Layered Double Hydroxides Sheets Research Articles

Synthesis of Advanced Nanoreinforced Polyurethane with Thiolene Photografted Organo‐Modified Layered Double Hydroxide

AbstractNew bionanocomposites with strongly intertwined structures based on polyurethane and layered double hydroxides (LDH) were obtained by using two different intermolecular thiolene coupling strategies in which the modification of LDH with oleic acid was either performed first and then followed by the thiolene photografting using 2‐mercaptoethanol, or the initial thiolene photografted oleic acid/2‐mercaptoethanol product was subsequently used to modify the LDH platelets. The resultant photografted oleate/LDH was condensed with diisocyanate and the addition of poly(ethyleneglycol) yielded a final hybrid polymer. The interleaved LDH materials were characterized by X‐ray diffraction and thermogravimetric analysis, underlining the possible commutative steps between photografting and organo‐modification processes. The resultant polyurethanes were characterized by FTIR spectroscopy, nuclear magnetic resonance, differential scanning calorimetry and rheology. Furthermore, the durability of the resultant polymers was assessed by photo‐FTIR. The results showed that strongly intermingled polyurethanes were processed by both strategies. Enhanced mechanical properties as well as more prolonged durability were found after anchoring LDH sheets and this was even more amplified when using the photografted acid direct intercalation into the LDH during the first synthesis step.

Amine‐Assisted Syntheses of Carbonate‐Free and Highly Crystalline Nitrate‐Containing Zn‐Al Layered Double Hydroxides

AbstractZn‐Al layered double hydroxides (LDHs), with nitrate as the charge balancing anion in the interlayer space, were synthesized by precipitation from homogeneous solution containing different amines [e.g., hexamethylenetetraamine (HMTA), diethylenediamine (DEDA), trimethylamine (TMA) and dimethylamine (DMA)]. The applied method does not require nitrogen atmosphere. The solution pH and concentration of different amines were varied in order to identify the controlling parameters and whether nitrate or carbonate are the interlayer anion. Particularly, the addition of amines turns out to be an effective tool for the synthesis of nitrate containing Zn‐Al LDHs independent from the nitrogen atmosphere. The structure, textural, composition, and morphological properties were investigated using the powder X‐ray diffraction (PXRD), thermogravimetric analysis (TGA), FT‐IR spectroscopy, and scanning electron microscopy (SEM). The analyses showed that the samples had high crystallinity and purity. The NO3‐ZnAl LDHs samples show that LDH sheets are predominantly smooth textured and the thickness of LDH sheets are found to be around 23 nm. The results also indicate that this method successfully produces a NO3– form Zn‐Al LDH that is almost identical to the one synthesized by conventional methods.

New Insights on the Incorporation of Lanthanide Ions into Nanosized Layered Double Hydroxides

Nanosized Layered Double Hydroxides (LDH) were prepared in confined environment through the microemulsion method in the presence of different lanthanide cations (Ln(III) = Eu(III), Yb(III), Tb(III), and Nd(III)). To investigate the effects of lanthanide insertion in the sheets of LDH materials, several samples were prepared upon progressively increasing the content of Ln ions and properly reducing the Al(III) amount; the samples were characterized in terms of metal content, structure, morphology, thermal behavior, and spectroscopic properties. The data revealed that Ln(III) content in the LDH samples depends on the ionic radius of the lanthanide cations and on its concentration in the starting microemulsion. X-ray powder diffraction (XRPD) indicated that Eu(III) can be inserted into the LDH structure in average atomic percentages lower than 2.7%, leading to the formation of a low symmetry phase, as confirmed by steady state luminescence spectra; while Yb(III) can be incorporated into the layer structure up to about 10% forming a pure layered phase containing the lanthanide in the sheet. The incorporation of Yb(III) and Eu(III) into the LDH sheets is also supported by FT-IR measurements. Coupled thermogravimetrical (TG) and differential scanning calorimetric (DSC) studies indicated that water molecules are essential in the coordination sphere of incorporated Ln cations; this observation accounts for the lower thermal stability of Ln-doped LDH compared to the undoped ones. Furthermore, Eu-luminescence measurements indicates that the lanthanide inclusion does not compromise its luminescence although the spectral position and brightness can be tuned by the loading.

