Surface properties of alumino-silicate single-walled nanotubes of the imogolite type

Abstract

An IR spectroscopy study is reported on the nature and accessibility of external and internal surfaces of single-walled alumino-silicate nanotubes (NTs) of the imogolite type. NTs form bundles with hexagonal symmetry, in which three kinds of surfaces may be figured out: surface A is the inner surface of NTs; surface B is that between three aligned NTs in the hexagonal packing; and surface C arises from slit mesopores between bundles. Two materials were considered: proper imogolite (IMO, (OH)₃Al₂O₃SiOH) and its methylated analogue, (Me-IMO, (OH)₃Al₂O₃SiCH₃). The chemical nature of the outer surface of NTs is the same in both materials, i.e. a curved gibbsite sheet with both Al–OH–Al and Al–O–Al groups and an amphoteric character. The inner surface is very hydrophilic in IMO NTs, lined by closely packed silanols, and hydrophobic in Me-IMO, all silanols being replaced by –SiCH₃ groups. The change in chemical composition is accompanied by an increment in pore size, about 1.0 nm in IMO, and ca. 2.0 nm in Me-IMO, which implies a change in the accessibility of the B surface, not available to any molecule in IMO, and accessible in Me-IMO to small molecules like water, due to larger pores between NTs. Aluminol species at the B surface display an acidic nature, in contrast with that of the same species at surface C, because of a confinement effect.