Atomistic simulation of nanoporous layered double hydroxide materials and their properties. I. Structural modeling

An atomistic model of layered double hydroxides, an important class of nanoporous materials, is presented. These materials have wide applications, ranging from adsorbents for gases and liquid ions to nanoporous membranes and catalysts. They consist of two types of metallic cations that are accommodated by a close-packed configuration of OH- and other anions in a positively charged brucitelike layer. Water and various anions are distributed in the interlayer space for charge compensation. A modified form of the consistent-valence force field, together with energy minimization and molecular dynamics simulations, is utilized for developing an atomistic model of the materials. To test the accuracy of the model, we compare the vibrational frequencies, x-ray diffraction patterns, and the basal spacing of the material, computed using the atomistic model, with our experimental data over a wide range of temperature. Good agreement is found between the computed and measured quantities.     

Characterization of self-assembled films of NiGa layered double hydroxide nanosheets and their electrochemical properties

In this study, we have demonstrated the synthesis and delamination of a rarely studied NiGa layered double hydroxide (LDH) system. Hydrothermal treatment under agitation conditions at 200 °C for 4 h resulted in the formation of highly crystalline NiGa LDHs in a shorter time than those synthesized without agitation. The LDH was delaminated into the individual nanosheets in formamide. The most significant finding in this study is the electrochemical behavior of interlayer ferricyanide anions intercalated with the layer-by-layer (LBL) assembly method. The morphology of LBL film with one layer is also monitored with atomic force microscopy. The cyclic voltammogram is similar to potassium metal hexacyanoferrate systems with its unique two-peak wave. Raman spectrum of the film revealed that the metal center of the interlayer cyano complex is in interaction with the Ni²⁺ of the host layer. It was concluded that the two-peak cyclic voltammogram of the film is a result of two different forms of the hexacyanoferrate in the interlayer.     

Nanostructured layered double hydroxide aerogels with enhanced adsorption properties

Aerogels of layered double hydroxides were prepared by a simple and eco-friendly method involving a quick coprecipitation followed by supercritical CO(2) drying. Such aerogels display high surface areas and enhanced adsorption behavior.     

Synthesis,characterization and catalytic properties of tetrachlorocuprate(II) immobilized on layered double hydroxide

Readily prepared copper(II) immobilized on layered double hydroxide has been found to effectively catalyse the 1,3‐dipolar cycloaddition (CuAAC) of a variety terminal alkynes and benzyl azides generated in situ from sodium azide and benzyl halides furnishing the corresponding 1,2,3‐triazoles in excellent yields. The advantages of the protocol are short reaction time, mild reaction conditions, reusability of the catalyst and applicability to a wide range of substrates.     

Computer simulation study of the structural stability and materials properties of DNA-intercalated layered double hydroxides

The intercalation of DNA into layered double hydroxides (LDHs) has various applications, including drug delivery for gene therapy and origins of life studies. The nanoscale dimensions of the interlayer region make the exact conformation of the intercalated DNA difficult to elucidate experimentally. We use molecular dynamics techniques, performed on high performance supercomputing grids, to carry out large-scale simulations of double stranded, linear and plasmid DNA up to 480 base pairs in length intercalated within a magnesium-aluminum LDH. Currently only limited experimental data have been reported for these systems. Our models are found to be in agreement with experimental observations, according to which hydration is a crucial factor in determining the structural stability of DNA. Phosphate backbone groups are found to align with aluminum lattice positions. At elevated temperatures and pressures, relevant to origins of life studies which maintain that the earliest life forms originated around deep ocean hydrothermal vents, the structural stability of LDH-intercalated DNA is substantially enhanced as compared to DNA in bulk water. We also discuss how the materials properties of the LDH are modified due to DNA intercalation.     

