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

The Interplay of Growth Mechanism and Properties of ZnO Nanostructures for Different Applications

This review provides a background on the structure and properties of ZnO nanostructures. ZnO nanostructures are advantageous for many applications in sensing, photocatalysis, functional textiles, and cosmetic industries, which are described in this review. Previous work using UV Visible (UV-vis) spectroscopy and scanning electron microscopy (SEM) for ZnO nanorod growth analysis in-solution and on a substrate for determination of optical properties and morphology is discussed, as well as their results in determining the kinetics and growth mechanisms. From this literature review, it is understood that the synthesis process greatly affects nanostructures and properties; and hence, their applications. In addition, in this review, the mechanism of ZnO nanostructure growth is unveiled, and it is shown that by having greater control over their morphology and size through such mechanistic understanding, the above-mentioned applications can be affected. The contradictions and gaps in knowledge are summarized in order to highlight the variations in results, followed by suggestions for how to answer these gaps and future outlooks for ZnO nanostructure research.

Effect of Polymer Length on the Adsorption onto Aluminogermanate Imogolite Nanotubes

This study evidences the adsorption of model nonionic polymers onto aluminogermanate imogolite nanotubes, attractive porous nanofillers with potential molecular loading and release applications. We resolve the underlying mechanisms between nanotubes and polyethylene glycols with different molecular weights by means of nanoisothermal titration calorimetry. The analysis of the results provides a direct thermodynamic characterization, allowing us to propose a detailed description of the energetics involved in the formation of polymer/imogolite complexes. The affinity toward the nanotube surface is enthalpy-driven and strongly depends on the polymer chain length, which significantly affects the polymer configuration and the flow properties of the resulting complexes, probed by small-angle neutron scattering and rheology, respectively. These findings open new avenues for the rational design of these hybrid mixtures for advanced applications.

Deciphering the evolution of supramolecular nanofibers in solution and solid-state: a combined microscopic and spectroscopic approach

Supramolecular self-assembly of small organic molecules has emerged as a powerful tool to construct well-defined micro- and nanoarchitecture through fine-tuning a range of intermolecular interactions. The size, shape, and optical properties of these nanostructures largely depend on the specific assembly of the molecular building units, temperature and polarity of the medium, and external stimuli. The engineering of supramolecular self-assembled nanostructures with morphology-dependent tunable emission is in high demand due to the promising scope in nanodevices and molecular machines. However, probing the evolution of molecular aggregates from the solution and directing the self-assembly process in a pre-defined fashion are challenging. In the present study, we have deciphered the sequential evolution of supramolecular nanofibers from solution to spherical and oblong-shaped nanoparticles through the variation of solvent polarity, tuning the hydrophobic-hydrophilic interactions. An intriguing case of molecular self-assembly has been elucidated employing a newly designed π-conjugated thiophene derivative (TPAn) through a combination of steady-state absorption, emission measurements, fluorescence correlation spectroscopy (FCS), and electron microscopy. The FCS analysis and microscopy results revealed that the small-sized nanofibers in the dispersion further agglomerated upon solvent evaporation, resulting in a network of nanofibers. Stimuli-responsive reversible interconversion between a network of nanofibers and spherical nanoaggregates was probed both in dispersion and solvent-evaporated state. The evolution of organic nanofibers and a subtle control over the self-assembly process demonstrated in the current investigation provide a general paradigm to correlate the size, shape, and emission properties of fluorescent molecular aggregates in complex heterogeneous media, including a human cell.

Termination Effects in Aluminosilicate and Aluminogermanate Imogolite Nanotubes: A Density Functional Theory Study

We investigate termination effects in aluminosilicate (AlSi) and aluminogermanate (AlGe) imogolite nanotubes (NTs) by means of semi-local and range-corrected hybrid Density Functional Theory (DFT) simulations. Following screening and identification of the smallest finite model capable of accommodating full relaxation of the NT terminations around an otherwise geometrically and electrostatically unperturbed core region, we quantify and discuss the effects of physical truncation on the structure, relative energy, electrostatics and electronic properties of differently terminated, finite-size models of the NTs. In addition to composition-dependent changes in the valence (VB) and conduction band (CB) edges and resultant band gap (BG), the DFT simulations uncover longitudinal band bending and separation in the finite AlSi and AlGe models. Depending on the given termination of the NTs, such longitudinal effects manifest in conjunction with the radial band separation typical of fully periodic AlSi and AlGe NTs. The strong composition dependence of the longitudinal and radial band bending in AlSi and AlGe NTs suggests different mechanisms for the generation, relaxation and separation of photo-generated holes in AlSi and AlGe NTs, inviting further research in the untapped potential of imogolite compositional and structural flexibility for photo-catalytic applications.

