Exfoliation of layered double hydroxide solids into functional nanosheets

Exfoliated layered double hydroxide (LDH) nanosheets are a class of positively charged 2D nanoparticles. These LDH nanosheets can be used for a wide range of functional polymer nanocomposites and hierarchically structured materials. In this review we summarize and evaluate latest scientific advances in exfoliating LDH compounds, including exfoliation methods, mechanisms, properties and applications of exfoliated LDH nanosheets. Then identify the prevailing challenges and the directions of future work. In the first section of this review we dicuss the liquid exfoliation of LDH compounds, free of intercalation of organic species, in formamide, saturated aqueous L-asparagine or aqueous NaOH/urea solution with the help of ultrasonic treatment, mechanical shaking or stirring. We then examine the liquid exfoliation of LDH compounds with preceding organic species-intercalation. Such organic species include carboxylate, sulfonate, dodecyl sulfate, and amphoteric biomolecular amino acids. Next, we dicuss the exfoliation of LDH in monomer or polymer to form LDH/polymer nanocomposites. The polymer nanocomposites include LDH/polyethylene, LDH/polyethylene-graft-maleic anhydride, LDH/poly (vinyl alcohol), LDH/polystyrene, LDH/poly (ethylene terephthalate), LDH/poly (butylene adipate-co-terephtalate), LDH/poly ((butylene succinate)-co-adipate), LDH/poly caprolactone, LDH/poly (l-lactic) acid, LDH/acrylic polymer, LDH/polyacrylamide, LDH/poly(amide-imide) and LDH/poly (methyl methacrylate). The properties of these LDH/polymer nanocomposites are also discussed. In addition to polymer nanocomposites, recent studies indicate that the exfoliated LDH nanosheets can be used in electrorheological media, magnetic materials, low infrared emissivity materials, catalysts, and biosensors. The literature survey and analysis suggests that future work should place emphasis on the innovation of green and cost-effective exfoliation methods, and deepen understanding of the interfacial interactions between the LDH nanosheets and organic species/polymers, and the new assembly tactics of exfoliated LDH nanosheets with functional molecules or nanomaterials to expand applications.