Use of Surfactant Micelles to Control the Structural Phase of Nanosize Iron Clusters

We have synthesized iron nanoclusters using two chemical types of inverse micelle systems to control the growth process. Surprisingly, we have found that the phase of the resulting nanosize iron depends strongly on the chemical nature of the surfactant used during the growth of the clusters. For example, use of nonionic surfactant micelles for this room-temperature synthesis produces the normally-high-temperature γ-phase with a fcc structure as determined by selected area electron diffraction (SAD) and high-resolution lattice fringe images. Use of a cationic quaternary ammonium surfactant micelle system, on the other hand, produces the normal bcc α-iron phase as determined by SAD. This strong dependence of the structure of nanosize clusters on surfactant type may reflect different binding-site preferences for the chemically different surfactants during the growth stage of the cluster synthesis. This is analogous to the well-known role of surfactants as habit modifiers during macroscopic crystalline growth processes in solution, but is the first example of such control of structural phase during nanocluster synthesis.