Single-walled Aluminosilicate Nanotubes Research Articles

Transparent Conductive Films Derived from Single-Walled Aluminosilicate Nanotubes

This study reports on an approach to enhancing the performance of silver nanowire (AgNW)-based transparent conducive films (TCFs) via introducing an emerging class of nanotubes, aluminosilicate nanotubes (AlSiNTs), which can reduce silver ions to form silver nanoparticles (AgNPs). This reaction was performed in a suspension of AlSiNTs and AgNO3(aq) under ambient conditions, i.e., without the need for additional reducing agents. The results of material characterization suggest that the AlSiNTs collapse after the formation of AgNPs. Computation based on density functional theory (DFT) was used to gain insight into the reducing capability of AlSiNTs. The computational results suggest that octahedral aluminum plays a key role in the transfer of charge from AlSiNTs to the adsorbed silver ions. A solution of AlSiNT–AgNP was mixed with a commercial AgNW suspension and deposited on a flexible PET substrate to form TCFs. The addition of AlSiNT–AgNP was shown to improve the connectivity of AgNWs and thus enhance th...

Utilizing imogolite nanotubes as a tunable catalytic material for the selective isomerization of glucose to fructose

The isomerization of glucose to fructose is an important step in the conversion of biomass to valuable fuels and chemicals. A key challenge for the isomerization reaction is achieving high selectivity towards fructose using recyclable and inexpensive catalysts. In this study, the isomerization of glucose to fructose is investigated through using imogolite nanotubes as a catalyst. Imogolite is a single-walled aluminosilicate nanotube characterized by surface areas of 200-400 m2/g and pore widths near 1 nm. Imogolite is a highly tunable structure and can be modified through substitution of Si with Ge or through functionalization of methyl groups to the inner surface. These modifications change the surface properties of the nanotubes and enable tuning of the catalytic performance. Imogolite nanotubes are successfully used as a heterogeneous catalyst for the isomerization of glucose to fructose. Of the compositions tested, aluminosilicate imogolite is the most active material for the conversion of glucose, achieving a glucose conversion of 30% and a fructose selectivity of 45%. Catalyst recycling experiments reveals that organic content accumulates on the nanotubes that results in a minor reduction in conversion while maintaining similar catalytic selectivity. The catalyst can be washed with aqueous ammonia, allowing the productivity of fructose to be recovered. Overall, imogolite nanotubes are an active and tunable catalytic platform with moderate selectivity for the isomerization of glucose to fructose.

Properties of Single-Walled Aluminosilicate Nanotube/Poly(vinyl alcohol) Aqueous Dispersions.

The properties of (synthesized) single-walled aluminosilicate nanotube (AlSiNT; light-scattering characterized length ∼2000 ± 230 nm and diameter ∼35 ± 4 nm) dispersed in an aqueous poly(vinyl alcohol) (PVA) solution (10 wt %) are systematically explored using a comprehensive combination of (polarized/depolarized) dynamic light scattering, rheological, rheo-optical, and scanning electron microscopy analysis schemes. The nanotube/polymer dispersions under investigation are promising for their fair nanotube dispersion in pristine aqueous media (e.g., without salt or acid addition), as well as for the optical transparency that greatly facilitates systematic exploration of structural features and dispersion state that are practically inaccessible for many of their (opaque) companions such as carbon nanotube dispersions. We provide the first in-depth analysis revealing excellent dispersion state of (unmodified) AlSiNT in the PVA matrix, giving rise to (critical) gel-like features and substantially promoted elasticity that can be utilized, as a practical assessment, to produce uniform and defect-free electrospun nanofibers. Additionally, there is unambiguous evidence of nematic liquid crystal-like "wagging" (strain-invariant, periodic oscillation) under steady shear flow, a phenomenon previously unreported for nanotube composite materials. Overall, the present findings suggest that AlSiNT/PVA dispersions possess promising rheological, optical, and electrospinning properties that are highly desirable for current nanotechnological applications, and may serve as an ideal model system for establishing structure-performance relationships for like nanotube/polymer composite materials.

