Depth-profiling XPS reveals a variation in the valence state of Fe across the surface and bulk regions of the SFMNR samples, influenced by the presence of FNR@FeOx nanoparticles (Figures ). These nanoparticles are formed through annealing in a reducing atmosphere (5%H2/Ar) at 800 o C whereas oxidizing conditions do not create them (Figure ). The binding energies for the sub-peaks corresponding to Fe 4+ , Fe 3+ , Fe 2+ , and Fe 0 are 713.0, 711.3, 710.2, and 707.0 eV, respectively. In the reduced SFMNR sample, the intensity of the Fe 0 peak increases with longer etching times (Figure ), indicating the presence of FeOx on the surface and metallic Fe inside. The averaged valence states of Fe on the surface of the reduced SFMNR is measured at 2.88, decreasing to approximately 2.7 after five etches before plateauing. In contrast, the averaged valence state of Fe in the oxidized SFMNR, which lacks nanoparticles, remains relatively constant at 3.1.