High Layered Double Hydroxides Research Articles

High Entropy Induced γ-NiOOH Formation in Layered Double Hydroxide as Highly-Active and Ultra-Stable Electrocatalyst for Oxygen Evolution Reaction

Developing a low-cost and highly efficient oxygen evolution reaction (OER) electrocatalyst and understanding the underlying reaction mechanism are critical for sustainable renewable energy conversion and storage. In this study, high entropy layered double hydroxide (LDH) was self-supported on nickel foam using a simple hydrothermal method. The resulting binder-free high entropy LDH consisting of five different transition metals of Fe, Ni, Co, Mn, and Cr exhibits an excellent OER activity in alkaline condition with a low overpotential of 218 mV at a current density of 50 mA cm-2. It is superior to the binary, ternary, and quaternary-metal LDH counterparts. We demonstrate that the high entropy LDH possesses ultra-stable electrochemical stability at a high current density of 400 mA cm-2 for 600 h. The modulation of electronic structure facilitates the formation of the highly active γ-NiOOH and the shifting of the d-band center, resulting the outstanding OER performance. The excellent catalytic activity and stability of the high entropy LHD make it is as a highly potential candidate for commercial use.

High entropy promoted active site in layered double hydroxide for ultra-stable oxygen evolution reaction electrocatalyst

In this study, high entropy layered double hydroxide (LDH) grown on nickel foam using a simple hydrothermal method is reported. The high entropy LDH consisting of five different non-noble transition metals of Fe, Ni, Co, Mn, and Cr (denoted as FeNiCoMnCr) exhibits an excellent OER activity in alkaline condition with a low overpotential of 218 mV at a current density of 50 mA cm−2. It is superior to the binary, ternary, and quaternary-metal LDHs. We demonstrate that the high entropy FeNiCoMnCr LDH possesses ultra-stable electrochemical stability at a high current density of 400 mA cm−2 for 600 h, which is the best stability of all the reported high entropy material electrocatalysts so far. The interaction among the multi-metal components along with the resulting electronic structure modulation facilitates the formation of highly-active γ-NiOOH species, significantly boosting catalytic activity; while the high entropy induced phase stability contributes to the superior durability.

An improved method of MgFe-layered double hydroxide/ biochar composite synthesis

Layered double hydroxides (LDHs) and their composites are a promising platform for a wide range of applications, especially in the environmental field. However, the facile and efficient synthesis of high-quality LDHs and their composites remains a challenge. Here, an improved co-pyrolysis method has been developed to synthesize LDH/biochar composites. A composite material was prepared by directly anchoring MgFe-LDH on corn stover by co-precipitation with FeCl3 and Mg(OH)2, and then through pyrolysis. The main features of this improved method are the use of the solubility product Mg(OH)2 to maintain the pH of the mixture at ∼10 (the optimum for LDH precipitation), eliminating the cumbersome pH control steps of the traditional co-precipitation method, and the low rate of solid-liquid reaction to obtain high LDH crystallinity and purity. This produces improved properties of the MgFe-LDH/biochar composite after pyrolysis: higher loading of MgFe-LDH, larger specific surface area of the composite, a more stable layered structure, and a more customizable LDH. The theoretical maximum adsorption capacity of phosphorus for the composite was 379.4 mg L−1 according to an adsorption isotherm study, and the dominant adsorption mechanism is chemical adsorption accompanied by physical adsorption. In actual wastewater applications, >80% of total phosphorus in biogas slurry was removed via adsorption, indicating that the composite is highly efficient in phosphorus selectivity in wastewaters with complex components. Here, a more sustainable synthesis method of LDH/biochar composite has been developed with high application potential.

Mesostructuring layered materials: self-supported mesoporous layered double hydroxide nanotubes.

