Recent Advances in the Synthesis and Application of Layered

Review pubs.acs.org/CR

Recent Advances in the Synthesis and Application of Layered Double Hydroxide (LDH) Nanosheets Qiang Wang and Dermot O’Hare* Chemistry Research Laboratory, Department of Chemistry, University of Oxford, 12 Mansfield Road, Oxford, OX1 3TA, U.K. with an interlayer region containing charge compensating anions and solvation molecules. The metal cations occupy the centers of edge sharing octahedra, whose vertexes contain hydroxide ions that connect to form infinite 2D sheets. The most widely studied LDHs contain both divalent and trivalent CONTENTS metal cations, a generic formula for these LDHs may be written 1. Introduction and Scope of Review 4124 as; [M2+1−xM3+x(OH)2][An−]x/n·zH2O, where M2+ may be 2. Synthesis of LDH Nanosheets 4125 common; Mg2+, Zn2+, or Ni2+ and M3+ may be common; Al3+, 2.1. Delamination or Top Down Methods 4125 Ga 3+ , Fe 3+ , or Mn 3+ . A n− is a nonframework charge 2.1.1. Delamination in Butanol 4125 compensating inorganic or organic anion, e.g. CO32−, Cl−, 2.1.2. Delamination in Acrylates 4126 SO42−, RCO2−, and x is normally between 0.2−0.4.1−8 LDHs 2.1.3. Delamination in CCl4 and Toluene 4126 may also contain M+ and M4+ cations but these are limited to 2.1.4. Delamination in Formamide 4127 specific examples such as Li+ and Ti4+. The chemistry of LDHs 2.1.5. Delamination in N,N-Dimethylformais now widely studied and this is in part driven by the use of mide−Ethanol Mixture 4130 these materials as precursors for preparing CO2 adsorbents,9−13 2.1.6. Delamination in Water 4130 catalysts,14 or directly as ion exchange hosts,15−17 fire retardant 2.1.7. Partial Delamination in Dimethyl Sulfadditives,18,19 polymer/LDH nanocomposites,20 drug delivery oxide and N-Methylpyrrolidone 4131 hosts,21 and as cement additives.22 2.2. Controlled Nucleation or “Bottom Up” Today the synthesis of low dimensional solids is of Methods 4132 tremendous importance in both fundamental research and for 3. Practical Applications of Delaminated LDHs 4132 their application in electronic, photonic, magnetic and 3.1. Synthesis of Polymer/LDH Nanocomposites 4133 mechanical materials. In particular, the anisotropy of a two3.1.1. Intercalation of the Monomers and in dimensional (2D) nanosheet, with a thickness on the order of Situ Polymerization 4133 around one nanometer and a lateral size ranging from a 3.1.2. Direct Intercalation of Extended Polysubmicrometer to several tens of micrometers, allows them to mer Chains 4136 serve either as an ideal 2D quantum system for the study of 3.1.3. Pre-exfoliation Followed by Mixing with fundamental physics or as a basic building block in the synthesis Polymer 4137 of functional solids.23 The charge-bearing inorganic macro3.2. Synthesis of Core/Shell Multifunctional Mamolecule-like nanosheets can be assembled or organized terials 4140 through various solution-based processing techniques (e.g., 3.3. Synthesis of Thin Films 4140 flocculation, electrostatic sequential deposition, or by the 3.4. Synthesis of Catalysts 4145 Langmuir−Blodgett method) to produce a range of nano3.5. Synthesis of Electrode Materials 4146 composites,3,24−26 multilayer nanofilms,27 and core−shell 3.5.1. Application in Supercapacitors 4147 nanoarchitectures,28,29 which have significant potential for 3.5.2. Application in Lithium Ion Batteries 4147 electronic, magnetic, optical, photochemical, and catalytic 3.5.3. Application in Dye-Sensitized Solar Cells 4148 applications. Dispersions of clay mineral particles on the 3.6. Synthesis of Hybrid Magnets 4148 nanometer have been much more extensively studied owing to 3.7. Synthesis of Bioinorganic Hybrid Materials 4150 their facile swelling characteristics in addition to a capacity to 4. Conclusions 4151 undergo surface modification with organophilic cations. 30 Author Information 4152 Nanosheets of other layered structures such as metal Corresponding Author 4152 chalcogenides,31 metal phosphates and phosphonates,32 as Notes 4152 well as layered metal oxides,33−36 have also been synthesized by Biographies 4152 manipulation of the interlayer interactions. Acknowledgments 4152 Delamination of LDHs is an interesting route for producing References 4152 positively charged thin platelets with a thickness of a few atomic layers, which can be used as nanocomposites for polymers,24,25,37 or as building units for making new designed

1. INTRODUCTION AND SCOPE OF REVIEW Layered double hydroxides (LDHs) are a class of ionic lamellar compounds made up of positively charged brucite-like layers © 2012 American Chemical Society

Received: November 18, 2011 Published: March 27, 2012 4124

dx.doi.org/10.1021/cr200434v | Chem. Rev. 2012, 112, 4124−4155

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Figure 1. Synthesis schemes of top down and bottom up for LDH single layers.

Table 1. Comparison of the Charge Density of Lamellar Solids and Some Typical LDHsa

organic−inorganic or inorganic−inorganic nanomaterials.28,38,39 Compared to cationic clays such as montmorillonite and laponite which may be exfoliated into single clay sheets in aqueous suspension, LDHs are more difficult to be delaminated. The high charge density of the LDHs layers and the high anion content result in strong interlayer electrostatic interactions between the sheets and significant hydrophilic character. Often extensive interlamellar hydrogen bonding networks lead to a tight stacking of the lamellae. The rigid spheroidal “sand rose” morphology of intergrown platelets prevents both accessibility to the major part of the surface or exfoliation of the sheets in water or in any other nonaqueous solvents.40,41 Therefore, in order to successfully delaminate LDHs into nanosheets (which may be single layers or just several layers thick) new procedures need to be developed. In the past few years there has been a rapid growth in publications related to the synthesis and application of LDH nanosheets. In 2006, Ma et al.42 briefly reviewed the exfoliation of LDHs in formamide. Since 2006, there have been significant developments in the field and no review paper summarizing all the different synthesis methods and applications of LDHs nanosheets has been published. We believe it is therefore timely to review this area, we have focused on describing the current delamination technologies, their advantages and disadvantages and their potential applications. We summarizes the latest progress in delamination methods, describe the necessary characterization technologies that have now been used to study delaminated LDH monolayers or nanosheets, such as X-ray diffraction (XRD), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The review concludes by summarizing the current potential applications of prepared LDH nanosheets. It should be noted that, in the whole paper, LDHs are abbreviated in the forms of M2+-M3+-A (M2+ denotes divalent cations, M3+ denotes trivalent cations, A denotes the interlayer anions), M2+-M3+ (if anions are not specified), or LDH-A (if cations are not specified).

layered solids LDHs

liponite hectorite montmorillonite

general chemical composition Mg3Al−NO3 Zn2Al−NO3 Ca2Al−Cl LiAl2−NO3 Zn2Cr−NO3 Ni2Al−NO3 Na0.7+[(Si8Mg5.5Li0.4) O20(OH)4]0.7− Na0.3(Mg,Li)3(Si4O10)(F,OH)2 (Na,Ca)0.33(Al,Mg)2(Si4O10) (OH)2•nH2O

typical charge density (e per Å2) 0.03141,43 0.04141 0.03541 0.04141 0.04041 0.04044 0.01445,46 0.008−0.01047 0.011−0.01747

a

A unit cell area of 50.7 Å2 was used for hectorite;48 a unit cell area of 46.7 Å2 was used for montmorillonite.49

force. Delamination/exfoliation then occurs when it is dispersed in a highly polar solvent, which is able to solvate the hydrophobic tails of the intercalated anions. For the bottom up synthesis, the traditional aqueous coprecipitation system is introduced into an oil phase with DDS as surfactant and 1-butanol as cosurfactant. The reverse micelles act as nanoreactors, in which LDH single layers can be formed due to limited space and nutrients. In the following section, we summarize all these approaches. 2.1. Delamination or Top Down Methods

2.1.1. Delamination in Butanol. The first complete delamination of LDHs was reported by Adachi-Pagano and co-workers in 1999.40 In their study they demonstrated the total delamination of the Zn−Al−NO3 LDH using DDS as an anionic surfactant and butanol as the dispersant. Other solvents including water, methanol, ethanol, propanol and hexane were also investigated, however these dispersions of Zn−Al−DDS led to unstable suspensions which settled after standing for a few hours and only a minor part of the material was dispersed. Dispersion in methanol appears to be kinetically controlled by the slow replacement of water molecules by methanol. After one week of stirring, ∼50% of the LDH can be exfoliated. In contrast, refluxing in butanol leads to a translucent colloidal solution, which remains stable for at least 8 months. Up to 1.5 g of Zn−Al−DDS per liter of butanol can be dispersed. Similar results have been obtained with higher alcohols such as pentanol and hexanol. It was noted that the hydration state of the modified LDHs is a critical parameter. Delamination was only achieved when the organo-LDH compounds were dried under vacuum at room temperature. The procedure would fail if a freshly prepared (wet) LDH or a LDH thoroughly dried under vacuum at 80 °C was used. The intense drying will lead to a new DDS containing

2. SYNTHESIS OF LDH NANOSHEETS The synthesis of nanosized LDH platelets can be generally classified into two approaches: “bottom-up” and “top-down” (see Figure 1). To date, the “top-down” synthesis is the most widely developed method. However, since the charge density of LDHs is significantly higher than that of other lamellar solids (Table 1), it has been proven to be much more difficult to achieve.41,43−49 It requires modification of the LDH interlamellar environment and then selection of an appropriate solvent system, for example, ion-exchange intercalation of the LDH with anionic surfactant such as dodecyl sulfate (DDS). The aliphatic tails of DDS are elongated and present a high degree of interdigitation, which could consequently enlarge the brucite interlayer distance and weaken the brucite interlayer 4125


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Figure 2. (A) Sketched structures of LDH−DDS, swollen LDH−DDS, and delaminated LDH−DDS. (B) Simplified representation of the edge− edge interaction of the exfoliated lamella; DDS−anions anchored onto basal planes and solvent molecules are omitted. 52 Reproduced with permission from reference 52. Copyright 2004 Elsevier, Inc.

Figure 3. (a) Photograph of a dispersion of Co−Al LDH. (b) A red laser beam directed through the dispersions to show the Tyndall effect. (c) Test tube inversion test demonstrating the formation of toluene gels for dispersions of (i) Mg−Al LDH, (ii) Co−Al LDH, (iii) Ni−Al LDH, and (iv) Zn−Al LDH. (d) The same dispersion as that in panel a after 7 days. 54 Reproduced with permission from reference 54. Copyright 2011 American Chemical Society.

LDH phase with a smaller basal spacing of 1.68 nm corresponding to a tilted and intertwined DDS interlayer arrangement. This LDH displays a densely packed structure which is resistant to exfoliation. A possible mechanism is that butanol with a boiling point higher than that of water under reflux conditions allow for a rapid replacement of all the intercalated water molecules by the solvent molecules. This seems to be the key process to completely exfoliate of the solid material.40,50 However, it should be noted that not all the DDS intercalated LDHs can be delaminated in butanol. Singh et al.51 investigated the delamination of Li−Al LDHs intercalated with different surfactants including DDS, sodium octyl sulfate (SOS), sodium 4-octylbenzenesulfonate (OBS), and sodium dodecylbenzenesulfonate. Among them, only Li−Al− C12H25C6H4SO3 and Li−Al2−C8H17C6H4SO3 were successful. They concluded that the delamination of the unique Li−Al LDH is dependent on the guest surfactant structure in terms of both chain length and headgroup moiety. And generally the alkyl chain is necessary for a successful delamination.51 2.1.2. Delamination in Acrylates. O’Leary et al.24 studied the delamination of LDHs in organic solvents, particularly

polymer monomers. When Mg−Al−DDS was added to a polar acrylate monomer at loadings of between 1 and 10 wt %, followed by heating the mixtures to 70 °C and then subjecting the mixtures to a high shear, delamination of the layers occurred as a result of LDH layers being forced to slide over each other. After it was stirred for 20 min, the majority of the solid remained in suspension after 24 h. It was found that there was some variation in behavior according to the monomer used. Reactions carried out in 2-hydroxyethyl methacrylate (HEMA) resulted in the formation of suspensions which were stable for several weeks at loading levels up to 10 wt %. The exfoliation of Mg−Al−DDS LDH in other acrylate monomers (ethylmethacrylate, methyl methacrylate, ethyl acrylate, methyl acrylate) was also investigated. Initially they all formed homogeneous suspensions. However, it separated to two bands after several hours, one pure monomer and the other a gelatinous suspension of Mg−Al-DDS LDH in the monomer. The two phases that formed were found to be stable for several weeks. It is believed that this technique could be extended to provide delaminated LDHs for a range of applications, for example, the preparation of oriented thin films of LDHs by spin coating the dispersed LDH on a flat substrate.24 4126


