title: Short, highly ordered, single-walled mixed-oxide nanotubes assemble from amorphous nanoparticles.
abstract: 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 ( approximately 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 approximately 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 ( approximately 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.
authors.affiliation: School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332-0100, USA.
authors.forename: Sanjoy
authors.initials: S
authors.lastname: Mukherjee
authors.forename: Keesuk
authors.initials: K
authors.lastname: Kim
authors.forename: Sankar
authors.initials: S
authors.lastname: Nair
mesh_terms: D000536:Aluminum Hydroxide / Q000737:chemistry; D005857:Germanium / Q000737:chemistry; D008027:Light; D043942:Nanotubes / Q000737:chemistry*; D010316:Particle Size; D012542:Scattering, Radiation; D013056:Spectrophotometry, Ultraviolet; D013059:Spectrum Analysis, Raman
chemical_list: D005857:Germanium; D000536:Aluminum Hydroxide