Synthesis of layered materials exhibiting hierarchical porosity remains challenging, but nevertheless worthwhile because it turns such solids into functional materials with high specific surface area. Using a soft-templating strategy in combination with the incorporation of 8-fold coordinated Eu3+, self-assembly of self-supported layered double hydroxide (LDH) nanotubes has been achieved. Heteromorphic equimolar substitution of Al3+ by Eu3+ in Zn2+/Al3+ LDH solids intercalated with 1,3,5-benzenetricarboxylate anions (BTC) assists precipitation of the double hydroxide layers onto the convex surface of Pluronic® P-123 worm-like micelles, yielding multilayer cylinders of BTC-intercalated LDHs. Removal of the micellar template is easily achieved by liquid extraction with methanol, yielding a network of interconnected, well-defined, self-supported, multi-walled, hollow cylindrical nanotubes. Removal of Eu3+ from the synthesis disables formation of the nanotubular morphology, but still yields LDHs containing a network of embedded mesopores, resulting in a specific surface area that is 5-fold higher as compared to standard LDHs.

Modifying the electrochemical process of Li–O2 battery by integrated NiMn-layered double hydroxides substrate

As typical two-dimension material, layered double hydroxides (LDHs) attract great interest and are believed to be promising catalysts in Li–O2 battery due to their facile tunability of composition, structures and properties. However, the low electric conductivity and limited catalytic active site exposure caused by the large particle size and severe lamella stacking are the main challenges. Herein, an interconnected, highly active and ultra-thin NiMn-LDHs is deposited on 3D Ni foam and used as integrated porous catalyst substrate (NiMn-LDHs substrate), the NiMn-LDHs substrate is functional as both current collector and catalyst layer, and the NiMn-LDHs substrate exhibits high catalytic activity and robust stability, the substrate successfully plays the function of tunning the electrochemical process of Li–O2 battery. The Li–O2 battery uses this kind of substrate exhibits much improved discharge capacity and long cycle performance compared with pristine Ni substrate. The benefit of the porous NiMn-LDHs in improving the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic activities is also supported by rotating disc electrode techniques. This work highlights the strategy of fabricating high efficient LDHs catalyst and provides a new direction for achieving highly efficient integrated catalytic cathode for practical use.

Eco-friendly strategy to a dual-2D graphene-derived complex for poly (lactic acid) with exceptional smoke suppression and low CO2 production

Poly (Lactic acid) (PLA) as a biodegradable biopolymer is remarkably limited for the engineering applications due to its inherent flammability and brittleness. The conventional polymer toughener and flame retardants severely damaged the environment-friendliness and sustainability of PLA. Particularly, the carbon dioxide (CO2) production from the combustion of PLA composites can easily induce the global warming. In this work, a viable strategy was developed to engineer a dual two-dimension (2D) graphene-derived complex as an effective environment-benign flame retardant and interface modifier for PLA materials. Graphene oxide was first functionalized with sustainable long-chain polyester as a multitasking modifier of flame retardant and interfacial compatibilizer. The 2D layered double hydroxide (LDH) was successfully anchored with polyester-functionalized graphene (PG) as a predefined carbonization-catalyzed promoter. The favorable distribution of the dual 2D LDH-anchored PG negated the poor dispersion of high loading LDH, providing PLA matrices with desirable toughness and engineered functional features. The experimental results revealed that the dual 2D graphene-derived complex could enhance flame retardancy with the heat release rate decreased 50% and 29% for poly(propylidene carbonate)-modified and polybutylene succinate-modified composites. The smoke production rate, total smoke production and CO2 production rate of PLA binary composites were also reduced significantly, which could benefit the environment protection. The developed strategy of eco-friendly dual 2D graphene-derived flame retardant not only provided a new insight into modifying the brittle polymer matrices for the functional exploration but also a cleaner process for biodegradable composites with exceptional smoke suppression and reduced carbon dioxide emission.

Synthesis of ionic liquid intercalated layered double hydroxides of magnesium and aluminum: A greener catalyst of Knoevenagel condensation

Due to their structural merits that arise from their stability and high surface area, the layered double hydroxide (LDH) materials have caused strong attention. These characteristics provided intriguing possibilities with improved efficiency for catalytic applications. In this work, the preparation of 1-butyl-3-methylimidazolium hydroxide ([BMIM]+OH−) intercalated by a facile approach in a layered double hydroxide (LDH) matrix is reported and its implementation as a greener catalyst is shown. Different physico-chemical techniques such as XRD, FTIR, TGA, and N2-physisorption, HRTEM, and CO2 adsorption are implemented to characterize the structure of the fabricated catalysts. The [BMIM]+OH−/LDH exhibit outstanding catalytic performance in Knoevenagel condensation, resulting from the high LDH surface area and synergistic effects between both the intercalated ionic liquid and LDHs matrix. Knoevenagel’s fabricated catalysts can be exploited to catalyze different condensations and can be reused well. This work therefore generates good opportunities in the field of catalysis for the preparing and implementation of LDH-based catalysts.