Intercalation of Fe(III) complexes into layered double hydroxides: Synthesis and structural preservation

In this study, intercalation of Fe(III) complexes [Fe(Ox)3]3− and [Fe(Cit)2]3− in Mg2Al layered double hydroxides (LDHs) had been carried out by different routes: direct self-assembly, reconstruction and anion exchange. The result of XRD indicated that these two kinds of Fe(III) complexes intercalated LDHs (LDHs/Fe-complexes) could be successfully synthesized by anion exchange obtaining two well crystallized LDHs both with a large basal spacing. The chemical environment of iron ion in the intercalated Fe complexes was determined by FTIR, XPS and XAFS, the results of which concertedly proved that the intercalated Fe complexes were well preserved in the interlayer of LDHs. The arrangement of the Fe(III) complexes anions in the galleries of LDHs was studied by one-dimensional electron density projection. The results showed that the [Fe(Ox)3]3− anions were probably accommodated in the gallery of LDHs as an interdigitated bilayer arrangement, and the [Fe(Cit)2]3− were perpendicularly arranged against the LDHs sheets. Through the research, it is proved that the obtained LDHs/Fe-complexes markedly enhanced the UV–vis absorption ability of Fe complexes.

Plane-Wave Density Functional Theory Study on the Structural and Energetic Properties of Cation-Disordered Mg−Al Layered Double Hydroxides

Periodic solid models have been employed to investigate the structural and energetic properties of cation-disordered Mg-Al layered double hydroxides (LDHs), including composition, ordering, and stacking pattern of layers. The geometry, lattice energy and density of states (DOS) of the periodic models were computed using the plane-wave pseudopotential implementation of density functional theory (DFT) with the virtual crystal approximation (VCA). The calculation results for the MgAl-Cl-LDH model show that a pure LDH phase is difficult to sustain when the Mg/Al ratio (R) is R > or = 4. The stability of the cation-disordered MgAl-Cl-LDH unit cell increases upon increasing R, owing to the decrease in the number of Al(III)-O-Al(III) linkages in the LDH sheets. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of MgAl-Cl-LDH receive contributions from the 3s and 3p orbitals of Mg and Al and the 2p orbitals of O and Cl, respectively, implying significant host-guest interactions. The increase of R leads to the decrease in electron density of 3s and 3p of metal cation and that of 2p in Cl. Consequently, the HOMO-LUMO energy gap as well as the systematical stability increase upon increasing R. From this point of view, it has been demonstrated that the cations distribute in an ordered arrangement with the absence of Al(III)-O-Al(III) linkages if R is in the low range (R = 2-3). It was also found that the 3R polytype is the most stable stacking pattern with the same Mg/Al ratio, owing to the fact that the conductive band energy levels decline with the increase of electron density of 3s and 3p of the metal cation in the 3R-stacking system. These findings agree well with the experimental results and provide a profound understanding of critical factors influencing the structure of LDHs, including metal composition and ordering, the stacking sequence, as well as the host-guest interactions.

Determining materials properties of natural composites using molecular simulation

Layered double hydroxides (LDHs) have a wide range of potential uses due to their ability to intercalate anionic species, including poly-anionic biopolymers. Atomistic simulations can provide considerable insight into these nano-structured materials, particularly given the recent advance in high-performance computing facilities and scalable simulation codes that has enabled simulations virtually free of finite size effects. In this work we present our findings of large-scale (>100 000 atoms) molecular dynamics simulations of Mg–Al LDHs intercalated with alginate oligomers. We have investigated the effect of two different alginate oligomer chain lengths upon the materials properties of these LDH composites. In addition to this we have explored finite size effects through the use of three different system sizes for each alginate oligomer, the largest of which contains ∼240 000 atoms. We estimate the average bending modulus of the systems to be 3 × 10−19 J. However, we find the smallest alginate oligomer in our study dampens the undulations of the LDH sheets at long wavelengths, which confers a greater interlayer compressibility due to the small alginate molecules bridging the interlayer spacing. The initial orientation of larger alginate oligomers is found to have an impact on the Young's moduli of the composite materials over the timescales considered in this work. We find the average in-plane Young's modulus to be approximately 40 GPa for the total composite materials and 135 GPa for the LDH sheets alone.

