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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.

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Sequential adsorption of polyanions and polycations was used to make a five-component energy/electron-transfer cascade, which mimics some of the functions of natural photosynthetic assemblies. The photon antenna part of the system consists of coumarin- and fluorescein-derivatized poly(allylamine hydrochloride) (Coum-PAH and Fl-PAH), palladium(II)tetrakis(4-N,N,N-trimethylanilinium) porphyrin (PdTAPP4+) or palladium(II)tetrakis(4-sulfonatophenyl) porphyrin (PdTSPP4-) layers, interleaved with anionic Zr(HPO4)2·H2O (α-ZrP) sheets. α-ZrP or HTiNbO5 sheets separate the porphyrin electron donor from a polyviologen electron acceptor layer. Layer-by-layer growth of these thin film assemblies was characterized by atomic force microscopy (AFM) and ellipsometry on planar supports, and by elemental analysis, surface area measurements, and transmission electron microscopy high on surface area silica supports. UV−vis absorption and steady-state emission spectroscopies showed that the overall energy/electron-transfer reaction (Coum → Fl → PdTSPP4- → viologen) occurs with approximate quantum yields of 0.47 and 0.61 for systems containing α-ZrP and HTiNbO5 sheets, respectively. Transient diffuse reflectance spectroscopy established that a porphyrin−viologen charge separated state is formed in the reaction, and that it has an exceptionally long-lived component (τ ≈ 900 μs) with the HTiNbO5 spacer. It is inferrred that the semiconducting HTiNbO5 sheets play an active role in relaying the electron from photoexcited PdTSPP4- to the viologen electron acceptor.

Thin films were grown on amine-primed Si and glass substrates by sequential adsorption reactions of polyallylamine hydrochloride (PAH) and anionic colloids derived from HTiNbO5 and HCa2Nb3O10. The acid−base chemistry of polycation/polyanion adsorption was studied in detail for PAH/HTiNbO5. The pK a of PAH, defined as the pH at which it is half protonated, is 8.7. Titanoniobate colloids, prepared by reaction of HTiNbO5 with tetra(n-butylammonium) hydroxide, TBA+OH-, are unilamellar at pH ≥ 8.5 and restack below pH 7.0. Efficient tiling of a PAH-terminated surface by a layer of unilamellar titanoniobate sheets occurs only at intermediate pH values (8.5−9.0). At lower pH, the colloid restacks on the surface, and at higher pH, only partial coverage by single sheets is observed by atomic force microscopy (AFM). At pH 8.5, high-quality multilayer films can be grown by sequentially adsorbing PAH with either the titanoniobate or niobate colloid. TGA/DTA studies of bulk PAH/titanoniobate intercalation compounds show that they decompose oxidatively to form HTiNbO5 at 310−350 °C and that this decomposition is followed by interlayer condensation to make Ti2Nb2O9. A similar process occurs in the PAH/titanioniobate multilayer films at 350 °C.