The Role of Cation-Vacancies for the Electronic and Optical Properties of Aluminosilicate Imogolite Nanotubes: A Non-local, Linear-Response TDDFT Study

We report a combined non-local (PBE-TC-LRC) Density Functional Theory (DFT) and linear-response time-dependent DFT (LR-TDDFT) study of the structural, electronic, and optical properties of the cation-vacancy based defects in aluminosilicate (AlSi) imogolite nanotubes (Imo-NTs) that have been recently proposed on the basis of Nuclear Magnetic Resonance (NMR) experiments. Following numerical determination of the smallest AlSi Imo-NT model capable of accommodating the defect-induced relaxation with negligible finite-size errors, we analyse the defect-induced structural deformations in the NTs and ensuing changes in the NTs' electronic structure. The NMR-derived defects are found to introduce both shallow and deep occupied states in the pristine NTs' band gap (BG). These BG states are found to be highly localized at the defect site. No empty defect-state is modeled for any of the considered systems. LR-TDDFT simulation of the defects reveal increased low-energy optical absorbance for all but one defects, with the appearance of optically active excitations at energies lower than for the defect-free NT. These results enable interpretation of the low-energy tail in the experimental UV-vis spectra for AlSi NTs as being due to the defects. Finally, the PBE-TC-LRC-approximated exciton binding energy for the defects' optical transitions is found to be substantially lower (up to 0.8 eV) than for the pristine defect-free NT's excitations (1.1 eV).

Chemically Selective Alternatives to Photoferroelectrics for Polarization-Enhanced Photocatalysis: The Untapped Potential of Hybrid Inorganic Nanotubes

Linear-scaling density functional theory simulation of methylated imogolite nanotubes (NTs) elucidates the interplay between wall-polarization, bands separation, charge-transfer excitation, and tunable electrostatics inside and outside the NT-cavity. The results suggest that integration of polarization-enhanced selective photocatalysis and chemical separation into one overall dipole-free material should be possible. Strategies are proposed to increase the NT polarization for maximally enhanced electron-hole separation.

The potential of imogolite nanotubes as (co-)photocatalysts: a linear-scaling density functional theory study

We report a linear-scaling density functional theory (DFT) study of the structure, wall-polarization absolute band-alignment and optical absorption of several, recently synthesized, open-ended imogolite (Imo) nanotubes (NTs), namely single-walled (SW) aluminosilicate (AlSi), SW aluminogermanate (AlGe), SW methylated aluminosilicate (AlSi-Me), and double-walled (DW) AlGe NTs. Simulations with three different semi-local and dispersion-corrected DFT-functionals reveal that the NT wall-polarization can be increased by nearly a factor of four going from SW-AlSi-Me to DW-AlGe. Absolute vacuum alignment of the NT electronic bands and comparison with those of rutile and anatase TiO2 suggest that the NTs may exhibit marked propensity to both photo-reduction and hole-scavenging. Characterization of the NTs' band-separation and optical properties reveal the occurrence of (near-)UV inside-outside charge-transfer excitations, which may be effective for electron-hole separation and enhanced photocatalytic activity. Finally, the effects of the NTs' wall-polarization on the absolute alignment of electron and hole acceptor states of interacting water (H2O) molecules are quantified and discussed.

Spectroscopic and Electrochemical Studies of Imogolite and Fe-Modified Imogolite Nanotubes

Carbon nanotubes and other forms of carbon nanoparticles, as well as metal nanoparticles have been widely used in film electrochemistry because they allow for the immobilization of larger amounts of catalyst (either biological or inorganic) on the top of the modified electrodes. Nevertheless, those nanoparticles present high costs of synthesis and of separation and purification that hamper their employment. On the other hand, imogolites (Im), with the general formula (OH)₃Al₂O₃SiOH, are naturally-occurring nanomaterials, which can be obtained from glassy volcanic ash soils and can also be synthesized at mild conditions. In this research paper, we characterize through spectroscopic techniques (i.e., fourier transform infrared spectroscopy (FTIR) spectroscopy, powder X-ray diffraction (XRD) and transmission electron microscopy (TEM)) synthetized Im and Fe-modified imogolite (Im(Fe)). Moreover, the Im and Im(Fe) were physically adsorbed on the top of a graphite electrode (GE) and were characterized electrochemically in the potential region ranging from -0.8 to 0.8 V vs. the saturated calomel electrode (SCE). When the film of the Im or of the Im(Fe) was present on the top of the electrode, the intensity of the charging/discharging current increased two-fold, but no redox activity in the absence of O₂ could be appreciated. To show that Im and Im(Fe) could be used as support for catalysts, iron phthalocyanine (FePc) was adsorbed on the top of the Im or Im(Fe) film, and the electrocatalytic activity towards the O₂ reduction was measured. In the presence of the Im, the measured electrocatalytic current for O₂ reduction increased 30%, and the overpotential drastically decreased by almost 100 mV, proving that the Im can act as a good support for the electrocatalysts.