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2.1.3. Delamination in CCl4 and Toluene. In DDS intercalated LDHs, the aliphatic tails of the DDS− anions exhibit a high degree of interdigitation in order to maximize guest−guest dispersive interactions, see Figure 2. The basal plane spacings are typically expanded to 2.5−3 nm, depending on the orientation of the DDS chains. In this arrangement, the attraction between the positively charged brucite layers is at a minimum. Therefore, if the solvent molecules are able to sufficiently solvate the hydrophobic tails of the intercalated anions, exfoliation may become thermodynamically favorable. Jobbágy and Regazzoni52 studied the delamination of Mg−Al− DDS in toluene and CCl4. Their studies indicated that, in toluene, swelling of the hydrophobic interlamellar space was produced, expanding the interbasal distance from 2.63 to 3.76 nm.53 In the presence of CCl4, on the other hand, the inorganic sheets lose their short-range spatial correlation. The peak associated with the 001 Bragg reflection disappears completely, indicating that under their conditions delamination of LDH− DDS in CCl4 is a thermodynamically favorable process.52 However, in very recent reports, Naik et al.54,55 claimed that LDH-DDS (Mg−Al, Co−Al, Ni−Al, Zn−Al) can be delaminated in toluene. The delamination of this range of LDH-DDS materials was carried out by stirring and sonicating a known mass of the solid in high purity toluene with different volume fractions (Φv). At lower volume fractions (Φv ≤ 0.005) of LDH, a clear transparent colloidal dispersion is obtained (see Figure 3(a) and (b)). With increasing concentration of the dispersed layered solid in toluene, a gel-like state is obtained (Figure 3(c)). Whereas after a week, the clear dilute dispersions separated into two phases: a clear region and a gel-like phase (Figure 3(d)). It was suggested that the gel is the preferred state of the dispersion. The delamination of Mg−Al−DDS was also studied using molecular dynamics simulations, which suggested that the exfoliation of LDH in toluene is a consequence of the modification of the cohesive dispersive interactions between surfactant chains anchored on opposing inorganic sheets by the toluene molecules. The toluene molecules function as “molecular glue”, holding the surfactant-anchored LDH sheets together, leading to gel formation. The results of the simulation are shown in Figure 4. The variation in interaction energy evaluated per unit surface area has been plotted as a function of interlayer separation. It can be seen that the total interaction energy decreases sharply from the separation of 12.72 to 5.5 nm, with a minimum value at 3.72 nm. It is obvious that the interaction energy per unit surface area in the presence of toluene is always lower than the value in vacuum. The difference represents the strength of the interaction between DDS chains tethered to opposing sheets mediated via toluene molecules and indicates strong association between the toluene molecules and the DDS surfactant chains anchored to the Mg−Al sheets. By combining their experimental result, they concluded that the surfactantintercalated Mg−Al−DDS would spontaneously delaminate in toluene and subsequently associate with a tactoidal microstructure.54 2.1.4. Delamination in Formamide. The delamination of amino acid intercalated LDHs in formamide was first reported by Hibino and Jones.56 In this method, the use of amino acid anions and polar solvents are key aspects. The delamination mechanism is that the strong hydrogen bonding between the intercalated anions and polar solvent as well as between the solvent molecules themselves could lead to the penetration of

Figure 4. Plot of the interaction energy between two surfactant anchored sheets of Mg−Al LDH at different interlayer separations in vacuum and in toluene. Einter is the interaction energy contributing from Coulomb energy, van der Waals energy, internal energy, and kinetic energy. Einter (vdW) is the interaction energy contributing from van der Waals energy only.54 Reproduced with permission from reference 54. Copyright 2011 American Chemical Society.

large amounts of solvent between the layers and hence facilitate delamination. Hibino and Jones have investigated a range of amino acid/polar solvent combinations including glycine, serine, and L-aspartic acid with water, ethanol, acetone, formamide, ethylene glycol, and diethyl ether. The optimum combination for successful delamination was found to be glycine/formamide, which may result from an optimization of the interactions between host, anions and solvent.56,57 The Mg−Al-glycine were prepared by coprecipitation, with a basal spacing of 0.81 nm, is shown Figure 5a. Then synthesized

Figure 5. PXRD patterns for (3:1) Mg−Al−glycine (a) after preparation and drying in air, (b) 1:1 mixture with formamide indicating absence of LDH reflections, and (c) of a material obtained by repeated addition of droplets of a completely delaminated mixture.56 Reproduced with permission from reference 56. Copyright 2001 The Royal Society of Chemistry.

Mg−Al−glycine was dispersed in formamide at room temperature, resulting in a clear colloidal dispersion. The reaction is extremely fast, and is complete within a few minutes. Only a broad reflection associated in the XRD because of the support glass slide is observed, with the absence of LDH Bragg reflections indicating loss of the original crystal plane stacking structure (Figure 5b). Repeated deposition of droplets of a 0.03 g/10 mL mixture onto a glass slide (dried at 25 °C), however, 4127


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resulted in the creation of a crystalline LDH as evidenced in Figure 5c. The absence of classes of Bragg reflections other than the 00l Bragg reflections indicates that the deposited LDH platelets are highly oriented parallel to the surface of the glass slide. It is clear that dispersion in formamide had resulted in the delamination of the LDH layers. For the (3:1) and (4:1) Mg− Al−glycine, up to 3.5 g of LDH could be delaminated per liter of formamide.56 In a later publication, Hibino58 expanded this method to include other interlayer amino acids anions for the purpose of finding other LDHs (e.g., Ni−Al, Co−Al, Zn−Al) that can be delaminated and that have better affinity for polymers. The affinity of glycine, the simplest amino acid, for polymers may not be sufficient for use as a modifier for LDHs. It has been reported that polymers could be extracted from polymer−LDH hybrids by solvents when delaminated LDH−glycine is used.37,59 In this study, 14 types of amino acid were intercalated in LDHs. Some of the amino acid intercalates were successfully delaminated in formamide, but others were not. The intercalates that could not be delaminated had a high amino acid content, exceeding 15−20% of the charge occupation rate. At that rate, closely packed amino acids were likely to be tightly connected to one another and to the host layers via hydrogen bonds, and therefore, formamide presumably could not open or penetrate the interlayers in large volume. In contrast, there was not a clear lower threshold for charge occupation of amino acid; even LDH intercalates with charge occupations of less than 1% underwent delamination. Since they have a much greater affinity with polymers, this could be a promising method for the preparation of polymer−LDH nanocomposites. They also demonstrated that, in addition to Mg−Al LDHs, the M2+−M3+ systems with other cations such as Ni−Al, Co−Al, and Zn−Al could undergo delamination in formamide completely or to some extent when glycine was intercalated. 58 The successful delamination of Mg−Ga−glycine was reported by Unal.60 Li et al.27 were the first to report the successful delamination of Mg−Al−NO3 in formamide. In their experiment, Mg−Al− NO3 was mixed with formamide, followed by aging using a mechanical shaker. After vigorous shaking, an apparently transparent solution was produced, and no sediment was observed upon standing. The XRD data recorded without drying (Figure 6a and b) exhibited a pronounced halo in a 2θ range of 20−30°, which was ascribed to the scattering of liquid formamide. Therefore, it was concluded that the host sheets were not sufficiently stacked together to give coherent scattering of X-rays, and therefore Mg−Al−NO3 must be exfoliated. The exfoliation was confirmed by TEM analysis, see Figure 6c. A typical two-dimensional object was observed with the delaminated LDH sheets, which had a lateral size, usually >1 μm. Because of the fracture or breakage of the sheets during delamination the nanoplatelets lost their hexagonal shape and had a reduced size in comparison to the original LDHs. 27 The mechanism for the delamination of Mg−Al−NO3 in formamide has also been proposed. It is well-known that there is a dense network of hydrogen bonds in the interlayer space of LDHs; interlayer water molecules are hydrogen-bonded to hydroxyl groups of the LDH host and, at the same time, are coordinated to interlayer anionic guests.61 Because formamide is highly polar, its carbonyl group would have a strong interaction to the LDH host, replacing the interlayer water molecules. The other end of the formamide molecule contains an NH2 moiety and this group may not be able to form strong

Figure 6. (a) XRD pattern of starting LDH. (b) XRD pattern for the colloidal aggregate centrifuged from the suspension (top). The bottom trace denotes the blank data for formamide itself. (c) TEM image of the LDH nanosheet (top) and its selected area diffraction pattern (bottom).27 Reproduced with permission from reference 27. Copyright 2005 American Chemical Society.

interactions with the interlayer anions. Therefore, once the replacement of water by formamide takes place, this weakens the interlayer attraction force through the destruction of the strong hydrogen-bonding network and promotes delamination.27 At this time, Wu et al.39 reported the delamination of Mg− Al−NO3 in formamide using ultrasonic treatment at room temperature. Samples of Mg−Al−NO3 at concentrations of 1− 40 g/L were dispersed in formamide. To facilitate exfoliation, these dispersions were treated using an ultrasonic water bath in successive intervals of 30 min, until the turbidity of the dispersion approached a constant value. It was found that the colloidal dispersions in a concentration range from 1 up to 40 g/L (mass of LDH added to volume of solvent) are stable and transparent. AFM images showed that a large proportion of the LDH sample was delaminated into single and double brucite layers (0.7−1.4 nm in thickness). These nanosheets had disklike shapes with a diameter of ∼20−40 nm. However, after resting at room temperature for several days to weeks, transparent gels are formed in dispersions at concentrations higher than 5 g/L. The delamination of LDH-NO3 was also compared with LDH−CO3, as shown in Figure 7. In contrast, the turbidity of a suspension of LDH−CO3 in formamide remained nearly unchanged, having a turbidity in the range between 1700 and 2000 NTU during ultrasonic treatment. 39 Direct delamination of Mg−Al−NO3 in formamide without incorporation of organic molecules greatly simplifies the delamination procedure and opens up for exfoliation of LDHs containing transition metal cations, for example, Ni, Co, and Fe, which will easily form stable complexes with amino acids. Subsequently, this method has been expanded to many other LDHs, with either different cations (divalent and/or trivalent) or different inorganic anions. Liu et al. 62 studied the delamination of Co−Al LDHs intercalated with a variety of anions such as NO3−, ClO4−, acetate, lactate, dodecyl sulfate, and oleate, and found that most of these intercalation products exhibited a delamination behavior in formamide. Among them, Co−Al−NO3 was found to have the best delamination behavior. The delamination yielded well-defined nanosheets 4128


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while almost no sediment was observed for Ni−Al LDH. The exfoliation degree of Zn-containing LDH was estimated to be only ∼40%, which was far less than those (nearly 100%) of Co−Al and Ni−Al LDHs. However, this may be due to a zinc hydroxycarbonate impurity in the samples.65 Ni−Fe−NO3 LDH was also successfully delaminated into nanosheets by Abellan et al.66 Ma et al.67 developed a novel topochemical synthetic approach for the synthesis and exfoliation of Co−Fe LDHs. Micrometer-sized hexagonal platelets of brucite-like Co2/3Fe1/3(OH)2 were first prepared by a homogeneous precipitation of an aqueous solution of divalent cobalt and ferrous ions through hexamethylenetetramine (HMT) hydrolysis under a nitrogen gas atmosphere. A subsequent oxidative intercalation process, by the action of iodine (I2) in chloroform (CHCl3), transformed the brucite-like precursor Co2+−Fe2+ hydroxides into hydrotalcite-like Co2+−Fe3+ LDHs, in which the oxidation of Fe2+ into Fe3+ introduced positive charges into the octahedral hydroxyl layers while iodide anions were intercalated into the interlayer space to retain charge neutrality. After a conventional ion-exchange process, Co2+−Fe3+ LDHs accommodating perchlorate anions were exfoliated into unilamellar nanosheets in formamide by an ultrasonic treatment (Figure 9).67 Using a similar approach and exfoliating the Co2+−Co3+−ClO4 and Co−Ni−NO3 LDHs in formamide, positively charged Co(OH)2 and Co−Ni hydroxide nanosheets with tunable composition nanosheets were produced.68,69 One drawback of these colloidal LDH nanosheets is that they underwent restacking upon aging/drying, which further limits its practical applications.27 Kang et al.70 investigated the influence of water washing on the restacking of LDH nanosheets, see Figure 10. The samples become opaque after washing with water. After only one wash with 40 mL water, Bragg reflections in the XRD at 2θ = 24°, 36°, and 41° were observed with the sample, suggesting the nanosheets could be immediately restacked in water. After being washed three times, the halo shifted from 20−30° to 25−45°, indicating formamide was being removed by the water. So, it was concluded that the colloid of exfoliated LDH nanosheets is not stable in water, and the removal of formamide can result in an immediate restacking of the nanosheets. The XRD pattern of the dried sample (Figure 10e) shows that the restacked material has a layered structure with a basal spacing of 0.87 nm, which corresponds to the interlayer distance of Mg−Al−NO3.70 To prevent the restacking in water, Kang et al.70 added carboxymethyl cellulose (CMC) into the colloid to make a LDH/CMC nanocomposite, as shown in Figure 11. When CMC is introduced into the colloid, the nanosheet could attach to CMC surfaces without destroying the colloidal state. It was found that the nanosheets encapsulated by CMC chains remain in a stable exfoliated state in water. Drying at 40 °C led the

Figure 7. Turbidity as a function of ultrasonication time for Mg−Al− NO3 in formamide at a concentration of 10 g/L. The turbidity as a function of ultrasonication time of Mg−Al−NO3 and Mg−Al−CO3 in formamide at concentrations of 10 g/L is shown in the inset. 39 Reproduced with permission from reference 39. Copyright 2005 The Royal Society of Chemistry.

with a lateral size in the micrometer length range. The authors reported that delamination in formamide involves two separate stages: rapid swelling into a highly swollen phase, and then progressive exfoliation into single sheets (see Figure 8). The

Figure 8. Schematic illustration of the possible delamination mechanism for LDHs in formamide.62 Reproduced with permission from reference 62. Copyright 2006 American Chemical Society.

swelling takes place almost instantly, and the exfoliation of the highly swollen phase proceeds progressively with the assistance of continuous shaking. This delamination behavior is very similar to that observed for layered titanates and manganese oxide. It is believed that this mechanism should be common with other layered systems.34,63,64 Liu et al.65 also studied the delamination of LDHs with a variety of divalent cations, for example, Zn−Al, Zn−Co−Al, Ni−Al. These LDHs containing a range of monovalent anion forms were found to exhibit a range of delamination behavior in formamide, the best results were again achieved with NO3− intercalated LDHs. For Zn−Al and Zn−Co−Al, LDHs a considerable amount of undelaminated precipitates were found,

Figure 9. Schematic illustration of topochemical synthesis and exfoliation of Co 2+−Fe3+ LDHs.67 Reproduced with permission from reference 67. Copyright 2007 American Chemical Society. 4129


