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We demonstrate new molecular-level concepts for constructing nanoscopic metal oxide objects. First, the diameters of metal oxide nanotubes are shaped with angstrom-level precision by controlling the shape of nanometer-scale precursors. Second, we measure (at the molecular level) the subtle relationships between precursor shape and structure and final nanotube curvature. Anionic ligands are used to exert fine control over precursor shapes, allowing assembly into nanotubes whose diameters relate directly to the curvatures of the 'shaped' precursors., (© 2012 American Chemical Society)

The fabrication, detailed characterization, and molecular transport properties of nanocomposite membranes containing high fractions (up to 40 vol %) of individually-dispersed aluminosilicate single-walled nanotubes (SWNTs) in poly(vinyl alcohol) (PVA), are reported. The microstructure, SWNT dispersion, SWNT dimensions, and intertubular distances within the composite membranes are characterized by scanning and transmission electron microscopy (SEM and TEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), XRD rocking curve analysis, small-angle X-ray scattering (SAXS), and solid-state NMR. PVA/SWNT nanocomposite membranes prepared from SWNT gels allow uniform dispersion of individual SWNTs in the PVA matrix with a random distribution of orientations. SAXS analysis reveals the length (∼500 nm) and outer diameter (~2.2 nm) of the dispersed SWNTs. Electron microscopy indicates good adhesion between the SWNTs and the PVA matrix without the occurrence of defects such as voids and pinholes. The transport properties of the PVA/SWNT membranes are investigated experimentally by ethanol/water mixture pervaporation measurements, computationally by grand canonical Monte Carlo and molecular dynamics, and by a macroscopic transport model for anisotropic permeation through nanotube-polymer composite membranes. The nanocomposite membranes substantially enhance the water throughput with increasing SWNT volume fraction, which leads to a moderate reduction of the water/ethanol selectivity. The model is parameterized purely from molecular simulation data with no fitted parameters, and shows reasonably good agreement with the experimental water permeability data.

This study developed a user-friendly program, cif2tube, to facilitate the construction of a nanotube or a nanoscroll model using an input crystallographic information file ( * .cif file). This program makes it possible for users to determine several parameters used in the construction of the nanotube/nanoscroll model, including the chirality, the directions in which it will roll-up, and the interlayer spacing of the nanoscroll. The ability to vary these parameters enables the customization of the resulting models. The capability of the cif2tube program was demonstrated by constructing models of existing as well as hypothetical nanotube/nanoscroll materials. The as-generated models from cif2tube were then compared with models obtained from commercial molecular simulation packages and as-generated models that were subjected to geometric optimization and molecular dynamics simulations. We also demonstrated the capability of cif2tube in the creation of models comprising hypothetical nanotubes of perovskite, zeolite, and metal organic frameworks (MOFs). cif2tube enables the creation of a large variety of existing and novel nanotube models to facilitate the computational study of nanotube materials. [ABSTRACT FROM AUTHOR]

Abstract: Single‐walled aluminosilicate nanotubes (AlSiNTs) are expected to possess mechanical strength comparable to that of single‐walled carbon nanotubes (SWCNTs). Most existing theoretical studies on the mechanical properties of AlSiNTs are based on defect‐free models, despite the fact that experimental results have revealed a variety of defects in AlSiNTs. Herein we developed a method for the modeling of defective AlSiNTs to enable the quantitative investigation of relationships among defect structures, structural stability, and mechanical properties of AlSiNTs. The defect structures dealt with in the proposed models are based on experimental findings. Our assessment of the stability and mechanical strength of nanotubes is based on multiscale computational tools, including density functional theory, molecular modeling, and nanoscale continuum modeling. Our study also identified the defect structure with the most pronounced impact on the stability and mechanical properties of AlSiNTs. [ABSTRACT FROM AUTHOR]

Zeolitic imidazolate framework-L (ZIF-L) is an emerging ZIF material possessing a unique two-dimensional layered crystal structure and a leaf-like crystal morphology. This paper reports a novel approach towards the incorporation of single-walled aluminosilicate nanotubes (AlSiNTs) into the interlayer of ZIF-L for the synthesis of AlSiNT@ZIF-L nanocomposites. The interlayer in ZIF-L is formed by hydrogen bonding between adjacent imidazoles. The resulting nanocomposites were subjected to a number of solid-state characterization techniques (SEM, TEM, powder XRD, EDS, TGA, nitrogen physisorption, and FT-IR) to elucidate the conformation of their microstructure. It was found that the AlSiNT@ZIF-L composite possesses an average crystal size considerably smaller than that of pure ZIF-L (1.1 μm for AlSiNT@ZIF-L and 7.8 μm for pure ZIF-L). Interestingly, ZIF-L and AlSiNTs are both microporous; however, the mesoporosity was achieved through the incorporation of AlSiNTs into ZIF-L crystals. A control experiment related to the synthesis of AlSiNT@ZIF-8 composites was performed to gain insight into the assembly pathway towards the formation of AlSiNT@ZIF-L crystals. Failure in the formation of AlSiNT@ZIF-8 supports the supposition that the assembly pathway involves the incorporation of AlSiNTs into the interlayer of ZIF-L, thereby facilitating the nucleation of ZIF-L crystals. [ABSTRACT FROM AUTHOR]