Pressure-induced amorphization of YVO₄:Eu³⁺ nanoboxes

A structural transformation from the zircon-type structure to an amorphous phase has been found in YVO4:Eu(3+) nanoboxes at high pressures above 12.7 GPa by means of x-ray diffraction measurements. However, the pair distribution function of the high-pressure phase shows that the local structure of the amorphous phase is similar to the scheelite-type YVO4. These results are confirmed both by Raman spectroscopy and Eu(3+) photoluminescence which detect the phase transition to a scheelite-type structure at 10.1 and 9.1 GPa, respectively. The irreversibility of the phase transition is observed with the three techniques after a maximum pressure in the upstroke of around 20 GPa. The existence of two (5)D0-->(7)F0 photoluminescence peaks confirms the existence of two local environments for Eu(3+), at least for the low-pressure phase. One environment is the expected for substituting Y(3+) and the other is likely a disordered environment possibly found at the surface of the nanoboxes.

Direct synthesis of single-walled aminoaluminosilicate nanotubes with enhanced molecular adsorption selectivity

Internal functionalization of single-walled nanotubes is an attractive, yet difficult challenge in nanotube materials chemistry. Here we report single-walled metal oxide nanotubes with covalently bonded primary amine moieties on their inner wall, synthesized through a one-step approach. Conclusive molecular-level structural information on the amine-functionalized nanotubes is obtained through multiple solid-state techniques. The amine-functionalized nanotubes maintain a high carbon dioxide adsorption capacity while significantly suppressing the adsorption of methane and nitrogen, thereby leading to a large enhancement in adsorption selectivity over unfunctionalized nanotubes (up to four-fold for carbon dioxide/methane and ten-fold for carbon dioxide/nitrogen). The successful synthesis of single-walled nanotubes with functional, covalently-bound organic moieties may open up possibilities for new nanotube-based applications that are currently inaccessible to carbon nanotubes and other related materials.

Nanotubes from chalcogenide misfit compounds: Sn-S and Nb-Pb-S

Carbon fullerenes and nanotubes revolutionized understandingof the reactivity of nanoscale compounds. Subsequently, our group and others discovered analogous inorganic compounds with hollow, closed nanostructures. Such inorganic nanostructures offer many applications, particularly in the energy and electronics industries. One way to create inorganic nanostructures is via misfit layer-ed compounds (MLC), which are stacks of alternating two-dimensional molecular slabs, typically held together via weak van der Waals forces. They contain "misfits" in their a-b plane structures that can make them unstable, leading to collapse of the slabs into tubular nanostructures. For example, metal chalcogenide MLCs of the general formula (MX)1+y/TX2 (M = Sn, Pb, Bi, Sb, and other rare earths; T = Sn, Ti, V, Cr, Nb, Ta, etc.; X = S or Se) consist of a superstructure of alternating layers where the MX unit belongs to a (distorted NaCl) orthorhombic symmetry group (O), the TX2 layer possesses trigonal (T) or octahedral symmetry, and the two layers are held together via both van der Waals and polar forces. A misfit in the a axis or both a and b axes of the two sublattices may lead to the formation of nanostructures as the lattices relax via scrolling. Previous research has also shown that the abundance of atoms with dangling bonds in the rims makes nanoparticles of compounds with layered structure unstable in the planar form, and they tend to fold into hollow closed structures such as nanotubes. This Account shows that combining these two triggers, misfits and dangling bond annihilation in the slab rims, leads to new kinds of nanotubes from MLCs. In particular, we report the structure of two new types of nanotubes from misfits, namely, the SnS/SnS2 and PbS/NbS2 series. To decipher the complex structures of these nanotubes, we use a range of methods: high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS), selected area electron diffraction (SAED) analyses, scanning electron microscopy (SEM), and Cs-corrected scanning transmission electron microscopy (STEM) in the high-angle annular dark-field mode (HAADF). In both new types, the lattice mismatch between the two alternating sublayers dictates the relative layer-stacking order and leads to a variety of chiral tubular structures. In particular, the incommensuration (a type of misfit) of the SnS2/SnS system in both the (in plane) a and b directions leads to a variety of relative in-plane orientation and stacking orders along the common c-axis. Thus the SnS/SnS2 nanotubes form superstructures with the sequence O-T and O-T-T, and mixtures thereof. We also report nanotubes of the misfit layered compound (PbS)1.14NbS2, and of NbS2 intercalated with Pb atoms, with the chemical formula PbNbS2. Thus, the possibility to use two kinds of folding mechanisms jointly offers a new apparatus for the synthesis of unique 1-D nanostructures of great complexity and a potentially large diversity of physicochemical properties.

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.

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 (pH_rx) 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.

