*2.2. XPS Analysis*

XPS measurements were carried out onto all single metal oxides and even a set of selected mixed oxides, in order to investigate their surface elemental composition, measured as atomic percentage, with oxidation state speciation. Such a study is fundamental in the present work, for evaluating how the different preparation techniques influence the properties of the materials. In binary metal oxide samples, regions relative to specific core levels and Auger transitions are here the object for detailed analysis of high resolution XPS spectra.

Copper oxidation state and speciation are commonly studied looking at high resolution spectra for Cu 2p3/<sup>2</sup> core level in the Binding Energy (BE) range 925–950 eV and for Cu L3M4,5M4,5 (Cu LMM) Auger transition (BE 555–600 eV). Considerations about peak shape, curve fitting, shake-up peaks and the value of the proper modified Auger parameter are needed [41,42]. The latter equals 1849.0 eV in both commercial (C) and synthesized (S) samples. This value and the inspection of the Auger peak shape exclude any occurrence of Cu metal. Moreover, the lack of shake-up peaks in S-Cu2O confirms the almost exclusive presence of the Cu(I) oxidation state for copper atoms, while shake-up peaks observed in C-Cu2O, support the presence of CuO [41,42]. These considerations and curve fitting of O 1s XPS peak are used to determine the ratio Cu(I)/Cu(II), that equals 95/5 in S-Cu2O and 70/30 in C-Cu2O, showing that the surface of commercial Cu2O sample is significantly oxidized.

Similar arguments are used for speciation of Indium atoms: analysis involves the In 3d core level doublet (BE 440–460 eV) and the In M5N4,5N4,5 (In MNN) Auger transition (BE 1060–1090 eV) [43]. Energy positions of these are used to calculate a proper modified Auger parameter, which equals 850.8 eV in S-In2O3 and 851.1 eV in C-In2O3. The core level peaks resulted broad and symmetric, and no plasmon loss feature at higher binding energies were observed, as it would be expected if metallic Indium was present [43,44]. It can be thus concluded that In2O3 (both C and S) samples are formed exclusively of In(III)-oxide.

In samples containing iron atoms, the study about the Fe-oxidation state focused onto Fe 2p core level multiplet, occurring in the 705–730 eV region [45,46]. It is recorded as the envelope of different signals coming from electrostatic and spin interactions, from crystal field interactions and from spin-orbit coupling between the 2p and 3d states. This signal requires careful curve fitting operation able to identify the 2p3/<sup>2</sup> main peak center of gravity and satellite peaks structures, due to shake-up and charge transfer processes, whose binding energy separation is used as parameter [45]. The recorded signal indicates that only the Fe3<sup>+</sup> oxidation state is present in the prevalent α-Fe2O3 form in both C- and S-samples, which, nevertheless, show a different multiplet splitting and a different energy separation. The latter equals 7.4 eV in the S-Fe2O3 and 8.4 eV in the C-Fe2O3, with the higher value being indicative for the presence of Fe(OH)3 [45]. From the data above, one can conclude that S-samples are more reliable than commercial ones.

In mixed metal oxides samples, the surface composition (atomic percentage) was obtained through quantification from the single high resolution XPS spectra for specific peaks: C1s, O1s, and Cu2p3/<sup>2</sup> elements were detected in all samples. Table 2 accounts for atomic percentage concentration data, which are specific for surface composition and can thus differ from those measured by EDX, which are instead relative to bulk concentration. By comparison with data in Table 1, a higher Cu atoms occurrence at the surface is observed for all samples, except for the CP-Cu/Fe-2 sample.


**Table 2.** Surface composition in mixed oxide samples determined by XPS. Atomic percentages are reported as mean values ± 1S (values averaged out of at least three replicates).

High resolution spectra for Cu speciation in mixed metal oxides samples are shown in Figure 1. By comparing samples prepared by HEM (Figure 1a,b) or CP (Figure 1c,d), a nearly perfect trace overlapping is observed in both Cu/In and Cu/Fe pairs, showing no difference for the preparation techniques.

**Figure 1.** X-ray Photoelectron spectra for Cu 2p3/<sup>2</sup> and Cu L3M4,5M4,5 (Cu LMM)Auger transition in (**a**,**b**) Cu/In and in (**c**,**d**) Cu/Fe pairs.

