Engineering nanocatalysts for selective growth of carbon nanotubes.

摘要

The unique physical properties of carbon nanotubes (CNTs) are determined by their atomic-scale structure. This structure-property relationship is clearly observed in single-walled carbon nanotubes (SWCNTs) where changes in atomic structure alter their electronic and optical properties. However, current synthetic approaches are not capable of producing well-defined CNTs. Since most practical technological applications require predictable and uniform performance, researchers have devoted enormous efforts towards the preparation of homogeneous CNTs.;This thesis describes a novel method for selective synthesis of CNTs using a two-stage, sequential, floating-catalyst synthesis route based on the preparation of metal nanocatalysts via microplasma techniques and gas-phase CVD growth of CNTs. Dimensionally- and compositionally-tuned metal nanocatalysts are synthesized using a continuous-flow, atmospheric-pressure microplsama reactor. The as-grown metal nanoparticles are subsequently introduced into a heated flow furnace reactor to catalyze CNT growth. Aerosol instrumentation is used to monitor the CNT catalytic growth on line, allowing kinetic parameters including growth rate and activation energy for CNT growth to be extracted.;Detailed material microcharacterization including transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD) shows that the size, composition, and crystal structure of metal nanoparticles can be engineered using the microplasma synthesis technique. By controlling the catalyst composition, we show that CNTs can be grown at temperatures as low as 300 °C with an activation energy of 37 kJ/mol for Ni0.67Fe 0.33 nanocatalysts. Additionally, independent preparation of dimensionally-tuned Ni nanoparticles in a microplasma allows precise control over the inner and outer diameter, as well as the number of walls in the CNTs. Reducing the Ni catalyst size from 3.1 to 2.2 nm is found to increase the growth rate by as much as 13 times and to achieve higher purity of SWCNTs (75%) than the commercial HiPCO process (65%).;One of the critical challenges for applications of CNTs is chirality control (e.g. metallic vs. semiconducting). Based on the methodology developed here, we have performed a systematic study with varying compositions of NiFe nanocatalysts. The results show that the composition of the nanocatalysts significantly influences the chirality distribution of as-grown SWCNTs. Detailed microcharacterization of the NiFe nanocatalysts suggests a link between the composition and, therefore, crystal structure of NiFe nanocatalysts and the final SWCNT chirality distribution, independent of particle size.