Synthesis of two-Dimensional layered double hydroxide: A systematic overview

Two-dimensional (2D) layered double hydroxides (LDH) are classic materials in fundamental research and practical application. 2D LDH have unique structural features, such as high aspect ratio, high specific surface area,...

Two-dimensional biomaterials: material science, biological effect and biomedical engineering applications

To date, nanotechnology has increasingly been identified as a promising and efficient means to address a number of challenges associated with public health. In the past decade, two-dimensional (2D) biomaterials, as a unique nanoplatform with planar topology, have attracted explosive interest in various fields such as biomedicine due to their unique morphology, physicochemical properties and biological effect. Motivated by the progress of graphene in biomedicine, dozens of types of ultrathin 2D biomaterials have found versatile bio-applications, including biosensing, biomedical imaging, delivery of therapeutic agents, cancer theranostics, tissue engineering, as well as others. The effective utilization of 2D biomaterials stems from the in-depth knowledge of structure-property-bioactivity-biosafety-application-performance relationships. A comprehensive summary of 2D biomaterials for biomedicine is still lacking. In this comprehensive review, we aim to concentrate on the state-of-the-art 2D biomaterials with a particular focus on their versatile biomedical applications. In particular, we discuss the design, fabrication and functionalization of 2D biomaterials used for diverse biomedical applications based on the up-to-date progress. Furthermore, the interactions between 2D biomaterials and biological systems on the spatial-temporal scale are highlighted, which will deepen the understanding of the underlying action mechanism of 2D biomaterials aiding their design with improved functionalities. Finally, taking the bench-to-bedside as a focus, we conclude this review by proposing the current crucial issues/challenges and presenting the future development directions to advance the clinical translation of these emerging 2D biomaterials.

Remediation of chromium-contaminated soil using delaminated layered double hydroxides with different divalent metals

Soil is commonly polluted by chromium, and layered double hydroxides (LDHs) are widely used for chromium removal due to their strong adsorption capacity and the unique properties of their delaminated products. In this study, delaminated LDHs (S-Mg-LDH and S-Ca-LDH) and their original LDHs were used to remediate Cr(VI)-contaminated soil. A series of characterizations confirmed the successful synthesis of delaminated LDHs whose sheet structure was thinner with a greater surface energy than the original LDHs. The remediation results indicated that delaminated LDHs could more efficiently immobilize Cr(VI) in soil. The immobilization rate of S-Mg-LDH was 64.32%, while Mg-LDH was only 8.09%. However, at low dosages, the efficiency of S-Ca-LDH was 28.1% while Ca-LDH was 5.16%, but they had similar effects at high doses. Moreover, soil pH had little effect on their removal efficiencies. The toxicity characteristic leaching procedure (TCLP) results showed that the leaching of Cr(VI) in soil after treatment with S-Mg-LDH, S-Ca-LDH, Mg-LDH, and Ca-LDH was reduced by 75.43%, 72.43%, 86.55%, and 75.90%, respectively. The phytotoxicity tests of soil treated by S-Mg-LDH and S-Ca-LDH revealed that they effectively reduced the toxicity of chromium and lowered its bioaccumulation. Overall, this study confirms the feasibility of delaminated LDHs for Cr(VI) immobilization in soils.

CuAl LDH/Rice Husk Biochar Composite for Enhanced Adsorptive Removal of Cationic Dye from Aqueous Solution

The preparation of CuAl LDH and biochar (BC) composite derived from rice husk and its application as a low-cost adsorbent for enhanced adsorptive removal of malachite green has been studied. The composite was prepared by a one-step coprecipitation method and characterized by X-ray Diffraction (XRD), Fourier Transform Infra Red (FTIR), Brunauer-Emmett-Teller (BET), and Scanning Electron Microscopy - Energy Dispersive X-ray (SEM−EDX). The result indicated that CuAl LDH was successfully incorporated with the biochar that evidenced by the broadening of XRD peak at 2θ = 24° and the appearance of a new peak at 1095 cm−1 on the FTIR spectra. The BET surface area analysis revealed that CuAl/BC composite exhibited a larger surface area (200.9 m2/g) that the original CuAl LDH (46.2 m2/g). Surface morphological changes also confirmed by SEM image, which showed more aggregated particles. The result of the adsorption study indicated the composite material was efficient in removing malachite green with Langmuir maximum adsorption capacity of CuAl/BC reaching 470.96 mg/g, which is higher than the original CuAl LDH 59.523 mg/g. The thermodynamic analysis suggested that the adsorption of malachite green occurs spontaneously (ΔG < 0 at all tested temperature) and endothermic nature. Moreover, the CuAl/BC composite showed strong potential as a low-cost adsorbent for cationic dye removal since it showed not only a high adsorption capacity but also good reusability. Copyright © 2020 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0). 

