Novel Flame-Gradient Method for Synthesis of Metal Oxide Nanomaterials
W. Merchan-Merchan, A.V. Saveliev, M. Desai
The University of Oklahoma, US
Keywords: flame synthesis, metal oxides, nanostructures
Abstract:Various transition metal oxide nanostructures are synthesized using a novel probe-flame gradient method. An opposed flow flame of methane and oxygen enriched air provides a hightemperature reacting environment forming various structures directly on the surface of metal probes [1,2,3,4]. In our study of metal oxides the unique thermal profile and chemical composition of the generated flame tends to convert almost pure bulk (99.9%) metallic materials into 1-D and 3-D oxide nanostructures [5,6]. The synthesized molybdenum, tungsten, and iron oxide structures exhibit unique morphological characteristics. The application of Mo probes results in the formation of micron size hollow Mo-oxide channels. SEM images of the synthesized molybdenum oxide nanostructures on the surface of the probe are shown in Fig.1. The slender, prismatic, four-faced structures are hollow with large cavities completely devoid of any other materials. The structures have large cavities, nano-sized wall thickness, sharp edges, and high specific surface area. The large cavities can be useful in a wide variety of applications such as the storage of fluids or nanoparticles, material reinforcements, or as a component in the fabrication of MEMs devices, etc. TEM imaging and high resolution HR- TEM imaging was performed on the molybdenum oxide structures forcharacterization and analysis. The lattice spacing was measured as 0.36 nm and corresponds to a (-111) plane or a monoclinic MoO2 cell. The formation of elongated iron-oxide nanorods is observed on iron probes. SEM imaging analysis on the surface of the probe shows a high density of the as-synthesized nanostructures protruding from the probe’s surface (Fig. 2). HR-SEM imaging analysis reveals that the materials formed are composed of elongated structures characterized by high aspect ratios. The diameters of the iron oxide nanorods vary from ten to one hundred nanometers with a typical length of a few microns. Among the straight and uniform iron oxide structures, nanostructures with more complex morphologies are also synthesized. The bending and branching phenomena present in some of the structures are highlighted by the white and black arrows in Fig. 2, respectively. In the area of nanomaterials, the modified “branched” and “bent” structures are excellent candidates for fabricating composite materials with enhanced mechanical or electronic properties due to the formation of a network-like phase within the matrix. The introduction of W probes results in the synthesis of 1-D carbon coated metal-oxide nanowires (Fig. 3). HR-TEM reveals that the grown structures are composed of two types of materials. The inner material or core is composed of a very crystalline metallic-like material which is covered by an outer sheath layer of crystalline carbon. The carbon layers are parallel to the central axis of the nanowire with a layer distance of 0.34 nm. HR-TEM at the end of the metallic wire shows material with a high degree of crystallinity and a lattice spacing of 0.38 nm. X-ray energy dispersive spectroscopy analysis conducted on the structures revealed the presence of carbon, oxygen, and tungsten.