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Recent Patents on Nanotechnology, 2012, 6, 159-168 159 Synthesis of LDH Nanosheets and their Layer-by-Layer Assembly

Renzhi Ma and Takayoshi Sasaki*

International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

Received: October 10, 2011 Revised: February 09, 2012 Accepted: June 02, 2012

Abstract: Layered double hydroxides, a rare family of anion-exchangeable functional materials with broad application

prospects, have received much attention both in academic research reports and technological patents. It was recently dis-

covered that they could be exfoliated into unilamellar nanosheets in different solvents. Especially, well-defined LDH

nanosheets can be obtained through total delamination, in formamide, of large-sized crystallites prepared from homogene-

ous precipitation using hydrolysis agents. Bearing a positive charge at a two-dimensional molecular scale, layered double

hydroxide nanosheets can be used as a model system for electrostatic layer-by-layer assembly in combination with poly-

anionic counterparts (polymer, DNA, oxide nanosheets, etc.) for various functional nanoarchitectures, such as lamellar

aggregates, multilayer nanofilms and nanoshells.

Keywords: Chemical exfoliation, layered double hydroxides, nanosheets, self-assembly

INTRODUCTION

Layered double hydroxides (usually referred to as LDH)

possess a lamellar structure with positively charged host

slabs intercalating exchangeable anions [1-5]. The basic host

units are metal-oxygen/hydroxyl octahedra centered by metal

cations, which are connected through shared edges to form

an infinite planar sheet. There are typically two metal ele-

ments with different valences (MII, MIII) in the host slabs.

The chemical composition may be expressed in a general

formula [MII1-xMIIIx(OH)2]x+[An-x/n]x-??mH2O (abbreviated as

[MII1-x/x-MIII] or [MII1-x/x-MIII-A] thereafter. For x = 1/3, it is

simply formulated as [MII-MIII]). In theory, any divalent or

trivalent metal cations may be accommodated in the LDH

host sheets, provided that the ionic radius is not too different

from that of Mg2+. Different combinations of divalent and

trivalent metal cations impart a wide variety of interesting

properties for anion exchange and adsorption [6-13], as well

as catalytic [14-18], (photo)electrochemical [19,20], mag-

netic [21] and pharmaceutical [22-30] applications.

Over the last decade, there has been a widespread interest

in the attempt to exfoliate LDH crystallites, under certain

conditions, into single-layer host units, i.e., unilamellar

nanosheets with ultimate two-dimensional (2D) features

[31,32]. As new elementary building blocks or inorganic

“macromolecules” bearing a positive charge, the obtained

LDH nanosheets may be self-assembled via electrostatic

interaction into novel nanocomposites, ultrathin films and

other nanoarchitectures. In this article, we outline some re-

cent progress on the synthesis of LDH nanosheets and their

assembly into functional nanoarchitectures.

*Address correspondence to this author at the International Center for

Materials Nanoarchitectonics (WPI-MANA), National Institute for

Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan;

Fax: 81-29-860-4950; E-mails: MA.Renzhi@nims.go.jp or

SASAKI.Takayoshi@nims.go.jp

2212-4020/12 $100.00+.00 EXFOLIATION PROCEDURES There is a strong electrostatic interaction, as a result of the high charge density of LDH host slabs, and an integrated hydrogen bonding network developed in the interlayer gal-lery. Exfoliation of LDH was once considered a difficult challenge. Not until very recently did the research for exfoli-aiton claim to success. Depending on the solvents used, exfo-liating methods can be divided into different categories: (1) in alcohol; (2) in formamide; and (3) in water, etc. 甲酰胺Exfoliation in Alcohol Short-chain alcohols were among the first kind of sol-vents tried to exfoliate LDH. The interlayer gallery of LDH crystallites may be substantially expanded after the insertion of anionic surfactants, and then it may become feasible to promote delamination via heat treatment. In 2000, Adachi-Pagano et al. first reported that refluxing [Zn-Al] LDH inter-calating dodecyl sulfate (DS) in butanol at 120°C led to a translucent colloidal suspension, indicative of exfoliation [33,34]. It was found that other solvents such as water, methanol, ethanol, isopropanol and hexane could not well exfoliate LDH under the same conditions. On the other hand, amyl alcohol and hexanol exhibit similar effects to those of butanol. The authors thus suggested that a successful exfolia-tion might depend on whether or not interlayer water could be replaced by the short-chain alcohol. When alcohol mole-cules can quickly replace the interlayer water, for example under refluxing conditions, the exfoliation is initiated. In another study in 2004, Singh et al. treated [LiAl2(OH)6]Cl·nH2O, after modification with different sur-factants, also under refluxing in butanol at 120°C [35]. They observed that samples containing sodium octyl sulfate (SOS) and sodium dodecyl sulfate (SDS) could not be exfoliated, while the intercalation of sodium octyl benzene sulfonate (SOBS) and sodium dodecyl benzenesulfonate (SDBS) was helpful in promoting delamination. Based on these observa-? 2012 Bentham Science Publishers