Sugar-anionic clay composite materials: intercalation of pentoses in layered double hydroxide

The intercalation of non-ionized guest pentoses (ribose and 2-deoxyribose) into the Mg-Al and Zn-Al layered double hydroxides (LDHs) was carried out at 298 K by the calcination-rehydration reaction using the Mg-Al and Zn-Al oxide precursors calcined at 773 K. The resulting solid products reconstructed the LDH structure with incorporating pentoses, and the maximum amount of ribose intercalated by the Mg-Al oxide precursor was approximately 20 times that by the Zn-Al oxide precursor. The ribose/Mg-Al LDH was observed to have the expanded LDH structure with a broad (003) spacing of 0.85 nm. As the thickness of the LDH hydroxide basal layer is 0.48 nm, the interlayer distance of the ribose/Mg-Al LDH is 0.37 nm. This value corresponds to molecular size of ribose in thickness (0.36 nm), supporting that ribose is horizontally oriented in the interlayer space of LDH. The maximum amount of ribose intercalated by the Mg-Al oxide precursor was approximately 5 times that of 2-deoxyribose. Ribose is substituted only by the hydroxyl group at C-2 position for 2-deoxyribose. Therefore, the number of hydroxyl group of sugar is essentially important for the intercalation of sugar molecule into the LDH, suggesting that the intercalation behavior of sugar for the LDH was greatly influenced by hydrogen bond between hydroxyl group of the intercalated pentose and the LDH hydroxide basal layers.     

Hollow nanoshell of layered double hydroxide

Hollow nanoshells of layered double hydroxide (LDH) have been fabricated using exfoliated LDH nanosheets as a shell building block and polystyrene beads as a sacrificial template.     

Intercalation of an oxalatooxoniobate complex into layered double hydroxide and layered zinc hydroxide nitrate

A Zn/Al layered double hydroxide with molar ratio of 3 was prepared by coprecipitation in alkaline pH and used as a matrix to intercalate the ionic complex diaquadioxalatooxoniobate(V) (DDON), derived from NH₄[NbO(C₂O₄)₂(H₂O)₂]2H₂O. In a similar way, the layered zinc hydroxide nitrate, Zn₅(OH)₈(NO₃)₂2H₂O, was synthesized, preexpanded with azelate ions (⁻OOC(CH₂)₇COO⁻), and then intercalated with the niobium complex. For both layered matrices, the results from X-ray powder diffractometry, Fourier transform infrared spectroscopy, and thermal analysis (TG/s-DTA) indicate the presence of the oxalate ion. In addition, results from X-ray photoelectron and Raman spectroscopy indicate the presence of the niobium center bonded to oxygen atoms. Finally, diffuse reflectance UV–vis spectroscopy suggests that the niobium centers are coordinated to oxalate ions. This is the first report of the intercalation of niobium into a layered matrix.     

LDPE/Mg-Al layered double hydroxide nanocomposite: Thermal and flammability properties

Layered double hydroxides (LDHs) are new nanofillers which exhibit improved thermal and flammability properties in various kinds of polymer matrices. These materials have certain advantages over conventional metal hydroxides and also layered silicates so far as the flame retardancy is concerned. In this article, flammability and thermal properties of the nanocomposite based on low density polyethylene (LDPE) and Mg-Al based layered double hydroxide (Mg-Al LDH) are reported in detail. The nanocomposites containing different LDH concentrations were prepared by melt-compounding using a tightly intermeshing co-rotating twin-screw extruder. The morphological analysis reveals an exfoliated/intercalated type LDH particle morphology in these nanocomposites. The thermogravimetric analysis (TGA) shows that even a small amount of LDH improves the thermal stability and onset decomposition temperature in comparison with the unfilled LDPE. The heat release rate (HRR) and its maximum (PHRR) during cone-calorimeter investigation are found to be reduced significantly with increasing LDH concentration. The nanocomposites not only exhibit reduced total heat released (measure of propensity to produce long duration fire), but also lower tendency to fast fire growth (measured by the ratio of PHRR and time of ignition). The limited oxygen index (LOI) and the dripping behavior are also improved with increasing LDH concentration.     

