Shape-controlled synthesis of iron oxide nanoparticles

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Crystals often exhibit particular shapes. It is known that different crystallographic surfaces have different surface properties [1, 2]. For a given crystalline structure of the same material, different crystallographic surfaces have different surface energies (σ ). Generally speaking, for crystallographic planes {111}, {100} and {110}, the relationship σ{111} < σ{100} < σ{110} holds [1]. In bulk materials the effect of differences in surface energies may be of limited significance, but in nanoparticles it can be greatly magnified because of the extremely high specific surface areas of nanoparticles. Shape-controlled nanoparticles may find many applications [1–3]. One application of shape-controlled nanoparticles would be the development of new, property-tailored catalysts [3]. Since surface energies affect adsorption of molecules and their chemical reactions, the shape of nanoparticles may become an important factor in determining reaction pathways and selectivity. The shape of nanoparticles is also important in the fabrication of nanoparticle superlattices [4]. Since they are zero dimension (0-D) materials, nanoparticles can be used as the building blocks for higher dimension materials, such as wires (1-D), monolayers (2-D), and multi-layer structures (3-D). These nanostructured materials are ordered assemblies of nanoparticles whose shapes determine their packing geometry and the ways of packing can affect the properties of the resulting assembly (e.g., its stability) [4, 5]. Recently, several research groups have reported the synthesis of shape-controlled nanoparticles [6–9]. All of those studies were based on liquid colloidal techniques; shape control was achieved by preferential growth of a particular crystallographic surface, either by controlling its growth kinetics or by blocking the un-preferred surfaces with adsorbed species (e.g., polymers). In this letter, we report an aerosol technique for the synthesis of shape-controlled iron oxide nanoparticles. Iron oxide was chosen for this initial study for its important applications in both optical-magnetic recording [10] and catalysis [11]. There exist a variety of techniques for iron oxide nanoparticle synthesis including the flame aerosol process, but shape-control has never been reported. In fact, nanoparticles produced in aerosol processes are typically a mixture of shapes [12–14]. We used a specially designed counterflow diffusion flame (CDF) reactor to synthesize the iron ox-