Investigating the Potential of Single-Walled Aluminosilicate Nanotubes in Water Desalination.

Water shortage has become a critical issue. To facilitate the large-scale deployment of reverse-osmosis water desalination to produce fresh water, discovering novel membranes is essential. Here, we computationally demonstrate the great potential of single-walled aluminosilicate nanotubes (AlSiNTs), materials that can be synthesized through scalable methods, in desalination. State-of-the-art molecular dynamics simulations were employed to investigate the desalination performance and structure-performance relationship of AlSiNTs. Free energy profiles, passage time distribution, and water density map were also analyzed to further understand the dependence of transport properties on diameter and water dynamics in the nanotubes. AlSiNTs with an inner diameter of 0.86 nm were found to fully reject NaCl ions while allowing orders of magnitude higher water fluxes compared to currently available reverse osmosis membranes, providing opportunities in water desalination.

Cover Picture: Defective Single‐Walled Aluminosilicate Nanotubes: Structural Stability and Mechanical Properties (ChemNanoMat 3/2016)

Single-walled aluminosilicate nanotubes (AlSiNTs) are an emerging form of nanotube with mechanical strength that is expected to be on par with that of single-walled nanotubes (SWCNTs). D.-Y. Kang et al. developed practical models of defective AlSiNTs to enable a quantitative investigation of the relationships among defect structures as well as the structural stability and mechanical properties of AlSiNTs. The assessment employed multi-scale computational tools: density functional theory, molecular modeling, and nanoscale continuum modeling. The developed methodology could be extended to other nanotube systems to the discovery of new high-strength nanotube materials. More information can be found in the Full paper by D.-Y. Kang et al. on page 189 in Issue 3, 2016 (DOI: 10.1002/cnma.201500185).

Solution-processed ultra-low-k thin films comprising single-walled aluminosilicate nanotubes.

Nanotube-based ultra-low-k thin films with high Young's modulus composed of single-walled aluminosilicate nanotubes (AlSiNTs) and a trace amount of poly(vinyl alcohol) (PVA) have been developed. The dehydrated AlSiNT film possesses a relative permittivity of 1.05, and a Young's modulus of 25 GPa, which exceed those of most existing ultra-low-k materials.

Incorporation of single-walled aluminosilicate nanotubes for the control of crystal size and porosity of zeolitic imidazolate framework-L

The incorporation of single-walled aluminosilicate nanotubes (AlSiNTs) into the interlayer of zeolitic imidazolate framework-L (ZIF-L) results in novel AlSiNT@ZIF-L nanocomposites.

Isomorphic substitution of aluminium by iron into single-walled alumino-silicate nanotubes: A physico-chemical insight into the structural and adsorption properties of Fe-doped imogolite

