| 其他摘要 | Owing to their unique one-dimensional (1D) structure, excellent physical and chemical properties, carbon nanotubes (CNTs) show great promise for a wide range of applications in electronics, composites, aerospace, etc. However, the applicatons of CNTs are still at laboratory stage, mainly due to the currently unavoidable structural heterogeneity of as-synthesized CNTs. To achieve various practical applications, we must implement controllable synthesis and assembly of uniform CNTs. In this dissertation, we synthesized CNTs with tunable microstructures and in situ assembled individual CNTs into macrostructures for special applications. The following results have been achieved.
High-quality single-walled CNTs (SWNTs) with tunable diameters were synthesized by an improved floating catalyst chemical vapor deposition (FCCVD). The as-synthesized samples showed a SWNT percentage of over 95 % without amorphous carbon. The diameter of the SWNTs were finely tailored in the range of 1.3±0.2, 1.6±0.2, 1.7±0.2, 1.9±0.2 and 2.1±0.2 nm, dependent on the experimental conditions. It was found that the selective etching effects of high hydrogen flow stabilized the decomposition of ultralow CH4 flow and considerably suppressed the deposition of amorphous carbon and small nanotubes, leading to very pure SWNT samples with high structural homogeneity. In addition, the abundance of specific (n, m) SWNTs could be selectively enriched simultaneously along with the diameter modulation. For example, the abundance of metallic tubes was dramatically increased in the SWNTs with diameters in the range of 1.3±0.2 nm.
The shape-engineering of 1D CNTs to macrostructures with well-defined geometry was realized, which is greatly advantageous for various practical applications. (i) By using a specially-designed porous substrate, individual SWNTs were in situ assembled into uniform thin SWNT sheets with tunable thickness in the range of 0.2-50 m, and then into a novel 3D book-like macrostructure (buckybook) with good control of the nanotube diameter, the sheet packing density and the book thickness (up to several millimeters), and (ii) with the assistance of high carrier gas flow, individual SWNTs were directly assembled into super long SWNT strands of up to 65 cm in length. In addition, the promise of such SWNT buckybooks was highlighted through demonstrating their high-efficiency molecular separation as a filter.
Nickel formate dihydrate was demonstrated as an effective catalyst precursor for synthesizing double-walled CNTs (DWNTs) with a narrow diameter distribution (the inner diameter: 1.32-2.81 nm, and the outer diameter: 1.98-3.47 nm) by hydrogen arc discharge technique. The as-fabricated DWNTs have excellent oxidation resistance up to ~800 oC, higher than that of DWNTs prepared by other methods. The high thermal stability of DWNTs is attributed to the cleaning effect of active radicals such as hydrogen and oxygen originated from the decomposition of nickel formate dihydrate and the in situ defect-healing effect induced by high arc current. In addition, appropriate oxidation conditions were effective to remove amorphous carbon and SWNTs in the DWNTs, which demonstrates that oxidation is an effective method for purification of the DWNTs.
An improved FCCVD method was proposed for preparing short cup-stacked CNTs (CSCNTs) with lengths of 0.2-3.5 m, a large hollow channel along the nanotube axis and little amorphous carbon deposition on their outer surface by the pyrolysis of the mixture of ferrocene and sulfur. In addition, a simple purification method was developed to remove iron particles and obtain short CSCNTs with two open ends, which are soluble in aqueous and organic solutions. Measured in situ inside a transmission electron microscope, individual CSCNTs were found to exhibit unexpectedly semiconducting behaviors with the band gap of about 0.44 eV, which results from their special stacking microstructure of graphene layers.
Various magnetic nanostructures such as Fe nanoparticles (Fe-NPs) adhering to SWNTs, carbon-encapsulated Fe-NPs, Fe-NP decorated multi-walled CNTs (MWNTs), and Fe-filled MWNTs were selectively synthesized by the pyrolysis of pure ferrocene at different sublimation temperatures, while keeping all other experimental parameters unchanged. Magnetic characterization reveals that these nanostructures have an enhanced coercivity, much higher than that of bulk Fe at room temperature. As for the Fe-NP decorated MWNTs, the Fe-NPs were completely shielded with graphitic shells and anchored on the surface of MWNTs by strong chemical linkage. Moreover, with 0, ~1 wt %, ~2 wt % sulfur as growth promoter, the size of the Fe-NPs can be controlled with an average diameter of ~5, ~22 and ~42 nm, respectively. The 5-nm-diameter Fe-NP decorated MWNTs not only exhibited sufficiently strong magnetic response for ease of separation, but also dispersed well in solutions, which makes them to be an ideal catalyst support in liquid-phase reactions, especially under acid/basic conditions. |
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