Powerful combination of g-C3N4 and LDHs for enhanced photocatalytic performance: A review of strategy, synthesis, and applications

The utilization of solar energy with photocatalytic technology has been considered a good solution to alleviate environmental pollution and energy shortage. Constructing 2D/2D heterostructure photocatalysts with layered double hydroxide (LDH) and graphitic carbon nitride (g-C₃N₄) is an effective approach to attain high performance in solar photocatalysis. This paper provides a review of recent studies about 2D/2D LDH/g-C₃N₄ heterostructure photocatalysts. Main strategies for constructing the desired 2D/2D heterojunction are summarized. The planar structure of LDH and g-C₃N₄ offers a shorter transfer distance for charge carriers and reduces electron-hole recombination in the bulk phase. The face-to-face contact between the two materials can promote the charge transfer across the heterostructure interface, thus improving the electron-hole separation efficiency. The performance and mechanisms of LDH/g-C₃N₄ photocatalysts in hydrogen production, CO₂ reduction, and organic pollutant degradation are analyzed and discussed. Incorporating reduced graphene oxide or Ag nanoparticles into LDH/g-C₃N₄ heterojunction and fabricating calcined LDH/g-C₃N₄ composites are effective strategies to further facilitate charge transfer at the interface of LDH and g-C₃N₄ and improve the absorption capacity for visible light. This review is expected to provide basic insights into the design of 2D/2D LDH/g-C₃N₄ heterojunctions and their applications in solar photocatalysis.

Organic Intercalant-Free Liquid Exfoliation Route to Layered Metal-Oxide Nanosheets via the Control of Electrostatic Interlayer Interaction

A scalable organic intercalant-free liquid exfoliation route to 2D nanosheets (NSs) of layered transition-metal oxides (TMOs) is developed by employing hydronium-intercalated derivatives as precursors. The replacement of interlayer alkali metal ions with larger hydronium ions via acid treatment makes possible the efficient liquid exfoliation of TMOs without any assistance of organic intercalant cations. Not only a weakening of interlayer electrostatic interaction upon hydronium intercalation but also an efficient solvation of deintercalated hydronium ions via hydrogen bonding with polar solvents is mainly responsible for the high efficacy of hydronium-intercalated TMOs as precursors for liquid exfoliation. The nature of the solvent employed also has a profound effect on the exfoliation yield of these TMO NSs; viscosity, surface tension, density, and Hansen solubility parameter as well as the capability to solvate the exfoliated NSs and hydronium ions are crucial factors for determining the exfoliation efficiency of the hydronium-intercalated precursor. All the obtained Ti_{1- x}O₂, MnO₂, and RuO₂ NSs show highly anisotropic 2D morphologies and distinct negative surface charges with a zeta potential of -30 to -50 mV. Such distinct surface charges of these NSs render them versatile hybridization matrices for the synthesis of novel nanohybrids with enhanced functionalities. The hybridization with the liquid-exfoliated TMO NSs is quite effective in improving the photocatalytic activity of CdS and the electrode functionalities of graphene and graphene-layered double hydroxide nanohybrids. The present study underscores the usefulness of the present liquid exfoliation method in synthesizing organic-free TMO NSs and their nanohybrids as well as in widening the application field of exfoliated TMO NSs.

