Positively Charged Nanosheets Derived via Total Delamination of Layered Double Hydroxides

Abstract

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.