Fabrication of nanocomposites by collagen templated synthesis of layered double hydroxides assisted by an acrylic silane coupling agent

The purpose of this study was to control the fabrication of nanocomposites at the nanoscale interface by collagen templated synthesis of Zn–Al layered double hydroxides (LDHs) assisted by γ-methacryloxypropyl trimethoxy silane (KH570) with further treatment of graft polymerization. The results show that collagen directs the growth of LDHs into curved nanorods by length of 300 nm in perfect consistency with collagen chain in both the size and flexility under the essential hydrophobic environment on the solid surface provided by KH570. The nanorods are aggregated into thin curved platelets due to strong interaction between collagen molecules themselves and strong interaction between collagen and LDH sheets. By further treatment of graft polymerization, the adjacent curved platelets encircle into numerous hollows via chemical linkage, achieving polyporous nanocomposites. Nanohybrid materials with this structure are especially interesting for applications as biosensors or supported catalysis.

Intercalation of l-Dopa into Layered Double Hydroxides: Enhancement of Both Chemical and Stereochemical Stabilities of a Drug through Host−Guest Interactions

This paper describes a systematic study on the intercalation of an unstable chiral drug l-dopa into layered double hydroxides (LDHs) and the enhancement of its chemical and stereochemical stabilities through host−guest interactions, by virtue of combining experimental and theoretical investigations. l-Dopa has been intercalated into a magnesium−aluminum LDH, and the structural characterization reveals that l-dopa anions are accommodated vertically in the interlayer region as a monolayer of partially superimposed species. Thermogravimetry−mass spectroscopy and polarimetry indicate that both the chemical and stereochemical stabilities of l-dopa are enhanced significantly in a confined region between LDH sheets compared with pristine l-dopa. The racemization phenomenon of pristine l-dopa has been rationalized by quantum mechanical calculations at the B3PW91/6-31G(d,p) level. The computed results suggest that in the solid state l-dopa undergoes racemization via an enol intermediate formed by hydrogen transfer from the chiral carbon atom to the carboxylate group. After intercalation in the interlayer galleries of the LDH, the carboxylate group of the l-dopa becomes involved in strong host−guest interactions with the layers, with the computed binding energy of l-dopa to the LDH layers being ca. −1100 kJ/mol. The host−guest interactions, including electrostatic and hydrogen bonding between l-dopa and LDH host, thus inhibit the racemization reaction of the guest because the carboxylate group is no longer able to act as a hydrogen acceptor. Therefore, the results of theoretical study are in good agreement with those of the experimental ones. The results show that these layered materials have potential applications as the basis of a novel storage and delivery system for l-dopa or other unstable chiral pharmaceutical agents.

Multiple Effects of Dodecanesulfonate in the Crystal Growth Control and Morphosynthesis of Layered Double Hydroxides

The morphology of Mg/Al layered double hydroxides (LDHs) has been manipulated by hydrothermal treatment following the nucleation by coprecipitation or urea hydrolysis method in sodium dodecanesulfonate (SAS) aqueous solution. It is observed that the dodecanesulfonate (AS) serves as both of the interlayer anion and the template for the formation of LDH sheets. The intercalation of the dodecanesulfonate anion occurring in each case enlarges the basal spacing and weakens the interaction of LDH sheets. Thus the vesicle-like phase formed by the surfactant in aqueous solution in the presence of metal cations induces the curved growth of AS-interecalated LDH nuclei and the special spherical aggregate of LDH particles.