Flame retardancy and smoke suppression properties of transparent intumescent fire-retardant coatings reinforced with layered double hydroxides

A series of novel layered double hydroxide (LDH)-modified phosphate esters (LPPBs) flame retardants were synthesized by the reaction of flexible phosphate ester (PPB) and LDHs with different mass ratio and then well characterized by Fourier transform infrared spectroscopy (FTIR) and 1H nuclear magnetic resonance spectroscopy (1H NMR). The obtained LPPBs were mixed with melamine formaldehyde resin to produce transparent fire-retardant coatings applied on wood substrates. The coatings containing LDHs exhibit a high degree of transparency even at high LDH contents due to the well dispersed and completely exfoliated state of LDHs in the amino matrix, as determined from the results of X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images. The fire protection tests show that the values of weight loss, char index, and flame spread rating of the coatings are remarkably decreased with the introduction of LDHs concomitant with an increase in the intumescent factor. The cone calorimeter and smoke density tests show that the flame retardancy and smoke suppression properties of the coatings are significantly improved with the introduction of LDHs due to the formation of a more compact, continuous, and intumescent char during combustion. The results of thermogravimetric (TG) analysis show that the thermal stability and residual weight of the coatings are gradually increased with increasing LDH content. Char residue analysis shows that the introduction of LDHs into the coatings contributes to generating more phosphorus-rich crosslinking structures and aromatic structures in the condensed phase that produce a more compact and thermally stable char against the release of heat and smoke, thus exhibiting excellent flame retardancy and smoke suppression properties.

Formation of high crystalline LDH sludge for removing Cu and Zn from wastewater by controlled double-jet precipitation.

In order to improve the heavy metal wastewater treatment by avoiding formation of amorphous sludge phase, we develop a faster formation of high crystalline layered double hydroxide (LDH) sludge to remove Cu and Zn from wastewater by controlled double-jet precipitation (CDJP) without hydrothermal or heat aging post-treatment. A series of experimental procedures are conducted to determine the optimal parameters. Results show that the optimal adding rate, pH value, and stirring rate is 0.5mLmin-1, 9.0, and 500rpm, respectively. The CuZnAl-LDH phase sludge is formed in a well-crystallized hexagonal platelet, which assembled into a flower-like architecture. Comparative studies show that the formation of amorphous LDH sludge in conventional precipitation (CP) could be divided roughly into two stages-from the mixed copper hydroxide, zinc hydroxide, and scarbroite to the mixed low crystallinity CuAl-LDH and ZnAl-LDH. However, in CDJP method, the high crystalline LDH sludge evolved from a new four-step evolution process that is the formation of an amorphous (quasi-)multinary metastable ternary CuZnAl-LDH phase, followed by the indiffusion of cations and substitution of anions to fabricate crystalline LDH, the integrated LDH hexagonal platelets assembled into a flower-like architecture by the screw dislocation growth mechanism, the coarsening growth of each ternary LDH platelet, respectively. Thereinto, the formation of (quasi-)multinary metastable LDH phase instead of metal hydroxide in initial stage would be an obvious advantage of the CDJP method compared to CP method due to the former skipping the sequential precipitation.

Calcium chloride addition to overcome the barriers for synthesizing new Ca-Ti layered double hydroxide by mechanochemistry

In this study, a new binary Ca-Ti layered double hydroxide (LDH) has been synthesized for the first time by a two-step (dry and wet) milling of calcium hydroxide, hydrated titanium dioxide and a third phase of calcium chloride. No reactions were observed while milling calcium hydroxide and hydrated titanium dioxide unless the further addition of a third phase of calcium chloride which triggered the solid state reaction to form LDH phase. The traditional co-precipitation process failed to synthesize Ca-Ti LDH due to the barriers of the precipitation pH and ion radius differences between Ca2+ and Ti4+. The X-ray diffraction (XRD), Thermogravimetric Analysis (TGA), X-ray photoelectron spectroscopy(XPS) and Scanning electron microscopy (SEM) techniques were applied to characterize the prepared samples. The characterization results demonstrated the successful production of high crystalline Ca-Ti LDH phase. This work proved the feasibility of mechanochemistry to produce novel binary LDH which may be of help for the synthesis of other new layered materials in general.

