Short, highly ordered, single-walled mixed-oxide nanotubes assemble from amorphous nanoparticles

Nanotubes are important building block materials for nanotechnology, but a synthesis process for short (sub-100-nm) solid-state nanotubes with structural order and monodisperse diameter has remained elusive. To achieve this goal, it is critical to possess a definitive mechanistic framework for control over nanotube dimensions and structure. Here we employ solution-phase and solid-state characterization tools to elucidate such a mechanism, particularly that governing the formation of short (similar to 20 nm), ordered, monodisperse (3.3 nm diameter), aluminum-germanium-hydroxide (aluminogermanate) nanotubes in aqueous solution. Dynamic light scattering (DLS), vibrational spectroscopy, and electron microscopy show that pH-control of chemical speciation in the aluminogermanate precursor solution is important for producing nanotubes. A combination of DLS, UV-vis spectroscopy, and synthesis variations is then used to study the nanotube growth process as a function of temperature and time, revealing the initial condensation of amorphous nanoparticles of size similar to 6 nm and their transformation into ordered aluminogermanate nanotubes. The main kinetic trends in the experimental data can be well reproduced by a two-step mathematical model. From these investigations, the central phenomena underlying the mechanism are enumerated as: (1) the generation (via pH control) of a precursor solution containing aluminate and germanate precursors chemically bonded to each other, (2) the formation of amorphous nanoscale (similar to 6 nm) condensates via temperature control, and (3) the self-assembly of short nanotubes from the amorphous nanoscale condensates. This mechanism provides a model for controlled low-temperature (< 373 K) assembly of short, monodisperse, structurally ordered nanotube objects.