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treatment of LDH−CO3 with DMF further leads to the delamination of LDH, resulting in colloidal aspect samples. FTIR and Raman analyses confirmed that the final product is LDH−HCOO− material. The possible mechanism for this phenomenon has been proposed. It is believed that the process involves the hydrolysis of DMF promoted by basic centers of hydrated LDH and, consequently, decomposition of carbonate into CO2 (and H2O). Formate ions are produced in the hydrolysis and intercalated into brucite-like layers. LDH− formate phase should be swelled in DMF−ethanol medium, promoting the separation and stabilization of a colloidal dispersion of LDH nanosheets. It is proposed that the DMF is essential for the LDH−CO3 decarbonation and exfoliation processes, and ethanol enhances the DMF hydrolysis since the extension of the reaction is low in presence of the pure amide. 2.1.6. Delamination in Water. Gardner et al.72 have reported a nearly transparent colloidal LDH suspension obtained from the hydrolysis of Mg−Al−methoxide in water at ambient temperature. Although the term of “delamination” was not used in their work, it is believed that the LDHs in their study may have been delaminated.73 They obtained colloidal solutions in two steps: preparation of alkoxide-intercalates of LDHs in nonaqueous media (alcohols) and mixing of the LDH derivatives with water. It should be noted that to achieve the delamination in water, the parent alkoxide−LDHs must be synthesized in nonaqueous media. Later, Hibino et al. 73 described a new method for delaminating LDHs in water. Water is the only solvent used throughout the procedure, and thick colloidal solutions of delaminated LDHs are obtained. This new approach involves preparing Mg−Al−lactate in water, washing them with water, and subsequently storing and maturing them in water until they delaminate. This facile, effective, and environmentally friendly approach has the following advantages: (1) water is the only solvent used in the procedure, from preparation of the LDHs to delamination, so that the parent LDHs do not have to be separated from synthesis or washing media before delamination; (2) the amounts of LDHs (10−20 g/L) that delaminate to form stable colloidal solutions are higher than the amounts that have previously been reported (1.5−3.5 g/L); and (3) no heat treatment is needed during the preparation and delamination processes. The possible mechanism for such delamination is believed to be the interaction between intercalated lactate and water. Mg−Al−lactate was synthesized by a coprecipitation method. The lactate anions in the reaction media were present in a large stoichiometric excess (more than 20-fold), so that they would be preferentially intercalated into the interlayers. The resulting white Mg−Al−lactate slurries were washed several times by repeated centrifuging and replacement of the supernatant water. Suspensions of Mg−Al−lactate were opaque just after washing, but they gradually changed into nearly transparent colloidal solutions. When the amount of Mg−Al−lactate in the water was 10−20 g/L, the suspensions took 3−5 days to become translucent. When the temperature was increased, delamination occurred more rapidly. The AFM analysis in Figure 12 provides the evidence that Mg−Al−lactate delaminated into single sheets. Large sheets (100−150 nm in the horizontal direction) showed an average thickness of about 2.5 nm. In contrast, smaller sheets generally showed a thickness of 0.55−0.95 nm, which nearly corresponds to the smaller basal spacing measured by XRD.73

Figure 10. XRD patterns of (a) colloidal suspension of Mg−Al−NO3 in formamide, and the samples (b−d) water-washed 1, 2, and 3 times, respectively, and (e) dried; d-values in nanometers. 70 Reproduced with permission from reference 70. Copyright 2009 American Chemical Society.

Figure 11. Sketch presentation of the formation process of CMC− LDH nanocomposite.70 Reproduced with permission from reference 70. Copyright 2009 American Chemical Society.

nanosheets and CMC to restack to a layered nanocomposite with a basal spacing of 1.75 nm, indicating a bilayer arrangement of CMC in the interlayer.70 2.1.5. Delamination in N,N-Dimethylformamide− Ethanol Mixture. In addition to formamide, it was reported by Gordijo et al.71 that Mg−Al−CO3 LDH can also be delaminated in N,N-dimethylformamide (DMF)−ethanol solvent mixture. In their work, the release of CO 2 from the Mg−Al−CO3 LDH suspension in certain DMF−ethanol solvent mixtures was observed. The results indicate that Mg− Al−CO3 LDH undergoes decarbonation when being suspended in DMF and the presence of ethanol enhances the decarbonation process. Decarbonation is more pronounced when DMF and ethanol are mixed in equal amounts (i.e., 1:1 (v/v) proportion). In addition to the decarbonation process, the 4130


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Hou et al.76 reported that Ni−Fe LDHs containing aminoundecanoic acid can be successfully exfoliated into elementary LDHs nanosheets by using a host−guest repulsive intercalation induced by the protonation of amino groups in aminoundecanoic acid molecules in an HCl solution. It utilized the electrostatic repulsive force between guest species and inorganic host layers, which might be suitable for the exfoliation of positive metal oxide layers with both poor swelling properties and high layer charge density. Manohara et al.77 synthesized formate-intercalated Ni−Al LDHs by formamide hydrolysis. They found that the Ni−Al− formate can be delaminated in water. It was explained that, when immersed in water, the formate ion grows its hydration sphere (osmotic swelling), eventually leading to the exfoliation of the metal hydroxide layers into lamellar particles. These nanoplatelets restack to form thicker tactoids upon evaporation of the dispersion. It was suggested by the authors that the intercalation of anions with a high enthalpy of hydration could present a new strategy for realizing the aqueous exfoliation of layered double hydroxides. This is a green alternative to the traditional method of exfoliation with the use of organic solvents. However, the incorporation of anions with a high hydration enthalpy in the interlayer of the LDH is a major challenge because the intercalation of such anions involves an enthalpy loss that has to be compensated for by other means. 77 2.1.7. Partial Delamination in Dimethyl Sulfoxide and N-Methylpyrrolidone. Zhao et al.78 reported that LDH either having nitrate counteranions or intercalated with organic molecules, can be partially exfoliated in dimethyl sulfoxide (DMSO) to form a transparent suspension. Before being delaminated, the average thickness of the LDH platelets was ∼13.2 nm, which corresponding to 15 stacked single layers. During the delamination process, the change in the thickness of the LDH platelets was monitored by AFM. It was found that the thickness of the platelets decreased to 1.8−5.3 nm,

Figure 12. (a) AFM image of delaminated 3:1 Mg−Al−lactate sheets on a mica substrate and (b) height profile along the white line in image (a).76 Reproduced with permission from reference 76. Copyright 2005 The Royal Society of Chemistry.

Subsequently, Jaubertie et al.74 synthesized Zn−Al−lactate and attempted to delaminate it in both butanol and decarbonated water by either reflux or ultrasonic treatment or both. The result indicated that butanol is not a suitable solvent for delamination; whereas an almost complete delamination was observed in decarbonated water with the assistance of sonication. Similar experiments were also performed with Mg− Al−lactate LDH.75 Dispersion in decarbonated water along with powerful ultrasound-assisted treatment caused delamination of all the samples. They also noticed that, upon drying, all samples restacked to give a better ordered layer stacking arrangement.

Figure 13. Schematic representation of the formation of LDH nanoplatelets in a reverse microemulsion: (a) Three microemulsions with different water to surfactant ratios; (b) the mixture of metal salts and urea inside a micelle; (c) the urea begins to undergo hydrolysis; (d) the LDH phase starts to crystallize; (e) the final nanoplate product. 85 Reproduced with permission from reference 85. Copyright 2010 The Royal Society of Chemistry. 4131


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corresponding to 2−6 stacked single layers. They also investigated other polar solvents including acetone, acetonitrile, tetrahydrofuran (THF), dimethylformamide (DMF), ethanol, and N-methylpyrrolidone (NMP), and noticed that NMP also led to the formation of transparent LDH suspension. When acetone, acetonitrile, and THF were used, sedimentation occurred just after shaking. For DMF and ethanol, the LDHs flocculated after standing 2 h. It is well-known that, in the interlayer space of LDHs, there is a dense network of hydrogen bonds formed through the interlayer water molecules, which are both hydrogen-bonded to hydroxyl groups of the LDH layers and coordinated to interlayer anionic guests. The possible mechanism is that formamide replaces the interlayer water molecules and destructs the strong hydrogen-bonding network, thus inducing the exfoliation. In fact, formamide and DMSO have been found to be able to intercalate into layered silicate kaolinite through hydrogen bonding to the hydroxyl surfaces of kaolinite.79,80 Herein, the exfoliation of LDHs using DMSO and NMP could follow a similar mechanism to that of formamide, presumably because of their strong hydrogen bonding interaction with the hydroxyl groups of the LDH layers.78

Figure 14. Powder XRD patterns of (a) empty sample holder; (b) the gel-like product separated by centrifuge; (c) the same product after drying for 30 min; (d) 180 min drying. The 2.5−10° region is highlighted as inset.81 Reproduced with permission from reference 81. Copyright 2006 The Royal Society of Chemistry.

2.2. Controlled Nucleation or “Bottom Up” Methods

with different sizes can be synthesized with a narrow size distribution. Structural and phase transitions of the DDS aggregates in the water-in-oil system are believed as the driving force to adjust the size of the LDH particles.83 The synthesized LDHs were also characterized by AFM, as shown in Figure 15. The very small average thickness confirmed the conclusions based on the XRD pattern that the limited number of layers in each particle results in no long-range coherence along the c dimension which results in the absence of basal reflections in the XRD pattern. For instance, particle profile analyses indicated that the thickness and lateral width dimensions are 14.5 ± 1.7 Å and 574 ± 46 Å respectively for LDH-RM2 and 129.6 ± 11.4 Å and 585 ± 58 Å for LDH-RM3. Crystallographic studies have already shown that the thickness of an Mg−Al−LDH monolayer is 4.7 Å. Thus the platelet with 14.8 ± 1.2 Å thickness for LDH-RM1 and 14.5 ± 1.7 Å for LDHRM2 can consist of up to 3−4 brucite layers without considering the interlayer spacing expanded by the chargebalancing DDS anions. However, elemental analysis results indicate that DDS groups are covering the platelets as chargebalancing species. Finally they proposed that the chargebalancing DDS groups form a flexible shell around the layer which, under AFM observation, makes the particle thicker than the 4.7 Å.83 More recently, this “bottom up” approach has been expanded to other LDHs systems such as Ni−Al84 and Co−Al LDHs,85 and to other reverse microemulsion systems such as cetyltrimethylammonium bromide/n-butanol/isooctane/M II and AlIII nitrate aqueous solution.86

In 2005, Hu et al.81,82 reported a facile one-step synthesis of Mg−Al LDH monolayers. It was the first time a reverse microemulsion method had been used to prepare LDHs. In their method, the traditional aqueous coprecipitation system (Mg(NO3)2·6H2O + Al(NO3)3·9H2O at pH ≥ 10) is introduced into an oil phase of isooctane with DDS as surfactant and 1-butanol as cosurfactant. The aqueous phase which contains the nutrients for the growth of LDH crystals is dispersed in the oil phase to form droplets surrounded by DDS. These droplets act as nanoreactors which may only provide limited space and nutrients for the growth of LDH nanoplatelets. The size of the particles can thus be effectively controlled both in diameter and thickness by the water to surfactant ratio. This system also allows the negatively charged DDS chains to interact with the LDH platelets as chargebalancing anions. A schematic representation of the process is shown in Figure 13. Employing the XRD and AFM analyses, Hu et al.81,83 showed clear evidence that single layer LDH nanosheets can be synthesized using this method. After collecting the gel-like materials from centrifuge, XRD patterns were recorded with respect to drying time, as shown in Figure 14. For the sample without drying, two broad Bragg reflections were observed at ∼2θ = 7.5° and 20° respectively (pattern b). However, all the strong basal plane Bragg reflections which are the characteristic features of LDHs are unobserved, suggesting that highly exfoliated LDH layers were formed in these reverse microemulsion systems. When the sample was dried in air, remarkable changes appeared (pattern (c) and (d)). The broad reflection at about 7.5° split and another weak Bragg reflection grew in intensity at 2θ = 18°. Moreover, other Bragg reflections at higher angles also grew in intensity. The inset of Figure 14 indicates that a weak and broad reflection was discerned at about 2θ = 3° (indicated by an arrow). It showed a gradual growth upon drying, which indicates that the sample starts to gain some structural order. A follow up paper presented a more comprehensive study of the effect of water to surfactant ratio (w) on LDH nucleation and growth. They showed that Mg−Al−DDS LDH particles

3. PRACTICAL APPLICATIONS OF DELAMINATED LDHS Research into the uses of LDHs continues to expand. 87 To date, the known applications of LDHs are widespread and cross many disciplines, such as catalysts, 88,89 catalyst precursors,14,90,91 anion exchangers,15−17 CO2 absorbents,9−13 bioactive nanocomposites,20,92,93 electroactive and photoactive materials.94−96 However, many of the applications are limited in scope due to the inaccessibility to the inner surfaces of the host layers. An effective solution to this problem would be the 4132


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Figure 15. 3-Dimensional AFM images of illustrative samples of Mg−Al−DDS LDH−RMn (n = 1−4): (A) LDH-RM1 (w = 12), (B) LDH-RM2 (w = 24), (C) LDH-RM3 (w = 36), and (D) LDH-RM4 (w = 48). In each case the samples were spin coated onto a HOPG substrate. 83 Reproduced with permission from reference 83. Copyright 2007 The Royal Society of Chemistry.

delamination of LDHs into single highly anisotropic layers ie. nanosheets.40 Nanosheets are also expected to be used as building blocks for the construction of various functional nanocomposites or nanostructures. In this work, all the possible applications of delaminated LDHs nanosheets are summarized and classified into seven groups, which are (1) synthesis of polymer/LDHs nanocomposite, (2) synthesis of core−shell multifunctional materials, (3) synthesis of thin films, (4) synthesis of catalysts, (5) synthesis of electrode materials, (6) synthesis of hybrid megnets, and (7) synthesis of bioinorganic hybrid materials.

