Although the smallest nanoparticles are usually round, many useful nanosized particles are large enough to show crystallographic structures with crystal facets. Surface-related chemical and magnetic properties of nanoparticles can be strongly influenced by the nature of the specific surfaces exposed and, for reactive metals such as iron, the oxides that form on them. Particles produced by a low-temperature gas-aggregation process have primarily non-equilibrium surface facets exposed. These contrast with the equilibrium shapes of particles produced or processed at higher temperatures. The ability to control particle morphology will enable tailoring of the properties of iron nanoparticles.
Work by researchers at Pacific Northwest National Laboratory (PNNL) and University of Idaho (UI) has shown that the morphology of iron nanoparticles prepared by a sputtering-gas-aggregation method depends on deposition temperature. Room-temperature-deposited iron nanoparticles form morphological structures ranging from cubes confined only by the 6 {100} planes to truncated rhombic dodecahedron confined both by the 6 {100} planes and 12 {110} planes. No particle was found to have a shape of regular rhombic dodecahedron (confined only by the 12 {110} planes). This differs from reports in the literature of iron particles deposited at high temperature, where particles were found to form structures ranging from regular rhombic dodecahedron (all {110} planes) to truncated rhombic dodecahedron confined both by the 6 {100} and 12 {110} planes. Particles deposited at high temperatures did not show cube morphology (only confined by the 6 {100} planes). These results, combined with those already reported, suggest that by using a low-temperature process, synthesis parameters can be altered to select particle morphology with possibility of optimizing particles for specific chemical or magnetic properties.