Emergence of Undulations and Determination of Materials Properties in Large-Scale Molecular Dynamics Simulation of Layered Double Hydroxides

Layered double hydroxides (LDHs) have generated a large amount of interest in recent years because of their ability to intercalate a multitude of anionic species. Atomistic simulation techniques such as molecular dynamics have provided considerable insight into the behavior of these materials. The advent of supercomputing grids allows us to explore larger-scale models with considerable ease. Here, we present our findings from large-scale molecular dynamics simulations of Mg2Al-LDHs intercalated with chloride ions. The largest studied system size consists of one million atoms with lateral dimensions of 588 Å × 678 Å. The system exhibits emergent properties, which are suppressed in smaller-scale simulations. Undulatory modes are caused by the collective thermal motion of atoms in the LDH layers. At length scales larger than 20.7 Å, these thermal undulations cause the LDH sheets to interact and the oscillations are damped. The thermal undulations provide information about the materials properties of the system. In this way, we obtain values for the bending modulus of 8.3 ± 0.4 × 10-19 J with in-plane Young's moduli of 63.4 ± 0.5 GPa for a hydrated system and 139 ± 1 GPa for the LDH sheets alone.

Morphology and size control of Ni–Al layered double hydroxides using chitosan as template

Layered double hydroxides (LDHs) synthesized by conventional methods are usually hexagonal platelet morphology. Using chitosan as template for synthesis of Ni–Al–CO3 LDHs, the LDH particles are curved in the a direction, deviating from the flat platelets. The crystallite growth in a and c directions is both inhibited. Especially in the c direction, the stacking thickness of LDH sheets decreases to half of the thickness of sample synthesized by conventional method.

Textural and electrochemical properties of carbon replica obtained from styryl organo-modified layered double hydroxide

Negative carbon replicas were obtained after pyrolysis of organo-modified layered double hydroxides (LDH) and dissolution of the inorganic moiety. The hybrid organic–inorganic precursors consisted of polystyrene sulfonate incorporated directly between LDH sheets of cationic composition Zn2Al or after in situ polymerisation of the monomers. The preparation conditions of the hybrid phase and the constraint imparted by the inorganic framework were found to affect strongly the textural properties. The carbons sequestered into the LDH intersheet gap present a textural hierarchy, i.e. a pronounced microporous character (micropore volume up to 1.07 mL g−1 and SBET = 2300 m2 g−1) and a mesoporosity. The nanotexture of the carbon replica was characterized by Raman spectroscopy. Finally, the carbons tested by cyclic voltammetry and complex impedance spectroscopy in two-electrode electrochemical supercapacitors in sulfuric acid medium present a capacitive behavior, i.e. a quasi rectangular response of intensity versus potential and a phase angle in the Z″–Z′ impedance plane close to π/2.

Approach Toward Nanoplate Poly(styrene sulfate): Synthesis of Nano-Polymers with Special Morphology by Using LDHs as Template

A simple chemical method to obtain nanopolymers with special morphology is reported. The key to the success of this preparation was the use of layered double hydroxides (LDHs), a two-dimensional host material, as template. Typical procedures include in-situ polymerization of the monomers in the interlayers of the LDH template, followed by the dissolution of LDH sheets so that the formed polymers can be free and collected. The obtained results demonstrate potential of LDHs as tools for the controlled synthesis of nanopolymers with platelike morphology.

Effect of surface states of layered double hydroxides on conductive and transport properties of nanocomposite polymer electrolytes

All solid-state poly(ethylene oxide) (PEO) nanocomposite electrolytes were made containing nanoscale fillers of layered double hydroxides (LDHs). Two kinds of LDHs with different surface states were prepared by aqueous co-precipitation method. The LDHs were added into PEO matrix to study the structures, conductivities and ionic transport properties of nanocomposite electrolytes. The structures of LDHs were characterized by infrared spectra, thermogravimetric analysis and wide-angle X-ray diffraction. With enhanced compatibility of LDH sheets by oligo(ethylene oxide) surface modification, the PEO/OMLDH nanocomposite electrolyte exhibits an amorphous morphology and an enhancement of conductivity by three orders of magnitude as compared to pure PEO electrolyte. The lithium ion transference number T Li + of PEO/LDH nanocomposite electrolyte measured with a value of 0.42 is two times higher than the one of pure PEO electrolyte, which can be attributed to the Lewis acid–base interaction between surface states of metal hydroxides and counter anions of lithium salts.

In situ polymerization of interleaved monomers: a comparative study between hydrotalcite and hydrocalumite host structures ☆☆Keynote Lecture.

