44074-G10
Self Assembly of Short, Single-Walled, Hydrophilic Aluminosilicate and Aluminogermanate Nanotubes
Nanoscale materials (specifically, objects of size less than 100 nm) are important ‘building blocks’ of an emerging nanotechnology based on synthesis of functional nanoparticles and their assembly into nanoscale or nanostructured devices. Such materials and devices are increasingly forming the foundation of transformational developments in areas as diverse as electronics, photonics, chemical processing, biotechnology, and energy generation/storage. Despite the considerable progress in nanoscale materials processing, a vast potential for constructing nanoscopic materials of unprecedented complexity and functionality remains almost entirely unexplored due to limitations in our understanding of nanoscopic molecular assembly processes. There are currently no generalizable approaches that provide the basis for generating complex nanoscopic objects such as hollow cylinders (nanotubes) and hollow nanospheres, with nanometer/subnanometer control over their dimensions as well as composition and atomic structure.
The goal of this project was to develop a deeper understanding of the formation of complex nanotube objects at very small length scales (i.e., below 100 nm in length and 2-15 nm in diameter), by combining principles of liquid-phase materials chemistry with molecular-level insights into energetics and structure. These initial insights would lead to more generalizable “design rules” to engineer the “shape and size”, “curvature”, and chemical structure of nanoscopic objects. We chose a unique model system based on synthetic single-walled aluminosilicate (AlSi) nanotubes whose ordered wall structure is identical to a layer of aluminum (III) hydroxide (gibbsite); with silanol (Si-OH) groups bound on the inner wall. It has a diameter of 2.2 nm and can be synthesized in aqueous phase at mild conditions. An aluminogermanate (AlGe) analog has been synthesized with a larger outer diameter of 3.3 nm. Both the AlSi and AlGe nanotubes are very short (about 100 nm and 20 nm in length, respectively). In Year 1 (2006-2007) of the project, we made substantial progress in elucidating their mechanism of formation. We proposed a new “nanoscale condensation-and-rearrangement” mechanism, and presented evidence that a very different self-assembly process driven by amorphous nanoparticle condensation and internal rearrangement can lead to nanotubes of more precisely controllable dimensions.
In Year 2 (2007-2008), we made two major contributions:
(1) We demonstrated the important concept of engineering the “curvature” of nanotubes at the atomic level by control of interatomic potential energies. Employing computational tools along with necessary theoretical developments, we explored the molecular-level thermodynamic foundation for self-assembly of these nanoscale materials, and validated our findings with our materials synthesis and characterization results. We used molecular dynamics simulations to study the energetics of the mixed-oxide AlSiGe nanotubes. In doing so, we have extended the parameterization of an interatomic force field (CLAYFF) to also cover Ge atoms. We have shown that the internal energy of the nanotubes exhibits a minimum as a function of the diameter. Furthermore, the energy minimum shifts with the composition (Si/Ge ratio). Finally, we showed that the predicted diameter of the minimum-energy nanotube was strongly correlated with experimentally observed diameter. We also developed an analytical molecular model that uses harmonic potential energies of the Al-O, Si-O and Ge-O bonds, and the detailed atomic structure, to explain and predict certain properties. An important overall finding is that the ability to control the curvature of the nanotube is strongly correlated to the difference in the wall structure, and hence the bond potential energies, on its inner and outer surfaces. The proposed mechanism of self-assembly (Year 1 work), and the energetics of self-assmebly (Year 2 work), together create the potential for a powerful method of assembling inorganic nanoscale materials of very small dimensions and controlled structure.
(2) We made the first comprehensive study of water adsorption and transport in single-walled aluminosilicate nanotubes. These are attractive materials for construction of nanofluidic devices. They have a well defined structure, a hydrophilic interior, precisely tunable length and diameter, and a functionalizable interior for tuning mass transport and adsorption properties. Our investigation highlighted their unique adsorption and diffusive water transport properties. Axial self-diffusivities of water molecules (at loadings ranging from near infinite dilution to near-saturation) were calculated using molecular dynamics simulations, whereas adsorption properties were computed with grand canonical Monte Carlo simulations and also compared to experimental data. Water transport at room temperature was observed to occur via Fickian diffusion. The self-diffusivity decreases with an increase in water content, whereas the transport diffusivity exhibited a maximum at intermediate water content. The diffusivities were comparable to the diffusivity of bulk liquid water and hence are considerably higher than in other nanoporous aluminosilicates such as zeolites. The computed adsorption isotherms a exhibited remarkable hydrophilicity of the pores of the nanotube. As a result of fast Fickian diffusion, hydrophilicity of the nanotubes, and short nanotube lengths; the diffusive water flux through an aluminosilicate nanotube film is predicted to be very high (~ 1000 mol/m2/s), even at very low pressure differentials. These results are significant for preparation of nanotube-based membrane devices.
