Synthesis of layered double hydroxides through continuous flow processes: A review

Layered double hydroxides (LDHs) are a family of two-dimensional (2D) materials with layered crystal structures that have found many applications. Common strategies to synthesize LDHs lead to a wide variety of morphologies, from discrete 2D nanosheets to nanoflowers. Here, we report a study of carefully controlled LDH nanoplate syntheses using zinc aluminum (ZnAl) and cobalt aluminum (CoAl) LDHs as examples and reveal their crystal growth to be driven by screw dislocations. By controlling and maintaining a low precursor supersaturation using a continuous flow reactor, individual LDH nanoplates with well-defined morphologies were synthesized on alumina-coated substrates, instead of the nanoflowers that result from uncontrolled overgrowth. The dislocation-driven growth was further established for LDH nanoplates directly synthesized using the respective metal salt precursors. Atomic force microscopy revealed screw dislocation growth spirals, and under transmission electron microscopy, thin CoAl LDH nanoplates displayed complex contrast contours indicative of strong lattice strain caused by dislocations. These results suggest the dislocation-driven mechanism is generally responsible for the growth of 2D LDH nanostructures, and likely other materials with layered crystal structures, which could help the rational synthesis of well-defined 2D nanomaterials with improved properties.

A composite material, containing magnetite particles, graphene and layered double hydroxides (LDHs) was fabricated through a simple two-step reaction. Graphene was used as the matrix for supporting magnetite particles and LDH nanoplates. The synthesized magnetite-graphene-LDH (MGL) composites were characterized by field emission scanning electron microscopic (FE-SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transformed infrared (FTIR) spectroscopy, N2adsorption-desorption, and X-ray photoelectron spectroscopy (XPS). The MGL composites were applied to remove arsenate from aqueous solutions and could be easily separated by magnetic separation process. The results showed enhanced adsorption capacity of arsenate on the MGL as compared to that of pure Mg/Al LDHs. The surface area of MGL is greatly enhanced through the incorporation of magnetite particles and graphene, which provides more active sites for arsenate uptake. Moreover, LDHs were hybridized with mechanically and chemically stable graphene materials, providing an accessible diffusion pathway in the macropore domain, and therefore their adsorption capacity was enhanced. The fast and efficient adsorption of arsenate from solution to MGL suggests that the MGL composites are potential and suitable materials in the preconcentration of arsenate from large volumes of aqueous solutions in wastewater treatment.

We report the controlled synthesis of NiCo layered double hydroxide (LDH) nanoplates using a newly developed high temperature high pressure hydrothermal continuous flow reactor (HCFR), which enables direct growth onto conductive substrates in high yield and, most importantly, better control of the precursor supersaturation and, thus, nanostructure morphology and size. The solution coordination chemistry of metal-ammonia complexes was utilized to synthesize well-defined NiCo LDH nanoplates directly in a single step without topochemical oxidation. The as-grown NiCo LDH nanoplates exhibit a high catalytic activity toward the oxygen evolution reaction (OER). By chemically exfoliating LDH nanoplates to thinner nanosheets, the catalytic activity can be further enhanced to yield an electrocatalytic current density of 10 mA cm(-2) at an overpotential of 367 mV and a Tafel slope of 40 mV dec(-1). Such enhancement could be due to the increased surface area and more exposed active sites. X-ray photoelectron spectroscopy (XPS) suggests the exfoliation also caused some changes in electronic structure. This work presents general strategies to controllably grow nanostructures of LDH and ternary oxide/hydroxides in general and to enhance the electrocatalytic performance of layered nanostructures by exfoliation.

Layered double hydroxides (LDH) have shown to improve the zinc electrodeposition efficiency at the negative electrode of aqueous zinc‐ion batteries. In this work, a copper‐doped Zn−Al−CO3 layered double hydroxide (LDH) has been synthesized by co‐precipitation method under constant pH, and investigated as suitable solid‐state additive in zinc‐based negative electrodes. X‐ray diffraction patterns in combination with scanning electron microscope images show that the as‐synthesized LDHs are well crystalline and hexagonal platelet‐like. LDH was mixed with zinc powder in different ratios and the electrochemical performances of the mixtures were characterized by galvanostatic cycling with potential limitation (GCPL) at different current rates. The results show that an appropriate combination of zinc and LDH can be used to reach electrodeposition efficiency equal to 98 %. Thereafter, the performance of this electrode in a full cell is studied. In such configuration, the electrode shows that the electrodeposition efficiency remains high even at high current rates, which is an important characteristic for grid‐scale energy storage.

