Controlled Synthesis, Manipulation of Surface Hydrophobicity and Self-Assembly of Hematite Nanocrystals

The preparation of uniform iron oxide monolayer thin films is of great interest not only in advanced materials synthesis but also in studies of microbial cell and iron oxide mineral interface interactions by techniques such as neutron reflectometry, which requires the use of uniform thin films of the mineral on a substrate. Here we report three techniques for fabricating such uniform hematite (α-Fe2O3) nanocrystal thin films. The nanocrystals with controlled sizes and morphologies were first synthesized by thermal hydrolysis of Fe3+ ions in acidic aqueous solution. Three methods were tested to enhance the surface hydrophobicity of the synthesized nanoparticles, including thermal dissociation of surface hydroxyl groups, stepwise solvent exchange, and surfactant-assisted phase transfer. FTIR and Raman spectroscopic analyses were used to elucidate the mechanisms of surface hydrophobicity changes on hematite nanoparticles. Results indicate that techniques of thermal treatment and stepwise solvent exchange caused the dissociation of surface -Fe-OH groups into -Fe-O-Fe structures, whereas surfactant-assisted phase transfer gave rise to surface hydrophobicity due to sorbed hydrocarbon chains. These surface modified hematite nanoparticles were found to readily self-assemble into monolayers at the water-air interface, which provided an effective means to obtain hematite nanoparticle thin films with controlled packing density and layer thicknesses in a Langmuir-Blodgett trough. Langmuir isotherm, SEM, TEM, AFM, UV-visible spectroscopy, and neutron reflectometry were used to characterize these thin films for their stability and uniformity before they were used for studies of the interactions between hematite and macromolecules.