LDHs), previously
known as anionic clays, are one family of 2D materials that have
attracted significant interest in recent years. LDHs comprise of
positively charged edge-shared octahedral coordinated metal
hydroxide layers sandwiched by charge compensating interlayer
anions with optional solvation eg. water. The general formula of an
LDH is [M
a+
1–x
M’
b+
x
(OH)
2
]
y+
[A
c–
y/c
]
y+
·zH
2
O ;y = a(1–x) + bx –2; A
c–
is an
interlayer anion; M and M’ are metal cations. Most frequently LDHs
conform to the formula [M
2+
1–x
M’
3+
x
(OH)
2
]
x+
[A
c–
x/c
]
x+
·zH
2
O (where
M
2+
and M’
3+
are divalent and trivalent metal cations typically of
Mg
2+
and Al
3+
respectively; A
c–
are inorganic or organic anions)
(Figure 1).
4, 5,6, 7
In a few special cases the M
2+
site can be
substituted by Li
+
and the M
3+
site can be substituted by M
4+
cations.
8
In this review, an LDH containing the metal components,
M
1/2+
and M’
3/4+
are abbreviated as MM’-LDH. LDHs became the
focus for both fundamental research and practical applications due
to their unique structures, tunable chemical compositions, and a
wide variety of material properties.
6, 9-13
Despite the unique intrinsic
properties of LDHs, various functionalization strategies have been
applied to LDHs that yield even more exciting performance
opportunities. That is, functionalization can drastically improve the
performance of LDHs or develop new properties for use in a wide
range of applications. For instance, the fabrication of three-
dimensional hierarchical nano-architecture of an NiFe-LDH
remarkably facilitated its rate of electron transport and channel
diffusion for catalytic water splitting.
14
Hybridizing with conductive
textile fibers made NiCo-LDH a promising candidate for high-
performance pseudocapacitors.
15
Intercalation of polysulfide
developed the potential of MgAl-LDH for highly selective and
efficient capture of radionuclide, e.g., uranium.
3
Hybridization of
LDHs with semiconductors to form heterojunction can markedly
inhibit the recombination of charge carriers, and thus significantly
enhance photocatalytic activity.
16, 17
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Figure 1. Representative structure of an LDH; [M
2+
1–x
M’
3+
x
(OH)
2
][A
c–
x/c
],
where M
2+
and M
’3+
are divalent and trivalent metal cations, respectively; A
c–
is an interlayer anion.
Functionalization of LDHs is a process involving the control and
manipulation of their surface zone, shape, size, and composition to
activate them, or introduction of foreign species or defects on LDHs
to enhance or create new functions.
1, 9, 18-20
The modification of
surface zone includes intercalation of functional species and
alteration of surface functional groups. Introduction of foreign
species emphasizes anchoring functional substances onto the
surface of LDHs to generate LDH-based hybrids. Apart from the
modification of surface zone and introduction of functional species,
LDHs can also be functionalized by manipulation of layer
composition including the type, ratio, and chemical valence of the
metal components, by regulation of the size and morphology of
LDHs, and by creation of defects resulting from surface and layered
composition regulation. Herein, we summarize the common
functionalization strategies that have been developed for LDHs in
the following major categories: (1) intercalation,
2, 3
(2) surface
modification,
21, 22
(3) hybrid assembly,
23, 24
(4) layer composition
tuning,
25, 26
(5) size and morphology regulation,
14, 27
(6) defect
introduction,
26, 28
among others. Each of these functionalization
strategies demonstrates that with the appropriate modification
LDHs can deliver novel and/or enhanced features, e.g., enhanced
photoelectronic,
23, 29
magnetic,
30
catalytic,
29, 31, 32
and energy
storage
15, 33, 34
properties, offering unique perspectives and
advantages for both fundamental and applied research.
The fundamentals why these strategies can effectively
functionalize LDHs are outlined in this review article. For example,
the reasons that intercalation is such a versatile and effective
approach in tuning the properties of LDHs are: (1) intercalation
provides the highest possible doping and/or phase change to the
pristine LDHs; (2) the intercalation process, and the concomitant
changes in the properties of LDHs is typically reversible; (3)
intercalation is controllable; (4) changes in intercalated LDHs during
material preparation can be monitored in situ and in real time; (5)
intercalation can induce structural changes, such as lattice
expansion or even phase changes, for improved or new
physiochemical properties; (6) intercalation adds a new degree of
freedom for tuning LDHs, which can be combined with other
modification methods.
