This paper describes a systematic study on the synthesis, anion exchange, and delamination of Co-Al layered double hydroxide (LDH), with the aim of achieving fabrication and clarifying the properties of LDH nanosheet/polyanion composite films. Co-Al-CO3 LDH hexagonal platelets of 4 mum in lateral size were synthesized by the urea method under optimized reaction conditions. The as-prepared CO3(2-)-LDH was converted to Cl- -LDH by treating with a NaCl-HCl mixed solution, retaining its high crystallinity and hexagonal platelike morphology. LDHs intercalated with a variety of anions (such as NO3-, ClO4-, acetate, lactate, dodecyl sulfate, and oleate) were further prepared from Cl- -LDH via an anion-exchange process employing corresponding salts. Exchanged products in various anion forms were found to show different delamination behaviors in formamide. Among them, best results were observed for NO3- -LDH in terms of the exfoliating degree and the quality of the exfoliated nanosheets. The delamination gave a pink transparent suspension containing well-defined nanosheets with lateral sizes of up to 2 microm. The resulting nanosheets were assembled layer-by-layer with an anionic polymer, poly(sodium styrene 4-sulfonate) (PSS), onto quartz glass substrates to produce composite films. Magnetic circular dichroism (MCD) measurements revealed that the assembled multilayer films exhibited an interesting magneto-optical response.

Positively charged nanosheets with a lateral dimension of micrometers have been synthesized by directly delaminating a well-crystallized Mg-Al layered double hydroxide (LDH) in the nitrate form. The action of formamide on LDH crystals 10 µm in size led to a transparent solution. X-ray diffraction measurement on a glue-like colloid centrifuged from the solution detected a broad feature at small angular range, while completely losing the sharp reflections for the precursor crystalline compound. The broad profile was very similar to the square of the structure factor calculated based on the LDH structure, which provides strong evidence for the total exfoliation of the LDH crystals into their single sheets. Upon in situ aging in a stream of nitrogen gas, the broad pattern was gradually converted into a sharp basal diffraction pattern, indicating the restacking of molecular LDH nanosheets to restore the original stacked form. Observations by transmission electron microscopy and atomic force microscopy revealed delaminated nanosheets with a lateral size of several micrometers and a thickness of 0.8 nm. The electron diffraction data and elemental microanalysis results as well as these morphological features supported the formation of unilamellar LDH sheets. The LDH nanosheets could be assembled layer-by-layer with an anionic polymer, poly(sodium styrene 4-sulfonate) (PSS), onto the solid surface to produce ultrathin nanocomposite films, demonstrating their usefulness as a positively charged lamellar nanoblock. The formation of nanostructured multilayer assemblies was confirmed by the progressive enhancement of UV absorbance due to PSS and the evolution of X-ray diffraction peaks showing a repeating distance of ∼2.0 nm.

No abstract

This paper describes the topochemical synthesis of Co-Ni layered double hydroxides (LDHs) from brucite-like Co-Ni hydroxides through a novel oxidative intercalation process employing bromine as an oxidizing agent, and their exfoliation into positively charged unilamellar nanosheets in formamide after anion-exchange treatment. In this protocol, hexagonal microplatelets of brucite-like Co-Ni hydroxides in variable composition were prepared by homogeneous precipitation of a mixed solution of divalent cobalt and nickel ions via hexamethylenetetramine hydrolysis. Subsequent treatment of the brucite-like Co-Ni hydroxides with excessive bromine in acetonitrile promoted partial oxidation of Co 2þ into Co 3þ , producing Br --intercalated Co-Ni LDHs inheriting the hexagonal morphology. This rational topochemical approach was applicable for realizing a pure phase of Co-Ni LDHs with nickel content up to 50% (metal content). Chemical analyses indicated that as-prepared Co-Ni-Br LDHs were unexceptionally characterized by a general chemical formula as(x e 0.5), a thermodynamically stable LDH structure with a M 2þ /M 3þ ratio of 2:1. We developed an ethanol-assisted anionexchange approach, which was effective in preventing carbonate contamination in preparing a variety of inorganic and organic anionic forms of Co-Ni LDHs. As-prepared NO 3 intercalated Co-Ni LDHs without substantial carbonate contamination were successfully exfoliated into unilamellar nanosheets bearing positive charges upon contact with formamide. The translucent nanosheet suspensions exhibited characteristic colors depending on the variable Co/Ni ratio.

This paper describes a topochemical synthetic approach to Co2+-Fe3+ layered double hydroxides (LDHs). Micrometer-sized hexagonal platelets of brucite-like Co2/3Fe1/3(OH)2 were first prepared by a homogeneous precipitation of an aqueous solution of divalent cobalt and ferrous ions through hexamethylenetetramine (HMT) hydrolysis under a nitrogen gas atmosphere. A subsequent oxidative intercalation process, by the action of iodine (I2) in chloroform (CHCl3), transformed the precursory brucite-like Co2+-Fe2+ hydroxides into hydrotalcite-like Co2+-Fe3+ LDHs, in which the oxidization of Fe2+ into Fe3+ induced positive charges to the octahedral hydroxyl layers while anions (I-) were intercalated into the interlayer space. Co2+-Fe3+ LDHs inherited the high crystallinity and hexagonal platelet morphology from their brucite-like precursor due to the topotactic nature of the transformation, which was verified by abundant microscopic and spectroscopic characterizations. After a normal ion-exchange process, Co2+-Fe3+ LDHs accommodating perchlorate anions were exfoliated into unilamellar nanosheets in formamide by an ultrasonic treatment.