Mechanical coupling limits the density and quality of self-organized carbon nanotube growth

Abstract

Aligned carbon nanotube (CNT) structures are promising for many applications; however, as-grown CNT "forests" synthesized by chemical vapor deposition (CVD) are typically low-density and mostly comprise tortuous defective CNTs. Here, we present evidence that the density and alignment of self-organized CNT growth is limited by mechanical coupling among CNTs in contact, in combination with their diameter-dependent growth rates. This study is enabled by comprehensive X-ray characterization of the spatially and temporally-varying internal morphology of CNT forests. Based on this data, we model the time evolution and diameter-dependent scaling of the ensuing mechanical forces on catalyst nanoparticles during CNT growth, which arise from the mismatch between the collective lengthening rate of the forest and the diameter-dependent growth rates of individual CNTs. In addition to enabling self-organization of CNTs into forests, time-varying forces between CNTs in contact dictate the hierarchical tortuous morphology of CNT forests, and may be sufficient to influence the structural quality of CNTs. These forces reach a maximum that is coincident with the maximum density observed in our growth process, and are proportional to CNT diameter. Therefore, we propose that improved manufacturing strategies for self-organized CNTs should consider both chemical and mechanical effects. This may be especially necessary to achieve high density CNT forests with low defect density, such as for improved thermal interfaces and high-permeability membranes.

Exohedral Physisorption of Ambient Moisture Scales Non-monotonically with Fiber Proximity in Aligned Carbon Nanotube Arrays

“…[26,49,50] Such wall defects are wellknown in multiwalled CNTs synthesized via chemical vapor deposition. [51] While Raman spectroscopy is a very useful tool for studying defects in carbon materials, [52] the quantitative study of wall defects in multiwalled CNTs using Raman scattering is very challenging, [26] so a secondary technique was therefore utilized: evaluation of the specific surface area of the CNTs using the adsorption isotherms of Kr via the theory developed by Brunauer, Emmett, and Teller (known as BET). [53] Figure 1d shows that the experimentally determined specific surface area of the CNTs is (776.8 ± 16.3 m 2 /g), a value consistent with a monolayer of Kr adsorbing onto both the exohedral and endohedral surfaces of the CNTs, defined as unrestricted adsorption.…”

Data-driven understanding of collective carbon nanotube growth by in situ characterization and nanoscale metrology

“…9,16,18,36,[98][99][100] Studying the mechanochemical aspects of CNT growth, wherein the effects of mechanical forces on the catalytic process are analyzed both experimentally and numerically, is an area of current research and will enable better understating and control on the CNT growth process. A deeper understanding of all the competing deactivation mechanisms and identifying the dominant ones will open the door for approaches to overcome them.…”

“…5,7,8 Moreover, the interactions among growing CNTs play an important role in creating the self-supporting aligned morphology, as well as in the eventual growth selftermination. 5,18 In a typical CNT forest, billions of CNTs grow simultaneously per square centimeter and interact together as they collectively build the vertically aligned structure. As shown in Fig.…”

Review—Electrochemical Growth of Carbon Nanotubes and Graphene from Ambient Carbon Dioxide: Synergy with Conventional Gas-Phase Growth Mechanisms

“…The structural characteristics of CNTs grown by CVD methods strongly depend on the experimental conditions under which they are prepared, including catalyst properties such as size, 59,60 composition, 32,33,[61][62][63] reduction techniques, 34,54 migration and particle ripening, [42][43][44][45] and catalyst support layers; [36][37][38] and global parameters such as time, 54 temperature, [39][40][41] and rate of carbon source. 54 These are shown in Figure 3.…”

“…Concepts critical to the catalytic growth of CNTs have been highlighted as precursor chemistry, 31 catalyst composition and/or oxidation state, [32][33][34] catalyst size, 35 physical and chemical properties of catalyst supports, [36][37][38] growth temperature, [39][40][41] and physical processes during growth such as Ostwald ripening, catalyst diffusion, and mechanically driven collective growth termination processes. [42][43][44][45] In contrast to this mature field, the growth of carbon nanostructures from the liquid-phase electrochemical reduction of CO 2 remains only a new field of research, with the most recent work demonstrating growth of large-diameter (>100 nm) CNTs 46 and fewlayer graphene flakes from CO 2 conversion. 15,16 These initial works demonstrate the capability to leverage CO 2 as a precursor in carbon nanostructure growth, even though forward-looking efforts to achieve high quality, precisely tuned materials such as single-walled CNTs or single-layered graphene at high yields will require control of the process beyond the systems-level approaches reported so far.…”