expand the basal spacing and/or ionogenic modification of the polymer to graft anions onto the polymers or monomers. 111 Although there are many possible strategies to synthesize exfoliated polymer/LDH nanocomposites, generally the methods can be classified into three principal options: (1) intercalation of the monomer molecules and in situ polymeration, (2) direct intercalation of extended polymer chains, (3) pre-exfoliation and followed by mixing with polymer, as shown in Figure 16.20,112 For method (1), the monomers are first intercalated into LDHs gallery. During the polymerization of these interlayer monomers, LDHs nanosheets will be exfoliated and distributed evenly within polymer substrates. For method 2, polymer instead of monomer as a whole is intercalated into LDH gallery, and these big molecules will lead to the exfoliation of LDHs nanosheets. With this method, LDHs are generally modified by organic anions, for example, DDS, to enlarge the layer distance. For method 3, LDHs were first exfoliated into nanosheets in a colloidal solution, followed by the mixing with polymer to obtain nanocomposite.113−115 In the following section, the recent advances of the above-mentioned three methods will be summarized in detail. 3.1.1. Intercalation of the Monomers and in Situ Polymerization. Because of the high exchangeability of the interlayer anions, not only inorganic anions (e.g., CO32−, NO3−, Cl−, SO42−, etc.) but also organic anions including monomers can be intercalated into the LDHs gallery. If this is followed by in situ polymerization of the intercalated monomers then this may lead to complete exfoliation to LDHs nanosheets. In 1989, Tanaka et al.116 investigated the anion exchange of Mg−Al−X LDH (X = CO32−, Cl−, and NO3−) with acrylate anions with the purpose of preparing polymer/LDHs nanocomposites. It was found that when Mg−Al−CO3 was used, the anion exchange reaction did not occur. When Mg−Al−NO3 was employed, the reaction proceeded to form a LDH-acrylate

3.1. Synthesis of Polymer/LDH Nanocomposites

In recent years, polymer/LDH nanocomposites have attracted considerable interest in the field of material chemistry.24,97−100 Owing to their novel mechanical, optical and thermal properties which are rarely present in pure polymers or microscale composites, these hybrids may have wide range of applications such as organoceramics, biomaterials, electrical and mechanical materials.101−107 In general, the performance of a nanocomposite strongly depends on the degree of dispersion (intercalation or exfoliation) of LDH layers in the polymer matrices.108 Among different types of nanocomposites, the exfoliated one usually attracts more interest since better dispersion of LDH layers in polymer matrix results in enhanced properties as compared to the intercalated nanocomposites.109 However, LDH layers possess a high charge density (∼300 mequiv/100 g) thus strong interlayer electrostatic interaction makes the exfoliation of LDH much more unfavorable.110 In addition, pristine LDH is not suitable for the penetration of giant polymer chains or chain segments into their gallery space unless its interlayer distance is significantly increased. Accordingly, the intercalation of LDH involves organic modification of LDH to 4133


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intercalation compound. The formation of the intercalation compound from Mg−Al−Cl occurred only with a concentrated acrylate solution. When the intercalation compounds were heated at 80 °C with the addition of an initiator, the interlayer acylate anions were polymerized to form a LDH/polyacrylate nanocomposite.116 Later, many different LDHs and polymers were selected for the preparation of polymer/LDHs nanocomposites. Hsueh et al.105 prepared Mg−Al LDHs/epoxy nanocomposites by mixing the amino laurate intercalated LDHs, EPON 828 resin, and Jeffamine D400 as a curing agent. The intercalation of amino laurate into LDHs makes the LDHs nanolayers more hydrophobic, enabling them to be exfoliated by epoxy molecules, this is analogous route reported for organo-clays in the synthesis of exfoliated clay/epoxy nanocomposites. A further reason for the use of amino laurate as the intercalated species is that the reaction between the amine groups of the intercalated amino laurate and the epoxy groups generates adhesion between the LDHs nanolayers and epoxy molecules and makes the exfoliated LDHs more compatible with the epoxy matrix. Hydrophobic organo-modified LDHs easily disperse in epoxy resin, and the amino laurate intercalated LDHs with large gallery space allow the epoxy molecules and the curing agents to easily diffuse into the LDHs galleries at elevated temperature. After the thermal curing process, exfoliated LDHs/epoxy nanocomposites were formed. Figure 17 shows the formation process of the LDHs/epoxy nanocomposites. Polyimide/LDHs can also be prepared using a similar route.117 Qiu et al.118 prepared an exfoliated Zn−Al LDH/polystyrene nanocomposite by in situ atom transfer radical polymerization reaction using an initiator-modified LDH. The Zn−Al LDH was initially intercalated with DDS, which resulted in interlayer separation increasing to about 2.64 nm. The delamination of LDH is obtained by the swelling process arising from the transport of the styrene into the galleries of LDH host. During polymerization, the styrene was polymerized by the αbromobutyrate which then became grafted to the surface of the Zn−Al LDH nanolayers. Finally the gallery space swelled and delaminated increasing of the amount of polystyrene (PS) in the galleries. Figure 18 illustrates the process of the exfoliating LDH layers in PS matrix. The modification of the LDH basal surfaces by dodecyl sulfate provides a hydrophobic

Figure 16. Pathway of nanocomposite preparation by (a) monomer exchange and in situ polymerization, (b) direct polymer exchange, and (c) restacking of the exfoliated layers over the polymer.20 Reproduced with permission from reference 20. Copyright 2001 American Chemical Society.

Figure 17. Formation process of the LDHs/epoxy nanocomposites.105 Reproduced with permission from reference 105. Copyright 2003 Elsevier Inc. 4134


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Figure 18. Process of exfoliating LDH by in situ atom transfer radical polymerization (ATRP) reactions. 118 Reproduced with permission from reference 118. Copyright 2005 Springer.

environment and an enlarged interlayer distance to allow catalyst molecules and monomers to enter the LDH galleries. Exfoliated Zn−Al-DDS LDH sheets with a thickness of less than 1 nm were dispersed individually in the PS matrix. The thermal stability of the nanocomposite displayed a marked improvement. Using the 50 wt % weight loss as a comparison point, the decomposition temperature of PS/LDH nanocomposite is about 45 °C higher than that of pure PS.118 Ding et al.119 synthesized exfoliated poly(methyl methacrylate) (PMMA)/Mg−Fe−DDS LDH polymer nanocomposites by in situ polymerization. The typical morphology of PMMA nanocomposites with 1.6 wt % and 8.0 wt % Mg−Fe−DDS LDH are shown in Figure 19. It is evident that the distribution of Mg−Fe−DDS LDH is homogeneous in the whole polymer matrix, in which modified inorganic layers are in partially exfoliated. The TEM micrograph of the sample containing 1.6 wt % Mg−Fe−DDS LDH is very different from those of polymers/layered silicates exfoliation nanocomposites in which the exfoliated clay layers are often face−face orientated because of the very high aspect ratio.120 In the case of PMMA/Mg− Fe−DDS−LDH exfoliation nanocomposite, the exfoliated Mg−Fe hydroxide sheets combined with DDS anions are dispersed disorderly in the PMMA matrix. Most of the exfoliated Mg−Fe hydroxide nanolayers are tilted with respect to the cutting section. These results provide positive evidence

of well molecular dispersion of Mg−Fe−DDS LDH layers in the PMMA matrix. The participation of Fe3+ ions is found to play an important role in the improvement of the thermal stability of polymer nanocomposites containing a small inorganic loading and welldispersed inorganic components. The possible reason is that Fe3+ cations prevent nanocomposites from decomposing by radical trapping at the onset temperature. It is well-known that Fe3+ ions are an effective free radical trap in solution for PMMA radicals. Early work pointed to the role of the Fe 3+ ions in radical trapping within clay in polymer/MMT nanocomposites.121,122 The thermal oxidative degradation steps of PMMA correspond to the end-initiation and random scission of PMMA chain under air atmosphere.123 Fe3+ ions within layers behave as an ideal radical scavenger and consequently may be used to determine the rate of initiation in free radical polymerization when radicals are formed by initiation of these unsaturated end groups of PMMA.119 Ding et al.124 and Qiu et al.125 reported that exfoliated polymer/LDHs nanocomposites can also be synthesized by emulsion polymerization. For instance, exfoliated PS/Zn−Al LDH nanocomposites have been synthesized via emulsion polymerization in the presence of N-lauroyl-glutamate (LG) surfactants and long-chain n-hexadecane. LDH layers are swelled or even exfoliated with the help of the LG surfactant 4135


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Figure 19. TEM images for nanocomposites (a) 1.6% Mg−Fe−DDS LDH and (b) 8.0% Mg−Fe−DDS−LDH. 119 Reproduced with permission from reference 119. Copyright 2008 Elsevier Inc.

Figure 20. Schematic diagram of the formation procedure of exfoliated PS/LDH nanocomposites via emulsion polymerization. 124 Reproduced with permission from reference 124. Copyright 2005 Elsevier Inc.

Figure 21. Scheme of the PS/Zn−Al LDH nanocomposites prepared by solution intercalation. 25 Reproduced with permission from reference 25. Copyright 2005 Elsevier Inc.

melt mixing. The Qu group were the first to report the direct intercalation of a macromolecule into an LDH in xylene solution to obtain a series of exfoliated polymer/LDHs nanocomposites, such as PE-g-MA/Mg−Al−LDH, 126 LLDPE/Zn−Al−LDH,111,127 PMMA/Mg−Al−LDH.128 The general synthesis scheme is shown in Figure 21. DDS ions were intercalated into the LDHs to weaken the electrostatic forces among the hydroxide sheets and render the LDH layers hydrophobic. In a typical preparation procedure, Zn−Al−DDS was first refluxed in 100 mL xylene and then PS was added

and the long-chain n-hexadecane within the emulsion. Subsequently, when the styrene was added and polymerized, the exfoliated LDH layers can be fixed in the PS matrix to form the exfoliated PS/LDH nanocomposites. The hexadecane also seems to play a key role in assisting the exfoliation of LDH layers, see Figure 20. 3.1.2. Direct Intercalation of Extended Polymer Chains. In addition to the intercalation of organic monomers followed by in situ polymerization, polymer molecules can also be directly intercalated into LDHs, either by solvent mixing or 4136


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directly to the Zn−Al−DDS suspension. After stirring for 3 min at 140 °C, the mixture was poured into 300 mL ethanol for rapid precipitation. The XRD and TEM results suggested that the exfoliation of LDH can be divided into solvent swelling and layer breaking processes and is affected by several reaction parameters. Completely exfoliated LDH layers can be achieved by decreasing the content of LDH, elongating the refluxing time, and rapid precipitation. In addition to xylene, cyclohexanone was also used to prepare exfoliated polymer/LDHs nanocomposites. Du et al. 129 synthesized exfoliated poly(propylene carbonate)/Mg−Al LDH (PPC/Mg−Al-LDH) by solution intercalation of PPC into the galleries of organic modified Mg−Al−LDH (OMg− Al−LDH) in cyclohexanone. In a typical preparation procedure, OMg−Al−LDH was first refluxed in cyclohexanone, and then PPC was added to an OMg−Al−LDH suspension with stirring. The simple mixture sample was obtained by dissolving the desired amount of PPC resin and 5% OMg−Al− LDH in chloroform. The solution was then poured out into ethanol. The precipitates, PPC/Mg−Al−LDH composites, were filtered and dried under vacuum. The TEM image in Figure 22 shows that the exfoliated Mg−Al nanosheets

simply refluxing the mixture of DDS modified Co−Al−LDH (OCo−Al−LDH) and PCL in cyclohexanone solution. Melt compounding is one of the most important and widely used approaches for the preparation of commercial polymers, and it allows the direct formation of nanocomposite during extrusion, without employing solvents. However, complete exfoliation is usually more difficult to achieve by melt compounding when compared to other techniques. Furthermore, the available literature suggests that LDH exfoliation is hard to achieve compared to phyllosilicates because of the high electrostatic stacking forces between layers and intercalated anions.131 However, both Zammarano et al.132 and Du et al.133 reported that organic modified LDHs can be exfoliated by melt mixing with polymers. Zammarano et al. 132 synthesized exfoliated polyamide 6/LDHs nanocomposites by melt processing with a twin screw microextruder at a variety of processing conditions. The anionic exchange capacity of LDH was varied in order to investigate its influence on the degree of exfoliation. It was found that exfoliated nanocomposites were successfully prepared by melt processing with a low exchange capacity LDH, whereas residue tactoids were observed with a high exchange capacity LDH. Shear, together with the exchange capacity, seems to be the key factor for the delamination in LDH/polyamide 6. No major change in the crystalline phase or in the rate of crystallization was observed in the nanocomposite as compared to the neat polymer. Du et al.133 demonstrated that the LLDPE/Mg−Al LDH exfoliated nanocomposites can be prepared via melt intercalation of LLDPE in the partly organic modified Mg−Al−LDH. This kind of exfoliation nanocomposite has excellent thermal stability. The abovementioned melt mixing is believed to be promising for the preparation of other exfoliated polymer/LDH nanocomposites, such as polypropylene, polystyrene, etc. 3.1.3. Pre-exfoliation Followed by Mixing with Polymer. With the development of the LDHs delamination/ exfoliation technologies (see section 2), several novel methods have been developed for the preparation of polymer/LDH nanocomposites from pre-exfoliated LDH nanosheets. The first report in the use of pre-exfoliated LDH for nanocomposites is by O’Leary et al.24 Mg−Al−DDS was delaminated in acrylate monomers under high speed stirring, and the subsequent polymerization of the suspensions gave the nanocomposite with the LDH still in the delaminated form. Later, Li et al. 134 synthesized PMMA/Mg−Al−LDH nanocomposite using the exfoliation−adsorption approach. Mg−Al−glycine LDH was first delaminated in formamide resulting in translucent colloidal solutions, to which an equal volume of acetone was added. This mixture was then mixed with a solution of PMMA in acetone under vigorously stirring. The final mixture solution was dried in the air-flowing oven at appropriate temperature. During the rapid evaporation of the organic solvents the polymer matrix impedes the reaggregation of the dispersed individually layers, forming exfoliated nanocomposite. Both XRD and TEM indicated that the lamellar of LDH dispersed individually in the polymer matrix.134 With the same method, they also synthesized poly(vinyl alcohol) (PVA)/Mg−Al−LDH nanocomposite. Typically, microcrystalline LDH−glycine was added formamide to give a translucent colloidal solution. An aqueous solution of PVA (10 g/L) was then added to the formamide colloid. This mixture is then cast as a film in the air-blast oven at 80 °C. Both the PMMA/LDH and PVA/LDH nanocomposites showed significant increased thermal stability compared to the pristine polymer.37,134

Figure 22. TEM image of PPC/Mg−Al−LDH nanocomposite sample.129 Reproduced with permission from reference 129. Copyright 2006 Elsevier Inc.

combined with DDS anions are dispersed disorderly in the PPC matrix. Some single LDH layers nearly vertical to the cutting section of the TEM specimen (pointed by arrows) were observed. The thickness and the lateral size of the exfoliated LDH layers are about 1 and 30−150 nm, respectively. In the same way, Peng et al.130 synthesized highly exfoliated polycaprolactone (PCL)/Co−Al−LDH nanocomposites by 4137


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Figure 23. Schematic description of assembling exfoliated LDH/CNT hybrids: (a) CNT formamide suspension, (b) LDH formamide suspension, and (c) mixture of CNT and LDH suspensions.135 Reproduced with permission from reference 135. Copyright 2010 American Chemical Society.