Flame-Retardant Mechanism of Layered Double Hydroxides in Asphalt Binder.

The flame retardancy of asphalt binders with layered double hydroxides (LDHs) was investigated using limiting oxygen index (LOI) and cone calorimeter tests. The flame-retardant mechanism of the LDHs was also studied with thermogravimetry and differential scanning calorimetry (TG–DSC), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The cone calorimeter testing results indicated that 2 wt.% of the LDHs can decease the peak heat and smoke release rate of asphalt binders. Because a low dose of LDHs can be well dispersed in asphalt binder and favor the formation of polyaromatic structures during combustion, the thermal oxidation resistance and compactness of the char layer can be improved. The LOI of asphalt binder can be increased and the heat and smoke release during combustion can be decreased with 25 wt.% LDHs. The decomposition of LDHs can absorb the heat release of the initial two stages of asphalt combustion and reduce the burning rate of asphalt. Due to the loss of loosely bound water in the LDHs during the blending process and the decrease of dispersibility at a high LDH dose, the improvement of thermal stability is limited.

Solid-state speciation of interlayer anions in layered double hydroxides

Layered double hydroxides (LDH) have been proposed for phosphate (PO4) recovery and recycling from waste streams due to their high anion exchange capacity, good stability and high affinity towards PO4. The high affinity towards PO4 strongly relates to the electrostatic interaction with PO4, and thus the charge of PO4. However, the anion speciation of intercalated PO4, i.e. either H2PO4−, HPO42− or PO43− is often overlooked. This study was set up to measure solid phase PO4 speciation through ion exchange stoichiometry, X-ray diffraction (XRD), attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) and modelling. Six phase pure Mg/Al LDH materials were synthesized using co-precipitation of metal nitrate (NO3−) solutions with varying M2+/M3+ ratio at pH 12 and pH 10. The LDHs synthetized at pH 12 contained larger equivalent fractions of intercalated OH−, smaller fractions of NO3− and smaller interlayer distance than those prepared at pH 10, likely because of the higher OH− concentration in the more alkaline synthesis solutions. Two high charge LDHs prepared at pH 12 or 10 were selected, exchanged with PO4 (0–20 mM initial PO4, 24 h) at one starting pH (7.20); desorption was subsequently performed with carbonate (3 mM, initial pH 8.4) during 480 h. The resulting solution concentrations of NO3, PO4 and CO3 and the pH allowed the identification of the anion exchange stoichiometry. The LDH synthesized at pH 12, which had a large fraction of exchangeable OH−, adsorbed PO4 as HPO42−/PO43−, in exchange for both NO3− and OH− anions. The material synthesized at pH 10 containing a lower fraction of exchangeable OH−, therefore, adsorbed mainly HPO42− in exchange for NO3− anions. The carbonate exchange was consistent with adsorption of divalent CO32−. The pH dependent speciation modelling showed that the exchanged PO4 ions have higher charge compared to those in the contacting solution. This study suggests that the highest P content of LDH is obtained in high charge materials holding divalent PO4 anions, i.e. materials synthesized at lower pH and/or exchanged in solutions with low alkalinity.

Nature-inspired delamination of layered double hydroxides into monolayered nanosheets in water

Inspired by the natural frost weathering of rocks and minerals, layered double hydroxides (LDHs) were delaminated by a facile temperature shock method, without the usual involvement of organic solvents and sonication. The temperature shock was carried out with cycles of freezing the LDH solution in liquid nitrogen and subsequent melting the frozen solution in a 80°C water bath. Around 67% LDHs were delaminated into nanosheets after 16 such cycles. Moreover, the LDH monolayers thus obtained retain their lateral size at least three times better than that prepared in formamide under sonication. The present work provides a green and general approach to obtain high quality LDH nanosheets, which are finding application across a wide spectrum of areas.

