In recent years, iron and other metal oxides have attracted substantial attention as promising inexpensive sensing materials. Iron oxide-based gas and humidity sensors have already been reported. The sensitivity of thick-film iron oxide sensors is rather low; however, it can be significantly improved by various doping schemes and by reducing film thickness. The enhancement is even greater when a nano-sized material is used due to increase in the metal oxide surface area, where the gas sensing takes place. Various methods have been used to produce nano-sized iron oxide powders, including chemical co-precipitation, sol-gel processes, pulsed laser evaporation, sputtering and spray pyrolysis. Here, in this work, we studied iron oxide nanoparticles prepared by a combustion method, with particle sizes varying in the range of 10-40 nm. Electron energy loss spectroscopy (EELS) in the transmission electron microscope was used to study the oxidation state of iron ions in these nanoparticles, with experiments performed in a nano-probe mode (with a probe size of 1.4 nm), allowing for very high spatial resolution. The studies showed that, depending on the formation conditions, a decrease in the oxidation state of iron could occur at the nanoparticle surface. This was indicated by changes in both the iron L₂₃- and the oxygen K-edges. These highly spatially resolved measurements showed broadening and a red energy-shift of both L₃ and L₂ iron lines at the surface of the particles. This was accompanied by a significant reduction of L₃:L₂-intensity, suggesting that there are mostly Fe²⁺ ions at the surface. A reduction in the pre-edge of oxygen K-edge also confirmed these results. This study shows that the surface oxidation state of iron oxide nanoparticles synthesized through combustion routes is highly dependent on formation conditions, and that EELS is an important research tool for investigating the surface-specific chemistry of nano-sized gas-sensing materials.

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