Controllable Synthesis and Scale-up Production Prospect of Monolayer Layered Double Hydroxide Nanosheets

As a type of layered material, layered double hydroxides (LDHs) exhibit high development potential and application prospects, and have been used widely in adsorbents, catalysts, ion exchangers, flame retardants, biology, sensing, medicine, and other fields. With the continued development in nanoscience and nanotechnology, it has been established that monolayer LDHs contain an abundance of exposed highly unsaturated coordination sites, and so display unexpected functionality. However, due to the higher charge density of the LDHs layers, the strong interactions between the layers, and the hydroxyl groups on the surface of the layers, the result is a compact stacking of the layers. Consequently, it is still a great challenge to synthesize high-quality monolayer LDHs. Despite various methods of preparing monolayer LDHs having been developed, which can generally be divided into top-down and bottom-up strategies, most of these approaches have used organic solvents, which take a long time to achieve the exfoliation of LDHs, or require special equipment. Furthermore, high costs and the low yields have prevented large-scale production of monolayer LDHs. With the rapid development of the national economy, the industrial preparation of monolayer LDHs has become an inescapable trend. The separate nucleation and ageing method for the preparation of nanostructured LDHs is a feasible method, the key features of which are a very rapid mixing and nucleation process in a colloid mill, followed by a separate ageing process. This method has been successfully applied to a pilot plant in China for the industrial-scale synthesis of LDHs materials. It should be noted that the particle size distribution of LDHs obtained by this method can be well controlled. Moreover, the synthesis operation is simple, and quick (with a short duration of only several minutes). Through new in-depth technology studies on two-dimensional layered materials, large-scale preparation, and industrial application of monolayer LDHs will certainly be increasingly realized, and ultimately transformed into economic benefits. In this review, we summarize the synthesis method of monolayer LDHs, describe the necessary characterization technologies that have been used to study monolayers LDHs nanosheets, such as X-ray diffraction, transmission electron microscopy, and atomic force microscopy. Then we discuss the applications in various fields, such as photocatalysis, electrocatalysis, batteries, supercapacitors, membrane materials, and biomedical fields. We further discuss the recent breakthroughs in the synthesis of monolayer and ultrathin LDHs and the advance of production scale-up of LDHs. Finally, the performance of monolayer/ultrathin LDHs is summarized to provide a basis for the ensuing design of high-performance monolayer LDHs.