Characterizing the Morphologies of Mechanically Manipulated Multiwall Carbon Nanotube Films by Small-Angle X-ray Scattering (original) (raw)
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2007
Films of multiwall carbon nanotubes (MWCNTs) grown by thermal chemical vapor deposition were studied using small-angle X-ray scattering (SAXS). We assessed the extent of alignment of carbon nanotubes (CNTs) by examining relative SAXS intensities as a function of azimuthal angle. We also identified features in the SAXS patterns that correspond well to CNT diameters measured through high-resolution transmission electron microscopy. For the case of thick films, corresponding to CNTs with lengths on the order of a millimeter, we were able to study the morphology of the films as a function of distance from the catalyst substrate. We examined two different films in which the morphologies of CNTs range from vertically aligned to entangled and tortuous. We determined that the alignment of CNTs as well as their average diameter can vary significantly throughout the film, demonstrating the utility of SAXS for quantitative structural analysis of CNT films, indicating the potential to reveal new information about the CNT growth process, and relating variations in morphology to evolution of the catalyst and reaction conditions.
Journal of Nanoscience and Nanotechnology, 2011
We have used small angle X-ray scattering (SAXS) to quantitatively characterize the morphology of vertically aligned (VA) multiwall carbon nanotube (MWCNT) arrays. We examined the extent of alignment of MWCNTs in terms of order parameter by analyzing SAXS intensity as a function of azimuthal angle. The SAXS measurements at different heights of CNT arrays from the substrate reveal two distinct morphologies and increasing alignment. We are able to quantitatively characterize a real variation in CNT diameters of the VA-MWCNTs through model fitting of the SAXS spectra. It found that the average CNT diameter increases with increasing distance from the substrate.
High-Speed in Situ X-ray Scattering of Carbon Nanotube Film Nucleation and Self-Organization
ACS Nano, 2012
The production of high-performance carbon nanotube (CNT) materials demands understanding of the growth behavior of individual CNTs as well as collective effects among CNTs. We demonstrate the first use of grazing incidence small-angle X-ray scattering to monitor in real time the synthesis of CNT films by chemical vapor deposition. We use a custom-built cold-wall reactor along with a high-speed pixel array detector resulting in a time resolution of 10 msec. Quantitative models applied to time-resolved X-ray scattering patterns reveal that the Fe catalyst film first rapidly dewets into well-defined hemispherical particles during heating in a reducing atmosphere, and then the particles coarsen slowly upon continued annealing. After introduction of the carbon source, the initial CNT diameter distribution closely matches that of the catalyst particles. However, significant changes in CNT diameter can occur quickly during the subsequent CNT self-organization process. Correlation of time-resolved orientation data to X-ray scattering intensity and height kinetics suggests that the rate of self-organization is driven by both the CNT growth rate and density, and vertical CNT growth begins abruptly when CNT alignment reaches a critical threshold. The dynamics of CNT size evolution and self-organization vary according to the catalyst annealing conditions and substrate temperature. Knowledge of these intrinsically rapid processes is vital to improve control of CNT structure and to enable efficient manufacturing of high-density arrays of long, straight CNTs.
Non-destructive characterization of structural hierarchy within aligned carbon nanotube assemblies
Journal of Applied Physics, 2011
Understanding and controlling the hierarchical self-assembly of carbon nanotubes (CNTs) is vital for designing materials such as transparent conductors, chemical sensors, high-performance composites, and microelectronic interconnects. In particular, many applications require highdensity CNT assemblies that cannot currently be made directly by low-density CNT growth, and therefore require post-processing by methods such as elastocapillary densification. We characterize the hierarchical structure of pristine and densified vertically aligned multi-wall CNT forests, by combining small-angle and ultra-small-angle x-ray scattering (USAXS) techniques. This enables the nondestructive measurement of both the individual CNT diameter and CNT bundle diameter within CNT forests, which are otherwise quantified only by delicate and often destructive microscopy techniques. Our measurements show that multi-wall CNT forests grown by chemical vapor deposition consist of isolated and bundled CNTs, with an average bundle diameter of 16 nm. After capillary densification of the CNT forest, USAXS reveals bundles with a diameter >4 lm, in addition to the small bundles observed in the as-grown forests. Combining these characterization methods with new CNT processing methods could enable the engineering of macro-scale CNT assemblies that exhibit significantly improved bulk properties.
