Hans-Joachim Muhr

Controlled Uptake and Release of Metal Cations by Vanadium Oxide Nanotubes

Vanadium oxide nanotubes (Cn-VOx-NTs) contain α-monoamines (CnH2n+1NH2 with 4≤n≤22) as templates intercalated between crystalline VOx layers comprising multilayer tube walls. The present study reveals that a large proportion of the amines can easily be exchanged by metal cations. The tubular morphology is not affected by this reaction, but the distance between the VOx layers, i.e., 2.8 nm in C12-VOxNTs, decreases in the reaction product to 0.9 – 1.2 nm, depending on the metal salt actually applied. Alkali (Na+, K+), alkaline-earth (Mg2+, Ca2+, Sr2+), and transition-metal salts (Fe2+, Co2+, Ni2+, Cu2+) have successfully been intercalated. This reaction is partly reversible since intercalated sodium cations can be resubstituted by dodecylamine. This exchange produces again C12-VOx-NTs with the original inter-layer spacing. However, this release is successful only when sodium is complexed by a crown ether. Under these reaction conditions, even a cyclic uptake and release of Na+ and amine, respectively, accompanied by a corresponding shrinkage and widening of the inter-layer distance, is observed while the tubular structure is widely preserved. Furthermore, a distinct selectivity of the metal-cation exchange has been observed.

Flexible V7O16 Layers as the Common Structural Element of Vanadium Oxide Nanotubes and a New Crystalline Vanadate

The walls of vanadium oxide nanotubes (VOx-NTs) are built up by vanadate layers between which the structure-directing template, either a primary amine or a diamine with long alkyl chain, is located. The feasibility of various exchange reactions un- der preservation of the tubular morphology indicates a high struc- tural flexibility of the VOx-NTs. The structure of the vanadate lay- ers appears to be the same in all tubular vanadates, as revealed by the similarity of the diffraction patterns. Plate-like crystals of a new crystalline phase, structurally closely related to the nanotubes, have now been prepared with ethylene diamine, applying a route that is analogous to the VOx-NT synthesis. The single crystal X-ray struc- ture determination showed that this new phase has the composition

The Cross‐Sectional Structure of Vanadium Oxide Nanotubes Studied by Transmission Electron Microscopy and Electron Spectroscopic Imaging

Vanadium oxide nanotubes (VOx-NTs) are easily accessible in pure form from vanadium(V) alkoxides and amines by a sol-gel reaction and a subsequent hydrothermal treatment. The wall structure of VOx-NTs containing hexadecylamine or dodecylamine as the structure-directing template has been characterised by transmission electron microscopy (TEM). A standard method for preparing TEM specimens was modified in order to investigate the cross-sectional structure of the tubes. The elemental distribution in the layered structure inside the tube walls has been visualised by electron spectroscopic imaging: vanadium oxide builds up the layers that appear with dark contrast in the TEM images while carbon, i. e., the organic template, is present in between. The bent VOx layers inside the tube walls are preferentially scrolls rather than concentric cylinders. Moreover, some tubes are formed by a combination of both types. The layer structure inside the tube walls is frequently disordered, and several types of defects appear. Querschnitte von Vanadiumoxid-Nanorohren und ihre Charakterisierung mittels Transmissions-Elektronenmikroskopie und elektronenspektroskopischer Abbildung Vanadiumoxid-Nanorohren (VOx-NTs) sind aus Vanadium(V)-Alkoxiden und Aminen durch eine Sol-Gel Reaktion und eine anschliessende hydrothermale Umsetzung leicht in reiner Form zu erhalten. Die Wandstruktur von VOx-NTs, die entweder Hexadecylamin oder Dodecylamin als Struktur-dirigierendes Templat enthalt, wurde mittels Transmissionselektronenmikroskopie (TEM) charakterisiert. Fur die Untersuchung des Rohrenquerschnitts wurde eine Standardmethode fur die Praparation von TEM-Proben modifiziert. Die elektronenspektroskopischen Abbildungen zeigen die Elementverteilung in den Schichten der Rohrenwand: Die in den TEM-Aufnahmen mit dunklem Kontrast erscheinenden Schichten enthalten Vanadiumoxid, wahrend das organische Templat dazwischen eingebaut ist. Die Rohrenwande bestehen oft aus gerollten VOx-Schichten; geschlossene, konzentrische Rohren sind dagegen selten. Einige Rohren stellen Kombinationen dieser beiden Grundtypen dar. In der Schichtstruktur der Wande treten haufig verschiedene Arten von Defekten auf.

The first oxide nanotubes with alternating inter-layer distances

Abstract A novel type of vanadium oxide nanotube (VO x -NT) has been discovered in the course of a comprehensive study investigating the influence of ammonia on the tube formation. Compared to the conventional vanadium oxide nanotubes prepared at pH values between four and eight, the new tubes were obtained under slightly basic conditions (pH 9–10), adjusted by ammonia. As revealed by transmission electron microscopy images, the wall structure comprises two different inter-layer distances that occur alternately. Such a wall structure is without precedent in any other tubular material.

The First Vanadium Oxide Nanotubes Containing an Aromatic Amine as Template

A new vanadate has been prepared in high yield by reacting vanadium(V) triisopropoxide and 3-phenylpropylamine in solution, followed by hydrolysis and hydrothermal treatment of the intermediate product. For the first time, an aromatic amine has successfully been applied as structure-directing template for the synthesis of vanadium oxide nanotubes. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images demonstrate the tubular morphology of the phenylpropylamine vanadium oxide nanotubes obtained. The size and structure are similar to that of vanadium oxide nanotubes formed with aliphatic amines. The tube walls comprise layers of vanadium oxide with the organic template intercalated in between. The interlayer distance is ca. 2.1 nm, and the structure of the VOx layers can be described by a square lattice with a≈0.61 nm. Furthermore, the TEM investigation has revealed the presence of many defects in the wall structure.

Solid-state NMR investigation of the novel microporous gallophosphate cloverite

Abstract The investigation of the gallophosphate molecular sieve cloverite by solid-state 13 C, 31 P, and 71 Ga NMR techniques is described. Comparison of the 13 C CP/MAS spectra of cloverite quinuclidine and quinuclidinium chloride give further evidence that the templating amine is protonated. The non-spinning CP experiment shows the full chemical shift anisotropy and reveals no motion of the template molecules. The 1 H-decoupled 31 P MAS spectrum of cloverite exhibits very poor resolution. However, the resonance lines of the five crystallographically independent 31 P atoms are resolved by applying a dipolar-dephasing technique which utilizes differences in dipolar 31 P— 1 H interactions. CP/MAS experiments with variable contact times and a depolarization experiment support this result and allow partial assignment of resonance lines to two groups of 31 P atoms. The experimental conditions for 71 Ga CP/MAS experiments were evaluated using Ga 2 (SO 4 ) 3 ·18H 2 O as a model substance. Application to cloverite with decreasing contact times shifts the maximum of the main signal from − 41 to − 26 ppm, thus indicating the presence of gallium sites with different cross-polarization dynamics.

Ca15(CBN)6(C2)2O- a new cubic structure type containing CBN4−-anions together with acetylide and oxo anions

Abstract Ca 15 (CBN) 6 (C 2 ) 2 O contains CBN 4− anions as well as C 2 − units and isolated O 2− anions. The compound is obtained by heating a stoichiometric mixture of CaO, C and BN with an excess of Ca in sealed niobium ampoules to 1270 K. It crystallizes in the space group Ia 3 d with a = 1656.84(9) pm. Preparation, crystal structure, NMR and IR-spectroscopic properties are discussed.