Ørnulv B. Vistad

Delamination and restacking of a layered double hydroxide with nitrate as counter anion

A layered double hydroxide (LDH) with nitrate as the counter anion (LDH–NO3 with Mg/Al = 3) was, for the first time, successfully delaminated in formamide under ultrasonic treatment. Atomic force microscopy (AFM) images showed that a large part of the LDH was delaminated into single and double brucite layers (0.7–1.4 nm in thickness). The nano-sheets had disk-like shapes with a diameter of ca. 20–40 nm. Findings from AFM were in good agreement with the average hydrodynamic diameter determined using dynamic light scattering. Powder X-ray diffraction pattern of LDH dispersed in formamide also confirmed that LDH–NO3 was exfoliated. The dispersions of LDH in formamide were stable and transparent up to a concentration of 40 g L−1. However, formation of transparent gels was observed at concentrations higher than 5 g L−1. Delaminated LDH could be restacked by adding sodium carbonate or ethanol.

Characterization of exfoliated layered double hydroxide (LDH, Mg/Al = 3) nanosheets at high concentrations in formamide

For the first time procedures for the direct characterization of exfoliated nanosheets of LDH (Mg/Al = 3) in suspension are reported. The shape, size and layer thickness of the nanosheets were determined using small angle X-ray and neutron scattering (SAXS/SANS) in combination with dynamic light scattering (DLS) and atomic force microscopy (AFM). Furthermore, by using ultrasonic treatment we were able to delaminate a glycinate containing LDH up to a concentration of 42.5 g L−1, which is ten times higher than the value reported in the literature for the same system. The exfoliated LDH nanosheets were directly characterized in suspension by SAXS/SANS and gave an average layer thickness of 1.4 nm for both tested concentrations (10 and 42.5 g L−1). This result provides strong evidence that the LDH, even at concentrations as high as 42.5 g L−1, is totally delaminated into single or a few stacked brucite layers. AFM images of exfoliated LDH, after dilution and deposition on mica, confirm that exfoliated nanosheets mainly consist of single and double layers, corresponding to thicknesses of 0.6 ± 0.1 and 1.3 ± 0.1 nm, respectively. The diameters of the nanosheets in suspension determined using SAXS and SANS are very similar (between 30 and 40 nm), and are in good accordance with the value observed by AFM (between 10 and 40 nm). Hydrodynamic diameters determined by dynamic light scattering (DLS) were 35 and 60 nm at 10 and 42.5 g L−1, respectively.

Preparation of Nb-substituted titanates by a novel sol-gel assisted solid state reaction.

Single-phase layered Nb-substituted titanates, Na(2)Ti(3-x)Nb(x)O(7) (x = 0-0.06) and Cs(0.7)Ti(1.8-x)Nb(x)O(4) (x = 0-0.03), were for the first time synthesized by a novel sol-gel assisted solid state reaction (SASSR) route. Conventional solid state reactions as well as sol-gel synthesis did not succeed in producing phase pure Nb-substituted titanates. In the SASSR synthesis route we combine the advantages of traditional sol-gel technique (i.e., homogeneous products formed at low temperatures) and solid state reaction (i.e., formation of stable, crystalline phases) for preparing single-phase niobium-substituted layered titanates. The obtained products were characterized by X-ray powder diffraction, scanning electron microscopy, inductively coupled plasma-atomic emission spectrometry, Raman spectroscopy, and thermogravimetric analysis. Results indicate that the Ti(IV) in the host layer of the samples could be partially replaced by Nb(V) without structural deterioration. After proton-exchange, more water molecules were intercalated into the interlayer of H(0.7)Ti(1.8-x)Nb(x)O(4) x nH(2)O with increasing niobium content, whereas the interlayer distance of H(2)Ti(3-x)Nb(x)O(7) (x = 0-0.06) was unchanged.