Pretreatment Control of Carbon Nanotube Array Growth for Gas Separation: Alignment and Growth Studied Using Microscopy and Small-Angle X-ray Scattering

Abstract

Aligned multiwalled carbon nanotube (CNT) arrays were prepared using chemical vapor deposition of C2H4 on Fe catalyst at 750 °C. CNT array height and alignment depends strongly on the duration of H2 pretreatment, with optimal height and alignment achieved using 10-15 min pretreatment. Small-angle X-ray scattering (SAXS) was used to quantify the alignment, distribution, and size of the CNTs in arrays produced from varying pretreatment times and the results correlated with microscopy measurements. SAXS analysis revealed that the higher section of the CNT arrays exhibited better alignment than the lower section. Combining these insights with transmission electron microscopy measurements of the CNT defects within each array enable a mechanism for the CNT growth to be proposed, where the loss of alignment arises from deformation of the CNTs during their growth. Gas permeation test across densified CNT arrays indicated that the alignment of the CNT array plays an important role in the gas transport, and that the gas diffusion across the well-aligned CNT arrays was enhanced by a factor of ~45, which is much more than that across the poorly aligned CNT arrays, with an enhancement factor of ~8.

Biomimetic Carbon Nanotube Films with Gradient Structure and Locally Tunable Mechanical Property

“…HOF ( f ) calculated based on the FFT images (see the Experimental Section) was used to quantitatively describe the CNTs orientation. The f value in the aligned region equals to 0.54, consistent with other aligned multiwalled nanotube arrays ( f = 0.55) and lower than the superaligned single‐walled nanotube forests ( f = 0.85) . The f value gradually decreases across the gradient structure until 0.13 at the bottom of the film (Figure g), meaning a very random arrangement and comparable to single‐walled CNT sponge ( f = 0.13) .…”

A Catalytic Etching-Wetting-Dewetting Mechanism in the Formation of Hollow Graphitic Carbon Fiber

“…The synthetic strategy for hollow graphitic carbon nanofibers (HGCNFs) with nanometer-sized pores or channels is believed to be important for many applicationsfor example, as catalyst or sensor supports, [1][2][3][4][5][6][7] as hydrogen-storage materials, 8 for gas storage, 9 and in electronic and electrochemical devices 10,11 -because of their distinctive one-dimensional (1D) structure, high structural and thermal stability, high surface area, and high electric and thermal conductivity. Several methods, such as the chemical vapor deposition (CVD) approach, [12][13][14] hydrothermal method, [15][16][17][18] and electrospinning technique, have been developed for the synthesis of HGCNFs.…”

Quantifying alignment in carbon nanotube yarns and similar two‐dimensional anisotropic systems

“…The Hermans factor, which is equivalent to the average of the second Legendre polynomial, 〈 P 2 〉, is now widely accepted as an orientational order parameter suitable for systems with uniaxial symmetry. It is usually calculated from crystallite ODFs determined using wide/small angle X‐ray diffraction (WAXS/SAXS) 22–24 . Due to their high penetration depths in organic materials, X‐rays can give information about 3D orientation distributions; however, to fully analyze such distributions, the traditional pole figure method is preferred 23,25 .…”