A novel method to delaminate nitrate-intercalated MgAl layered double hydroxides in water and application in heavy metals removal from waste water

Nitrate-intercalated MgAl layered double hydroxide (LDH) was successfully delaminated in water by a facile and effective method upon reflux at 120 °C for 24 h followed by sonication at 40 °C for 5 h. This process is environmentally friendly since water is the only solvent used. The delaminated nanosheets were characterized by microscopic, spectroscopic, and particle size analyses. The delamination process successfully produced octahedron-shaped single-layer nanosheets 50-150 nm in size. X-ray photoelectron spectroscopy (XPS) data confirmed that the surface elements and their chemical status are consistent with the basic layer of MgAl LDH. The delaminated nanosheets displayed higher adsorption capacity for removing heavy metals from waste water than the original powdered LDH. After treating the waste water, a sharp and intense peak in the X-ray powder diffraction (XRD) pattern of the precipitate confirms the restacking of the LDH nanosheets.

Preparation of two dimensional layered double hydroxide nanosheets and their applications

Layered double hydroxides (LDHs) with their highly flexible and tunable chemical composition and physical properties have attracted tremendous attention in recent years. LDHs have found widespread application as catalysts, anion exchange materials, fire retardants, and nano-fillers in polymer nanocomposites. The ability to exfoliate LDHs into ultrathin nanosheets enables a range of new opportunities for multifunctional materials. In this review we summarize the current available LDH exfoliation methods. In particular, we highlight recent developments for the direct synthesis of single-layer LDH nanosheets, as well as the emerging applications of LDH nanosheets in catalyzing oxygen evolution reactions and preparing light emitting devices, supercapacitors, and flame retardant nanocomposites.

Ultra-high aspect ratio hybrid materials: the role of organic guest and synthesis method

Hybrid layered double hydroxide materials with high aspect ratio have been prepared by slow hydrolysis of metal hydroxides with hydrophobic anions.

Morphology-Conserved Transformations of Metal-Based Precursors to Hierarchically Porous Micro-/Nanostructures for Electrochemical Energy Conversion and Storage

To meet future market demand, developing new structured materials for electrochemical energy conversion and storage systems is essential. Hierarchically porous micro-/nanostructures are favorable for designing such high-performance materials because of their unique features, including: i) the prevention of nanosized particle agglomeration and minimization of interfacial contact resistance, ii) more active sites and shorter ionic diffusion lengths because of their size compared with their large-size counterparts, iii) convenient electrolyte ingress and accommodation of large volume changes, and iv) enhanced light-scattering capability. Here, hierarchically porous micro-/nanostructures produced by morphology-conserved transformations of metal-based precursors are summarized, and their applications as electrodes and/or catalysts in rechargeable batteries, supercapacitors, and solar cells are discussed. Finally, research and development challenges relating to hierarchically porous micro-/nanostructures that must be overcome to increase their utilization in renewable energy applications are outlined.

A luminescent ultrathin film with reversible sensing toward pressure

A flexible ultrathin film based on alternate assembly of a sodium polyacrylate (PAA) modified styrylbiphenyl derivative (BTBS) and layered double hydroxide nanosheets is fabricated, which exhibits pressure-responsive photoluminescence with a high sensitivity and good reversibility.

One pot synthesis of Mg2Al(OH)6Cl·1.5H2O layered double hydroxides: the epoxide route

Pure Mg2Al(OH)6Cl·1.5H2O layered double hydroxide (LDH) has been synthesized at room temperature by a one-pot method, homogeneously driven by chloride-assisted glycidol rupture (epoxide route). Well-defined nanoplatelet texture was achieved and the LDH crystallization mechanism discussed. Nanoplatelets self-assemble in the form of highly oriented films with excellent optical properties. LDH films exhibited stability toward detaching in aqueous solutions and allowed a fast anionic exchange preserving a high transparency.

Orderly ultrathin films based on perylene/poly(N-vinyl carbazole) assembled with layered double hydroxide nanosheets: 2D fluorescence resonance energy transfer and reversible fluorescence response for volatile organic compounds

Neutral poly(N-vinyl carbazole) (PVK) and perylene are coassembled within the interlayers of layered double hydroxide (LDH) nanosheets to form (perylene@PVK/LDH)(n) ultrathin films by the hydrogen-bond layer-by-layer assembly method. An efficient 2D fluorescence resonance energy transfer (FRET) process from PVK to perylene is demonstrated, and this FRET process can be inhibited/recovered reversibly by the adsorption/desorption of common volatile organic compounds (VOCs).