Morphological, textural and surface properties are reported of Fe-doped aluminosilicate nanotubes (NTs) of the imogolite type (IMO, (OH)3Al2O3SiOH). Two samples with 1.4 wt% Fe were obtained by either direct synthesis (Fe-1.4-IMO) or post synthesis loading (Fe-L-IMO). Since formation of Fe2O3 clusters was observed in both cases, a third sample with a lower Fe content corresponding to 0.70 wt% Fe was prepared by direct synthesis, with the aim of preventing clusters formation (Fe-0.70-IMO). The samples were characterized by: High Resolution Transmission Electron Microscopy (HRTEM); X-ray Diffraction (XRD); N2 sorption isotherms; Thermo-Gravimetric Analysis (TGA); Diffuse Reflectance (DR) UV–vis and IR spectroscopies; electrophoretic mobility in water (ζ-potential) and adsorption of the azo-dye Acid Orange 7 (NaAO7) from water solutions. With Fe-0.70-IMO, Al3+ isomorphic substitution by Fe3+ was the main process occurring, leading to a material with formula (OH)3Al1.975Fe0.025O3SiOH, in which formation of Fe(OH)Al groups occurred at NTs outer surface. With Fe-1.4-IMO, some of the formed Fe(OH)Al groups probably acted as nucleation seeds, due to the natural tendency of iron to form Fe–O–Fe bridges, being responsible of the (undesired) formation of some Fe2O3 clusters. Comparison with Fe-L-IMO, in which only the formation of Fe2O3 clusters was, in principle, expected, showed on the contrary that some isomorphic substitution occurred also by post-synthesis loading.The intrinsic acidity of Fe(OH)Al groups was marginally different from that of Al(OH)Al groups, as shown by both IR spectroscopy and ζ-potential measurements, but the presence of iron modified the adsorption properties of IMO. Interaction with AO7− anions in water, indeed, occurred in different ways: i) with proper IMO, besides electrostatic interaction with the positively charged external surface of nanotubes, AO7− anions preferentially adsorb via H-bonding on inner SiOH groups; ii) with both Fe-0.70-IMO and Fe-1.4-IMO, Fe3+ cations of Fe(OH)Al groups act as coordination centres for N atoms in the AO7− moiety; iii) with Fe-L-IMO, larger Fe2O3 clusters likely hinder AO7− adsorption.

Relationships among the structural topology, bond strength, and mechanical properties of single-walled aluminosilicate nanotubes.

Carbon nanotubes (CNTs) are regarded as small but strong due to their nanoscale microstructure and high mechanical strength (Young's modulus exceeds 1000 GPa). A longstanding question has been whether there exist other nanotube materials with mechanical properties as good as those of CNTs. In this study, we investigated the mechanical properties of single-walled aluminosilicate nanotubes (AlSiNTs) using a multiscale computational method and then conducted a comparison with single-walled carbon nanotubes (SWCNTs). By comparing the potential energy estimated from molecular and macroscopic material mechanics, we were able to model the chemical bonds as beam elements for the nanoscale continuum modeling. This method allowed for simulated mechanical tests (tensile, bending, and torsion) with minimum computational resources for deducing their Young's modulus and shear modulus. The proposed approach also enabled the creation of hypothetical nanotubes to elucidate the relative contributions of bond strength and nanotube structural topology to overall nanotube mechanical strength. Our results indicated that it is the structural topology rather than bond strength that dominates the mechanical properties of the nanotubes. Finally, we investigated the relationship between the structural topology and the mechanical properties by analyzing the von Mises stress distribution in the nanotubes. The proposed methodology proved effective in rationalizing differences in the mechanical properties of AlSiNTs and SWCNTs. Furthermore, this approach could be applied to the exploration of new high-strength nanotube materials.

Interfacial polymerization of polyamide-aluminosilicate SWNT nanocomposite membranes for reverse osmosis

A new method of single-pass flow which incorporates aluminosilicate single-walled nanotubes (SWNT) in a polyamide matrix was developed to fabricate thin film nanocomposite (TFN) membranes for low pressure reverse osmosis (RO) via interfacial polymerization. The TFN membranes were characterized using attenuated total reflectance–Fourier transform infrared (ATR–FTIR) and X-ray photoelectron spectroscopy (XPS) for the analysis of functional groups as well as composition of SWNTs. The typical morphology of polyamide layers was observed using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The introduction of SWNTs in polyamide active layer is evident with the proliferation of aluminum and silicon elements from XPS analysis. All membranes show the rugose structure with “leaf-like” outgrowths and the “ridge-and-valley” structure commonly observed in polyamide RO membranes. The hydrophilicity was increased as observed in the enhancement in water flux and pure water permeance, due to the presence of hydrophilic nanotubes. With the incorporation of the single-walled aluminosilicate nanotubes, higher permeate flux was achieved while sustaining high rejection of monovalent and divalent ions, typical of polyamide RO membrane.

