Reverse Micelle Synthesis of Co-Al LDHs: Control of Particle Size and Magnetic Properties

Nanoplatelet Co - Al layered double hydroxides (LDHs) {Co 1.8Al1.1(OH)6(DDS)x•yH 2O (DDS = dodecyl sulfate; x ≈ 0.9; y = 2 - 6)} have been prepared using a homogeneous precipitation approach in water pools formed by a water-in-oil reverse microemulsion. The size of the LDH particles is controlled by adjusting the water to surfactant ratio (ω). The particle sizes vary from about 60 × 60 × 13 nm3 to 160 × 160 × 40 nm3 with increasing ω. This contrasts with platelets of Co1.89Al(OH)6(DDS)0.8•4.3H2O prepared by conventional homogeneous precipitation which had dimensions about 2 - 3 μm wide and 0.5 - 1 μm thick. All the Co - Al LDH samples were paramagnetic above 50 K. The molar magnetic susceptibilities of each sample were very similar and they could all be fitted to the Curie - Weiss law with μeff ca. 5.2 μB per Co2+ and θca. 6 K. Below 50 K, we observed a rapid rise in χMT indicative of cooperative ferromagnetic interactions. At very low temperature the LDH nanoplatelets show magnetic behavior which is different to Co 1.8Al1.1(OH)6(DDS)x•yH2O made by conventional homogeneous precipitation. The nanoplatelets show bifurcation in their field-cooled (FC) and zero-field-cooled (ZFC) magnetizations and also a frequency dependence of the in-phase (χ′) and out-of-phase (χ″) ac susceptibilities, which is dependent on the size of the nanoplatelets. © 2010 American Chemical Society