1
The modification of surface functional
groups of LDHs can be one of the most common strategies in
altering the surface properties of LDHs, because the introduction of
foreign functional groups renders LDHs to possess different or
enhanced functions. For instance, the introduction of amine
terminal groups onto MgAl-LDH makes the MgAl-LDH a potential
biomaterial without causing any hemolysis,
35, 36
and enhances its
functions in the environmental field.
37
The reason that the
introduction of functional materials onto LDHs can alter the
properties of LDHs is ascribed to the fact that it can combine the
strength of each component in the structure and functions. I n
addition, the components in hybrids may even generate synergies,
which will result in the enhancement of the properties.
19, 20, 34
23, 24,
38
For example, in the electrochemical field, LDHs have been
reported to be promising electrode materials for the next-
generation supercapacitors, but the relatively low conductivity of
LDHs constrains their performance.
39, 40
It’s suggested that the
electrochemical properties of LDHs can be further improved by the
hybridization with conductive materials.
29
This is because the
conductive materials, e.g., graphene, can significantly enhance the
electrical conductivity and accelerate the electron transfer,
resulting in excellent charge and discharge capability.
41-45
Many
other reasons for the enhanced or newly created functions of LDHs
produced by various functionalization strategies are summarized in
Section 2.
In this review, we focus on the functionalization strategies for
LDHs, and highlight the significance of functionalization in tuning
the physiochemical properties of LDHs and their subsequent
applications particularly in the environmental, energy, catalysis, and
biomaterials sectors. We also provide some insights into the
challenges and future opportunities of LDHs.
2. Strategies to functionalize LDHs
In general, functionalization of LDHs can be achieved by
modifying their surface zone, assembling hybrids, and regulating
layer composition, size and morphology. The modification of
surface zone includes intercalation of functional species and
alteration of surface functional groups. Hybrid assembly emphasizes
anchoring functional substances onto the surface of LDHs to
generate LDH-based hybrids. The assembly usually occurs at the
interface between LDHs and the loaded functional substances.
Manipulation of layer composition focuses on the regulation of type,
ratio, and chemical valence of the metal components. The size and
morphology section highlights the impact of size and morphology
on the properties of LDHs. Apart from the above aspects, other
factors such as creation of defects resulted from surface and
layered composition regulation can also be utilized to functionalize
LDHs.
2.1 Intercalation
The phenomenon of intercalation was first discovered ca. 600-
700 A.D. in China.
46
At that time, alkali metal ions were intercalated
into natural minerals, such as kaolin, to make porcelain.
47
The first
intercalation phenomenon in the literature was reported by
Schafhäutl in 1840, in which the authors attempted to dissolve
graphite in sulphuric acid.
48
The modern era of intercalation
research was initiated in 1926 by Fredenhagen et al., in which the
uptake of potassium vapor into graphite was reported.
49
Over the
subsequent decades, intercalation strategies for 2D materials have
significantly advanced and various mechanisms have been
clarified.
18, 50-63
Exploring the benefits of intercalation in 2D
materials for various applications remains a very active
contemporary field of research.
The intercalation of 2D LDHs is a process of inserting a foreign
species between the edge-shared metal hydroxide layers of the
LDHs (Figure 2). In many instances, intercalation can lead to
drastically improved performance for the LDHs. For example,
incorporation of homogeneous catalysts (e.g., inorganic anions,
organic acid/base, organic complexes) into LDH interlayer space
offers the chance to heterogenize the homogeneous reaction
process over LDH-based catalysts with enhanced lifetime and
thermal stability, as well as facile separation/purification.
9
Intercalation can also make LDHs more favorable for the use of
energy storage and conversion. In one example, the intercalation of
bistrifluoromethane sulfonamide into galleries of CoTi-LDH
endowed CoTi-LDH with the ability in making up low-overpotential
Li-O
2
batteries with superior cycling stability.