We report a method for estimating the effective molecular diffusivity of mixed matrix membranes (MMMs) with tubular fillers. This method is based on constructing three-dimensional MMM models for solving Fick׳s diffusion equation to obtain the concentration profiles. The effective diffusivity of the MMMs was thus derived from the concentration distributions. Using this approach, the effects of various structural parameters, including the tubular filler volume fraction, spatial distribution, orientation, aspect ratios, and diffusivity ratios, were assessed. The results suggest that the tubular filler alignment and spatial distributions are critical to the mass transfer in MMMs. We compared the diffusivity estimated by the presented method to the experimental data. Our approach shows improved predictions compared with the Maxwell model and the Kang–Jones–Nair (KJN) model. Overall, this work presents a useful tool for understanding and designing MMMs with tubular fillers. [ABSTRACT FROM AUTHOR]

Abstract: The fabrication of ultrathin (300 nm) aluminosilicate nanotube–poly(vinyl alcohol) composite membranes with partial vertical alignment of the nanotubes (up to 33 %), by solution‐casting methods on porous polymeric substrates, is reported. A high loading (up to 60 vol %) of nanotubes is achieved. A comprehensive microstructural characterization of the membranes is performed by a combination of SEM, TEM, grazing‐incidence wide‐angle X‐ray scattering measurements, and simulations. This investigation shows that the nanotubes are individually dispersed and partially aligned in the polymeric matrix by the use of appropriate matrix and substrate materials. Permeation measurements of gas probe molecules (CO2 and CH4) on two different types of membranes, one containing bare nanotubes and the other containing amine‐functionalized nanotubes, also support the proposed microstructure of the thin nanocomposite membranes. [ABSTRACT FROM AUTHOR]

Abstract: We demonstrate a generalized kinetic modeling and experimental approach to describe the formation and growth of single-walled metal oxide nanotubes. These materials can now be synthesized using solution-based methods at temperatures as low as 95°C. Ultimately, it is desired to produce nanotubes on a larger scale (e.g., kilogram or ton) for technological applications. However, a quantitative multiscale understanding of nanotube growth, via a detailed growth model, is critical in order to predict and control key properties such as the length distribution and concentration of the nanotubes. Such a model can then be used to design liquid-phase reactors for scale-up of nanotube synthesis. The present model considers three types of species present in the synthesis solution: molecular precursors, amorphous nanoparticles, and nanotubes. The nucleation of a nanotube embryo is due to the rearrangement of an amorphous nanoparticle into an ordered tubular structure, as revealed by recent works. The nanotube growth model is represented by up to 800 population-balance differential equations. Parameterization, validation, and predictive evaluation of the model is carried out via detailed statistical TEM measurements of nanotube length distributions throughout the synthesis. This model is capable of explaining and predicting the evolution of nanotube populations as a function of kinetic parameters. It also allows insight into mesoscale and microscale nanotube growth processes. For example, it shows that two different mechanisms operate during nanotube growth: (1) growth by precursor addition, and (2) oriented attachment of nanotubes to each other. [Copyright &y& Elsevier]

Abstract: Nanotubes have been shown to possess intriguing mass transport properties, and are being incorporated into polymeric membranes for molecular separations. Although models have been developed to predict the effective permeability and selectivity of composite membranes with non-spherical fillers, they only apply to fillers with isotropic transport properties. However, molecular transport in tubular fillers is essentially one-dimensional. Here, an analytical model is developed – including the orientational distribution, aspect ratio, volume fraction, permeability of the fillers, and membrane non-idealities – to describe molecular transport in composite membranes with tubular fillers. Using the model, the effects of various structural and operational parameters of the composite membrane are assessed. It is found that the filler volume fraction, as opposed to the filler orientation, intrinsic permeability, and aspect ratio, has the most significant impact on the membrane permeability. Several case studies of binary mixture separations by ideal and non-ideal composite membranes with tubular fillers are discussed. Finally, an advanced composite membrane configuration that allows exploitation of the unique transport properties of tubular fillers is discussed. [Copyright &y& Elsevier]

Increasing demand for affordable desalinating water has stimulated the vigorous development of reverse osmosis (RO) membranes. To date, the performance of polymeric membranes is limited by the permeability–selectivity trade-off: more permeable membranes lead to a less ideal selectivity and vice versa. Significant efforts have been made to identify new classes of ultra-permeable nanoporous membranes, but such trade-off can still be commonly observed. In this study, we identify the key role of the surface uniformity and chemistry of permeation channels inside nanoporous membranes in mitigating the trade-off. By using highly tunable aluminosilicate nanotubes (AlSiNTs) as a model system, this study demonstrates that, by properly designing the interior surface of AlSiNTs, the materials can offer not only higher water permeability but also a better ability to reject salts. Detailed investigations on the capacity, dynamics, energetics, and distribution of water as well as the free energy landscapes for water molecules and ions are also carried out to understand the permeation mechanisms of species at an atomic level. The outcomes obtained herein can provide guidelines for the rational design of nanotube-based membranes, or porous materials in general, to achieve more efficient and effective desalination processes. • Permeability-selectivity trade-off can be mitigated by a proper membrane design. • Surface uniformity and chemistry of permeation channels inside membranes play a key role. • Permeation channels of uniform and positively charged surface may result in faster water transport and better selectivity. • Homogeneous water energy landscape for faster permeation and unfavorable interaction with sodium ions for higher selectivity. [ABSTRACT FROM AUTHOR]