Figure 24. (A) Schematic illustration of the processes involved in the fabrication of the HMBS@LDH/EVOH composites. (B) TEM and (C) AFM images of the as-synthesized LDHs. The inset in panel C illustrates the height profile along the marked white line. 78 Reproduced with permission from reference 78. Copyright 2011 The Royal Society of Chemistry.

Recently, several new approaches have been reported. For example, Huang et al.135 reported a simple method for preparing exfoliated LDH/CNT hybrids through mixing positively charged LDHs and negatively charged CNTs. In their work, Co−Al−NO3 LDH was delaminated in formamide, forming a transparent colloidal suspension. Similarly, the CNT−COONa was also dispersed in formamide. The interactions between LDH and CNT were investigated by changing the initial volume ratio of LDH suspension to CNT solution. The sedimentation process of LDH suspension can be

easily observed from the color change. With the initial proportion of LDH nanosheets to the CNTs in the range from 1:1 to 1:3, a simultaneous sedimentation was observed, which is explained by the electrostatic force between positively charged LDHs nanosheets and the negatively charged CNTs (see Figure 23). Furthermore, the homogeneously exfoliated LDH/CNT hybrids thus obtained were used to prepare highperformance polyamide 6 nanocomposites. Zhao et al.78 report the partial exfoliation of LDHs in dimethyl sulfoxide (DMSO) as a route for the synthesis of 4138


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Figure 25. Illustration diagram for preparing the exfoliated polymer/LDH nanocomposite via pre-exfoliated organic LDH. 136 Reproduced with permission from reference 136. Copyright 2011 Elsevier Inc.

Figure 26. Low (a) and high (b) magnification SEM images of core−shell composites coated with 20 bilayers of LDH/PSS, SEM (c) and TEM (d) images of oxide shells after calcinations.149 Reproduced with permission from reference 149. Copyright 2006 The Royal Society of Chemistry.

TiO2 and ZnO), that is, the photocatalytic oxidation effect on the polymer matrix.78 A novel approach to prepare the exfoliated polymer/LDH nanocomposites, which involves the synthesis of a preexfoliated organic LDH first and the subsequent blending into an acrylic resin and irradiation by a medium pressure mercury lamp was proposed by Yuan et al.136 First, the LDH was intercalated by DDS using the coprecipitation method to expand the layer separation. A silane coupling agent, γ-(2,3epoxypropoxy)propyl trimethoxysilane (KH560) was then grafted onto the interlayer of LDH to supply the epoxy group. Finally trimethylolpropane thioglycolic acetate (TMPT) as a trithiol terminal A3 monomer was induced to the epoxidized LDH (LDH-EP) via the reaction of epoxy with thiol group, obtaining a thiol-end-capped LDH (LDH-SH) hybrid with the pre-exfoliated microstructure. Moreover, the

transparent polymer nanocomposites. After the exfoliation in DMSO, the intercalation characteristics of organic-LDHs were maintained. Therefore, the partially exfoliated LDH nanosheets in DMSO could be used as functional fillers for polymer nanocomposites. In their paper, organic UV absorber, 2hydroxy-4-methoxybenzophenone-5-sulfonic acid (HMBS), was chosen as a model functional anion to be incorporated into LDH to prepare a transparent ethylene-vinyl alcohol copolymer (EVOH) composite film (see Figure 24). The versatile nature of this system enables to overcome the disadvantage of low thermal stability to naked organic UV absorbers and improve the transparency of the final composite. The obtained composite film had a visible light transmittance of 90% (comparable to that of the pure matrix). In addition, as an organic/inorganic hybrid, the HMBS-intercalated LDH, does not bear the main drawback of inorganic UV absorbers (e.g., 4139


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by Zhang et al.152 In this work, the nonsteroid antiinflammatory drug diclofenac (DIC) was intercalated into Mg−Al LDH. Drug release from the magnetic nanohybrid in the absence of an applied magnetic field increased significantly compared to that of the DIC−LDH because of the much smaller particle size. Several similar works also proved that such core−shell structured LDHs are highly efficient for drug delivery.21 However, it should be noted that all these work did not take advantage of delaminated LDHs. Recently, Li et al. 29 demonstrated that it is also possible to coat the delaminated LDH nanosheets onto a magnetic core such as Fe3O4. The synthesis scheme is shown in Figure 27. Magnetic Fe 3O4

UV irradiation approach as a “clean, green and efficient” technology was introduced to prepare polymer/LDHnanocomposites. The synthesized pre-exfoliated LDH-SH hybrid was blended with an acrylic resin in the presence of 1hydroxycyclohexyl-phenyl ketone as a photoinitiator, and then exposed to a medium pressure mercury lamp, forming a completely exfoliated polymer/LDH nanocomposite (see Figure 25).136 3.2. Synthesis of Core/Shell Multifunctional Materials

Core/shell multifunctional materials are now firmly established as an important class of advanced solids. They offer superior performance in areas, such as catalysis,137,138 photovoltaics,139 microwave absorption,140 giant magnetoresistance and electrical conductivity,141−144 drug delivery,145 gene delivery,146 and environmental remediation.147,148 Because of the dominant layered structural feature, the synthesis of nanoshells containing LDH nanosheets involves an additional challenge, and it has not been realized until the success of the delamination of LDHs. The first core−shell material with the nanoshell of LDH nanosheets was reported by Li et al.149 Mg−Al−NO3 LDH was first delaminated in formamide to make a colloid solution with exfoliated LDH nanosheets. Polystyrene (PS) beads were dispersed in a formamide suspension containing LDH nanosheets, the suspension was then ultrasonically agitated to promote the adsorption of the LDH nanosheets onto the PS surface. The sample was recovered by centrifugation (6000 rpm, 30 min) washed with ultrapure water and then redispersed in an aqueous solution of PSS. Core−shell composites coated with multilayer shells of (PSS/LDH)20 were synthesized by repeating this procedure 20 times. Figure 26 shows the SEM images of the obtained core−shell composite. The homogeneous curvature of the spherical PS beads was preserved after the deposition of the LDH/PSS shell. The only noticeable difference between the spheres with or without shells was the surface roughness. The LDH nanosheets were scarcely perceived even in a high-magnification image (Figure 26b). By calcining at 480 °C, the PSS core was burnt away to leave a hollow mixed metal oxide (Figure 26c and d). Finally the calcined material was exposed to humid air (relative humidity of 95%) to recover the LDH structure. Because of their high composition flexibility and good biocompatibility, numerous LDH materials containing intercalated drugs and biomolecules have been prepared via anion exchange and coprecipitation methods, in the expectation for used in developing a new classes of drug delivery system. 150,151 This microreservoir-type system (drug−LDHs) provides clear advantages, such as high drug loading capacity and better biocompatibility, suggesting that these materials may have potential applications as the basis of a novel tunable drug delivery device. However, the use of bulk drug−LDH in drug delivery system especially in targeted drug delivery mechanisms as carriers has been profoundly restricted due to lack of special affinity toward the pathological sites.152 In the 1970s, a magnetic drug targeting approach for the selective delivery of chemo-therapeutic agents was proposed by Widder et al. 153 Magnetic drug targeting allows the concentration of the drugs at a defined target site with the aid of an external magnetic field, thus the released drug can react exclusively with the pathological sites to reduce the side effects and enhance the bioavailability of the drug.154,155 The first magnetic nanohybrid using a drug-intercalated LDH nanoshell supported on magnesium ferrite core was reported

Figure 27. Synthesis of the magnetic core/anionic functionalized LDH shell composite structure.29 Reproduced with permission from reference 29. Copyright 2009 John Wiley and Sons.

crystalline particles (∼400 nm) were coated with a layer of silica by a sol−gel process reported previously by Shi and coworkers (Figure 28a and b).156 Silica-coated magnetite core composites were dispersed in a formamide suspension containing LDH nanosheets and then ultrasonically agitated to promote the adsorption of LDH nanosheets onto the silica surface. The sample was recovered by centrifugation and washed with ultrapure water. In the next step, the sample was redispersed in an aqueous solution of Na2CO3. The product was recovered by centrifugation. Fe3O4/SiO2 cores coated with 20 layer pairs of carbonate and LDH nanosheets ((CO 32−/ LDH)20) were synthesized by repeating the above procedures 20 times. The obtained sample was heated to 480 °C for 4 h to remove CO32− and water. Finally, the calcined material was dispersed into aqueous solution to recover its original LDH structure and in the meantime absorb the functional anions into the LDH galleries. SEM image in Figure 28c shows that the spherical morphology of monodisperse Fe3O4 core/silica shell beads was preserved well after the deposition of the 20 layers of carbonate LDH shell, and there is no visible unwrapped core or separate irregular particles. The thickness of CO32−−LDH shell is estimated from Figure 28d to around 15 nm. The Fe3O4 core is first coated with a thin layer of silica, then further coated with a multilayer CO32−−LDH composite shell. Afterward, a calcination process is performed to remove CO 32−, and the shell become amorphous. Finally, the LDH structure can be restored and anions can be simultaneously intercalated in between the LDH galleries upon immersion in an aqueous solution containing the desired anions.29 4140


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Figure 28. SEM images of (a) Fe3O4 core, (b) Fe3O4 core covered with SiO2 layer, (c) Fe3O4/SiO2 core/CO32−−LDH shell composite, and (d) TEM images of Fe3O4/SiO2 core/carbonate LDH shell composite.29 Reproduced with permission from reference 29. Copyright 2009 John Wiley and Sons.

Figure 29. (a) AFM image of the first LDH nanosheet layer on a Si wafer precoated with PEI and PSS. (b) UV−vis absorption spectra of multilayer films of (PSS/LDH)n prepared on a quartz glass substrate precoated with PEI. 27 Reproduced with permission from reference 27. Copyright 2005 American Chemical Society.

3.3. Synthesis of Thin Films

from conventional LDH preparation methods (e.g., coprecipitation or hydrothermal) typically result in powdery translucent aggregates that do not adhere well to the substrate. To achieve continuous transparent LDH films, a general method for the quantitative preparation of colloidal LDH particles is needed. This would avoid both the loss of product and the extra steps of fractionating the LDH to obtain the desired particle size. In one case, films as thin as 100 nm were prepared from an LDH synthesized by coprecipitation, but the particles had to be prefractionated by centrifugation to select only the colloidalsized particles.163 Among different methods that have been investigated, the delamination of LDHs into nanosheets is regarded as the most efficient way to obtain the colloidal LDHs

Owing to their structural anisotropy, most layered materials can be cast as thin films, making them attractive for use as functional coatings and membranes.157−160 However, developments in this area have not been significantly realized for LDH compositions. For instance, although LDHs has been recognized as very suitable for use in sensor technologies and optical devices, 161,162 progress in these areas has been hampered by the difficulty in making thin and continuous LDH films. Methods of LDH preparation include the coprecipitation of the component cations and anions, the hydrothermal structuring of mixed metal oxides, and the induced hydrolysis of metal hydroxides. Efforts to prepare films 4141


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solutions, which are perfect precursor for the synthesis of thin films.72 Li et al.27 were the first to assemble Mg−Al LDH nanosheets layer-by-layer (LBL) with an anionic polymer, poly(sodium styrene 4-sulfonate) (PSS), onto the solid surface to produce ultrathin nanocomposite films. Mg−Al−NO 3 LDH was delaminated in formamide to obtain a colloid solution. The LBL preparation procedure is as follows: substrate primed with polyethylenimine (PEI) was first treated with a PSS aqueous solution for 20 min and washed with water, by which a monolayer of corresponding polyelectrolytes can be coated; the substrate was then dipped in a colloidal suspension of LDH nanosheets for 20 min and washed with water again. The series of deposition operations for the PSS and LDH nanosheets was repeated n times to produce multilayer films of (PSS/LDH)n. The resulting films were dried with a nitrogen gas flow. The surface topography of the first LDH nanosheet layer on a Si wafer precoated with PEI and PSS was investigated by AFM, as shown in Figure 29a, from which PEI and PSS themselves were not perceptible. The substrate surface was densely tiled with the LDH nanosheets with a lateral size ranging from hundreds of nanometers to several micrometers. Although there were some overlaps and uncovered gaps, the monolayer region was predominant. The overall coverage was determined to be 88%, and the overlapped percentage was 11%. The growth of PSS/LDH multilayer films under the optimized conditions was monitored by UV−vis absorption spectra (Figure 29b) measured immediately after each deposition cycle. An absorption band around 200 nm is diagnostic of PSS. The nearly linear increment of its absorbance indicates the successful LBL assembly of multilayer ultrathin films of LDH nanosheets and PSS. An absorbance of 0.34 at 193 nm was reached after the deposition of 10 bilayers, which is comparable to that for a multilayer film of (PSS/PDDA)10 (PDDA = polydiallydimethylammonium chloride),164 demonstrating that the LDH nanosheets are as effective as organic linkers serving as a positively charged nanomodule.27 Employing the same LBL method, a Co−Al LDH/PSS composite thin film was also prepared on a quartz glass substrate.62 The magneto-optical effects of the multilayer films of Co−Al LDH nanosheets were investigated with magnetic circular dichroism (MCD) spectroscopy. The room temperature MCD spectra of the as-assembled multilayer films are shown in Figure 30, which shows that these multilayer films with larger stacking layer numbers (n > 10) exhibited notable positive magneto-optical signals at wavelengths of >300 nm. The magnetic field dependence of the MCD signal clearly showed the ferromagnetic behavior of the nanocomposite films at room temperature. As the PSS does not show any prominent absorption in this spectral range, the observed MCD feature was attributed to Co−Al LDH nanosheets. The ferromagnetic effect of the LDH layers can be ascribed to the spin−orbit coupling in Co2+ in octahedral geometry. As indicated in Figure 30, three positive peaks are commonly observed in all the spectra. The MCD signal at 390 nm was attributed to the charge transition of O(2p) to Co(π*), and the absorptions at 470 and 615 nm fall close to the d-d* transition (A2g → T2g and A2g → T1g, respectively) of cobalt. Another notable feature of these spectra was that the MCD signal was significantly enhanced with the increase of layer stacking in the first 30 layers. The dramatic increment of MCD signal with increasing number of LDH layers indicates strong interlayer couplings between the electronically isolated LDH layers. It was also

