Size-dependent catalytic effect of magnetite nanoparticles in the synthesis of tunable magnetic polyaniline nanocomposites

Nanocomposites comprising magnetic nanoparticles (NPs) embedded in an organic conducting polymer are promising materials that may allow one to exploit synergic effects between the electrically conducting and the magnetically permeable components. Having already shown that magnetite NPs can be conveniently used as a catalyst for the oxidative polymerization of the aniline dimer resulting in NPs embedded in the final composite and how to modulate the magnetic coercivity of the composites, we now turn to investigate how the size of magnetite NPs affects the polymerization and the properties of the final composite. Magnetite NPs of diameter 2.3, 10, and 27 nm turned out to be effective catalysts with cheap oxidants such as H2O2 and O-2. Yield data show that the rate-determining step occurs on the NP surface. Extensive characterization shows that the NPs are well-dispersed in the composite with no significant morphological change. The static magnetic properties of the composites are widely different, e.g., the magnetic blocking temperature shifts from 290 K for 27 nm NPs to 54 K for 10 nm NPs, while composites with 2.3 nm NPs are virtually unblocked down to 5 K. The dynamic electromagnetic behavior studied up to the microwave range only shows energy absorptions associated with the ferromagnetic resonance, at frequencies around 1 GHz.