Electron energy loss spectroscopy andab initioinvestigation of iron oxide nanomaterials grown by a hydrothermal process

Abstract

We first describe with the help of reference experiments (at the $\text{Fe}\text{ }2p$ and $\text{O}\text{ }1s$ edges) and ab initio calculations how electron energy loss spectroscopy (EELS) can be used in order to characterize phases of iron oxide and/or hydroxide nanomaterials. In particular we show that dehydration of iron hydroxides such as goethite can easily appear under the electron beam but might be followed by monitoring the $\text{O}\text{ }K$ peak. Indeed both local spin-density approximation (LSDA) and $\text{LSDA}+U$ calculations confirm that intensity of the prepeak of $\text{O}\text{ }K$ should increase while H atoms are removed. We also demonstrate that different magnetic orders do not change significantly the $\text{O}\text{ }K$ EELS fine structure of goethite. Thus, nanomaterials (particles and wires) synthesized by a hydrothermal treatment of nanoscale (10--40 nm) magnetite particles have been conducted. Among them, crystalline iron oxide nanowires with average diameter of 20 nm and length of up to $10\text{ }\ensuremath{\mu}\text{m}$ are reported. The $\text{O}\text{ }K$ edge and $\text{Fe}\text{ }{L}_{2,3}$ edges were studied by EELS for these nanostructures. The results indicated that the valence of iron is $3+$ in the wires while it is the mixture of $2+$ and $3+$ in the particles. From these combined EELS, scanning transmission electron microscopy, diffraction, and high-resolution electron microscopy, the complexity of the produced phases from these hydrothermal treatments can be revealed. This work shows how EELS with high-energy resolution is a unique tool to differentiate iron oxide compounds such as the tricky magnetite-maghemite solid solution or the case of partially dehydrated phases, even on a nanometer scale.