Figure 30. MCD spectra for the multilayer films of (LDH/PSS)n (n = 10, 15, 20, 30, 40, and 50) measured in 10 kOe at 27 °C.62 Reproduced with permission from reference 62. Copyright 2006 American Chemical Society.

noted that the MCD signal reached the maximum at a layer number of 30 and further increasing the layer stacking led to a reduced response. Although further understanding of the thickness dependence awaits detailed experimental and theoretical studies, these results should stimulate extensive research on Co−Al LDH nanosheets as an important testing ground for 2D ferromagnetism.62 In the recent years, there have been rising research interests in the fabrication of thin films from LDH nanosheets by the LBL method, and such fabricated thin films have been used in many fields such as luminescence,165−172 electrocatalysis,173,174 fluorescence chemosensors,175 bioanalysis and biodetection,176 biomimetic materials,177 antireflection coatings,178 and inorganic sandwich-layered materials.179,180 For the preparation of luminescent materials, the general idea is to assemble the luminescent organic anions with the LDH nanosheets by the LBL method. The LDH-based ultrathin films with individual colors including red, green, and blue have been reported. 165−167,171,172 For example, by assembling the sulfonated polythiophene (SPT) and Mg−Al LDH nanosheets LBL, a red luminescent thin film can be fabricated. Figure 31 shows the UV−visible absorption spectra and the photographs after each bilayer cycle. The intensity of the absorption band at ∼437 nm (the π−π* transition of SPT) correlates linearly with n (Figure 31, inset), indicating a stepwise and regular film growth procedure. The color intensity of the thin film gradually increased with the increase if the bilayer numbers. By combining the possibilities of electrostatic assembly and the three primary color principle, Yan et al.168 further demonstrated the possibility of fabricating multicolor luminescent ultrathin film systems, which have great potential for use in optoelectronics, solid-state light-emitting materials, and optical devices. The rational combination of luminescent components affords precise control of the emission wavelengths and intensity, and multicolored luminescent ultrathin films can be precisely tailored covering most of the visible spectral region. These ultrathin films also exhibit well-defined multicolor polarized fluorescence with high polarization anisotropy, and the emissive color changes with polarization direction. By choosing the proper luminescent organic anions, reversible thermochromic and piezochromic luminescent thin films have also been fabricated.169 Yan et al.169 presented a 4142


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Figure 31. UV−visible absorption spectra of the (SPT/LDH)n (n = 4 - 32) ultrathin films, the inset shows the absorbance at 437 nm as a function of the number of bilayers (n), and photographs of ultrathin films with different numbers of bilayers when exposed to daylight. 166 Reproduced with permission from reference 166. Copyright 2011 John Wiley and Sons.

Figure 32. Assembly process for the (BSB/LDH)n ultrathin film. (a) Chemical structure and schematic representation of BSB. (b) Schematic illustrations of one Mg−Al LDH monolayer. (c) Quartz glass substrate. 169 Reproduced with permission from reference 169. Copyright 2011 John Wiley and Sons.

of the luminescence at two different wavelengths can be reversibly transformed by varying the temperature, thus altering the luminescent color of the film.169 By introducing (2,2′-(1,2ethenediyl)bis[5-[[4-(diethylamino)-6-[(2,5disulfophenyl)amino]-1,3,5-triazin-2-yl]amino]benzene sulfonate anion (BTZB), piezochromic luminescent has been synthesized. 170 Unlike the pristine BTZB, which shows no piezochromic luminescence, the BTZB/LDH system exhibited a sensitive piezochromic luminescence response and reversible changes in fluorescence, optical absorption spectra, and structure in the pressure range from 0.1 mPa to 18.8 GPa, see Figure 34 The reversible piezochromic luminescence of BTZB/LDHs suggests they have potential applications in luminescent sensors and switches. Theoretical calculations demonstrate that the piezochromic luminescence properties of BTZB/LDHs are related to the changes in packing mode, relative orientation,

supramolecular ultrathin film system with thermochromic luminescence based on the LBL assembly of anionic bis(2sulfonatostyryl) biphenyl (BSB) and positively charged Mg−Al LDH nanosheets, see Figure 32. In the temperature range 20− 100 °C, the ultrathin film exhibited fast luminescence response and reversible transformations, including color, fluorescence lifetime, and anisotropy, see Figure 33. The thermochromic luminescence behavior of the assembled BSB anions, which is absent for their pristine form, originates from the host−guest interactions within the ultrathin film system. It has been demonstrated that thermochromic luminescence process of the BSB/LDH system is related to the changes in the orientation and aggregation of BSB anions between LDH monolayers. Moreover, coassembly of BSB with other luminescent anions into a ultrathin film allows fabrication of responsive thermochromic luminescence film systems, in which the ratio 4143


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Figure 33. Fluorescence spectra of the (BSB/LDH)8 ultrathin film with thermochromic luminescence at 20 and 100 °C. Inset: The reversible fluorescence response over five consecutive cycles, and photographs of the ultrathin film before and after heating. 169 Reproduced with permission from reference 169. Copyright 2011 John Wiley and Sons.

Figure 35. LBL assembly process of the (LDH/AuNPs)n ultrathin film.173 Reproduced with permission from reference 173. Copyright 2011 The Royal Society of Chemistry.

anticipated that the method in this work can be used to immobilize other metal or metal oxide nanoparticles within a 2D inorganic matrix for the fabrication of nanoscale sensors and electronic devices.173 With the same method, a novel enzymefree electrochemical sensor was fabricated based on ultrathin films composed of Co−Al nanosheets and Naphthol green B (NGB) on a glass carbon electrode. Herein, the delaminated Co−Al LDH nanosheets function as building blocks, as well as electroactive substance. NGB, which is often used as a mediator in electrocatalysis serves as a negatively charged component. 174 The (NGB/LDH)6 ultrathin films modified electrode demonstrated a couple of well-defined reversible redox processes attributed to the electron transfer between Co−Al LDH nanosheets and GCE. Furthermore, the ultrathin film modified electrode exhibits excellent electrocatalytic activity toward H2O2 with a linear response as the concentration of H2O2 ranges from 8 × 10−6 to 1.8 × 10−4 M. Thanks to its long-term stability and excellent anti-interference performance, (NGB/ LDH)n ultrathin films are also promising in electrochemical sensor applications. The same group further demonstrated that a fluorescence chemosensor can be fabricated from delaminated LDH nanosheets. Shi et al.175 fabricated fluorescence indicator/ LDH ultrathin films by alternate assembly of 1,3,6,8pyrenetetrasulfonic acid tetrasodium salt (PTS) and Zn−Al LDH nanosheets on quartz substrates using the LBL deposition technique, and demonstrated their application as a fluorescence

and configuration of the intercalated chromophores on compression.170 Zhao et al.173 fabricated LDH nanosheets/Au nanoparticles (AuNPs) ultrathin films via the LBL assembly technique, and demonstrated its excellent electrocatalytic performance for the oxidation of glucose. The fabrication procedure is shown in Figure 35. Fluorine-doped tin oxide (FTO) substrates were cleaned in turn in an ultrasonic bath containing soapy water, deionized water, acetone, ethanol and deionized water for 10 min each, and then immersed in saturated NaOH aqueous solution for 20 min. After these procedures, the LBL assembly was performed by sequential dipping of FTO substrates in positively charged LDH nanosheets and negatively charged AuNPs colloidal suspension for 10 min each, followed by thorough rinsing with deionized water. The resulting films were dried in a vacuum oven at ambient temperature. The structural and morphological studies indicate that the (LDH/AuNPs) n ultrathin films exhibit long-range stacking order, in which the AuNPs are highly dispersed and immobilized with a monolayer arrangement in the LDH gallery. The resulting (LDH/AuNPs) n ultrathin films display improved electron transfer kinetics and excellent electrocatalytic activity toward glucose. Comparison studies demonstrate that the plasmon coupling of adjacent AuNPs layers in the (LDH/AuNPs)n ultrathin films plays a vital role in facilitating electron transfer across the multilayers. It is

Figure 34. Reversible piezochromic luminescence of BTZB/Mg−Al LDH at two typical pressures (0.1 mPa and 18.8 GPa). (a) Changes in fluorescence spectra over two cycles, (b) UV/vis spectra over two cycles (inset: color changes over two cycles). 170 Reproduced with permission from reference 170. Copyright 2011 John Wiley and Sons. 4144


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Figure 36. Mechanism of measurement-regeneration cycle of the (PTS/LDH)n ultrathin films.175 Reproduced with permission from reference 175. Copyright 2011 The Royal Society of Chemistry.

chemosensor for Cu2+. The fluorescence chemosensor with film thickness of 48 nm (24 bilayers) exhibited a broad linear response range for Cu2+ solution (0.6−50 mM), good repeatability, high photostability and storage stability (93.2% of its initial fluorescence intensity remains after one month) as well as excellent selectivity. The process of fluorescence quenching and regeneration for the fluorescence chemosensor is schematically shown in Figure 36. The fluorescence quenching results from the complexation of Cu2+ and PTS in the ultrathin film due to a high thermodynamic affinity of Cu 2+ for typical O-chelate ligands and fast metal-to-ligand binding kinetics; while the regeneration of the quenched ultrathin films originates from the complexation of Cu2+ and EDTA due to the much larger complex constant between EDTA and Cu2+ (log K ≈ 9) than pyrene and Cu2+ (log K ≈ 5).173 In a very recent report, Han et al.176 explored the application of the ultrathin films in bioanalysis and biodetection. Ultrathin films containing exfoliated Mg−Al LDH nanosheets and cobalt(II) phthalocyanine tetrasulfonate (CoPcTs) were assembled using the LBL technique on idium−tin−oxide (ITO) electrode substrate. The (LDH/CoPcTs) n ultrathin film modified ITO electrode exhibits significant electrocatalytic performance for the oxidation of dopamine, which is related to the Co(II)/Co(III) couple in the film. The dopamine biosensor shows rather high sensitivity, low detection limit, and excellent anti-interference properties in the presence of ascorbic acid. Furthermore, compared with pristine organic multilayer (poly(dimethyldiallylammonium chloride)/CoPcTs)n modified electrodes, the (LDH/CoPcTs)n electrodes showed superior reproducibility and long-term stability due to the immobilization and dispersion of electroactive CoPcTs molecules by LDH nanosheets. This work suggests a novel approach to immobilize electroactive species into an inorganic layered matrix with nanoscale level control, for the purpose of technological applications in electroanalysis and biosensors.176

precipitation and drying, restacking was observed (see Figure 37). In the dried sample, the iron porphyrin anions are strongly

Figure 37. Schematic representation for the process of exfoliation and restacking of the LDH single layers in the presence of FeTDFSPP. 57 Reproduced with permission from ref 57. Copyright 2003 Elsevier Inc.

bound to the layers, being adsorbed at the surfaces of the crystals instead of being intercalated. The way the charges are distributed in the iron porphyrin molecule is the key factor for this behavior. The obtained crystals are of submicrometer dimensions.57 Later, they further proved that the catalyst Fe(TDFSPP) supported on Mg−Al LDH nanosheets shows a superior catalytic activity in the oxidation of cyclooctene and cyclohexane using iodosylbenzene as oxidant, comparing to the results obtained under homogeneous conditions. The good catalytic activity was explained by the easy access of the substrate to the catalytic centers that were located in the interface between the solid and solution. This access is expected to be easier than that of the intercalated iron porphyrin systems, where the substrate and the oxidant must diffuse between the layers to react with the active center of the catalyst. The results suggest that a small modification to the iron porphyrin structure may drastically modify the catalyst efficiency in certain cases. Since the procedure is an easy and efficient way of immobilizing anionicmolecules of catalysts, it

3.4. Synthesis of Catalysts

The first catalyst that was synthesized using delaminated LDH nanosheets was the Mg−Al single layers immobilized iron porphyrin.57 Glycine intercalated Mg−Al LDH was first delaminated in formamide, for the immobilization of the iron porphyrin, a green solution of FeTDFSPP was added dropwise to the LDH−glycine suspension. After reaction mixture was left to rest for 4 days, a brown solid was obtained. The solid was washed with deionized water, centrifuged until the washing solution was colorless, and dried in a desiccator. During 4145


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Figure 38. α-Amino acids used as ligands attached to the brucite-like layers. (1) L-glutamic acid, (2) L-alanine, and (3) L-serine in pristine (homogeneous) state are labeled a; their anions (green) in intercalated (heterogeneous) or delaminated (colloidal) states are labeled as b and c, respectively.184 Reproduced with permission from reference 184. Copyright 2011 John Wiley and Sons.