Surface-charging behavior of Zn-Cr layered double hydroxide

A Zn-Cr layered double hydroxide (LDH) having the formula Zn(2)Cr(OH)(6)Cl(0.7)(CO(3))(0.15)2.1H(2)O was synthesized and characterized by powder X-ray diffraction, infrared spectroscopy, acid-base potentiometric titration, mass titration, electrophoretic mobility, and modeling of the electrical double layer. Adsorption of alizarin was also performed in order to show some particular features of the HDL. Net hydroxyl adsorption, which increases with increasing pH and decreasing supporting electrolyte concentration, takes place above pH 5. The electrophoretic mobility of the particles was always positive and it decreased when the pH was higher than 9. An isoelectric point of 12 could be estimated by extrapolating the data. The modified MUSIC model was used to estimate deprotonation constants of surface groups and different adsorption models were compared. Good fit of hydroxyl adsorption and electrophoresis could be achieved by considering both OH(-)/Cl(-) exchange at structural sites and proton desorption from surface hydroxyl groups. The modeling, in agreement with alizarin adsorption, indicates that most of the structural positive charge of the LDH is screened at the surface by exchanged anions and negatively charged surface groups. It also suggests that only structural charge sites initially neutralized by chloride ions are active for anion exchange. The remaining sites are blocked by carbonate and do not participate in the exchange.     

Preparation of sodium oleate/layered double hydroxide composites with acid-resistant properties

MgAlCO3 type layered double hydroxides (LDHs) with Mg/Al ratios ranging from 2 to 5 were synthesized by coprecipitation. Composites with sodium oleate/LDH were prepared by ion exchange and reconstruction of the LDH in sodium oleate solution. The amount of sodium oleate in the composites prepared by this reconstruction method was higher than that in samples prepared by the ion-exchange method. The basal spacings of the LDHs increased to 3.9 and 1.8 nm after synthesis of the composites, these spacings being in good agreement with models based on the assumption that the oleate ions are intercalated as bilayer and/or micelle structures, and as monolayers in the LDH interlayers, respectively. The number of sorbed oleate ions was higher than calculated from the anion-exchange capacities of the LDHs in most of the samples, increasing as the Mg/Al ratios of the LDHs were increased from 2 to 5. These results suggest that the oleate ions are present not only in the interlayers but also on the surfaces of the LDH particles. The acid-resistant properties of the composites were found to be much higher than for the pure LDHs. It is thus confirmed that the surfaces of the LDH particles in the composites are mostly covered with sorbed oleate ions and that the composites are good candidates as drug delivery materials.     

Adsorption and diffusion of argon in disordered nanoporous carbons

Application-specific optimization of disordered nanoporous carbons remains a formidable challenge due to the difficulty in accurately characterizing their microstructures with current empirical methods. Using molecular simulation techniques, we investigated the adsorptive and diffusive behavior of argon in three models of disordered nanoporous carbons. We found that the structural and morphological differences between these models gave rise to distinct phenomenological properties. The adsorptive behavior of argon in both the low and high pressure regimes was enhanced dramatically in the models with more crystalline microstructures. As for dynamic properties, we found that the adsorbent’s structure and energetic topology significantly alters the rates of diffusion as well as the characteristics of the underlying diffusion mechanisms.     

Layered assembly of graphene oxide and Co-Al layered double hydroxide nanosheets as electrode materials for supercapacitors

An innovative strategy of fabricating electrode material by layered assembling two kinds of one-atom-thick sheets, carboxylated graphene oxide (GO) and Co-Al layered double hydroxide nanosheet (Co-Al LDH-NS) for the application as a pseudocapacitor is reported. The Co-Al LDH-NS/GO composite exhibits good energy storage properties.     

Controllable photoluminescence properties of an anion-dye-intercalated layered double hydroxide by adjusting the confined environment

This article reports a novel method to tune the photoluminance properties of ammonium 1-anilinonaphthalene-8-sulfonate (ANS) in a 2D matrix of layered double hydroxide (LDH) by changing the interlayer microenvironment. ANS and a series of surfactants with different alkyl chain lengths (pentanesulphonate (PES), hexanesulphonate (HES), heptanesulphonate (HPS), decanesulphonate (DES), and dodecylsulphonate (DDS)) were respectively cointercalated into the galleries of ZnAl-LDH by the anion exchange method. Thin films of ANS/surfactant-LDHs obtained by the solvent evaporation method possess good c orientation as revealed by XRD and SEM. It was found that the ANS/HPS-LDH film showed the maximum fluorescence efficiency and the longest intensity-average lifetime among these ANS/surfactant-LDH composites owing to the "size-matching" rule between the organic dye and surfactant. Moreover, the fluorescence properties can be tuned by changing the relative molar ratio of ANS/HPS, and the film containing 20% ANS (molar percentage, expressed as ANS(20%)/HPS-LDH) exhibits the maximum fluorescence efficiency, the longest average lifetime, and significantly enhanced photo and thermal stability. In addition, the composite films show fluorescence anisotropy, attributed to the preferential orientation of ANS in the LDH gallery. Therefore, this work demonstrates a feasible approach to tuning the photoluminescence properties of a dye confined in an inorganic 2D matrix via changing the interlayer microenvironment, which may be considered to be a good candidate for solid photoluminescence materials, nonlinear optics, and polarized luminescence materials.     