64
Herein, we
summarize some common LDHs and intercalants reported in recent
years, and the benefits of the intercalation in improving various
performances of LDHs (Table 1). The data suggests that different
combinations of LDHs and intercalants can offer different
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performances, and intercalation is indeed a promising means to
functionalize LDHs and to expand their applications. The reasons
that intercalation is such a versatile and effective approach in
tuning the properties of 2D LDHs are: (1) intercalation provides the
highest possible doping and/or phase change to the pristine LDHs;
(2) the intercalation process, and the concomitant changes in the
properties of LDHs is typically reversible; (3) intercalation is
controllable; (4) changes in intercalated LDHs during material
preparation can be monitored in situ and in real time; (5)
intercalation can induce structural changes, such as lattice
expansion or even phase changes, for improved or novel
physiochemical properties; (6) intercalation adds a new degree of
freedom for tuning LDHs, which can be combined with other
modification methods.
1
Figure 2. Schematic illustration of the intercalation of ZnAl-LDH.
65
Reproduced with permission from ref.
65
, Copyright 2014, Elsevier.
Table 1. Representative examples reported in recent years of LDH intercalation, and the benefits of the intercalation in improving various performances of
LDHs.
LDH Intercalant
Performance
vs.
the pristine LDH
Ref.
MgAl Polysulfide Highly selective and efficient for radionuclide (UO
2
2+
) sequestration
3
MgAl MoS
4
2–
Highly selective and efficient for heavy metals and radionuclides removal; 5-8 folds increase of the capture
capacities
2, 66
MgAl
Hydroxyl ammonium
ionic liquids
~6 folds higher adsorptive capacity towards organic contaminants
67
MgAl Fe(CN)
6
4–
/S
2–
Endowed LDH with an ability of rapid detection of heavy metal ions
68
MgAl
Keggin
polyoxometalate
Highly efficient catalytic activity; produced the highest turnover number reported in Knoevenagel
condensation of benzaldehyde with ethyl cyanoacetate
69, 70
Silylate
d MgAl
MnO
2
nanowires High catalytic activity, stability, and reusability
71
MgAl MnO
2
Enhanced electrocatalytic activity; excellent stability, selectivity, and reproducibility
72
MgAl
Ruthenium
polypyridine complex
High thermal and photo stability; enhanced luminescence efficiency and lifetime
73
MgAl Decavanadate Enhanced corrosion-resistant properties, especially long-term corrosion resistance
74
MgAl CO
3
2–
/NO
3
–
Tunable electronic transport properties by changing intercalated ions
75
NiCr
Diphenylamine-4-
sulfonate
Highly selective and efficient for heavy metal removal
76
NiFe Cobalt Significantly enhanced catalytic activity for water splitting; long term stability
77
NiCo
n-alkylsulfonate
anions
Tunability of magnetic properties, enhanced coercivity
78
NiCo Ethylene glycol Ultrahigh specific capacitance and excellent cycling stability
79
NiMn MnO
2
Greatly improved supercapacitor behavior
80
2.2 Surface modification
Modification and control of surface properties of LDHs are of
crucial importance in the functionalization of LDHs. There are
numerous studies focusing on the surface modification of LDHs by
hybridizing functional materials onto the surface of LDHs. For
example, surfactants are the most popular functional species to be
anchored onto the LDH surface to functionalize an LDH.
18, 19
In this
section, we focus on the regulation of the properties of LDHs by
the introduction of various functional groups.
The modification of surface hydroxyl groups of LDHs can be
one of the most common strategies in altering the surface
properties of LDHs. For example, Oh et al. modified the surface
hydroxyl groups of MgAl-LDH by grafting
aminopropyltriethoxysilane, which introduced amine terminal
groups onto the surface of MgAl-LDH without affecting the LDH
layered structure (Figure 3a).
81
The amine terminal groups could
be utilized as an active site for further modification to render
MgAl-LDH as an effective drug-delivery carrier.
81
Hu et al. found
that the functionalized MgAl-LDH with amine terminal groups
(functionalized through a three-step surface grafting process)
possessed enhanced blood compatibility, which made it a
potential biomaterial without causing any hemolysis.
35, 36
. Besides
the applications in the medical field, amine groups functionalized
LDHs can also be used in the environmental field. For example,
Ezeh et al. found that after amine modification, MgAl-LDH
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presented an enhanced physical adsorption of CO
2
, and with an
increase of amine loading, the adsorption of CO
2
was further
enhanced.