IR spectroscopic study of the acidic properties of alumino-silicate single-walled nanotubes of the imogolite type

An IR spectroscopic study is reported of the surface acidic properties of two nanotube (NT) materials, imogolite (IMO, the aluminosilicate (OH)3Al2O3SiOH) and its hybrid organic/inorganic homologue methyl-imogolite (Me-IMO, (OH)3Al3O3SiCH3). Both are made of closely packed, intertwined bundles of NTs, forming two types of nanotunnels: (i) inner cylindrical ones, pertaining to single NTs, and (ii) intertube pores arising from the contact of adjacent NTs. Mesoporosity is present as a minor feature. The outer diameter of Me-IMO is larger than that of IMO (ca. 3nm against 2nm), and so are the intertube pores of Me-IMO and IMO (ca. 0.3 and 0.4.5nm, respectively). The latter are not accessible even to small molecules: the former become so after proper outgassing, as recently shown by molecular adsorption of CO2 (Zanzottera et al. (2012) [5]).The behaviour is reported with respect to basic molecules (water, CO and ammonia). As a whole, acidic properties are moderate, especially if compared with other aluminosilicates, and depend on the hydration state of the surface. The inner surface of Me-IMO NTs, featuring methyl groups, is inactive. The inner surface of IMO, lined with silanols, behaves closely to amorphous silica: hydronium ions are formed in the presence of water, and the IR frequency of CO molecules H-bonded to silanols after removal of water is very close to that observed with severely outgassed amorphous silica (2156cm−1). With IMO treated at 300°C, some Lewis acidic sites are detected, because of dehydration, as well as new, more acidic, OH groups arising from incipient nanotubes disruption, a prelude to the formation of layered structures (Zanzottera et al. (2012) [4]). The outer surface of a single NT (the same for both materials) features OH species bridged over two octahedral Al atoms, which display an amphoteric nature. In NTs packed into bundles, two types of surfaces occur, that at intertube cavities and the outer at the exterior of bundles. The latter does not exhibit acidic properties. With Me-IMO, AlOH groups in the intertube cavities reveal a moderate acidic character, as shown by ammonia (ammonium species formation) as well as CO adsorption (stretching frequency at 2151–2154cm−1).

Kinetic and Surface Study of Single-Walled Aluminosilicate Nanotubes and Their Precursors

The structural and surface changes undergone by the different precursors that are produced during the synthesis of imogolite are reported. The surface changes that occur during the synthesis of imogolite were determined by electrophoretic migration (EM) measurements, which enabled the identification of the time at which the critical precursor of the nanoparticles was generated. A critical parameter for understanding the evolution of these precursors is the isoelectric point (IEP), of which variation revealed that the precursors modify the number of active ≡Al-OH and ≡Si-OH sites during the formation of imogolite. We also found that the IEP is displaced to a higher pH level as a consequence of the surface differentiation that occurs during the synthesis. At the same time, we established that the pH of the reaction (pHrx) decreases with the evolution and condensation of the precursors during aging. Integration of all of the obtained results related to the structural and surface properties allows an overall understanding of the different processes that occur and the products that are formed during the synthesis of imogolite.

Imogolite-like nanotubes: structure, stability, electronic and mechanical properties of the phosphorous and arsenic derivatives

Imogolite is a single-walled aluminosilicate nanotube (NT) found in nature that can be easily synthesized, as well as its analogue aluminogermanate NT. Based on geometrical assumptions and pKa values, species such as H3PO4, H3PO3, H3AsO3, H3AsO4 could also be candidates to form imogolite-like structures. In the present work, we provide insights about the stability, electronic, structural and mechanical properties of possible imogolite like NTs by means of self-consistent charge density-functional tight-binding method (SCC-DFTB). Similarly to aluminogermanate, where the tetrahedral silicate groups are replaced by germanate, in this work tetrahedral silicate groups are substituted by phosphate, phosphite, arsenate and arsenite units in the imogolite structure. Detailed analysis is focused on structural properties, strain energy, band gap and Mulliken charges distribution. The calculated strain energy curves for all studied zigzag imogolite-like NTs present well-defined minima, which change as a consequence of composition variation. Moreover, the strain energy curves of armchair imogolite-like NTs also present minima, although in all cases less stable than zigzags by at least 2.2 meV per atom. The insulating NT behaviour changes after internal modification from silicate to phosphate, phosphite, arsenate and arsenite, as well as the charge distribution inside and outside the nanotubes.