Layered double hydroxides (LDHs) have a great potential in arsenic removal due to their large surface areas and high anion exchange capacities. In this study, a new synthesis method involving fast coprecipitation followed by hydrothermal treatment has been used to synthesize stable homogeneous nano-scale MgAl-LDHs for As (V) removal. The products have been characterized using CHN, BET, XRD, FTIR, SEM, and TEM. The nano-scale LDHs synthesized in this study showed a poorer crystallinity, higher interlayer space, and smaller crystallite size than the commercial LDHs. The photon correlation spectroscopy showed that the nano-scale LDHs have a narrow particle size distribution with an equivalent hydrodynamic diameter of 122 nm. The adsorption isotherm of As (V) on the nano-scale LDHs with initial As (V) concentration 0.2-8 mg·dm-3 was best fitted by Freundlich model, while the isotherm with initial As (V) concentration 12-200 mg·dm-3 was best described by Langmuir model. The maximum adsorption capacity of As (V) on nano-scale LDHs was 78.1 mg/g, which was about 10 times higher than that of commercial LDHs. As (V) adsorption on nano-scale LDHs was a rapid process that achieved 92% adsorption equilibrium in 10 min. The adsorption of As (V) on nano-scale LDHs was confirmed by EDX analysis, and FTIR by showing a typical FTIR band at 830 cm-1. As (V) adsorbed on the nano-scale LDHs was effectively desorbed using 5 w/v% NaOH solution and the LDHs were successfully regenerated using 20 w/v% Mg (NO3) 2.6H2O solution. The results suggest that nano-scale LDHs as potential adsorbent for As (V) from waters, and that its reusability is possible.

2D/2D NiAl LDH integrated graphitic carbon nitride with robust interfacial contact for driving photocatalytic hydrogen production

Effective concentration of dichromate anions using layered double hydroxides from acidic solutions

An integrated tandem photoelectrochemical (PEC) cell, composed of a three-dimensional (3D) ZnO/CdS/NiFe layered double hydroxide (LDH) core/shell/hierarchical nanowire arrays (NWAs) photoanode and a [Formula: see text]-Cu2O photocathode, was designed for unassisted overall solar water splitting in this study. The optical and photoelectrochemical characteristics of ZnO-based photoanodes and Cu2O photocathode were investigated. The results show that ZnO/CdS/NiFe LDH nanostructures offer significantly enhanced performances with a photocurrent density reaching 5.8[Formula: see text]mA[Formula: see text][Formula: see text][Formula: see text]cm[Formula: see text] at 0.9[Formula: see text]V and an onset potential as early as 0.1[Formula: see text]V (versus RHE). The enhancement can be attributed to the existence of CdS nanoparticles (NPs) which boosts the light absorption in visible region and enhances charge separation. Moreover, the introduction of NiFe LDH nanoplates, with unique hierarchical mesoporous architecture, promotes electrochemical reactions by providing more active sites as co-catalyst. On the above basis, the ZnO/CdS/NiFe LDH–Cu2O two-electrode tandem cell system was established. At zero bias, the device shows a photocurrent density of 0.4[Formula: see text]mA[Formula: see text][Formula: see text][Formula: see text]cm[Formula: see text] along with the corresponding solar-to-hydrogen (STH) conversion efficiency reaching 0.50%. Our results indicate that the tandem PEC cells consisting of metal–oxide–semiconductor photoelectrodes based on Earth-abundant and low-cost materials hold promising application potential for overall solar water splitting.

High-entropy hexacationic layered double hydroxides of the cationic composition MgNiCoAlFeY were obtained by five different methods: coprecipitation at constant pH, coprecipitation at constant or variable pH followed by hydrothermal treatment, microwave assisted solvothermal, hydrothermal, mechanochemical method followed by hydrothermal treatment. All samples, except for the one obtained by coprecipitation at variable pH, are phase pure, with a uniform distribution of cations. The samples were characterized by X-ray diffraction, infrared spectroscopy, Raman spectroscopy, transmission electron microscopy. Thermal transformations of the samples were studied. The synthesis method affects the characteristics of the samples. The sample obtained by hydrothermal synthesis at variable pH possesses magnetic properties. The largest particles and those morphologically close to the hexagonal shape are formed by coprecipitation followed by hydrothermal treatment. The sample obtained by the microwave assisted solvothermal method is characterized by lower thermal stability.

A controlled radical polymerization process using the Reversible Addition-Fragmentation Chain Transfer (RAFT) approach was scaled up by a factor of 100 from a small laboratory scale of 5 mL to a preparative scale of 500 mL, using batch and continuous flow processing. The batch polymerizations were carried out in a series of different glass vessels, using either magnetic or overhead stirring, and different modes of heating: Microwave irradiation or conductive heating in an oil bath. The continuous process was conducted in a prototype tubular flow reactor, consisting of 6 mm ID stainless steel tubing, fitted with static mixers. Both reactor types were tested for polymerizations of the acid functional monomers acrylic acid and 2-acrylamido-2-methylpropane-1-sulfonic acid in water at 80 °C with reaction times of 30 to 40 min. By monitoring the temperature during the exothermic polymerization process, it was observed that the type and size of reactor had a significant influence on the temperature profile of the reaction.

Tailoring the properties of polyamide thin film membrane with layered double hydroxide nanoclay for enhancement in water separation

Zinc-Aluminum layered double hydroxide: High efficient removal by adsorption of tartrazine dye from aqueous solution

Zn–Cr layered double hydroxide: Visible light responsive photocatalyst for photocatalytic degradation of organic pollutants

Degradation by irradiation of an Acid Orange 7 on colloidal TiO2/(LDHs)

Hyperfine interaction in Zn–Al layered double hydroxides intercalated with conducting polymers