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.
Original language | English (US) |
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Pages (from-to) | 407-418 |
Number of pages | 12 |
Journal | Chemistry of Materials |
Volume | 31 |
Issue number | 2 |
DOIs | |
State | Published - Jan 22 2019 |
Funding
We thank D. Copic and C. R. Oliver for assistance in building and testing the force-controlled CNT growth apparatus, originally at the University of Michigan. This research was supported by the Department of Energy, Office of Science (DESC0010795), and by the National Science Foundation Graduate Research Fellowship under grant no. 1122374 (to N.T.D.). SEM and TEM imaging was performed at Harvard University’s Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the National Science Foundation under NSF award no. ECS-0335765. Catalyst patterning and deposition was performed at Microsystems Technology Laboratories (MTL) at MIT. SAXS characterization was performed at the 12-ID-B beamline at the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357.
ASJC Scopus subject areas
- General Chemistry
- General Chemical Engineering
- Materials Chemistry