Volumetric flame synthesis of mixed tungsten–molybdenum oxide nanostructures

Abstract

Mixed-metal (Mo/W) oxide nanostructures were synthesized volumetrically in the oxygen-rich zone of a counter-flow diffusion flame. High purity Mo and W wires were used to generate Mo and W oxide vapors that are transformed into mixed-metal oxide nanostructures while transported by the gas flow in the upward direction toward the stagnation plane. The Mo wire was inserted in a lower position within the flame volume than the W wire. Samples of generated nanomaterials collected thermophoretically from the flame volume at various axial positions allowed tracing of the particle evolution. The following synthesis stages were identified. MoO3 structures are first formed in the lower part of the flame. As they are carried by the gas flow, W oxide vapors deposit on the surfaces of the early formed MoO3 nanostructures. W atoms diffuse at the interfaces of the MoO3 structures to form mixed W–Mo oxide nanoparticles. Fully grown structures, collected near the stagnation plane (SP), are nanocubes exhibiting well-defined edges with sizes less than 100nm. Energy-dispersive X-ray spectroscopy elemental mapping revealed that the mature structures have uniform distribution of W, Mo, and O atoms. The measured lattice spacing showed expanded lattice distance of MoO3 which may be attributed to the incorporation of W atoms in the MoO3 crystal and formation of W–Mo oxide structures.