Vinyl benzene sulfonate (styrene sulfonate, VBS) and polystyrene sulfonate (PSS) were incorporated into Layered Double Hydroxides (LDH) by templating reaction. The two host structures investigated are Zn 2Al(OH) 6Cl. nH 2O of hydrotalcite-type and Ca 2Al(OH) 6Cl. nH 2O of hydrocalumite-type. The electrostatic binding between sulfonate groups of VBS and the inner-surface of LDH host structure evaluated by 13C CP-MAS NMR spectroscopy causes some shifts in the resonance lines. In situ polymerization of VBS molecules between LDH sheets is reached after soft heat treatment. Its completion requires a good matching between the layer charge density and the projected surface area of the guest molecule achieved by a ratio M 2+ to M 3+ of 2. For the pristine material, the change from Oh to Td aluminum site appears to be a collective process for the hydrocalumite, whereas it is more progressive for the hydrotalcite-type material. For the corresponding organoceramics, the site conversion before the breakdown of the lamellar structure is substantially reduced, and especially for PSS/LDH nanocomposite obtained by in situ polymerization. CaAl–VBS intercalate crystallizes in C2/ c space group inducing a bilayer arrangement of VBS anions in the interlamellar space. Furthermore, distances between anionic sites allow an in situ zig-zag polymerization of syndiotactic type along the a axis. For hydrotalcite, the short Al 3+–Al 3+ distance at a√3 prevents also the formation of PSS other than syndiotactic. The thermal stability of the nanocomposites depends on the host structure but also on the synthesis pathway.

Enhanced Ionic Conductivity in Poly(ethylene oxide)/Layered Double Hydroxide Nanocomposite Electrolytes

All solid-state poly(ethylene oxide) (PEO) nanocomposite electrolytes were made containing nanoscale fillers of layered double hydroxides (LDHs). Two kinds of oligo(ethylene oxide) modified LDHs were prepared by template method, and added into PEO/LiClO4 matrix (with EO/Li molar ratio of 8) to study the effect on the ionic conductivity of PEO/LDH nanocomposite electrolytes. The structures of the modified LDHs were characterized by infrared spectra, thermogravimetric analysis and wide-angle X-ray diffraction. The results show that the oligo(ethylene oxide) with phosphonate anion can be effectively intercalated into the gallery region of LDHs and formed as an organic-inorganic hybrid (PLDH). With enhanced compatibility of LDH sheets by oligo(ethylene oxide) surface modification, the PEO/PLDH nanocomposite exhibits fully exfoliation morphology. The well dispersed LDH layers in PEO/LiClO4/PLDH nanocomposite electrolytes rendering the formation of amorphous phase, results in an enhancement of ionic conductivity by three orders of magnitude compared to the pure PEO/LiClO4 polymer electrolyte. This novel nanocomposite electrolytes system with high ionic conductivity will be benefited to fabricate the thin-film type of Li-polymer secondary battery.

Synthesis and characterization of a polystyrene sulfonate layered double hydroxide nanocomposite. In-situ polymerization vs. polymer incorporation

Vinyl benzene sulfonate (styrene sulfonate — VBS) and polystyrene sulfonate (PSS) are incorporated into layered double hydroxides (LDH) of nominal composition ZnnAl(OH)2(1+n)Cl·nH2O by exchange and templating reaction, respectively. PSS adsorption leads to a delamination process of the LDH 2D-host structure. A hydrothermal treatment improves greatly the crystallinity of the PSS–LDH nanocomposite. The interactions between VBS and the host structure are evaluated by 13C CP-MAS solid state NMR spectroscopy. The electrostatic binding between the sulfonate group and the inner-surface of the LDH host structure causes some shifts in the resonance lines, although the shifts are independent of the LDH layer charge density. In-situ polymerization of VBS molecules between LDH sheets is reached after soft heat treatment. Its completion requires a good matching between the layer charge density and the projected surface area of the guest molecule as exemplified by a Zn to Al ratio of 2. In comparison to the pristine material, the change from Oh to Td aluminium site is substantially reduced for PSS–LDH nanocomposite and absent for the material prepared after the polymerization reaction.

