Imogolites
Aluminosilicates and aluminogermanates nanotubes logo_tutelle logo_tutelle 

3D structure of an short imogolite nanotube.

Since the discovery of carbon nanotubes (NTs), there has been great interest in the synthesis and characterization of similar shaped structures like inorganic nanotubes, nanorods, or nanowires. However, limitations such as purity, complexity of the protocol, low-yield production, and size polydispersity still remain major impediments for industrial-scale applications. In this context, synthetic imogolites appear as an exception. Imogolites are singlewalled aluminosilicate NTs of 2 nm diameter and up to 1 μm in length with the empirical formula (OH)3Al2O3Si OH whose structure has been determined using X-ray Diffraction (XRD), solid state Nuclear Magnetic Resonance (NMR), and Transmission Electron Microscopy (TEM). Imogolite analogues where Ge replace Si atoms have been succesfully synthesized. Although early reports of their synthesis involved diluted (i.e., millimolar) conditions, these imogolite analogues were recently obtained from 100 times more concentrated solutions, thereby opening the route for large scale applications. These analogues have been described to be structurally identical to the Al-Si imogolite, except for a larger tube diameter (∼3.3 nm) and shorter length (less than 100 nm). We areng the imogolite  and Imogolite analogue formation mechanism. We are also studying the use of Imogolite and Imogolite analogue in self assembled new material.

 

contact : Antoine Thill

 

 

New developments in nanosciences and nanotechnologies are strongly dependent on our ability to synthesize well-controlled nanobuilding units, with specific properties. We report in this paper the first synthesis of hybrid single-walled imogolite nanotubes (OH)3Al2O3SixGe1–xCH3 with diameter-controlled hydrophobic nanopores varying from 1.8 to 2.4 nm. Methylation and nanotube dimensions are studied by combining infrared spectroscopy, cryo-TEM observations, and X-ray scattering measurements. We show that, in solution, the water density inside methylated nanotubes is decreased by a factor of 3 compared to the bulk value. Spontaneous confinement of bromopropanol molecules inside the nanotubes, when added to the solution, is demonstrated. These newly synthesized nanotubes may open up possibilities for water filtration or water decontamination.

 

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Ge-imogolites are short aluminogermanate tubular nanomaterials with attractive prospected industrial applications. In view of their nano-scale dimensions and high aspect ratio, they should be examined for their potential to cause respiratory toxicity. Here, we evaluated the respiratory biopersistence and lung toxicity of 2 samples of nanometer-long Ge-imogolites.

 

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Imogolite is known to form bundles when dried; but these nanotubes can also be perfectly dispersed in water. When the silicon precursor in the synthesis of imogolite nanotubes is changed from tetraethoxysilane (TEOS) to methyltriethoxysilane (TEMS), internal wall hydroxyls are replaced with methyl groups. This chemical modification also happens to change their dispersion properties; the nanotubes then form small bundles in solution. In this work, we explore modifications of the synthesis procedure in order to enhance the dispersion of such hybrid nanotubes. The study focuses on the effects of [Al]/[Si] ratio, acid types, ethanol addition or choice of silicon precursor. Using small-angle X-ray scattering (SAXS), infra-red spectroscopy (IR) and cryo-TEM, we show that it is possible to improve yield and length. However, none of the tested modifications allowed the prevention of bundle formation in solution.

 

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Imogolite nanotubes are promising building blocks for nanotechnologies with potential applications in molecular separation, molecular storage, or catalysis. We present an experimental study of the structure of germanium-based imogolite nanotubes Al2O3Ge(OH)4 arranged in bundles. It combines cryo-transmission electron microscopy, infrared spectroscopy, thermogravimetric measurements, and X-ray scattering experiments. Thanks to a systematic method developed to analyze X-ray scattering diagrams as a function of the nanotube shape, single-walled germanium-based imogolite nanotubes, known as cylindrical for more than 30 years, are shown to take an hexagonal base shape when arranged in bundles. Physical and chemical properties of hexagonal imogolite nanotubes should markedly differ from those of cylindrical ones, making hexagonal basis nanotubes a “new” member, of particular interest, of the rich family of imogolites.

 

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Micron-long germanium-based double-walled imogolite nanotubes were synthesized at high concentrations, as evidenced by cryo-TEM, AFM, SAXS and IR characterization methods. In addition, the spontaneous formation of a liquid-crystalline phase was observed. The novel synthesis route made it possible for the first time to obtain both long and concentrated germanium-based imogolite-like nanotubes in a single step.

