Bringing down barriers to the tunability, purity, and scalability of imogolite nanotubes

Abstract

Synthetic imogolite is an aluminosilicate nanotube 2.1 to 2.3 nm wide and up to 1000 nm long. The high aspect ratio and ability to modify specific functional groups are desirable attributes affording it a wide range of potential. Additionally, the isomorphic substitution of Ge for Si in the nanotube walls increases the tubular diameter in a controlled manner. However, the widespread adoption of this nanoparticle is restricted by: i) the complex relationship between imogolite and secondary phases; ii) uncertainty regarding the impact of precursor attributes on nanotube lengths and morphology; iii) scalable synthesis. The first project involves the synthesis of a suite of alumino(silicate) nanoparticles and their characterization using relatively accessible laboratory methods. The phase space for imogolite nanotubes and these secondary phases, including proto-imogolite, amorphous silica and pseudo-boehmite, are well established and delineated. It is determined that hydrolysis ratio is the most significant factor driving nanotube formation, followed by the initial concentration of reagents. In the next study, the impact of precursor attributes on tunable imogolite properties and nanotube growth are investigated. These precursors, proto-imogolites and the short nanotube sections they form, are modified by the isomorphic substitution of Ge and ageing to vary their width and length, respectively. Total nanotube counts and length monodispersity are found to increase with the addition of Ge and increasing precursor ageing. For the first time ever, the occurrence of multi-segmented imogolite nanotubes is also reported. Finally, the scalable synthesis of imogolite is tackled using a novel resin treatment approach to address ionic strength, which impedes nanotube formation at higher concentrations. High purity imogolite nanotubes are synthesized at 10 times higher concentrations than the standard. Nanotube formation is also observed at concentrations 100 times the standard, the first ever reported for Si imogolites. The findings from this body of research have implications for understanding imogolite nanotube growth, and improved tunability of nanotube physical attributes.