Moreover, Cu atoms on the surface are exclusively encountered as Cu(I), with a CuO component that is observed only in traces and not quantifiable [42]. This is true also for samples prepared using commercial-Cu2O, which contains 30% CuO (*vide infra*), showing that, where present, Cu(II) is in the bulk more than on the surface.

The spectra for In-3d core level doublet were recorded in HEM and CP mixed oxides samples and they are shown in Figure 2a: the only chemical state observed was In2O3 [43,44]. Figure 2b shows the Fe 2p core level spectra as recorded in mixed oxides samples. Curve fitting results and peak analysis show the presence of the solely α-Fe2O3 form of iron species [45,46] in both measured samples.

**Figure 2.** XP spectra for (**a**) In 3d and (**b**) Fe 2p core levels in mixed metal oxides samples.

In conclusion, samples prepared by HEM and CP do not show differences for what concerns the surface composition, and more interestingly C- and S-samples of ternary oxides show very similar surface composition and properties, even if C-Cu2O contains 30% of Cu(II) which remains confined in the bulk.

## Valence Band Maximum Evaluation by XPS

XPS analysis can be used to measure Valence Band Maximum (VBM) energy level, which is fundamental for subsequent band structure evaluation. This task can be accomplished adopting the procedure developed by Kraut and co-workers [47,48], then correctly extended to not covalent and oxide-based semiconductors by Chambers and co-workers [49]. The procedure requires acquisition

of the XP spectrum in the low binding energy region, near the zero-value which corresponds to the Fermi level. Here, the XP spectrum reflects the electron density in the low energy states [47–49]. The VBM energy is determined as the intersection of two straight lines, obtained from least square fitting: the first line fits the spectrum baseline and background over the Fermi level, the second line fits the leading edge of the spectrum towards increasing binding energy [49]. In this work, while the band edge fitting operation was optimized through the maximization of R<sup>2</sup> correlation coefficient, the background fitting operation was performed by inclusion of all data points measured at negative binding energies coordinates. Figure 3 shows XP spectra in the Valence Band region, between 11 and -3 eV, with comparison of Commercial and Synthesized binary metal oxide samples. In Cu2O samples, a 0.74±0.20 eV difference in extracted band edges is observed, though spectra are similar in shape and sharp steep band edges do appear. Spectra are shown in Figure 3a with values equal 1.65±0.20 and 0.86±0.14 eV in C-Cu2O and S-Cu2O samples, respectively, the difference being ascribed to the presence of CuO in the C-sample, for which a more positive VB edge is commonly attested [19,40,50]. Results agree with both theoretical [40,51] and experimental common literature references [19,20,50,52].

**Figure 3.** XP Valence Band Spectra in (**a**) Cu2O, (**b**) In2O3, and (**c**) Fe2O3 binary oxide samples.

XP spectra for valence band region in In2O3 samples are shown in Figure 3b, where a certain difference is observed: the synthesized sample shows a higher intensity peak than the commercial one. This feature is indicative of a higher number of localized electrons within the VB energy levels, and its origin has been identified in the occurrence of random O-vacancies, whose levels build up the VB ones by orbital mixing [53], and in the crystalline domains size. This last could result reduced enough to hinder efficient electron transfer to the CB and so decrease the naturally occurring *n*-type character of this semiconductor material [25,54]: in fact, VBM results are closer to the Fermi level. Extracted values equal 2.20±0.11 and 1.80±0.15 eV for Commercial and Synthesized samples respectively, and both result less positive than literature values [25,30,34,52], making the material closer to hydrogen evolution potential.

Figure 3c reports XP spectra for valence band region in Fe2O3 samples, and differences between samples are visible, similar to those observed in In2O3 samples. In this case, the Commercial sample is characterized by a higher number of electrons in the VB, supposed to derive from localized states within the energy gap, due to defective particles, especially oxygen vacancies [55], and agree with the presence of iron hydroxide traces shown by XPS speciation measurements.

A less intense signal is recorded in the synthesized sample, and it is thought to be caused by defective particles and small crystalline domains. XPS data for Fe-containing systems are typically difficult to fit/convolve. Extracted VBM position are 1.53 ± 0.22 and 1.67 ± 0.23 eV for C-Fe2O3 and S-Fe2O3 samples, respectively, with a small difference between them and at lower energy than some literature theoretical [55,56] and experimental data [36,39,57]. Table 3 lists VBM extracted values for all binary metal oxide samples described above.


**Table 3.** Valence Band Maxima extracted from XPS measurements. The error values were determined from the regression method extrapolation.