37
In another example, Li et al. introduced various
functional groups including phenolic hydroxyl groups, alcoholic
hydroxyl groups, and carboxyl groups onto the surface of ZnAl-LDH
by loading fulvic acid.
82
The fulvic acid anchored ZnAl-LDH showed
a great ability to simultaneously remove organic dyes and heavy
metal cations. In addition to the functional groups derived from
organic compounds, functional groups from inorganic substances
can also functionalize LDHs. For instance, Lima et al. prepared
fluorinated MgAl-LDH by using NaAlF
6
, and found that the
introduction of surface F terminal groups onto MgAl-LDH could
significantly modify the physicochemical, thermal, and adsorptive
properties.
83
Apart from the above discussed functional groups, many
other groups have been grafted onto the surface of LDHs, such as
epoxide group, disulfide bonds, Br groups, and silanol groups,
7, 18,
36
which led to remarkable improvement in a range of physical
properties of the LDHs.
Figure 3. (a) Schematic illustration of aminopropyltriethoxysilane (amine terminal groups) grafting onto the surface of MgAl-LDH.
81
Reproduced with
permission from ref.
81
, Copyright 2009, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Schematic illustration of (1) core-shell; (2) hollow core-shell; (3)
yolk or rattle core-shell nanostructures; (4) spherical core-shell nanoparticles; (5) hexagonal core-shell nanoparticles; (6) multiple small core materials
coated by single shell material; (7) nanomatryushka material; (8) movable core within hollow shell material.
84
Reproduced with permission from ref.
84
,
Copyright 2016, The Royal Society of Chemistry. (c) Schematic illustration of preparation of LDH microspheres with tunable interior architecture from core-
shell to hollow structure. (d) Cyclic voltammograms (CVs) curves (i); galvanostatic (GV) discharge curves (ii); current density dependence of the specific
capacitance (iii); Nyquist plots of the electrochemical impedance spectroscopy for the hollow, yolk-shell, core-shell LDH microspheres, and LDH
nanoparticles (reference sample) (iv).
85
Reproduced with permission from ref.
85
, Copyright 2012, American Chemical Society.
2.3 Hybridization
The combination of two or more distinct properties into a
unique composite is an exciting direction for the fabrication of
novel multifunctional materials. Construction of LDH-based
hybrids, especially one with a nanostructure, by interacting LDHs
with other materials (e.g., silica nanoparticles, magnetic
nanoparticles, semiconductors, rare earth and noble metal
elements), is an emerging active research vector which may serve
various fields, such as environmental remediation, energy
conversion and storage.
23, 24, 38
The introduction of functional
materials onto LDH can alter the properties of LDHs, offering
various enhanced or new functions to LDHs. Hybrid assembly
provides the advantages of combining the strength of each
component in the structure and functions. In addition, the
components in hybrids may generate synergies. The resultant
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hybrids may exhibit new properties depending on the interactions
between the different components. Herein, we mainly highlight
the construction of LDH-based hybrids by the fabrication of core-
shell structure and the loading of conductive materials,
semiconductors, and rare earth elements or noble metals onto
LDHs to enhance or extend the functions of LDHs. Notably, some
LDH-based hybrids can be grouped into different sections. These
materials will be discussed only in one of the sections below.
2.3.1 Introduction of a “core” or “shell”
The terminology of “core-shell” was first adopted in the early
1990s when researchers attempted to synthesize concentric
multilayer semiconductor nanoparticles to improve the property
of semiconductor materials.
86
Figure 3b shows different types of
core-shell structures. The general concept of core-shell presents
two materials with two functions in one structure (Figure 3b-1).
When the core is removed, hollow core-shell structural material
will be created (Figure 3b-2). And by combining the above two
types of core-shell structures, the yolk or rattle core-shell
architecture which has a core@void@shell configuration (Figure
3b-3) is generated.
86
Generally, core-shell nanoparticles are well-
known for better stability, for being able to protect the core
material from the surrounding environment, for improved
physiochemical properties, for improved semi-conductive
properties, for easy biofunctionalization. The shell could change
the functions and properties of the original core,
84
and the core
could also modify the functions and properties of the original
shell.