Evaluation of the microporosity of pillared [Fe(CN) 6]–MgAl–LDHs

Layered double hydroxides (LDHs) containing Mg 2+ and Al 3+ in the basic layers and NO − 3 as an interlayer anion were synthesized by the method of coprecipitation (pH 10). By changing the Mg 2+/Al 3+ ratio (1.5–4.5), the charge density on the (NO 3)–MgAl–LDH sheets was varied. After pillaring with Fe(CN) 3− 6, which was based on an anion exchange process, the interlayer space became accessible. This was reflected in the large created surface areas and micropore volumes. The applied models for the calculation of the micropore size distributions (Maes–Zhu–Vansant and Horvath–Kawazoe) gave matching results, revealing narrow distributions for all the samples, with the majority of the pores smaller than 0.71 nm. A correlation was found between the Mg 2+/Al 3+ ratio and the resulting microporosity after pillaring. The optimal ratio was situated around 3.3, resulting in a pillared [Fe(CN) 6]–MgAl–LDH with a Langmuir surface area of 499 m 2/g and a micropore volume between 0.158 ml/g (μPV min) and 0.177 ml/g (μPV max). As an alternative, direct coprecipitation of the pillared LDHs was evaluated. This one-step mechanism proved to be a method producing similar results. Taking all this into consideration, one can conclude that hexacyanoferrate(III) complexes form ideal anionic pillars for the creation of microporous layered double hydroxides.

Structural aspects and thermal properties of takovite-like layered double hydroxides pillared with chromium oxo-anions

Intercalated layered double hydroxides (LDHs) of chromate (CrO42–) and dichromate (Cr2O72–) have been synthesised and characterised. The parent chloride material ([Ni–Al–Cl]) was obtained by coprecipitation at constant pH and further anion-exchanged to incorporate the desired oxo-anions. The physico-chemical properties of all compounds were investigated using X-ray powder diffraction, infrared spectroscopy, thermogravimetric analyses and extended X-ray absorption spectroscopy (EXAFS). The structural evolution of the pristine and pillared materials, following calcination between 100 to 450 °C, was also thoroughly studied. It is shown that the intercalated guests, chromate and dichromate anions, interact with the host hydroxylated sheets at moderate temperatures. This so-called “grafting process” is characterised by a reorganisation of the anionic species within the interlamellar domain associated with a shrinking of the basal spacing. This phenomenon was clearly evidenced with powder X-ray diffraction (contraction of the interlamellar distance) and, more interestingly, by chromium K-edge EXAFS (variation in the local environment of the probe element). On the other hand, thermogravimetric studies indicated that the thermal stability was greatly enhanced following pillaring with oxo-anions. This is reflected through the dehydroxylation step of the LDHs sheets which shifted to much higher temperatures, from 280 °C in the [Ni–Al–Cl] precursor to over 450 °C in the dichromate intercalated compound. Rehydration and reverse exchanges with carbonate anions of the thermally treated materials were both unsuccessful, suggesting that the grafting phenomenon is irreversible. The specific surface areas were also negatively affected following anion exchange and moderate thermal treatment, indicating that the structural changes observed through the different analytical techniques also influence the microtextural properties.

Incorporation of poly(acrylic acid), poly(vinylsulfonate) and poly(styrenesulfonate) within layered double hydroxides

Poly(acrylic acid), poly(vinylsulfonate) and poly(styrenesulfonate) have been incorporated between the positively charged sheets of layered double hydroxides (LDHs)M1–xAlx(OH)2+(M = Mg, Ca, Co) and Zn1–xM′x(OH)2+(M′= Al, Cr) to form layered nanocomposites. The resulting nanocomposites contained the LDH sheet structures separated by 7.6–16.0 A, which is sufficient to accommodate polymer bilayers between the LDH sheets. Preparations were carried out in deaerated aqueous base by a template reaction, involving the formation and precipitation of nanocomposites from metal nitrate-salt precursors in the presence of the dissolved polymer. Structural and compositional details were provided by X-ray diffraction (XRD), FTIR spectroscopy, elemental analysis, differential scanning calorimetry (DSC) and thermogravimetry (TG). Scanning electron microscopy (SEM) indicates that the nanocomposition of LDHs with ionomers significantly alters the particle microstructure from that of the LDH carbonates derived from aqueous precipitation.