Investigation of the Structural Evolution of Thermal Treated Imogolite Nanotubes

Thermal treatment is one of the most commonly applied process to modify the structure, porosity, and surface reactivity properties of clay minerals. Among them, imogolite nanotubes, (OH)₃Al₂O₃(Si,Ge)OH, have a unique structure consisting of a curved octahedral [O₃Al(OH)₃] outer layer on which isolated [Si(Ge)O₃(OH)] tetrahedron units are bonded on the inner surface by sharing three mutual oxygen atoms [1]. A recent study by Monet et al. [2] has shown that three different intermediate stages occur during the thermal annealing of hydrophilic aluminogermanate imogolite nanotubes, until re-crystallization into a Ge-mullite compound beyond 950 °C. Interestingly, after dehydroxylation, a metaimogolite structure is formed with both six-fold and five-fold coordinated Al atoms, which further reorganizes into Al(VI) and Al(IV) at temperature higher than 600 °C. The family of imogolite nanotubes also includes methylated imogolite nanotubes, where inner hydroxyl groups are replaced by methyl units. In particular, it has been demonstrated that the rolling mode of methylated and hydroxylated nanotubes are different [3], which may result in different structural evolutions as a function of temperature.<br/>Here, we investigated the thermal transformation of methyl-modified germanium-based nanotubes (OH)₃Al₂O₃GeCH₃. Dehydration and dehydroxylation and demethylation have been observed with ex-situ Infrared spectroscopy up to 500 °C. Complex structural transformations and atomic re-organization occur at higher temperature, which are studied thanks to time-resolved in-situ X-ray Absorption Near Edge Spectroscopy (XANES) and X-ray scattering (XRS) experiments, , up to ~1000 °C. XRS diagrams give information on the evolution of the shape and organization of the nanotubes. Furthermore, Pair Distribution Function investigations have been performed to examine the transformation from long to short range order when heating, in the metastable states formed above 500 °C. Moreover, XANES spectra at Al K-edge coupled with the Multivariate Curve Resolution with Alternating Least Squares chemometrics quantitative analysis give the evolution of Al environments during heating, except for Al(V), which is usually elusive by XANES [2]. The Al coordination shows an evolution from an Al(VI) octahedral environment to a combination of two different Al coordination sites. Only octahedral Al(VI) and tetrahedral Al(IV) environments were evidence by XANES for hydroxylated aluminogermanate nanotubes [2] Interestingly, a very peculiar intermediate signature of Al is thus revealed for its methylated counterpart, whose metaphases appear to be different [2].<br/>Finally, the thermal treatment allows one to modify the physico-chemical properties of clays nanotubes. As hydrogen storage is an environmental issue of importance, we will illustrate the effect of thermal treatment of imogolite nanotubes on hydrogen storage capacities.<br/><br/>References<br/>[1] Cradwick P. D. G., Farmer V. C., Russell J. D., Masson C. R., Wada K. and Yoshinaga N. (<b>1972</b>). Imogolite, a Hydrated Aluminium Silicate of Tubular Structure. Nature Physical Science 240, 187-189.<br/>[2] Monet G., Rouzière S., Vantelon D., Coelho Diogo C., Maurin D., Bantignies J. L., Launois P. and Paineau E. (<b>2021</b>). Mechanisms of Structural Reordering During Thermal Transformation of Aluminogermanate Imogolite Nanotubes. The Journal of Physical Chemistry C 125, 12414-12423.<br/>[3] Monet G., Amara M.S., Rouzière S., Paineau E., Chai Z., Elliot J.D., Poli E., Liu L-M., Teobaldi G. and Launois P (<b>2018</b>). Structural resolution of inorganic nanotubes with complex stoichiometry. Nature Communications 9, 2033.