probably can be adapted for other molecules with catalytic activities.181 On the basis of the exfoliation and possibility of grafting the LDH single layers to produce alternative supports for immobilization of catalysts, Wypych et al.182 later developed another process for the preparation of immobilized catalysts. In this process, Mg−Al−DDS was first exfoliated into nanosheets in toluene, surface functionalized with (3-aminopropyl)triethoxysilane (3APTS) and then treated with iron(III) porphyrins. Both anionic iron(III) porphyrin (Fe(TDCSPP)) and neutral iron(III) porphyrin (Fe(TPFPP)) were studied in this work. The catalytic activity of the both iron porphyrinsupported catalysts, (Fe(TDCSPP)/LDH-3APTS and Fe(TPFPP)/LDH-3APTS), were investigated for their activity on the oxidation of weakly reactive alkanes, such as cyclohexane. It was found that their catalytic activity favored the oxidation of cyclohexane, this behavior was attributed to the high residence time of the substrate inside of the pores of the randomly stacked LDH single layers.182 Liu et al.183 reported that the delaminated Pd-containing LDH nanosheet colloidal dispersion can work as catalyst for Heck reactions. Owing to the largely enhanced accessibility for reactant molecules resulting from the nature of high inner surface area of LDHs, these palladium-bearing nanosheets showed excellent efficiency in Heck reactions for a wide range of substrates. The presence of Mg, Al, and OH− ions in the LDH-lamellae favors the control of the size of the in situ formed Pd(0) species. The implantation of the formed Pd(0) in the LDH layer matrixes prevents the formation of Pd-black. Moreover, the Pd sites and basic sites on the brucite-like nanosheets are combined at a molecular level and interact with each other closely. The basic sites on the LDH monolayers might function as basic ligands. By modifying Zn−Al LDH nanosheets with α -amino acids as ligands, a novel efficient chiral catalyst has been synthesized.184 Figure 38 schematically illustrates the synthesis procedure of the catalyst. The α-amino acids (Figure 38a) were first intercalated into the interlayer regions of LDHs as anions to produce the heterogeneous ligands (Figure 38b). The positively charged brucite-like layer interacts with the α-amino acid anions through electrostatic attraction with the carboxylate groups. The LDHs intercalated with α-amino acid anions were then delaminated in formamide16b or water to produce

transparent or translucent colloidal ligands (Figure 38c). It has been demonstrated that an impressive enhancement of enantioselectivity of the vanadium-catalyzed epoxidation of allylic alcohols can be achieved by attaching α-amino acid anions as ligands to LDH nanosheets. Thanks to the steric synergies of rigid inorganic layers, remarkable enhancement of chiral induction has been achieved. The huge inorganic layers can make a stable and rigid environment around the chiral center, and thus have significant impact upon the enantiomeric selectivity by restricting or directing the access trajectory of reactant molecules. The delamination of nanosheets allows the catalytic reactions to be carried out under pseudohomogeneous reaction conditions, thereby significantly increasing the reaction rate while preserving the enhancement of the enantioselectivity. With water as the solvent, the colloidal catalyst can be directly separated from the products by simple liquid/liquid separation. The catalysts can be therefore easily recycled without loss of catalytic activity and enantioselectivity.184 The delaminated LDH nanosheets have also been used in the synthesis of photocatalysts.185 Gunjakar et al.185 synthesized mesoporous layer-by-layer ordered nanohybrids by selfassembly between oppositely charged 2D nanosheets of Zn− Cr LDH and layered titanate. The obtained heterolayered nanohybrids show a strong absorption of visible light and a remarkably depressed photoluminescence signal, indicating an effective electronic coupling between the two component nanosheets. The self-assembly between 2D inorganic nanosheets leads to the formation of highly porous stacking structure, whose porosity is controllable by changing the ratio of layered titanate/Zn−Cr LDH. The resultant heterolayered nanohybrids are fairly active for visible light-induced O 2 generation with a rate of ∼1.18 mmol h−1 g−1, which is higher than that of the pristine Zn−Cr LDH material (∼0.67 mmol h−1 g−1). 3.5. Synthesis of Electrode Materials

Because of the rapid depletion of fossil fuels and severe environmental pollution, energy has become one of the most important topics in the 21st century. It requires not only renewable and clean energy sources but also more advanced energy storage and management devices, such as supercapacitors, lithium ion battery, and solar energy conversion. 186 For all the above-mentioned energy storage devices, the electrodes are always the most important components that 4146


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Wang et al.212 reported an innovative strategy of fabricating electrode materials by layered assembling two kinds of singleatom-thick sheets, carboxylated graphene oxide (GO) and Co− Al LDH nanosheet for the application as a pseudocapacitor. The LDH/GO composite was synthesized by mixing Co−Al LDH nanosheet dispersed in formamide and GO dispersed in water, Figure 39a. Precipitation was formed immediately once

determine the total performance. Therefore, one efficient route for improving the performance is to synthesize new electrode materials with special structures, such as mesoporous materials,187−189 nanorods or nanowires,190−192 and nanosheets.193−195 Nanosheets can, in fact, not only increase the specific area of materials but also decrease remarkably the diffusion distance for ions from the bulk electrolyte to the surface of active material owing to their very thin thickness in the scale of nanometer. In this section, the fabrication of electrode materials from delaminated LDHs nanosheets for the applications in (1) supercapacitor, (2) lithium ion battery, and (3) dye-sensitized solar cell (DSC) will be summarized. 3.5.1. Application in Supercapacitors. The first thin film electrode fabricated from delaminated LDH nanosheets was reported by Wang et al.196 in 2007. A highly oriented and densely packed thin-film was obtained by drying a transparent colloidal suspension of Co−Al LDHs nanosheets (delaminated in formamide) on a pretreated ITO-coated glass plate substrate. Electrochemical investigations showed that this thin-film electrode has good supercapacitor behavior with a high specific capacitance of up to 2000 F/cm 3 (667 F/g), a good electrochemical stability, and a high-rate capability. This good electrochemical behavior was attributed to its special microstructure of the thin-film electrode. Co−Al LDHs nanosheets have open frameworks and highly exposed electrochemically active Co sites, which can result in a massive utilization of electrochemical active materials and give a high specific capacitance. The edge-to-edge interactions can produce a continuous highly oriented thin film, which can decrease the interfacial resistance to improve rate capability of the electrode. Moreover, the face-to-face interactions can yield a densely packed structure, which can provide a good electrochemical stability of the thin-film electrode during the charge/discharge process.196,197 A more comprehensive investigation proved that the partial isomorphous substitution of Co2+ by Al3+ is the key factor in the improvement of the electrochemical behavior, and Co0.75Al0.25−LDH thin-film electrode shows the best performance (specific capacitance of 2500 F/cm3 (833 F/g). Increasing the thickness of the thin film from 100 to 500 nm has no significant effect on the specific capacitance of the thin-film electrode.198 Using a similar method, a Zn−Co LDH thin film electrode for supercapacitor application has also been fabricated.199 In addition to LDH-based electrode materials, there are some other promising materials for supercapacitor including carbonbased materials (activated carbons, carbon aerogels, carbon nanotubes, carbon fabrics, and reduced graphene oxide),200−203 transition-metal oxides and hydroxides (RuO2, MnO2, Co3O4, NiO, Ni(OH)2),204−206 and conducting polymers (polyaniline, polypyrrole, and polythiophene).207−209 Each material has its unique advantages and disadvantages for supercapacitor applications. For example, carbon material has high power density and long life cycle, but the small double-layer capacitance limits its application. Transition-metal oxides, hydroxides, and conducting polymers have been widely investigated because of their relatively higher capacitance and fast redox kinetics, while the relatively low mechanical stability and cycle life are the major limitations.210 Therefore, to fabricate better electrodes, carbon materials/LDHs composites have been explored, by which the unique advantages of these capacitive materials can be coupled, such as multiwall carbon nanotube/LDH,211 graphene oxide/LDH.210,212

Figure 39. (a) Schematic formation and structure of Co−Al LDH nanosheet/GO composite. (b) Digital photographs of (left) an aqueous dispersion of Co−Al LDH nanosheet, (middle) an aqueous dispersion of GO, and (right) a mixture of Co−Al LDH nanosheet and GO.212 Reproduced with permission from reference 212. Copyright 2011 The Royal Society of Chemistry.

the Co−Al LDH nanosheet dispersion was added to the GO dispersion, Figure 39b. The Zeta potentials of the two dispersions are 43 mV for Co−Al LDH nanosheet and 50 mV for GO, respectively. Both of the two dispersions are stable and well-dispersed and the two dispersions are matched well to assemble in a layered structure. This composite electrode exhibited excellent performance for energy storage, with high specific capacitance and long cycle life. The specific capacitance was calculated to be 1031, 854, 483, 250 F/g based on the total mass of the composite, respectively, corresponding to discharge current densities of 1, 2, 8, 20 A/g. The average capacitance, calculated from the cyclic voltammograms (CV) was 778, 536, 274, 190 F/g, respectively, corresponding to scan rates of 5, 10, 30, 50 mV in CV measurement. The high specific capacitance of the composite is due to the single atomic layered structure of Co−Al LDH nanosheets. All Co atoms occupy the surface of sheet and thus have an opportunity to contribute to redox reaction. In addition, the face-to-face assembly of GO and Co− Al LDH nanosheets optimizes their contact area, which is advantageous to efficient electron transport. The galvanostatic charge and discharge curves showed that there was no obvious decrease of specific capacitance at a current density of 20 A/g after more than 6000 cycles. 3.5.2. Application in Lithium Ion Batteries. Synthesis of electrode materials for the application of lithium ion battery from LDHs has been reported since 2006, for instance, pure NiFe2O4 spinel from Ni−Fe2+−Fe3+−LDHs,194 ZnO/ZnAl2O4 porous nanosheet film193 and carbon coated Co−Fe mixed oxide195 from vertically aligned LDH on film substrates. However, all the above electrodes were synthesized from bulk 4147


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LDHs. The first electrode synthesized from delaminaed LDH nanosheets was reported by Latorre-Sanchez et al.213 The preparation of the hybrid material started with persistent aqueous colloidal solutions of GO and Ni−Mn LDH nanosheets. When the aqueous dispersion of GO sheets is mixed with the one containing Ni−Mn LDH nanosheets, an instantaneous precipitation of a solid took place, Figure 40.

overcome some drawbacks of the organic dyes such as their relatively low heat stability and narrow absorption bands. One possible approach to improving the inherent light-harvesting ability of the organic dyes is to hybridize them with nanosized multifunctional inorganic materials, such as LDHs, which can provide a stable chemical environment, higher heat or photostability, and are environmentally friendly.219 Lee et al.219 reported a new hybrid LDH/organic nanosheet used as a light sensitizer in photovoltaic devices, in which the anthraquinone sulfonate (AQS) anion is selected as the organic sensitizer and the LDH nanosheets as the inorganic host. The LDH−AQS was exfoliated in formamide, resulting in a transparent and stable LDH nanosheet suspension, see Figure 41. Because of the high pH environment of the reactive hydroxide surfaces of the LDH framework, the photoexcited AQS− or AQS2− can be stabilized.220 The LDH−AQS nanosheets were deposited on a nanoporous TiO2 electrode by using a self-organization process with the solution of the exfoliated LDH−AQS (1 mg/mL) in formamide, then rinsed with ethanol and dried at 70 °C. Figure 42a and b clearly reveal that a continuous 2 nm thick layer of LDH-AQS nanosheets were deposited on the surface of TiO2 spheres. The elemental mappings show a homogeneous distribution of Al and Mg, indicating that the LDH nanosheets covered almost the entire surface of the TiO2 spheres. This hybrid sensitized cell showed improved conversion efficiency up to 160% of the initial value compared with the AQS-sensitized cell. This functional LDHbased sensitizer might provide a new platform for the development of light-harvesting sensitizers because of the variable compositions of the LDHs.

Figure 40. Idealized illustration of the preparation and structure of the hybrid GO/Ni−Mn LDH. The scheme remarks size of the sheets and the large particle surface of GO on which the smaller Ni−Mn LDH nanosheets are supported. (B) Photographs of the aqueous dispersions of GO (left), Ni−Mn LDH (middle) and the hybrid GO/Ni−Mn LDH formed immediately after mixing the previous components. 213 Reproduced with permission from reference 213. Copyright 2012 Elsevier Inc.

After the synthesis, the initial hybrid GO/Ni−Mn LDH was calcined at 450 °C in Ar atmosphere in order to convert the GO into graphene and LDH nanosheets into the corresponding mixed oxide nanoparticles. The resulting graphene/Ni−Mn hybrid material also exhibits room temperature superparamagnetism and can act as anode for Li-ion batteries. A capacity value as high as 400 mAh/g was still obtained after 10 charge/ discharge cycles at a rate of 50 mA/g. 3.5.3. Application in Dye-Sensitized Solar Cells. Dyesensitized solar cells (DSCs) have been extensively investigated for solar energy conversion by using various combinations of inorganic semiconductors and organic sensitizers because of their low cost, easy production, and high efficiency.214,215 Recently, inorganic semiconducting materials, such as quantum dots (CdSe,216 CdS,217) and organometal perovskites,218 have been proposed as inorganic sensitizers in photovoltaic cells to

3.6. Synthesis of Hybrid Magnets

The family of bimetallic oxalate-based 2D magnets has been shown to be a very versatile class of magnetic hybrid materials. They are formed by an anionic layer and cations that occupy the interlayer space and define the interlayer separation.221 This type of networks can be formed with many different types of cations. While maintaining the overall structure, a myriad of magnetic and transport properties has been observed for this type of material; from ferromagnetic to paramagnetic, and from metallic to semiconducting to insulating can be tuned by

Figure 41. Chemical structures of the LDH_AQS nanosheets, photographs of the LDH_AQS powder, and the Tyndall light scattering and lightinduced coloration of the exfoliated LDH_AQS nanosheets suspension. 219 Reproduced with permission from reference 219. Copyright 2010 John Wiley and Sons. 4148


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Figure 42. (a) SEM image, (b) HRTEM image, and (c) (e) elemental maps for the LDH_AQS nanosheets self-organized on a TiO 2 sphere. The SEM image was obtained from a TiO2 electrode on FTO glass.219 Reproduced with permission from reference 219. Copyright 2010 John Wiley and Sons.