Surface properties of alumino-silicate single-walled nanotubes of the imogolite type

An IR spectroscopy study is reported on the nature and accessibility of external and internal surfaces of single-walled alumino-silicate nanotubes (NTs) of the imogolite type. NTs form bundles with hexagonal symmetry, in which three kinds of surfaces may be figured out: surface A is the inner surface of NTs; surface B is that between three aligned NTs in the hexagonal packing; and surface C arises from slit mesopores between bundles. Two materials were considered: proper imogolite (IMO, (OH)3Al2O3SiOH) and its methylated analogue, (Me-IMO, (OH)3Al2O3SiCH3). The chemical nature of the outer surface of NTs is the same in both materials, i.e. a curved gibbsite sheet with both Al-OH-Al and Al-O-Al groups and an amphoteric character. The inner surface is very hydrophilic in IMO NTs, lined by closely packed silanols, and hydrophobic in Me-IMO, all silanols being replaced by -SiCH3 groups. The change in chemical composition is accompanied by an increment in pore size, about 1.0 nm in IMO, and ca. 2.0 nm in Me-IMO, which implies a change in the accessibility of the B surface, not available to any molecule in IMO, and accessible in Me-IMO to small molecules like water, due to larger pores between NTs. Aluminol species at the B surface display an acidic nature, in contrast with that of the same species at surface C, because of a confinement effect.

Synthesis of CdTe QDs/single-walled aluminosilicate nanotubes hybrid compound and their antimicrobial activity on bacteria

The use of molecular conjugates of quantum dots (nanocrystalline fluorophores) for biological purposes have received much attention due to their improved biological activity. However, relatively, little is known about the synthesis and application of aluminosilicate nanotubes decorated with quantum dots (QDs) for imaging and treatment of pathogenic bacteria. This paper describes for a first time, the use of single-walled aluminosilicate nanotubes (SWNT) (imogolite) as a one-dimensional template for the in situ growth of mercaptopropionic acid-capped CdTe QDs. This new nanohybrid hydrogel was synthesized by a simple reaction pathway and their enhanced optical properties were monitored by fluorescence and UV–Vis spectroscopy, confirming that the use of these nanotubes favors the confinement effects of net CdTe QDs. In addition, studies of FT-IR spectroscopy and transmission electron microscopy confirmed the non-covalent functionalization of SWNT. Finally, the antimicrobial activity of SWNT coated with CdTe QDs toward three opportunistic multi-resistant pathogens such as Salmonella typhimurium, Acinetobacter baumannii, and Pseudomonas aeruginosa were tested. Growth inhibition tests were conducted by exposing growing bacteria to CdTe QDs/SWNT hybrid compound showing that the new nano-structured composite is a potential antimicrobial agent for heavy metal-resistant bacteria.

Diffusion of Tetrafluoromethane in Single-Walled Aluminosilicate Nanotubes: Pulsed Field Gradient NMR and Molecular Dynamics Simulations

Diffusion of tetrafluoromethane (CF4) in single-walled aluminosilicate nanotubes was studied by means of pulsed field gradient (PFG) NMR and molecular dynamics (MD) simulations. The application of a high magnetic field and high magnetic field gradients allowed 13C PFG NMR measurements of diffusion to be performed under conditions of sufficiently large signal-to-noise ratios for a broad range of CF4 loadings. The PFG NMR data were analyzed to obtain the diffusivities for diffusion of CF4 inside the nanotube aggregates, in which the sorbate displacements exceeded the average length of individual nanotubes. In addition, the corresponding diffusivities under conditions of fast exchange of CF4 molecules between nanotubes or nanotube aggregates and the surrounding gas phase in a nanotube bed were also estimated. The experimental CF4 diffusivities inside the nanotube aggregates were found to be several times smaller than the corresponding diffusivities obtained by MD for diffusion inside the defect-free nanotube...

