Synthesis and characterization of ultra-small single-walled carbon nanotubes produced via template technique.

摘要

Synthesis of ultra-small SWNTs using AlPO4-5 (AFI) single as a template is a relatively new approach for carbon nanotubes synthesis (Tang, 1998). This method potentially has several advantages over the traditional synthesis and can be used to prepare SWNTs with pre-designed tube structures. The task of this thesis is to improve the density and study the formation mechanism of 0.4 nm SWNTs in AFI crystal channels, as well as produce and characterize even smaller SWNTs and iodine molecule arrays using AlPO 4-11 (AEL) single crystal as template.; I. To increase the density of 0.4 nm SWNTs in the AFI channels, two approaches are employed. The first approach is in-situ synthesis of AFI crystals with organic template that have a richer carbon content. Indirect evidence by micro-Raman spectra indicates that a higher filling factor of nanotubes can be reached by using carbon precursor of tetrapropylammonium hydroxide which contains 12 carbon atoms per molecule, in contrast to 9 carbon atoms of tripropylamine (TPA) precursor. The second approach is to generate negatively charged framework and Bronsted acid sites, by replacing P5+ using Si4+, or replacing Al3+ using divalent metallic cations (Me2+) such as Mn2+, Mg2+, and Co2+ in the AFI crystal lattices. The introduced metallic cation sites not only enhance the adsorption force of the channel walls to the guest molecules, but also play an important catalytic role in pyrolyzing the carbon precursor molecules.; II. We directly monitored the decomposition process of TPA in AlPO4-5, and SAPO-5 crystals. It has been found that the TPA precursors exist in the as-synthesized crystals in three different forms: tripropylammonium fluoride, hydroxide and tripropylammonium cation compensating the negative charge of the framework. The latter is bonded to the framework by strong chemical interaction and its decomposition undergoes by a series of ss-elimination reactions to give propylene and ammonia, with the stepwise formation of dipropylammonium and n-propylammonium cations. The 0.4 nm SWNTs filling density was found to be higher than that resulting from the carbon precursor of tripropylammonium fluoride and hydroxide, because of the strong adsorption force of the channel walls to pyrolyzate.; III. Mono-sized SWNTs with diameter only 0.3 nm are synthesized inside the channels of SAPO-11 crystals. The structure of this nanotube is confirmed by various techniques including Polarized optical microscopy, single crystal X-ray diffraction etc. Micro-Raman spectra shows that two possible structure (2,2) and (3,0) nanotubes co-exist inside the channels of SAPO-11 crystals. Strong curvature effects induce instability of freestanding nanotubes. The electronic structure of the (2,2) and (3,0) nanotube are studied by polarized adsorption spectra and resonant Raman spectra.; IV. The iodine species are introduced into the channels of AEL single crystals by physical diffusion method. Due to size confinement of the AEL channels, iodine molecules can only be oriented in two directions, either along the long-axis of the elliptical channel or along the channel direction. Polarized absorption spectra and polarized Raman spectra have been investigated. They show that both Iodine molecular ribbon sheets and molecular chains exist inside the channels of AEL crystals. And the predominant structure will be altered by changing the iodine loading density.