Physical Review B, 2009
The average structure of double-walled carbon nanotube ͑DWCNT͒ samples can be determined by x-ray diffraction ͑XRD͒. We present a formalism that allows XRD patterns of DWCNTs to be simulated and we give researchers the tools needed to perform these calculations themselves. Simulations of XRD patterns within this formalism are compared to experimental data obtained on two different DWCNT samples, produced by chemical vapor deposition or by peapod conversion ͑i.e., high-temperature peapod annealing͒. For each sample, we are able to determine structural aspects such as the number of walls, the diameter distribution of inner and outer tubes, the intertube spacing, and the bundled structure.
Progress in Natural Science: Materials International, 2012
In-situ thermal expansion tests on a series of carbon nanotube bucky-paper composites were performed with direct heating within a synchrotron SAXS source. The impact of the samples density and morphology as well as the chemistry and degree of decoration of the carbon nanotubes on the scattering patterns were investigated and correlated to the materials macro-properties. The results demonstrate that simple densification techniques, such as acetone dipping or gold electroless deposition, could reduce greatly the displacements of the carbon nanotubes within the structure and lead to more thermally stable material.
Nanotechnologies in Russia, 2008
Arrays of multiwalled carbon nanotubes (CNTs) aligned perpendicular to the substrate surface have been produced by the pyrolysis of a mixture of hydrocarbons with ferrocene as the source of the catalyst. Based on scanning electron microscopy data, the mechanism of formation of aligned CNT arrays is proposed. The potential of angle-resolved X-ray spectroscopy for determining the degree of disorder of graphite layers in CNTs is demonstrated. Aligned CNT arrays are characterized by anisotropy of magnetic properties due to encapsulation of metallic rods in the inner cavities of nanotubes.
Journal of Applied Physics, 2020
Nanometer thin single-walled carbon nanotube (CNT) films collected from the aerosol chemical deposition reactors have gathered attention for their promising applications. Densification of these pristine films provides an important way to manipulate the mechanical, electronic, and optical properties. To elucidate the underlying microstructural level restructuring, which is ultimately responsible for the change in properties, we perform large scale vector-based mesoscopic distinct element method simulations in conjunction with electron microscopy and spectroscopic ellipsometry characterization of pristine and densified films by drop-cast volatile liquid processing. Matching the microscopy observations, pristine CNT films with finite thickness are modeled as self-assembled CNT networks comprising entangled dendritic bundles with branches extending down to individual CNTs. Simulations of the film under uniaxial compression uncover an ultra-soft densification regime extending to a ~75% strain, which is likely accessible with the surface tensional forces arising from liquid surface tension during the evaporation. When removing the loads, the pre-compressed samples evolve into homogeneously densified films with thickness values depending on both the pre-compression level and the sample microstructure. The significant reduction in thickness, confirmed by our spectroscopic ellipsometry, is attributed to the underlying structural changes occurring at the 100 nm scale, including the zipping of the thinnest dendritic branches.
arXiv (Cornell University), 2019
Fullerene carbon nanotubes (CNTs) are stiff, all-carbon macromolecules with diameters as small as one nanometer and few microns in length. Whereas defective CNTs behave as colloids and do not form self-supporting macroscale materials, Fullerene CNTs behave as polymers, both in fluid phases as well as solid macro-materials. Solutions of Fullerene CNTs in chlorosulfonic acid (CSA) follow the phase behavior of rigid rod polymers interacting via a repulsive potential and display a liquid-crystalline phase at sufficiently high concentration. Here, we apply small-angle X-ray scattering, paired with polarized optical microscopy, to characterize the morphology of the liquid-crystalline phases formed in CNT solutions at concentrations from 3 to 6.5 % by volume. Theoretically, for highly polydisperse rods, such as CNTs, a direct transition is expected to occur from a nematic to a columnar phase only at high volume fraction (~50 %). Surprisingly, we find a hexagonal columnar phase at ~ 4.3 % volume fraction, implying a large inter-particle spacing. We attribute this early transition to thermal undulations of CNTs, which enhance their electrostatic repulsion, increasing the effective diameter of the CNTs in solution by an order of magnitude. We calculate the critical concentration where the mean amplitude of undulation of an unconstrained rod becomes comparable to the rod spacing and find that thermal undulations start to affect steric forces at concentrations as low as the isotropic cloud point.