Recent advance in ultrathin/ultrasmall layered double hydroxides

Abstract

Layered double hydroxides (LDHs), as a class of anionic intercalation materials, have been widely used in various fields such as photo/electro-catalysis, biology, medicine, etc. With the dramatic development of nanotechnology, LDHs-based multifunctional materials have drawn great attention due to their unique properties. This review particularly focuses on the fine control of the particle size and thickness of LDHs from three different perspectives including synthetic strategy, characteristic method, LDHs application, which may pave a new pathway for the rational design of LDHs-based functional materials. In this review, there are five individual synthetic methods summarized of ultrathin/ultrasmall LDH. The first one is involving separate nucleation and aging steps. LDHs have been prepared by the rapid mixing of the precursor solutions at the nucleation step and followed by a separate aging step. Moreover, the size distribution of LDH is narrow. The second method is exfoliation of LDHs through chemical agents such as formamide, acetone, alcohol or plasma. LDHs can be exfoliation into a thin layer or even a single layer. This method provides a new type of nanosheet with ultimate two-dimensional anisotropy and positive charge. The third method is the reverse microemulsions, which can be used to control the nucleation and growth of LDHs. The size can be precisely controlled between 30−200 nm by this method. The fourth is one step method. By introducing a layer growth inhibitor, the interactions between neighboring layers have been wakened and thus we can directly synthesize LDHs single-layer nanosheets. This facile one-step method offers significant savings in both cost and time; The fifth method is synthesis of LDHs with base materials (graphene oxide, nitrogen-doped graphene) by the surface confinement effect, and the size of LDHs synthesis by this method is 5−6 nm. To date, many advanced characterization methods have been used to give a better understanding of the LDHs structure and reaction mechanism. X-ray absorption fine structure spectroscopy (XAFS) is widely used to obtain structural information of LDHs that include the bond length, valence state, defect structure and structural disorder. In addition, X-ray photoelectron spectroscopy (XPS) has been used to probe information of LDHs about structural evolution and valence state through changing binding energy. Besides, positron annihilation spectrometry (PAS) is a well-established technique to convey information on the local electronic environment in LDHs and can give insight into the defect characteristics even at atomic scales. Mossbauer spectroscopy has been used to probe the iron-based LDHs cause it can provide a unique means by which to probe the Fe sites in these LDHs. What count is, DFT calculations have been applied to elucidate the reaction mechanism and exploration of the structure-property relationships of LDHs. With the advancement of technology, DFT calculation may provide more useful information and thereby guide the experiments. Through changing the particle size and thickness of LDHs, we can control the microstructure and electronic structure of LDH which plays an important role to influence the chemical and physical properties. A variety of ultrathin/ultrasmall LDHs functional materials have been used in whthin many fields such as catalytic property, adsorption property, optical property and biological application, etc. It is believed that in the future, the foundation and application fields of hydrotalcite-based functional materials will be more extensive. LDHs with different grain sizes, layer thicknesses, defect types, and electronic structures will receive more extensive attention and research.