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 physicochemical 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.

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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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.

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 n(OH)/n(Al) = 0.5 to 3) on the final product structure was examined using a combination of characterization tools. Thus, key hydrolysis ratios were identified: n(OH)/n(Al) = 1.5 for the formation of nanotubes with structural defects, n(OH)/n(Al) = 2 for the synthesis of a well crystallized Ge imogolite and n(OH)/n(Al) > 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.