Opposed Flow Flame Synthesis of Molybdenum Oxide Nanostructures (original) (raw)

An opposed laminar flow flame formed with methane/acetylene and oxygen enriched-air was employed to produce molybdenum oxide nanostructures directly in the gas phase. The composition of the fuel and oxidizer to form the flame was 96%CH 4 +4%C 2 H 2 and 50%O 2 /50%N 2 , respectively. Raw material was introduced into oxidizer side of the flame in the form of solid molybdenum wires with 99% purity. The high temperature and the oxygen rich chemical environment of the flame resulted in fast surface oxidation of the probes and material etching from their surfaces. Upon their interaction with the flame, the probes generated molybdenum trioxide vapors. The vapors were transported in the direction of the stagnation plane of the flame and reduced to molybdenum dioxide as they entered the low temperature fuel rich zone. The velocity gradient and thermophoretic forces in the flame affected the transport of the molybdenum dioxide precursors. These precursors in the gas phase formed nanostructures that were thermophoretically collected from the flame volume. Essential morphological variations of generated nanomaterials were observed depending on flame and probe parameters. The distance of the collection plane from the molybdenum probe also played an important role in the morphology of generated nanoforms. The molybdenum probes with diameters of 0.75 mm and 1 mm were used to achieve two distinct synthesis conditions. The variation of probe diameter affected probe temperature and resulted in different supersaturation levels of molybdenum dioxide vapors. Experiments with 1.0 mm diameter corresponded to lower supersaturation levels and resulted in the synthesis of well-defined convex polyhedron nanocrystals and nanorods. Higher material etching rates and, hence, supersaturation levels were obtained with 0.75 mm diameter probes. These conditions resulted in synthesis of mainly spherical molybdenum oxide nanomaterials agglomerated in soot-like fractal aggregates. The effect of flame parameters and material concentration on shape and structure of generated nanomaterial is also studied numerically. The underlying mechanisms governing the morphological variation of molybdenum oxide nanocrystals are analyzed using the following steps: monomers formation, nucleation, and growth. The nucleation model is based on the classical nucleation theory, and the growth model considers agglomeration and diffusion in the varying thermal environment using thermophoretic analysis. The model predictions are in good qualitative agreement with the experimental data.