Design of latex-layered double hydroxide composites by tuning the aggregation in suspensions.

Colloidal stability of polymeric latex particles was studied in the presence of oppositely charged layered double hydroxide (LDH) platelets of different interlayer anions. Adsorption of the LDH particles led to charge neutralization and to overcharging of the latex at appropriate concentrations. Mixing stable colloidal suspensions of individual particles results in rapid aggregation once the LDH adsorption neutralizes the negative charges of the polymer spheres, while stable suspensions were observed at high and low LDH doses. The governing interparticle interactions included repulsive electrical double layer forces as well as van der Waals and patch-charge attractions, whose strength depended on the amount of LDH particles adsorbed on the latex surface. The type of the LDH interlayer anions did not affect the colloidal stability of the samples. Structural investigation of the obtained latex-LDH composites revealed that the polymer spheres were completely coated with the inorganic platelets once their concentration was sufficiently high. These results are especially important for designing synthetic routes for hybrid systems in suspensions, where stable colloids are required for uniform film-formation and for the homogeneous distribution of the inorganic filler within the composite materials.

Preparation, characterization and antimicrobial applications of Zn-Fe LDH against MRSA

Facile and simple processes to get Zn-Fe layered double hydroxide (LDH) with nitrate as the interlayer anion are reported. The method of co-precipitation produced high crystallinity LDH that is marked by XRD, SEM, TEM and FT-IR. Results showed that 99.8% of Cd+2 removals were at pH11 and 4h. To get the adsorption isotherms, the concentration of metal ions extending from 6 to 18mg/L was utilized. Results supported the Langmuir adsorption model. In contrary, the adsorption process followed the pseudo-second-order reaction kinetics. Interestingly, the prepared LDH shows durable antimicrobial activities against Gram-negative (Proteous vulgaris, Klebsiella pneumoniae, Escherichia coli and Pseudomonas aeruginosa) and Gram-positive (Staphylococcus epidermidis, Staphylococcus aureus, Streptococcus pyogenes and MRSA) and fungi (Candida albicans, Aspergillus fumigatus, Geotricumcandidum, and Trichophyton mentagrophytes). The minimum inhibitory concentration (MIC) of Zn-Fe LDH varied from 0.49 to 15.60μg/mL according to the types of microorganisms. The prepared LDH achieved 90% at pH8.50 which is the pH of wastewater and at the same time exhibited durable antimicrobial activities against MRSA, Gram-negative, Gram-positive and fungi. Results have significant implications in the field of bioremediation of water with little cost, simple operation, high productivity and easiness of the equipment.

Dual Support System Ensuring Porous Co–Al Hydroxide Nanosheets with Ultrahigh Rate Performance and High Energy Density for Supercapacitors

Layered double hydroxides (LDHs) are promising supercapacitor electrode materials due to their high specific capacitances. However, their electrochemical performances such as rate performance and energy density at a high current density, are rather poor. Accordingly, a facile strategy is demonstrated for the synthesis of the integrated porous Co–Al hydroxide nanosheets (named as GSP‐LDH) with dual support system using dodecyl sulfate anions and graphene sheets as structural and conductive supports, respectively. Owing to fast ion/electron transport, porous and integrated structure, the GSP‐LDH electrode exhibits remarkably improved electrochemical characteristics such as high specific capacitance (1043 F g−1 at 1 A g−1) and ultra‐high rate performance capability (912 F g−1 at 20 A g−1). Moreover, the assembled sandwiched graphene/porous carbon (SGC)//GSP‐LDH asymmetric supercapacitor delivers a high energy density up to 20.4 Wh kg−1 at a very high power density of 9.3 kW kg−1, higher than those of previously reported asymmetric supercapacitors. The strategy provides a facile and effective method to achieve high rate performance LDH based electrode materials for supercapacitors.

