Surface atomic structures of Fe2O3 nanoparticles coated with cetyltrimethyl ammonium bromide and sodium dodecyl benzene sulphonate: an extended x-ray absorption fine-structure study

Abstract

Fe2O3 nanoparticles coated with sodium dodecyl benzene sulphonate (DBS) or cetyltrimethyl ammonium bromide (CTAB) were prepared by using a microemulsion method in the system water/toluene. The nanoparticles were characterized by means of transmission electron microscopy and average particle sizes of 5.0 nm and 6.0 nm were found for DBS-modified and CTAB-modified nanoparticles respectively. The local atomic structures of these iron(III) oxide nanoparticles were probed by using the extended x-ray absorption fine-structure technique. Fe K absorption spectra were collected at beam line 4W1B of Beijing Synchrotron Radiation Facility. A structural model was proposed for describing their atomic structures. The Fe-O bond length at the surface of DBS-coated Fe2O3 nanoparticles was found to be similar to that in bulk Fe2O3, but there was about 0.04 Å expansion for the CTAB-coated Fe2O3 nanoparticles. On the basis of the model proposed in this paper, the thicknesses of the surface layers were estimated to be 0.5 nm and 0.7 nm, respectively, for the DBS-coated and CTAB-coated Fe2O3 nanoparticles. The anharmonicity of the atomic vibration and the asymmetry of atom-pair distribution were found to be larger at the surface of the nanoparticles than in the bulk material, while the Debye-Waller factors are almost the same for the surface and the core parts of the nanoparticles. It can be concluded that the atomic structure of the nanoparticle surface is ordered, but the atom-pair distribution is asymmetric.