87
In other words, the core/shell can exhibit new chemical or
physical properties with shell/core formation. In addition, core-
shell composites may have properties that are synergistic between
the core and the shell and/or offer new properties depending on
the interactions between the core and the shell.
87, 88
Thanks to the above highlighted merits, the construction of
core-shell composites has become an effective avenue to
functionalize LDHs. For instance, to enhance the supercapacitor
behavior of NiAl-LDH, Shao et al. fabricated core-shell LDH
microspheres with tunable interior architecture using SiO
2
as a
core.
85
Via the regulation of interior architecture, core-shell,
hollow core-shell, and yolk core-shell types of NiAl-LDHs were
produced (Figure 3c), and variations in specific surface area and
pore-size distribution were achieved. Moreover, the prepared
core-shell microspheres, especially the hollow ones, exhibited
excellent pseudocapacitance performance, including high specific
capacitance and rate capability, high charge/discharge stability,
and long-term cycling life (Figure 3d). The improvement of the
supercapacitor performance of NiAl-LDH was due to the greatly
improved faradaic redox reaction and mass transfer. In another
example, Han et al. used CoAl-LDH as the core to produce flexible
CoAl-LDH@poly(3,4-ethylenedioxythiophene) core-shell
nanoplatelets for high-performance energy storage.
87
The
synthesized material exhibited high specific capacitance, excellent
rate capability, and long-term cycling stability, which were
superior to those of the conventional supercapacitors and the
CoAl-LDH without the shell of poly(3,4-ethylenedioxythiophene).
The largely enhanced pseudocapacitor performance of the
prepared material was related to the synergistic effect of its
individual components: the LDH nanoplatelet core provided
abundant energy-storage capacity, while the highly conductive
poly(3,4-ethylenedioxythiophene) shell and the porous
architecture facilitated the electron/mass transport in the redox
reaction. LDH-based core-shell materials have growing
applications because of the multifunctionality that is achieved
through the tailoring of core/shell materials. Table 2 summarizes
some representative LDH-based core-shell composites in the past
three years and the significance of fabrication of a core-shell
structure. Clearly, novel physiochemical properties, such as
medical, magnetic, catalytic, electrochemical, and electronic
characteristics can be imparted to LDHs through the formation of
a core-shell structure.
Table 2. Representative LDH-based core-shell hybrids reported in the past three years.
Core/Shell Synthesis method
Performance
vs.
the pristine LDH
Ref.
Fe
3
O
4
/CuAl-LDH
Hydrothermal and co-
precipitation
Endowed magnetism; endowed high sensitivity, good reproducibility, and long-term stability
as electrochemical sensors; endowed real-time monitoring for live cancer cells
89
Fe
3
O
4
/enrofloxacin
intercalated MgAl-LDH
Delamination–
reassembly
Endowed magnetism; enhanced stability; potential magnetic targeting drug delivery-
controlled-release system
90
Fe
3
O
4
/(Zn, Mg, Ni)Al-
LDH
Co-precipitation Endowed high superparamagnetism for easy separation
91
Co
3
O
4
/NiCoAl-LDH
Two-step
hydrothermal
synthesis
Enhanced electrochemical performance: exhibited high specific capacitance (1104 F g
–1
at 1
A g
–1
), adequate rate capability and cycling stability (87.3% after 5000 cycles)
92
NiCo
2
O
4
/NiCoAl-LDH
Hydrothermal
synthesis and a step-
by-
step in situ
structure fabrication
Enhanced specific capacitance of 1814.24 Fg
–1
at a current density of 1 Ag
–1
and 93%
retention after 2000 cycles at 10 Ag
–1
.
93
ZnO/CuZnAl-LDH
Deposition-
Precipitation
Enhanced activity in photoreduction of CO
2
to hydrocarbons
94
CuO/CoFe-LDH
Calcination and
electrodeposition
Largely improved specific capacitance, high rate capability and long cycling lifespans;
exhibited excellent supercapacitive performances with a high energy density (1.857 mWh
cm
-3
) and long-term cycling stability (99.5% device capacitance retention after 2000 cycles)
95
TiO
2
/CoNi-LDH Electrodeposition Remarkably enhanced performance for photoelectrochemical water splitting
96
ZnO and CdS/CoNi-LDH Electrodeposition Efficient solar water oxidation
97
