Thermal Collapse of Single-Walled Alumino-Silicate Nanotubes: Transformation Mechanisms and Morphology of the Resulting Lamellar Phases

Abstract

The thermally induced structural transformations are studied of three imogolite-type nanotube (NT) materials: (i) proper imogolite (IMO, (OH) 3 Al 2 O 3 SiOH) with outer surface covered by Al−OH−Al groups and inner surface lined by silanols); (ii) methyl-imogolite (Me-IMO, (OH) 3 Al 2 O 3 SiCH 3 ), in which the inner surface silanols have been replaced by methyl groups while the outer surface is unchanged, and (iii) the material Me-IMO-NH 2 , obtained by grafting the outer surface of Me-IMO with 3-aminopropylsilane (3-APS). TG-MS analysis on the parent IMO shows only loss of water (up to ca. 700 K), whereas XRD indicates the formation of a lamellar phase because of the mutual reaction of inner silanols. With both Me-IMO and Me-IMO-NH 2 , mass spectrometry and NMR analysis reveal the occurrence of a more complex collapsing mechanism, basically due to the reaction of outer Al-OH groups and inner Si-CH 3 , following the cleavage of the NTs structure, yielding methane and transient Al-O-CH 2 -Si species.All three materials show a limited decrease in the interlayer distance caused by collapse as well as a substantial residual porosity. It is concluded that the layered structure can be conceived as consisting of an overall buckled structure, the strong strain within the silico-alumina layer of the single-walled NT providing the driving force against a complete flattening. As a minor feature, decomposition of perchlorate species to chloride anions with the release of molecular oxygen is observed with IMO species that are trapped during the synthesis at the narrow interpores cavities.

Al/Fe isomorphic substitution versus Fe2O3 clusters formation in Fe-doped aluminosilicate nanotubes (imogolite)

“…With IMO, previous work (Zanzottera et al 2012) has indeed shown the occlusion of some perchlorate anions coming from the synthesis batch, since molecular oxygen was released at high temperature during TG-mass analysis (Zanzottera et al 2012). During the synthesis of Fe-x-IMO, chloride ions likely replace (larger) perchlorate anions, thereby decreasing the final a value.…”

“…To determine Brunauer-Emmett-Teller specific surface area (BET SSA) and porous volume values reported in Table 1, N 2 isotherms were measured at 77 K on samples previously outgassed at 523 K, in order to remove water and other atmospheric contaminants, still preserving NT structure (MacKenzie et al 1989;Zanzottera et al 2012). Non-local density functional theory method (NL-DFT) was used to determine pores size distributions (PSDs), by means of an N 2 -silica kernel for cylindrical pores.…”

“…The outer diameter is ca. 2.0 in natural IMO, whereas it varies in the range of 2.5-2.7 nm in the synthetic samples (Cradwick et al 1972): the presence of different inter-tube impurities may affect NTs packing (Zanzottera et al 2012) which also depends on the synthesis temperature. The synthesis temperature is usually 373 K, whereas the synthesis at 298 K yields NTs with a radius close to that of natural IMO (Wada 1987).…”

Effect of Ionic Strength on the Bundling of Metal Oxide Imogolite Nanotubes

“…13,14 Finally, both inner and outer hydroxylated surfaces offer interesting possibilities for surface functionalization. INTs can thus be rendered amphiphilic by replacing their inner silanol or germanol moiety by a functionalized one, 15 or by grafting molecular compounds on the outer aluminum wall. [15][16][17][18] Whatever the nature (SW or DW) or the nanotube nominal composition (OH) 3 Bundling is analyzed thanks to X-ray scattering (XRS) experiments.…”

“…INTs can thus be rendered amphiphilic by replacing their inner silanol or germanol moiety by a functionalized one, 15 or by grafting molecular compounds on the outer aluminum wall. [15][16][17][18] Whatever the nature (SW or DW) or the nanotube nominal composition (OH) 3 Bundling is analyzed thanks to X-ray scattering (XRS) experiments. Our experiments and their analysis allow us to propose the first phase diagram for the bundle assembling of metal-oxide imogolite nanotubes.…”