# XPS on Co\({}_{0.95}\)R\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) Nanoparticles with R = Gd or Ho
## 3 Results and Discussions
Figure 3: \(Cont.\)  
Figure 2: TEM images of Co\({}_{1-x}\)R\({}_{x}\)Fe\({}_{2}\)O\({}_{4}\) nanoparticles and size distributions for the samples with R = Gd (**left**) and R = Ho (**right**). XPS core-level spectra can provide valuable information about the valence state of the atoms. The O 1s core-level spectra, shown in Figure 4, are very similar for the two investigated samples and consist of a main signal at about 529.7 eV from the oxygen-metal bonds in the lattice [56] and a second contribution at higher binding energy, around 532 eV, due to less electron-rich oxygen species, like carbonyl and hydroxyls chemisorbed on the surface of the nanoparticles. The spectra recorded in the region of Gd and Ho 4\(d\) binding energies are presented in Figure 5. Gd 4\(d\) XPS signal shows two main features centered at about 142.4 eV and 147.7 eV, close to the values reported for Gd\({}_{0.04}\)Fe\({}_{2.96}\)O\({}_{4}\)[57], indicating the Gd\({}^{3+}\) presence in the Co\({}_{0.95}\)Gd\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) sample. The features in the Ho 4\(d\) XPS spectra of Co\({}_{0.95}\)Ho\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) are similar to those found for Ho-doped SPION [58], confirming the presence of Ho\({}^{3+}\) ions. Figure 6 shows the Fe 2\(p\) and Co 2\(p\) core-level XPS spectra for Co\({}_{0.95}\)Gd\({}_{0.05}\)Fe\({}_{2}\)O  
Figure 4: The XPS O 1s core-level spectra. Figure 3: SEM image (**a**), EDS spectra (**b**), and the elemental mapping (**c**–**f**) of Co\({}_{0.95}\)Gd\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) nanoparticles. and Co\({}_{0.95}\)Ho\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\). The signal in the Co \(2p\) region is very similar for the two samples and consists of two main peaks, corresponding to the \(2p_{3/2}\) and \(2p_{1/2}\) states, at about 780.2 eV and 795.6 eV, and two satellites located around 786 eV and 802.5 eV. The Co \(2p_{1/2}\)-\(2p_{3/2}\) spin-orbit splitting of about 15.4 eV indicates the presence of both Co\({}^{3+}\) and Co\({}^{2+}\) ions. However, since it is closer to the value of 15 eV found for Co\({}^{3+}\) compounds than to 16 eV for Co\({}^{2+}\) compounds [59], Co is predominantly present as the Co\({}^{3+}\) state in both Co\({}_{0.95}\)Gd\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) and Co\({}_{0.95}\)Ho\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\). The presence of the satellite peaks at about 6 eV higher energies than the main lines confirms the existence of divalent Co. The Fe \(2p\) core-electron spectra recorded for the two samples are similar, with the \(2p_{3/2}\) and \(2p_{1/2}\) peaks centered at 710.7 eV and 724.4 eV, close to the characteristic values of 711 eV and 724.7 eV for Fe\({}^{3+}\)[60]. The satellite peaks observed at approximately 8 eV higher binding energy than the main signals are characteristic of Fe\({}^{3+}\) species [61]. The asymmetric peak shape for the investigated samples suggests a complex structure of the Fe \(2p\) XPS spectra and the presence of a satellite closer to the main signals, originating from Fe\({}^{2+}\) ions, at about 714.5 eV. A detailed analysis is difficult due to the overlap with the Co LMM Auger signal centered at 713 eV. Figure 6: The Co \(2p\) (**left**) and Fe \(2p\) (**right**) core-level XPS spectra. In order to obtain further information on the oxidation state of Co and Fe ions in the doped cobalt ferrites, the XPS spectra in the 3s electron binding energy region of the transition metal ions were recorded. The Co 3s and Fe 3s core-level spectra presented in Figure 7 show a complex structure, arising from multiplet splitting due to the exchange interaction between the valence electrons and the unpaired electron remaining in the core-level after photoemission. Although the exchange coupling is the main mechanism behind the 3s splitting observed in the XPS spectra of Fe and Co compounds, the inter-atomic charge transfer effects should also not be ignored [62; 63].