Investigating local growth conditions in the flame synthesis of molybdenum-oxide nanoplates from molybdenum substrates

Abstract

Molybdenum-oxide nanoplates are grown directly from the surface of molybdenum substrates using a catalyst-free flame synthesis technique. To investigate the effects of oxidizing routes, e.g., from gas-phase O2, H2O, and CO2 as reactants, on the growth and morphology of the molybdenum-oxide nanostructures, quasi-1D counterflow diffusion flames are strategically utilized and probed at specific locations within their flame structures. Moreover, two separate flame structures are compared using different fuels (i.e., methane and hydrogen) and diluted with nitrogen to have near-identical temperature profiles. For example, the H2O oxidizing route is scrutinized by assessing growth at the same elevated temperature locations at the fuel sides of the reaction zones of a CH4 flame (comprising H2O and CO2 as synthesis reactants) and an H2 flame (comprising only H2O as synthesis reactant). The temperature effect on the resultant morphologies are examined at 1280 K, 1500 K, and 1720 K. Single-crystalline MoO2 nanoplates (of thicknesses of 60-80 nm, widths of 200-450 nm, and lengths of 1-2 µm) that are regularly distributed with high-surface-area-exposed packing are obtained at 1720 K on the air side of the CH4 flame (with O2, H2O, and CO2 as synthesis reactants) for 10 min growth durations. Based on the results and Gibbs free energy calculations, the CO2 route is deduced to seed the growth of the nanoplates at the nucleation stage heterogeneously, with subsequent condensation-based growth from volatile intermediate species, such as MoO2(OH)2, playing an important role. With such studies revealing fundamental growth mechanisms, scale-up of the process with different flame configurations is afforded.