Rapid Catalyst-Free Flame Synthesis of Dense, Aligned α-Fe2O3 Nanoflake and CuO Nanoneedle Arrays

Abstract

This paper describes a simple and yet rapid flame synthesis method to produce one-dimensional metal oxide nanostructures by directly oxidizing metals in the postflame region of a flat flame. Single and bicrystal α-Fe₂O₃ nanoflakes and CuO nanoneedles were grown in the postflame region by a solid diffusion mechanism and were aligned perpendicularly to the substrate with a surface coverage density of 10 nanostructures per square micrometer. The α-Fe₂O₃ nanoflakes reached lengths exceeding 20 μm after only 20 min of growth. This rapid growth rate is attributed to a large initial heating rate of the metal substrate in the flame and to the presence of water vapor and carbon dioxide in the gas phase that together generate thin and porous oxide layers that greatly enhance the diffusion of the deficient metal to the nanostructure growth site and enable growth at higher temperatures than previously demonstrated.