Figure 43. Schematic representation of the synthesis of the LDH-assisted growth of an extended Mn II−CrIII oxalate-bridged 2D magnet through the exfoliation/restacking approach.224 Reproduced with permission from reference 224. Copyright 2010 The Royal Society of Chemistry.

selecting or chemically designing the right building block.222 The highly tunable intralayer composition coupled with the wide possible choice of anionic organic moieties makes LDHs as a promising building block for hybrid materials. The first hybrid magnet material fabricated using LDHs was reported by Coronado et al.222 They demonstrated that polynuclear bimetallic oxalato complexes can be intercalated into a diamagnetic MgII−AlIII LDH. The insertion of oxalate-bridged Mn-ox-Cr complexes was achieved by ion exchange with Mg− Al−NO3 LDH. However, because the charge density of the [MIIMIII(ox)3]− anionic layer (−0.013 q·A−2) is significantly smaller than that of the LDH cationic layers (+0.028 q·A −2), the polymerization of these complexes in the interlamellar space is not favored. To overcome this problem, Coronado et al.223 focused on anionic 2D lattices based on the cyanide ligand (CN−), whose smaller size in comparison with the oxalate improved the fit between the density charge imposed by the positively charged LDH layers (−0.024 compared to +0.028

q·A−2). The use of a cationic LDH hosts templates the formation of an anionic 2D cyanide-based bimetallic molecular ferromagnet. The confined interlamellar space offered by the LDH, prevents the formation of the thermodynamically favored 3D system. Although ion-exchange reactions have been proved to be effective to the organized assembly of functional molecules into the interlamellar space of LDH host, the design of more complex hybrid materials through this synthetic route remains limited by several experimental conditions, such as long reaction times (since the guest substitution step is limited by diffusion processes) and the chemical stability, size or charge of the molecular guest. Recently, Coronado et al. 224 have developed an exfoliation/restacking approach to drive the growth of an extended MnII−CrIII oxalate-bridged 2D magnet into LDH interlamellar space. The positively charged nanosheets, resulting from the delamination of LDH, have been employed as macrocations to direct the self-assembly of 4149


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Figure 44. Schematic representation of the layered components and the restacked material. (a) View of the [TaS 2]0.33− superconducting layer (Ta, blue spheres; S, yellow spheres). (b) View of the [Ni 0.66Al0.33(OH)2]0.33+ magnetic layer (Ni, gray spheres; Al, white spheres; O, red spheres). (c) Representation of the restacked material along the c-axis showing the alternating superconducting/magnetic layers. 225 Reproduced with permission from reference 225. Copyright 2010 Nature Publishing Group.

Figure 45. Top: Electrostatic surface potentials for (a) PPL (pI = 5.0) at pH 7.5, (b) Hb(pI = 6.8) at pH 8.5, and (c) BSA(pI = 4.8) at pH 7.4 produced by GRASP 2 (licensed by Dr. Honig’s Lab from the Department of Biochemistry and Molecular Biophysics of Columbia University). Regions of positive charge are shown in blue and those of negative charge are shown in red. Bottom: schematic representations showing the possible orientations of negatively charged (a) PPL, (b) Hb, and (c) BSA on the positively charged two-dimensional LDH surface. 233 Reproduced with permission from reference 233. Copyright 2009 American Chemical Society.

oxalate-based low-dimensional anionic complexes in solution, see Figure 43. After adding the exfoliated LDH sheets into a freshly prepared aqueous solution of the soluble oxalate-based magnet [K(18-crown-6)]3[Mn3(H2O)4{Cr(C2O4)3}3], the LDH cationic nanosheets, will direct these anionic moieties to self-assemble in solution resulting in the precipitation of insoluble coordination polymers. This material exhibits ferrimagnetic ordering below 3 K that arises from dominant antiferromagnetic interactions between metallic centers through the oxalate linkers. This result opens the door for the design of a completely new sort of hybrid magnetic multilayers that combine molecule-based magnets and layered inorganic flexible hosts. In a similar way, a novel hybrid material that exhibits the coexistence of superconductivity and magnetism was fabricated by interleaving magnetic LDH cation layers with superconducting layers of [TaS2]0.33− anions.225−227 The hybrid material was synthesized by mixing two freshly prepared colloidal dispersions of [TaS2]0.33− and [MII0.66MIII0.33(OH)2]0.33+ nanosheets under continuous mechanical shaking in an inert atmosphere. The shiny black solids were left to stand at room temperature and filtered under vacuum. The superconducting component, NaxTaS2 has a superconductoring transition at 4.5 K. These layers can be

exfoliated in solvents with high dielectric strength, giving rise to colloidal suspensions of anionic [TaS2]0.33− nanosheets (Figure 44a). For the magnetic component, LDH nanosheets such as [Ni0.66Al0.33(OH)2](NO3)0.33 is chosen due to the chemical, structural and magnetic reasons (Figure 44b). The magnetic properties of this lamellar system can be tuned by changing the cations or the MIII/MII ratio.227 There is a perfect matching in the charge density of the two components (−0.022 e·Å−2 and +0.021 e·Å−2 for the [TaS2]0.33− and [MII0.66MIII0.33(OH)2]0.33+, respectively), and the system can then self-assemble to form the composite 1:1 layered material to achieve local electroneutrality (Figure 44c). The results indicated that this new hybrid material [Ni0.66Al0.33(OH)2][TaS2] possesses the coexistence of superconductivity and magnetism at ∼4 K. The method is further demonstrated in the isostructural [Ni0.66Fe0.33(OH)2][TaS2], in which the magnetic ordering is shifted from 4 to 16 K. Finally, it was remarked that this versatile approach is promising in the preparation of multifunctional superconductors by combining superconducting inorganic layers and functional molecules. It also opens the way to incorporate the unique properties of molecules (molecular bistability, molecular switching, single-molecule magnetism, and so on) into twodimensional host superconducting matrices. 4150


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3.7. Synthesis of Bioinorganic Hybrid Materials

employing a reverse microemulsion approach. Proteins can be then assembled onto the surface of LDHs, resulting in the formation of protein/LDH hybrid materials. In addition to the applications already discussed above, delaminated LDH nanosheets have also been used in some other promising areas.237−239 For example, starting from preformed monodispersed CdSe nanoparticles (4 nm) and delaminated LDH nanosheets, LDH-CdSe quantum dot composites were prepared through the restacking of LDH layers in the presence of CdSe nanoparticles in 1-butanol. Because of the interaction of the quantum dots with the LDH layers, the composites exhibit a blue shift in the CdSe absorption spectrum.237 By utilizing the osmotic swelling of LDH in formamide and the memory effect of LDH nanosheets, a useful technique to prepare regularly stacked LDHs with large bio- or organic molecules as guests in the interlayer has been developed.238 By mixing and restacking of delaminated LDH nanosheets, special nanostructured materials have been reported.239 With the rapid development of the delamination techniques, some novel applications of the delaminated LDH nanosheets are still emerging.

Bioinorganic hybrid materials represent a new generation of materials at the interface of biology and material science, which is able to display functionalities as complex as that of natural systems, such as drug vectorization and delivery, molecular machinery functions or sensing properties.228 The use of biocompatible inorganic host matrices for the immobilization of biological agents may not only play a protective role for the intercalated biomolecules or for cell therapy but these composites may also add their own intrinsic properties (electronic, optic, magnetic, redox or acid−base properties) leading to novel multifunctional bioinorganic materials. This novel generation of hybrid bioinorganic materials must then display new properties based on the synergic effect of both inorganic layers and biomolecules functions which must develop at the nanoscale interface. 229 LDHs have been considered a very favorable host for the preparation of bioLDH hybrid materials. 28,92,230−232 Biomolecules can be intercalated into LDHs by coprecipitation, anion exchange or reconstitution. In order to prepare more active materials, Vial et al.229 developed a delamination/restacking method for the confinement of biomolecules such as urease into Zn−Al LDHs. This bioinorganic hybrid urease/Zn−Al LDH material displayed a larger amount of embedded urease comparing to the traditional synthesis methods. This hybrid material has a potential application as a novel urea biosensor for medical diagnosis or as a continuous monitoring device for the environment. Delaminated LDH nanosheets derived from protein/LDH bioinorganic hybrid materials have also been reported.233−235 An et al. obtained the LDH colloid by delaminating LDHlactate in water, followed by the assembly in aqueous medium with three proteins (porcine pancreatic lipase (PPL), hemoglobin (Hb), and bovine serum albumin (BSA)), see Figure 45. For PPL whose negative charges are concentrated on the side surface opposite to active sites, its structure and conformation are well retained. However, the orientations of the PPL molecules on two-dimensional LDH nanosheets could be lying flat or standing up depending on the PPL/LDH ratio. The bioactivity of PPL lying flat is enhanced in both the hydrolysis and kinetic resolution in comparison with its soluble counterpart. For Hb, a tetrameric hemeprotein with relatively uniform distribution of surface negative charges, the interfacial assembly might result in the unfolding of its tertiary or quaternary structure, but its secondary structure and redoxactive heme groups are not denatured. For BSA, whose negative charges are distributed along the surfaces of linearly arranged domains I and II, although its secondary structure is unfolded, the loss of the ordered structure is less than previously found owing to the less curvature of the two-dimensional LDH nanosheet surface.233 Kong et al.236 reported a biprotein/LDH ultrathin film in which hemoglobin (HB) and horseradish peroxidase (HRP) molecules were assembled alternately with LDH nanosheets. Such bioinorganic hybrid exhibited a stable direct electrochemical behavior owing to the redox of biomolecules. Moreover, it showed remarkable electrocatalytic activity toward oxidation of catechol without involvement of H2O2, due to the synergistic effect of confining the two different proteins in close proximity. Therefore, this work provides a novel and efficient strategy for the immobilization of bioactive proteins into an inorganic layered matrix, for the purpose of electroanalysis and biosensing. Bellezza et al.234,235 reported that the LDH colloids can also been obtained by

4. CONCLUSIONS In this review, we have attempted to summarize the current methods that have been developed for the synthesis of dispersed LDH nanosheets. In general, these synthesis methods can be divided into two general strategies. The “bottom-up” method tries to control the nucleation and growth conditions in such a way as to only allow the formation of dispersed nanosheets. To date, this is best achieved using an oil−water inverse micelle/microemulsion scheme. The more developed method and perhaps the easiest to scale up to commercially relevant quantities is to develop suitably modified LDHs systems in which solvation forces enable the delamination of the LDHs to be thermodynamically favorable. However, the best systems tend to use highly polar solvents which makes it almost impossible to recover the nanosheets in bulk form. New and exciting applications for these materials are emerging all the time. We have reviewed the current literature in the areas of polymer/LDH nanocomposites, core−shell LDH materials, LDH thin films, nano dispersed LDH catalysts, LDH electrode materials, LDH hybrid magnets, and bioinorganic hybrid materials. Nanodispersed clay-based materials currently dominate the nanocomposite field, however we anticipate that the use of LDH materials will rapid accelerate to catch up as new and cost-effective methods of preparing stable LDHs dispersion are developed. Currently, isolating delaminated LDH nanosheets from a solvent dispersion without aggregation is still a major challenge. Utilizing the as synthesized LDH suspension directly, such as in the synthesis of core−shell structured materials and ultrathin films is a potential way forward. The fabrication of LDH-based core− shell structured materials is relatively simple; therefore, its future development should focus on the functionalization of the LDH shells and explore its specific applications. Fabrication of ultrathin films from delaminated LDH nanosheets is currently a very active field, particularly with the LBL method. Because of the wide applications of the ultrathin films, major successes in this field can be expected. Another application of the delaminated LDH nanosheets is flocculation with oppositely charged compounds, for instance, in synthesis of LDH catalysts, LDH electrode materials, LDH hybrid magnets, and bioinorganic hybrid materials. In all these applications, great 4151


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attention should be paid to the selection of appropriate LDH and the complementary compounds that can deliver the desired multifunctional response. Proper delamination methods should be chosen to optimize the mixing and matching of the two components. For the synthesis of catalysts, the delaminated LDH nanosheets can work either as support or as the active species. Since the syntheses of electrode materials, hybrid magnets, and bioinorganic hybrid materials from delaminated LDH nanosheets have just emerged over the last several years, Dermot O’Hare studied at Oxford University for both his undergraduate and graduate degrees. His doctoral thesis was in organometallic chemistry under the direction of Professor MLH Green FRS. In 1985 he was awarded a Royal Commission of 1851 Research Fellowship. He held a visiting postdoctoral fellowship at CR&D, Dupont in Delaware in 1996/7 he then returned to Oxford to join the faculty in 1997. He was made professor of Chemistry in 1998. He was the Royal Society of Chemistry Sir Edward Frankland Fellow in 1996/ 97. In 1996 the Institüt de France selected him as one of the top 50 leading scientists in Europe under 40 yrs. In 1997 he was awarded the Exxon European Chemical and Engineering Prize. In 1998 he was awarded the Royal Society of Chemistry Corday Morgan Medal and Prize. In 2009 the University of Oxford Teaching Excellence award and in 2010 the Royal Society of Chemistry Ludwig Mond prize.

more major success is anticipated. In many ways LDHs should be much more attractive prospects in years to come as they are chemically precise materials, which can be offer chemists and materials scientist fantastic compositional, structural, and morphological control.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

ACKNOWLEDGMENTS The authors thank SCG Chemicals, Thailand, for financial support.

Notes

The authors declare no competing financial interest.

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Biographies

Dr. Qiang Wang is currently a postdoctoral associate in the University of Oxford. He received his BSc (2003) and MSc (2005) from Harbin Institute of Technology (HIT) in China, and his PhD (2009) from Pohang University of Science and Technology (POSTECH) in South Korea. Before moving to Oxford, he had worked in the Institute of Chemical and Engineering Sciences (ICES) under A*STAR, Singapore for two years. His research interests are heterogeneous catalysis and materials chemistry, with a particular focus on energy and environmental issues. He was awarded the Chinese Government Award for Outstanding Self-Financed Students Abroad in 2008. 4152


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