No abstract

The exfoliation of acid-exchanged K4Nb6O17 with tetra(n-butyl)ammonium hydroxide in water produces a colloidal suspension of individual sheets, which roll into loosely bound tubular structures. The tubule shape can be made permanent via precipitation of the colloid with alkali cations. Atomic force microscopy and transmission electron micrographs reveal that the tubules have outer diameters ranging from 15 to 30 nm and that they are 0.1 to 1 μm in length. The observed curling tendency, preferential folding, and cleavage angles of the individual sheets are interpreted in terms of the crystal structure of the parent solid, K4Nb6O17. The driving force for tubule formation appears to be relief of strain that is inherent in the asymmetric single sheets. This driving force is absent in bilayer colloids formed early in the exfoliation process, which are found only as flat sheets. Tubules in colloidal suspensions that have been subjected to turbulence have a tendency to unroll into flat sheets on surfaces, indicating that the forces controlling rolling and unrolling are closely balanced.

Monolayer and multilayer thin films consisting of anionic α-zirconium phosphate (α-ZrP) sheets and polycations (poly(allylamine hydrochloride) (PAH), cytochrome c) were characterized by transmission electron microscopy (TEM), ellipsometry, UV−visible absorbance spectroscopy, reflectance FT-IR, XPS, and X-ray diffraction. Titration and powder X-ray diffraction experiments confirm that exfoliation of α-ZrP begins to occur when enough tetra(n-butylammonium) hydroxide (TBA+OH-) has been added to exceed single-layer packing of TBA+ ions (x ≈ 0.50) in the intercalation compound Zr(HPO4)2-x (TBA+PO4 -) x ·nH2O. The identical contrast of many sheets in TEM micrographs suggests that the suspension is unilamellar. Alternately dipping cationic substrates into α-ZrP-containing suspensions and aqueous PAH gives a multilayer film that resembles the corresponding bulk intercalation compound. X-ray photoelectron spectra of multilayer films show that they are Zr-rich, relative to α-ZrP, consistent with some corrosion during the exfoliation reaction. The α-ZrP/PAH layer pair thickness is 13/14.7 Å, as measured by ellipsometry/X-ray diffraction, respectively. A 13-layer pair film is sufficiently well-ordered in the stacking direction to give a Bragg peak in the diffraction pattern. The agreement between the bilayer thickness and the total film thickness, measured from Kiessig fringes in the low-angle part of the diffraction pattern, confirms that only a single dense α-ZrP or PAH monolayer is deposited in each adsorption step.

The intercalation and exfoliation reactions of α-zirconium phosphate, Zr(HPO4)2·H2O (α-ZrP), were studied microscopically by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The reaction of α-ZrP with tetra(n-butylammonium) hydroxide (TBA+OH-) initially produces intercalation compounds, which then transform to unilamellar colloids. The rate-determining step in intercalation is the opening of the interlamellar galleries. Subsequent diffusion of TBA+ ions into the opened galleries is rapid. The hydrolysis reaction of α-ZrP colloids proceeds from the edges inward, forming ∼4-nm hydrated zirconia particles that decorate the edges of the sheets. The reaction does not go to completion, as it is limited by equilibrium associated with the release of phosphate into the solution. The hydrolysis reaction is negligible at 0 °C, which permits the synthesis of hydrolysis-free unilamellar colloids. Remarkably, these colloids form monolayer films on amine-derivatized silicon surfaces with a high density that suggests significant surface mobility during the adsorption process. Addition of appropriate phosphonic acids to colloidal α-ZrP suspensions enables modification of the sheet edges, illustrated here by the anchoring of osmium oxide particles to the sheet edges by a vinylphosphonate linker.