Applying a combination of melt synthesis followed by long-term annealing a fluorohectorite is obtained which is unique with respect to homogeneity, purity, and particle size. Counterintuitively, the hectorite undergoes a disorder-to-order transition upon swelling to the level of the bilayer hydrate. Alkylammonium-exchanged samples show at any chain length only a single basal spacing corroborating a nicely homogeneous layer charge density. Its intracrystalline reactivity improves greatly upon annealing, making it capable to spontaneously and completely disintegrate into single clay lamellae of 1 nm thickness. Realizing exceptional aspect ratios of around 20,000 upon delamination, this synthetic clay will offer unprecedented potential as functional filler in highly transparent nanocomposites with superior gas barrier and mechanical properties.

ABSTRACT:Delamination is a key step to obtain individual layers from inorganic layered materials needed for fundamental studies and applications. For layered van-der-Waals materials like graphene the adhesion forces are small allowing for mechanical exfoliation, whereas for ionic layered materials like layered silicates the energy to separate adjacent layers is considerably higher. Quite counter intuitively, we show for a synthetic layered silicate (Na 0.5 -hectorite) that a scalable and quantitative delamination by simple hydration is possible for high and correlated. This is indicated by fulfilling the classical Hansen-Verlet and Lindeman criteria for melting. We provide insight into the requirements for layer separation and controlling the layer distances for a broad range of materials and outline an important pathway for the integration of layers into devices for advanced applications.

The large hydration enthalpy of inorganic interlayer cations sandwiched between moderately negatively charged silicate layers endows to smectites (e.g., hectorite) remarkably rich intracrystalline reactivity compared with most other layered materials. Moreover, they are transparent and inert in most potential suspension media. Upon suspension in water, smectites readily swell. For homogeneous, melt-synthesized smectites, the degree of swelling can be tuned by choice of interlayer cation and charge density of the layer. Because swelling renders the clay stacks more shear labile, the efficiency of exfoliation by applying shearing forces can in turn be adjusted. Certain smectites even spontaneously delaminate into clay platelets of uniform thickness of 1 nm by progressive osmotic swelling. Osmotic swelling can also be applied to produce well-defined double stacks when one starts with ordered, interstratified heterostructures. Nanocomposites made with high-aspect-ratio fillers obtained this way show superior mechanical, flame retardancy, and permeability properties.

One-dimensional crystalline heterostructures of two types of nanoreactors strictly alternating along the stacking direction are obtained by partial ion exchange into a synthetic clay mineral distinguished by an outstandingly homogeneous charge density. The driving force for the formation of these high interface architectures is proven to be founded in densely packed cationic molecules in the interlayer with a charge density deviating from the charge density of the host clay mineral. The established mechanism renders the approach generally applicable and will allow for synthesis of ordered heterostructures of any desired combination of functional molecules that moreover are separated by exactly 1 nm thick silicate walls. The ordered heterostructures were furthermore shown to represent the thermodynamic equilibrium suggesting the mechanism of formation to be robust.

Ordered heterostructures of layered materials where interlayers with different reactivities strictly alternate in stacks offer predetermined slippage planes that provide a precise route for the preparation of bilayer materials. We use this route for the synthesis of a novel type of reinforced layered silicate bilayer that is 15 % stiffer than the corresponding monolayer. Furthermore, we will demonstrate that triggering cleavage of bilayers by osmotic swelling gives access to a generic toolbox for an asymmetrical modification of the two vis-à-vis standing basal planes of monolayers. Only two simple steps applying arbitrary commercial polycations are needed to obtain such Janus-type monolayers. The generic synthesis route will be applicable to many other layered compounds capable of osmotic swelling, rendering this approach interesting for a variety of materials and applications.