Surface oxidation state of combustion-synthesized γ-Fe ₂O₃ nanoparticles determined by electron energy loss spectroscopy in the transmission electron microscope

Electron energy loss spectroscopy (EELS) in the transmission electron microscope (TEM) was used to compare the iron oxidation state at the surface and interior of γ-Fe₂O₃ nanoparticles produced by the combustion process under fuel conditions leading to low and high soot concentrations. These experiments were performed in the nanoprobe mode, which allowed for very high spatial resolution (the probe size was 1.4 nm). Here, low soot concentrations were obtained in a laminar ethylene-air diffusion flame seeded with iron pentacarbonyl, while high soot concentrations were achieved with the addition of acetylene to this fuel mixture. The studies showed that the surface oxidation state of iron was lowered with the addition of acetylene, although the core composition remained the same. This was indicated by changes in both the iron L₂₃- and the oxygen K-edges at the surface of the particles. These highly spatially-resolved measurements showed a chemical shift of both the L₃ and L₂ iron lines, accompanied by significant reduction of the L₃:L₂-intensity ratio, indicating Fe²⁺ at the particle surface. Reduction in the pre-edge peak of the oxygen K-edge at the particle surface also indicated iron reduction at the surface. These results suggest that the surface oxidation state, and therefore gas-sensing properties, of combustion-synthesized iron oxide nanoparticles is highly dependent on flame conditions. Furthermore, this study shows that EELS is an important research tool for the investigation of nanoscale gas-sensors, allowing differentiation of composition and oxidation state at the interior and surface of individual nanostructures in these materials.