Orientation-controlled assembly and solvothermal ion-exchange of layered double hydroxide nanocrystals

The orientation-controlled LDH crystals on Si substrates were intercalated by dicarboxylate ions to give the anisotropic layer expansion.     

Thermally stimulated luminescence of a natural layered double hydroxide

Layered double hydroxides (LDHs) and many of the related hydrotalcite-like minerals have been well studied from the chemical and structural point of view; however, their luminescence properties have been scarcely studied. We herein report on the thermoluminescence (TL) behaviour of a natural LDH (Mg₆Cr₂CO₃(OH)₁₆·4H₂O), previously characterized by X-ray fluorescence, X-ray energy-dispersive spectrometry, electron probe microanalysis, thermogravimetry and differential thermal analysis, that exhibited a very complex green-IR spectral emission. The broad waveband peaked at ~?640 nm can be mainly linked to the ⁴T₁?→?⁶A₁ (at 570 nm), ⁴A_2g?→?²E_g (~?685 nm), ⁴T₁?→?⁶A₁ (~?700 nm), and ¹T_2g?→?³A_2g (green) and ¹T_2g?→?³T_2g (red) transitions due, respectively, to the presence of Mn²⁺, Cr³⁺, Fe²⁺ and Ni²⁺. The weak red-TL emission can likely be attributed to the quenching effect due to Fe (~?8–11%) ions substituting for Mg²⁺.     

Benzocarbazole anions intercalated layered double hydroxide and its tunable fluorescence

Luminescent benzocarbazole anions (BCZC) intercalated into the interlayer region of Mg-Al-layered double hydroxides (BCZC/LDH) with different layered charge densities (LCD) were prepared. The structure and chemical composition of the composites were characterized by X-ray diffraction, elemental analysis, thermogravimetry and differential thermal analysis (TG-DTA), infrared spectra (FT-IR), UV-vis absorption and fluorescence spectroscopy. The photoemission behavior of BCZC in the LDH matrix with high (Mg/Al ratio = 1.801) and low (Mg/Al ratio = 3.132) LCD is similar to that of BCZC solid and aqueous solution states respectively, indicating that the luminescence performances of the intercalated dye anions can be tuned by adjusting the LCD of the LDH layer. Moreover, the thermal stability and stacking order of BCZC are largely improved upon intercalation, and the BCZC/LDH thin film exhibits well polarized luminescence with the luminescent anisotropy of 0.15-0.20. In addition, molecular dynamics (MD) simulation was employed to calculate the basal spacing and molecular arrangement of the intercalated BCZC within the LDH matrix. The simulation results show that the distribution of BCZC anions is much broader in the gallery of Mg-Al-LDH with high LCD, while BCZC anions exhibit a more ordered arrangement in LDH with low LCD. Furthermore, the radial distribution functions of interlayer water molecules were also studied. Based on the combination of experiment and theoretical simulation, this work provides a detailed understanding of the tunable photoluminescence, orientation and diffusion behavior of the luminescent molecules confined within the gallery of a 2D inorganic matrix.     

Molecular simulation of loading-dependent diffusion in nanoporous materials using extended dynamically corrected transition state theory

A dynamically corrected transition state theory method is presented that is capable of computing quantitatively the self-diffusivity of adsorbed molecules in confined systems at nonzero loading. This extension to traditional transition state theory is free of additional assumptions and yields a diffusivity identical to that obtained by conventional molecular-dynamics simulations. While molecular-dynamics calculations are limited to relatively fast diffusing molecules, our approach extends the range of accessible time scales significantly beyond currently available methods. We show results for methane, ethane, and propane in LTL- and LTA-type zeolites over a wide range of temperatures and loadings, and demonstrate the extensibility of the method to mixtures.