Investigating flame-based synthesis of carbon nanotubes and metal-oxide nanowires.

摘要

The synthesis of carbon nanotubes (CNTs) and metal-oxide nanowires (e.g. ZnO, WO2.9) are examined experimentally by inserting probes into various flame geometries at atmospheric pressure. The main probed-flame configurations are the inverse co-flow diffusion flame (IDF) and the counter-flow diffusion flame (CDF), which are compared with each other to assess the translatability of local synthesis conditions in producing the same growth attributes and morphologies. The CDF is characterized using laser-based spontaneous Raman spectroscopy (SRS), and validated with simulations using detailed chemical kinetics and transport. SRS is used to measure local conditions in the 2-D axisymmetric IDF. Properties of the as-synthesized nanostructures are determined by field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDXS), and resonance Raman spectroscopy (RRS).;Various morphologies of CNTs are grown catalytically on metal-alloy substrates of different compositions (i.e., Fe, Fe/Cr, Ni/Cu, Ni/Ti, Ni/Cr, Ni/Cr/Fe), as well as on metal-oxide solid solutions (i.e. NiAl2O4, CoAl2O4 and ZnFe2O4). Vertically well-aligned multi-walled CNTs (MWNTs) with uniform diameters are obtained from Ni/Cr/Fe and Ni/Ti alloys. CNTs produced from ZnFe2O 4 substrates are found to be a mixture of MWNTs and single-walled carbon nanotubes (SWNTs) with at least 30% SWNTs by number. Effects of local gas-phase temperature, substrate temperature, carbon-based precursor species concentrations, and substrate voltage bias on CNT formation, diameter, growth rate, yield, density, and morphology are investigated.;Aligned single-crystal tungsten oxide nanowires with diameters of 20-50nm are grown directly from tungsten substrates at high rates, with local gas-phase temperature and chemical species specified at the substrate for self-synthesis. Voltage bias is shown to dramatically alter the morphologies of the as-synthesized WOx nanomaterial. Singlecrystalline ZnO nanowires are grown directly on zinc-plated steel substrates at high rates with no catalysts. Larger-diameter (>100nm) nanowires are produced at higher temperatures; while smaller-diameter (25-40nm) nanowires are produced at lower temperatures, and only on the fuel side of the reaction zone. Reactions with H2O appear to be the dominant route for nanowire synthesis. Nanoribbons and other nanowire-based morphologies are also found and discussed.