In Situ Mechanochemical Modulation of Carbon Nanotube Forest Growth

Abstract

Ordered synthesis of one-dimensional nanostructures, such as carbon nanotubes (CNTs), involves competition between the growth kinetics of individual structures, their physical entanglement, and intermolecular forces that cause coupling of structures in close proximity. Specifically, CNT synthesis by chemical vapor deposition can directly produce films and fibers by providing CNT growth sites in close proximity such that the CNTs self-align into macroscopic assemblies. Because CNTs are mechanically coupled during these processes, the question arises as to whether or not mechanical forces intrinsic to the formation of CNT ensembles influence the growth kinetics and quality of CNTs, as can be expected from fundamental theories of mechanochemistry. Here, we study how mechanical forces influence CNT growth by applying controlled compression to CNT forests in situ; and relate the outcomes quantitatively to the CNT morphology and lengthening rate. We find that applied forces inhibit the self-organization of CNTs into a forest and accelerate the termination of collective growth. By correlating in situ kinetics measurements with spatial mapping of CNT orientation and density by X-ray scattering, we find that the average growth rate of individual CNTs is also mechanically modulated; specifically, a 100-fold increase in force causes a 4-fold decrease in average CNT lengthening rate. We attribute the slower growth kinetics to a stress-dependent increase of 0.02–0.16 eV in the effective activation energy for CNT growth. Via finite element modeling, we conclude that the force magnitudes that cause remodeling of the growing CNT network are less than the average strengths of adhesive contacts between CNTs. Last, we find that CNT growth rate and orientation can respond dynamically to changes in applied force, further demonstrating the mechanochemical nature of CNT growth and suggesting new approaches to control CNT quality and morphology in situ, with general application to other one-dimensional nanostructures.

Substrate adhesion evolves non-monotonically with processing time in millimeter-scale aligned carbon nanotube arrays

“…To support this trend, E has previously been observed to scale with NF diameter, 79 the number of CNT walls and contact area, 40,114 carbon deposition on the array, 45 and CNT V f . 42,45,114,115…”

Evaluation of process-structure-property relationships of carbon nanotube forests using simulation and deep learning

“…pointed out that a mechanical coupling between growing CNTs transmits forces that can create an energy barrier that ultimately exceeds energy available from chemical reaction at underlying catalyst particles [122], stunting CNT growth. Another recent report showed that CNT forests growing under a compressive mechanical decreased the observed CNT population growth rate [107]. These studies suggest that mechanical stress acting on catalyst nanoparticles alter the activation energy of CNT growth.…”

“…These studies suggest that mechanical stress acting on catalyst nanoparticles alter the activation energy of CNT growth. In these studies, the collective CNT forest grew at a decreased rate with Arrhenius-like dependence [107]:…”

“…Fundamental kinetics studies using small angle x-ray scattering (SAXS) have revealed that the areal density, alignment, and diameter distribution of CNT forests evolve with time and that CNT forest growth termination is often abrupt [84][85][86]106]. Recently, an external compressive force applied to the top surface of CNT forests was shown to decrease the ensemble forest growth rate, indicating that CNT growth kinetics are directly influenced by even relatively small external forces [107]. The forces generated by forest self-assembly and transmitted to CNT catalyst particles are expected to exceed those applied in the experiment while also manifesting as both tensile and compressive orientations.…”

“…The forces generated by forest self-assembly and transmitted to CNT catalyst particles are expected to exceed those applied in the experiment while also manifesting as both tensile and compressive orientations. By extension, it may be assumed that the mechanical forces generated during self-assembly similarly modulate the growth rate of individual CNTs during their growth and may contribute to the abrupt growth termination of CNT forests [82,83,85,107]. Measuring the evolution of forces within a forest at the level of individual CNTs is currently inaccessible by experimental techniques and requires a simulation capable of modeling both the selfassembly process of interacting CNTs and the mechanical response resulting from these interactions.…”