| The synthesis of metal-oxide nanowires (i.e. WO2.9, ZnO, Cu 2O, and Fe3O4) and nanoplates (i.e. MoO2 ) is examined experimentally with metal-substrate probes inserted into counter-flow diffusion flames (CDFs) at atmospheric pressure. The quasi-one-dimensional flow field allows for correlation between morphologies and local growth conditions, as well as the tailoring of the flame structure, through computational simulations with detailed chemical kinetics and transport, to provide conditions suitable for gas-phase growth of nanostructures. Comparisons of products synthesized between methane and hydrogen flames, as well as between locations probed on either the fuel side or the air side of the reaction zone, permit evaluation of the roles of O2 versus H2O versus CO2 in the oxidative route(s) involved. The as-synthesized nanostructures are characterized by field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and energy dispersive X-ray spectroscopy (EDXS).;Tungsten oxide nanowires are grown with diameters ranging from 50 to 200 nm at 1720K. The crystal structure is tetragonal WO2.9, but the growth directions vary with flame conditions. Single-crystal ZnO nanostructures are formed at 1000, 1300, and 1600K. All growth mechanisms are possible based on Gibbs free energy calculations. Molybdenum oxide nanoplates are grown in the methane flame at 2000K on both the air and fuel sides, where similar amounts of H2O and CO2 are found. In the hydrogen flame, oxidized structures are grown on the air side, and micron sized plates are synthesized on the fuel side. Cu2O nanowires are grown only on the air side of the methane and hydrogen flames, where large amounts of oxygen are present. The fuel sides of both flames show nucleation sites on the surface but no nanowire growth. Iron oxide nanowires are formed on the air side of the methane and hydrogen flames. Carbon nanotubes and nanowires are grown on the fuel side of the methane flame, while iron-oxide nodules are formed on the fuel side of the hydrogen flame. |