Physico-chemical control over the single- or double-wall structure of aluminogermanate imogolite-like nanotubes

It is known that silicon can be successfully replaced by germanium atoms in the synthesis of imogolite nanotubes, leading to shorter and larger AlGe nanotubes. Beside the change in morphology, two characteristics of the AlGe nanotube synthesis were recently discovered. AlGe imogolite nanotubes can be synthesized at much higher concentrations than AlSi imogolite. AlGe imogolite exists in the form of both single-walled (SW) and double-walled (DW) nanotubes, whereas DW AlSi imogolites have never been observed. In this article, we give details on the physicochemical control over the SW or DW AlGe imogolite structure. For some conditions, an almost 100% yield of SW or DW nanotubes is demonstrated. We propose a model for the formation of SW or DW AlGe imogolite, which also explains why DW AlSi imogolites or higher wall numbers for AlGe imogolite are not likely to be formed.

Shaping single-walled metal oxide nanotubes from precursors of controlled curvature

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.

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.

Self-diffusion of water and simple alcohols in single-walled aluminosilicate nanotubes

Understanding transport phenomena of fluids through nanotubes (NTs) is of great interest in order to enable potential application of NTs as separation devices, encapsulation media for molecule storage and delivery, and sensors. Single-walled metal oxide NTs are interesting materials because they present a well-defined solid-state structure, precisely tunable diameter and length, as well as a hydrophilic and functionalizable interior for tuning transport and adsorption selectivity. Here, we study the transport properties of hydrogen-bonding liquids (water, methanol, and ethanol) through a single-walled aluminosilicate NT to investigate the influence of liquid-surface and liquid-liquid interactions and the effects of competitive transport of different chemical species using molecular dynamics (MD) simulations. The self-diffusivities (D(s)) for all the three species decrease with increasing loading and are comparable to bulk liquid diffusivities at low molecular loadings. We show that the hydrogen-bond network associated with water makes its diffusion behavior different from methanol and ethanol. Mixtures of water and methanol show segregation in the NT, with water located closer to the tube wall and the alcohol molecules localized near the center of the NT. D(s) values of water in an analogous aluminogermanate NT are larger than those in the aluminosilicate NT due to a larger pore diameter.

Hydroxyl vacancies in single-walled aluminosilicate and aluminogermanate nanotubes

We report a theoretical study of hydroxyl vacancies in aluminosilicate and aluminogermanate single-walled metal-oxide nanotubes. Defects are introduced on both sides of the tube walls and lead to occupied and empty states in the band gap which are highly localized both in energy and in real space. Different magnetization states are found depending on both the chemical composition and the specific side with respect to the tube cavity. The defect-induced perturbations to the pristine electronic structure are related to the electrostatic polarization across the tube walls and the ensuing change in Lewis acid-base reactivity. A general approach towards a quantitative evaluation of both the polarization across the tube walls and the tube excluded volume is also proposed and discussed on an electrostatic basis.

Synthesis of large quantities of single-walled aluminogermanate nanotube

A simple aqueous synthesis yielded about 100 times more structurally well-organized single-walled aluminogermanate nanotubes than previously reported "standard" procedures. The structure analyses using XRD, IRTF, TEM, and XAS were greatly facilitated by the high concentrations available, and they ascertained the imogolite-like structure of the nanotubes. Simplicity and yield of the synthesis protocol are likely to favor commercial applications of theses materials as well as simplified syntheses of other nanophases.

Controlling nanotube dimensions: correlation between composition, diameter, and internal energy of single-walled mixed oxide nanotubes

Control over the diameter of nanotubes is of significance in manipulating their properties, which depend on their dimensions in addition to their structure and composition. This aspect has remained a challenge in both carbon and inorganic nanotubes, since there is no obvious aspect of the formation mechanism that allows facile control over nanotube curvature. Here we develop and analyze a quantitative correlation between the composition, diameter, and internal energy of a class of single-walled mixed oxide aluminosilicogermanate (AlSiGeOH) nanotubes. A series of synthetic AlSiGeOH nanotubes with varying Si/Ge ratio are characterized by X-ray photoelectron spectroscopy, vibrational spectroscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction to relate their compositions and diameters. We then study these nanotubes computationally by first parametrizing and validating a suitable interatomic potential model, and then using this potential model to investigate the internal energy of the nanotube as a function of diameter and composition via molecular dynamics simulations. There are minima in the internal energy as a function of diameter which progressively shift to larger nanotube diameters with increasing Ge content. An approximate analytical theory of nanotube diameter control, which contains a small number of physically significant fitted parameters, well describes the computational data by relating the composition and geometry to the strain energy of bending into a nanotube. The predicted composition-dependent shift in the energetically favored diameter follows the experimental trends. We suggest related methods of controlling nanotube energetics and their role in engineering nanotubes of controlled dimensions by liquid-phase chemistry.