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Imogolites are natural aluminosilicate nanotubes displaying an astonishing monodispersity in diameter.
The diameter is controlled by the structure and composition of the nanotube wall and can be tuned by several chemical manipulations. It has recently been discovered that the structure of imogolite nanotubes can change from single-walled (SW) to double-walled (DW) when Si is replaced by Ge during synthesis. Starting from the pure Ge composition, we show that the transition between DW and SW structures can be induced by the incorporation of a small quantity of Si in the synthesis. At that point, the suspension contains a mixture of structures with a nearly constant average diameter. In particular, we found evidence for the presence of a few nanoscrolls. Above 25% Si, SW nanotubes become more stable and present a continuously decreasing diameter with increasing Si. A model  is proposed to explain the stability of these different nanotubes and, more generally, the structures of other organic or inorganic nanotubes as a balance between rigidity, surface tension, and adhesion competitive energies.

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It is known that silicon can be successfully replaced by germanium atoms in the synthesis of imogolite nanotubes leading to shorter and larger AlGe nanotubes. Beside the change in morphology, two characteristics of the AlGe nanotube synthesis were recently discovered. AlGe imogolite nanotubes can be synthesized at much higher concentrations than AlSi imogolite. AlGe imogolite exists in the form of both single-walled (SW) and double-walled (DW) nanotubes whereas DW AlSi imogolites have never been observed. In this article, we give details on the physico-chemical control over the SW or DW AlGe imogolite structure. For some conditions, an almost 100% yield of SW or DW nanotubes is demonstrated. We propose a model for the formation of SW or DW AlGe imogolite which also explains why DW AlSi imogolites or higher wall numbers for AlGe imogolite are not likely to be formed.

 

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Formation mechanism of Imogolite nanotubes


The growth mechanisms of imogolite-like aluminogermanate nanotubes have been examined at various stages of their formation. The accurate determination of the nucleation stage was examined using a combination of local- (XAS at the Ge−K edge and 27Al NMR) and semilocal scale technique (in situ SAXS). For the first time, a model is proposed for the precursors of the nanotubular structure and consist in rooftile-shaped particles, up to 5 nm in size, with ca. 26% of Ge vacancies and varying curvatures. These precursors assemble to form short nanotubes/nanorings observed during the aging process. The final products are most probably obtained by an edge−edge assembly of these short nanotube segments.

 

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Evidence of two AlGe imogolite morphologies by SAXS and cryo-TEM.


 

It has been recently discovered that the synthesis of Al−Ge imogolite-like nanotubes is possible at high concentration. Despite this initial success, the structure of these Al−Ge imogolite-like nanotubes remains not completely understood. Using high resolution cryo-TEM and Small Angle X-ray Scattering, we unravel their mesoscale structure in two contrasted situations. On the one hand, Al−Ge imogolite nanotubes synthesized at 0.25 M are double-walled nanotubes of 4.0 ± 0.1 nm with an inner tube of 2.4 ± 0.1 nm. Moreover, SAXS data also suggest that the two concentric tubes have an equal length and identical wall structure. On the other hand, at higher concentration (0.5M), both SAXS and cryo-TEM data confirm the formation of single-walled nanotubes of 3.5 ± 0.15 nm. Infrared spectroscopy confirms the imogolite structure of the tubes. This is the first evidence of any double-walled imogolite or imogolite-like nanotubes likely to renew interest in these materials and associated potential applications.

 

 

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Formation kinetic of AlGe imogolite nanotubes by AFM and SAXS.


Atomic Force Microscopy (AFM) and in situ Small Angle X-ray Scattering (SAXS) were used to investigate the evolution of the aluminogermanate imogolite-like nanotubes concentration and morphology during their synthesis. In particular, in situ SAXS allowed quantifying the transformation of protoimogolite into nanotubes. The size distribution of the final nanotubes was also assessed after growth by AFM. A particular attention was focused on the determination of the single and double walled nanotube length distributions. We observed that the two nanotube types do not grow with the same kinetic and that their final length distribution was different. A model of protoimogolites oriented aggregation was constructed to account for the experimental growth kinetic and the length distribution differences.

 

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Effect of the hydrolysis ratio during the synthesis of AlGe Imogolite nanotubes.


The synthesis protocol for Ge-imogolite (aluminogermanate nanotubes) consists of 3 main steps: base hydrolysis of a solution of aluminum and germanium monomers, stabilization of the suspension and heating at 95 °C. The successful synthesis of these nanotubes was found to be sensitive to the hydrolysis step. The impact of the hydrolysis ratio (from nOH/nAl = 0.5 to 3) on the final product structure was examined using a combination of characterization tools. Thus, key hydrolysis ratios were identified: nOH/nAl = 1.5 for the formation of nanotubes with structural defects, nOH/nAl = 2 for the synthesis of a well crystallized Ge imogolite and nOH/nAl > 2.5 where nanotube formation is hindered. The capability of controlling the degree of the nanotube's crystallinity opens up interesting opportunities in regard to new potential applications.

 

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Maj : 01/08/2017 (1846)

 

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