Controllable synthesis of iron nanoparticles on polyethylenimine-modified montmorillonite: Dependence on the amine protonation extent

This work looked at controllably synthesizing iron nanoparticles using borohydride solution reduction strategy with polyethylenimine-modified montmorillonite clay mineral as a host. In the hybridization process the protonation extent of polyethylenimine had vital but complex influence on the size, morphology, dispersion, distribution location, and even phase identity of the resultant iron particles. At the 90% protonation extent, optimized iron particles were obtained as nearly spherical smallest-sized core-shell entities (63 nm mean diameter) well dispersed on the external clay mineral surface, wherein the shell (amorphous iron oxide) preserved the core (polycrystalline metallic iron) from complete oxidation in air atomosphere. This regulation largely depended upon (a) the polymer interfacial conformation at clay mineral surface, (b) the relevant card-house-type pore feature, and (c) the complexation of the unprotonated polymer amino groups with the precursor ferric ions. The formation of the clay mineral card-house-type pore structure (in dry state) would be caused by washing and drying the electrostatic-steric stabilized polymer/clay mineral composite particles from the final dispersion under suitable conditions (with ethanol and acetone diluents and by vacuum-drying at 25 degrees C). In the hybridization system, besides the anticipated polymer-clay mineral electrostatic attraction, the hydrogen-bonding interactions of polyethylenimine with montmorillonite and iron particles were also detected.