Layered double hydroxide-based fire resistant coatings for flexible polyurethane foam

Montmorillonite clay (MMT) is frequently used to create fire resistance in Layer-by-Layer (LbL) applied coatings. This manuscript reports that switching the MMT with layered double hydroxides (LDH) produced a more effective fire resistant coating on flexible foam. Most of the formulations required at most two trilayers (TL) to produce ignition resistant and very low heat release rate polyurethane foam. The best coating (greatest flammability reduction with the least amount of coating) delivered a 41% reduction in peak heat release rate. This was a 2TL, 10% mass coating produced using a coating solution with a high LDH content. This manuscript discusses the impact of the LDH-based coating formulations and number of monolayers on the LbL coating growth rate, composition, and fire resistance.

Interplay of Charge Density and Relative Humidity on the Structure of Nitrate Layered Double Hydroxides

The structure of several crystalline Ni(II)−Al(III) and Mg(II)−Al(III) layered double hydroxides (LDHs) intercalated with nitrate anions was studied as a function of the relative humidity. For low charge density LDHs, the electrostatic attraction between the anions and the positive LDH layers prevails over any other interaction, and the anions remain flat, F, with their C3 axis perpendicular to the xy planes, irrespective of water activity. For LDHs bearing higher charge densities, the incoming water molecules drive an abrupt phase expansion in which the anions tilt their C3 axis with respect to the interlamellar xy plane, thus resulting in an expanded, T, form. The structural F−T transition is discontinuous, and involves a second staging phase intermediate, with alternated F and T galleries. For intermediate charge density LDHs, the hydration process easily reverts, once the samples are resubmitted to a dry atmosphere. Dehydration of high charge density LDHs is however kinetically hindered; this behavior is explained in terms of the structural features of the involved phases. For all samples, the maximum net gallery expansion is proportional to the layer charge density, irrespective of the nature of the divalent cation.

Delamination of organo-modified layered double hydroxides in polyamide 6 by melt processing

Layered double hydroxides (LDH) are a class of readily synthesizable layered crystals that can be used as an alternative to the commonly used silicate crystals for the preparation of polymeric nanocomposites. In this work layered double hydroxide/polyamide 6 nanocomposites (LDH/PA6) were prepared from organo-modified LDH by melt processing. The anionic exchange capacity of LDH was varied in order to investigate its influence on the degree of exfoliation. LDH were dispersed by a twin screw micro-extruder at a variety of processing conditions. The nanocomposites were characterized by wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), transmission electron microscopy, dynamic scanning calorimetry, and thermogravimetric analysis. 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/PA6. No major change in the crystalline phase or in the rate of crystallization was observed in the nanocomposite as compared to the neat polymer. A reduction in the onset of thermal decomposition temperature was observed in PA6/LDH compared to neat PA6, likely due to a nucleophilic attack mechanism. The properties of this nanocomposite system are discussed with connections to the current understanding within the broader nanocomposite field.

Theoretical evaluation of the microporosity of pillared layered double hydroxides

Over the last ten years, the concept of pillaring has frequently been applied on layered double hydroxides (LDHs). Due to the variety of possible anionic pillaring species and the adjustable layer charge density, LDHs offer good perspectives with regard to the creation of porous adsorbents and catalysts. But despite these possibilities, their porosity properties can still not compete with those of industrially applicable materials like zeolites. In this study, theoretical calculations based on geometrical models and performed on both Fe(CN)6-MgAl-LDHs (A) and [PV2W10 O40]-ZnAl-LDHs (B) were reported. Properties such as the micropore volume and the interpillar distance were calculated, and compared to experimental data. For a M(II)/M(III) ratio in the layers of 3, the theoretical maximum micropore volumes were 0.3843 cm3/g (A) and 0.1497 cm3/g (B), respectively. By implementing parameters like the stack size, pillars on the outside of the stacks and the possibility of collapse, the model was adjusted in order to create a realistic picture of the microstructure of pillared LDHs. This led to a better understanding of the limiting factors, and gave an explanation for the relatively low micropore volumes of pillared LDHs. For the Fe(CN)6-MgAl-LDHs, small interpillar distances were responsible for the partial inaccessibility of the interlayer regions by N2. This effect was the most pronounced for high charge density LDHs. The situation for the [PV2W10O40]-ZnAl-LDHs is more complex. Probably due to an incomplete pillaring process, the theoretical maximum values are not reached.