Defect Structures in Aluminosilicate Single-Walled Nanotubes: A Solid-State Nuclear Magnetic Resonance Investigation

We report a detailed investigation of the defect structures in aluminosilicate single-walled nanotubes via multiple advanced solid-state NMR techniques. A combination of 1H–29Si and 1H–27Al FSLG-HETCOR, 1H CRAMPS, and 1H–29Si CP/MAS experiments were employed to evaluate the proton environments around Al and Si atoms in the final nanotube structure. The 1H CRAMPS spectra of dehydrated aluminosilicate nanotubes revealed the proton environments in great detail. Integration of these results with the findings from the 1H–29Si and 1H–27Al FSLG-HETCOR and 1H–29Si CP/MAS data allows the structural assignment of all the chemical shifts and the identification of various types of defect structures in the aluminosilicate nanotube wall. In particular, we identify five main types of defect structures arising from specific atomic vacancies in the nanotube structure. It is estimated that ∼16% of Si atoms in the nanotube inner wall are involved in a defect structure. The characterization of the detailed structure of the n...

High permeate flux of PVA/PSf thin film composite nanofiltration membrane with aluminosilicate single-walled nanotubes

A new type of thin film nanocomposite (TFN) membranes for nanofiltration was successfully prepared by incorporating aluminosilicate single-walled nanotubes (SWNTs) within the poly(vinyl alcohol) (PVA) matrix. The nanocomposite PVA film was composed of well dispersed synthesized aluminosilicate SWNT with up to 20% volume fraction cast on a polysulfone support. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) revealed that the TFN membranes have characteristic bands at 920–1010cm−1 corresponding to SiOH and SiOAl stretching vibration of the aluminosilicate SWNT. This insinuated the successful incorporation of aluminosilicate SWNT into the polymer matrix, which was further confirmed and quantified by X-ray photoelectron spectroscopy (XPS). The PVA layers, in the range of 0.99–1.36μm, are free from large defects or cracks as observed in the scanning electron microscopy (SEM) images. The membrane surface hydrophilicity increased as the membrane roughness decreased and as the contact angles decreased from 64.2° to 59.4–50.5°. The increase in water flux is due to the presence of hydrophilic nanotubes. With the incorporation of the aluminosilicate single-walled nanotubes, higher permeate water flux was achieved, while sustaining high rejection of divalent ions (97%) and monovalent ions (59%).

Single-Walled Aluminosilicate Nanotube/Poly(vinyl alcohol) Nanocomposite Membranes

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.

Osmotic ensemble methods for predicting adsorption-induced structural transitions in nanoporous materials using molecular simulations

Osmotic framework adsorbed solution theory is a useful molecular simulation method to predict the evolution of structural transitions upon adsorption of guest molecules in flexible nanoporous solids. One challenge with previous uses of this approach has been the estimation of free energy differences between the solid phases of interest in the absence of adsorbed molecules. Here we demonstrate that these free energy differences can be calculated without reference to experimental data via the vibrational density of states of each phase, a quantity that can be obtained from molecular dynamics simulations. We show the applicability of this method through case studies of the swelling behaviors of two representative systems in which swelling upon adsorption of water is of importance: single-walled aluminosilicate nanotube bundles and cesium montmorillonite. The resulting predictions show that the aluminosilicate nanotube bundles swell significantly with increasing interstitial adsorption and that the layer spacing of cesium montmorillonite expands up to about 12.5 Å, giving good agreement with experiments. The method is applicable to a wide range of flexible nanoporous materials, such as zeolites, metal-organic frameworks, and layered oxide materials, when candidate structures can be defined and a force field to describe the material is available.