Evidence of Double-Walled Al-Ge Imogolite-Like Nanotubes. A Cryo-TEM and SAXS Investigation

International audienceSince 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.1 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)3Al2O3SiOH2 whose structure has been determined using X-ray Diffraction (XRD),2 solid state Nuclear Magnetic Resonance (NMR), and Transmission Electron Microscopy (TEM).3 Imogolite analogues with the generic formula (OH)3X2O3YOH (where X ) Al, Ga, or In and Y ) C, Si, Ge, or Sn) have been considered, mainly from a theoretical point of view,4 to tailor these NTs to specific needs. However, to date, only Ge imogolite analogues5 have been successfully 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,6 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). In the present study, the structure of two sets of samples obtained following the synthesis protocol described by Levard et al.6 with initial Al concentrations of 0.25 and 0.5 M is obtained using different probes. Infrared spectroscopy (see Supporting Information (SI)) confirms that the local structure of the formed Al-Ge NTs is compatible with the one of imogolite. Their mesostructure has been examined by cryo-TEM and Small Angle X-ray Scattering (SAXS). We demonstrate here that Al-Ge imogolite-like NTs synthesized at 0.25 M are double-walled NTs with a low dispersion in diameter. SAXS data also suggest that the double-walled NTs consist of two concentric tubes of equal length and identical wall structure. At higher concentration (0.5 M), both SAXS and cryo-TEM data confirm the formation of single-walled NTs