2.1.5. Surface Properties of Materials

X-ray photoelectron spectroscopy (XPS) quantification results are shown in Table 3. The quantification is based on La 3d, O 1s and Mn 2p or Fe 2p peak areas. The La atomic percentages are higher than stoichiometry. The lanthanum surface enrichment is often reported for La-based perovskites [32]. For LaMn\_SSR sample, this enrichment can be explained by the presence of La(OH)3 at the outermost surface. With the disappearance of La(OH)3 in the course of ball milling and the surface contamination by Fe, the La enrichment became less pronounced for LaMn\_HEBM and LaMn\_LEBM samples. An opposite trend was observed over the LaFeO3 samples as a slight La enrichment of the surface was observed after the HEBM and LEBM steps, a phenomenon that cannot be explained at this stage considering that TPR experiment, and especially the low contribution reduction at 410 ◦C, suggesting the formation of surface FeOx cluster species that should have contributed to a decrease in La/Fe surface ratio.

**Table 3.** Quantification from X-ray photoelectron spectroscopy (XPS) analysis for LaMnO3.15 and LaFeO3 samples.


<sup>1</sup> issued from ΔE(Mn 3s) values.

The superposition of the O 1s spectra for the three LaMnO3.15 samples is shown in Figure 5. Satisfactory peak fitting can be achieved with three components in the three samples. The component OI at low BE (529.2 eV) is assigned to bulk O2<sup>−</sup> species [33]. At intermediate BE, the large OII component, centered at 531.3 eV, is ascribed to several species such as OH−, CO3 <sup>2</sup>−, O2 <sup>2</sup><sup>−</sup> and/or O<sup>−</sup> species [34]. The presence of carbonate species is confirmed on the C 1s core level spectra, with a signal located at 289.0 eV. The OIII component, at higher BE (533.3 eV), originates from adsorbed water [35]. The SSR sample exhibits an intense OII component, which can be related to the presence of La(OH)3 (XRD) and carbonate surface species. The contribution of this component to the O 1s signal significantly decreases after the HEBM process and remains rather constant after the LEBM process, in agreement with the disappearance of the La(OH)3 phase after the ball milling processes. Figure A1 (Appendix A) shows the La 3d region for LaMnO3.15 samples, the La 3d spectrum being split into a 3d5/<sup>2</sup> and a 3d3/<sup>2</sup> lines due to the spin-orbit interaction. The magnitude of the multiplet splitting can be useful for the chemical assessment. While the energy difference between the main peak and its satellite (ΔE(La3d)) is around 3.9 for La(OH)3, the one for La2O3 is higher (4.6 eV) [36]. Therefore the increase in ΔE(La3d) value observed after the HEBM process suggests a lower La(OH)3 contribution to the La3d signal, in agreement with the XRD results and O 1s spectra analysis. Mn 2p spectra (Figure A1) show two main peaks corresponding to the spin–orbit split of 2p3/<sup>2</sup> and 2p1/<sup>2</sup> levels, while the weak signal at lower BE from the main peak is assigned to the satellite of the 2p1/<sup>2</sup> peak. The 2p3/<sup>2</sup> peak satellite is not noticeable because it overlaps with the 2p1/<sup>2</sup> peak. Mn 2p3/<sup>2</sup> peak has its maximum at BE of 641.6 eV for the three LaMnO3.15 samples. This BE value is intermediate between those recorded for Mn2O3 and for MnO2 [37], confirming the presence of a mixture of Mn3<sup>+</sup> and Mn4<sup>+</sup> species in the LaMnO3.15 samples. In order to estimate the proportion of Mn3<sup>+</sup> and Mn4<sup>+</sup> species, the Mn 3s core level has been studied. The superposition of Mn 3s spectra is shown in Figure 5. Two peaks, originating from the coupling of non-ionized 3s electron with 3d valence-band electrons [38], are distinguished. From the energy difference between the two peaks, ΔE(Mn 3s), it is possible to estimate the Mn AOS [39]. A significant decrease in ΔE(Mn 3s) value was observed from LaMn\_SSR to LaMn\_HEBM sample, and then remains stable for LaMn\_LEBM sample (Figure 5). Therefore, the HEBM process resulted in a pronounced increase in Mn AOS on the material surface from 3.2 to 3.8, while the LEBM process does not induce significant additional change (Table 3). The surface AOS (obtained by XPS, Table 3) is

identical to the bulk AOS (obtained by TPR, Table 2) for the LaMn\_SSR material. However, while the bulk AOS was not affected during the HEBM and LEBM steps, it is evident from the XPS results that these steps result in an oxidation of the surface Mn ions.

**Figure 5.** High-resolution spectra of O 1s and Mn 3s core level spectra for LaMnO3.15 samples: (a) SSR, solid state reaction; (b) HEBM, high-energy ball milling; (c) LEBM, low-energy ball milling.

XPS results obtained for the LaFeO3 samples are presented in Figure 6. The superposition of the O 1s spectra is shown in Figure 6. As previously observed for Mn-based samples, the same three components can be extracted from the O 1s signal. However, the component contributions to the O 1s signal are not significantly impacted by the ball milling process, since the OI to OII component ratio is similar regardless the synthesis step. Figure A2 shows the La 3d region obtained for the LaFe samples. ΔE(La 3d) values remains constant for the three samples: the multiplet splitting amplitude of ~4.2 eV confirms the oxide form. Figure 6 shows the curve fitted Fe 2p spectra. Indeed, the similarly to Mn 2p spectra, Fe 2p signal presents multiplet structures which can be fitted in order to resolve the surface iron state. The Fe 2p spectral fitting parameters of Fe2O3 compound proposed by Biesinger et al. [37], i.e., binding energy, percentage of total area, full width at half maximum and spectral component separation, have been used to simulate the Fe 2p signal. The good concordance between the fitted integration and the Fe 2p experimental envelope suggests that iron species are mainly in +III oxidation state, as it is often described in the literature.

**Figure 6.** High-resolution spectra of O 1s and Fe 2p core level spectra for LaFeO3 samples: (a) SSR, solid state reaction; (b) HEBM, high energy ball milling; (c) LEBM, low energy ball milling.
