*2.3. UV-Visible Spectroscopy Characterization*

The first step for photocatalytic processes is light absorption, thus UV-Visible Diffuse Reflectance Spectroscopy (UV-VIS DRS) was used to measure the optical properties for all samples in 200–800 nm range. The UV-VIS DRS properties directly depend on band gap and electronic energy structure and they affect the photocatalytic activity too [10,11,18]. Changes of light absorption properties with composition were observed in mixed oxide samples in present work. In samples containing the Cu2O/In2O3 pair, absorption spectra (Figure 4) clearly change with composition. Lines corresponding to binary metal oxides are plotted for comparison. Whether HEM or CP preparation technique is adopted, the increase in Cu/In ratio induces a general redshift in absorption spectra and a corresponding significant absorption at wavelengths above 500 nm, where fundamental transition of Cu2O is [19]. While the absorption tail observed in CP prepared samples (Figure 4a) can be attributed mainly to scattering phenomena, due to aggregation of small particle size, the reduction in size for Cu2O component is at the origin of the blueshift observed in spectra of HEM prepared samples (Figure 4b). Such a shift and the marked absorption tails observed at long wavelengths for samples prepared using C-Cu2O are instead attributed to the not negligible CuO presence in commercial Cu2O, because CuO fundamental absorption occurs at lower energy [19].

**Figure 4.** UV-Visible Diffuse Reflectance Spectroscopy (UV-VIS DRS) for Cu/In mixed oxides pairs prepared by (**a**) CP and by (**b**) HEM.

Absorption spectra recorded for samples containing the Cu2O/Fe2O3 pair are shown in Figure 5, together with lines of binary metal oxides for comparison. Here, the increase in Cu/Fe is found to change absorption spectra for both preparation techniques, but in a different way accounting for absorption features of the two components at wavelengths longer than 500 nm [19,26]. In CP prepared samples (Figure 5a) the component oxides present fundamental absorption region different enough to observe a linear trend with increasing Cu/Fe ratio and thus a slight blueshift and increasing similarity towards the Cu2O spectrum. This feature has been attributed to typical α-Fe2O3 spectra characteristics which appear over 550 nm [26,55], well over than S-Cu2O fundamental absorption [19], thus, the addition of this specific component did not result in a redshift. On the contrary, in samples prepared by HEM using C-Cu2O (Figure 5b), spectra of precursor components (C-Cu2O and C-Fe2O3) show a similar fundamental absorption region and differences only in absorption intensity. Hence, an increase in Cu/Fe ratio corresponds to a higher absorption at longer wavelengths, which has been mainly attributed to CuO impurities in C-Cu2O precursor.

**Figure 5.** UV-Visible DRS for Cu/Fe mixed oxides pairs prepared by (**a**) CP and by (**b**) HEM.

However, DRS absorption measurements demonstrated that prepared nanocomposites are photoactive in almost the whole UV-Visible range in function of their composition. In particular, the introduction of Cu2O has given In2O3 a better response in the VIS-region, while the interaction between Cu2O and α-Fe2O3 has induced less predictable effects because of similar fundamental absorption. Studying these features is crucial because they are involved in enhancement of solar light harvesting properties for application in photocatalysis. Anyway, in both pairs, properties are very sensitive to nanocomposites preparation procedures, and thermal treatments.

Band-Gap Evaluation by UV-Visible Spectroscopy

The optical energy gap (Eg,opt) is a fundamental property in semiconductors and it equals the minimum energy required to excite an electron from VB to CB by means of light absorption. This energy gap can be directly measured through UV-Visible spectroscopy, if a single fundamental absorption is clearly distinguished. In case this is not possible or solid-state samples are studied, as in the present work, a simple and widely adopted data elaboration method can be used, which is described in detail elsewhere [58,59]. Briefly, the Kubelka–Munk function (K–M), F(R), is calculated starting from the experimental reflectance spectrum and is related to linear absorption coefficient α and to Eg,opt through a power law (1) describing the optical absorption strength in function of photon energy.

$$\mathbf{F(R)} \cdot \text{(hv)} = \mathbf{A} \cdot (\mathbf{h} \mathbf{v} - \mathbf{Eg})^n \tag{1}$$

The exponent *n* assumes different values depending on the type of electronic transition. Provided there is some knowledge about the occurring electronic transition, the plot of product (2) versus radiation energy, (hν), shows up a linear trend in the region corresponding to fundamental absorption and energy gap [58,59].

$$(\text{F(R)} \cdot \text{(hv)})^{1/n} \tag{2}$$

The linear least square fitting in this region allows for the extraction of the Eg,opt value as the intersection of straight line with the energy axis, according to the Tauc Plot extrapolation procedure. It can be applied to pure or lightly doped semiconductors, but it does not produce reliable results if fundamental absorption edges are not separable or if a simple combination of individual optical gaps cannot be assumed, as in highly doped semiconductors or in nanocomposites. In the present work, mixed oxide samples fall in the second case, therefore Eg,opt value was measured only for all single metal oxide samples. The linear regression operations have been optimized by merging two criteria: (a) maximization of correlation coefficient *R*2, ensuring a better description by linear model and (b) consideration of a calculation range containing a minimum of 20 data points, to give procedure a statistical validation. Matching those two criteria allows for a good extrapolation result. Tauc Plots for different single metal oxide samples are shown in Figure 6.

**Figure 6.** Tauc Plots for (**a**) Cu2O, (**b**) In2O3, and (**c**) Fe2O3 binary oxide samples.

In the case of Cu2O, absorption spectra are reported as light blue traces in both Figures 4 and 5 and large difference do appear between C-Cu2O and S-Cu2O samples. Absorption in the commercial sample extends up to 600 nm, while the S-Cu2O sample shows a significant absorption in the UV region, decreasing significantly over 500 nm, where some absorption is guaranteed by a pronounced tail extending at longer wavelengths. The extended presence of CuO (30%) in the commercial precursor and particle size cause the differences. Figure 6a shows Tauc Plots for Cu-oxide samples. These show a steeper rise for S-Cu2O sample and two very different values for estimated Eg,opt energies are observed, and both have been calculated assuming a direct allowed electronic transition occurs between VBM and CBM [19,40], and *n* coefficient is, thus, set equal to 0.5. Values within 2.0 and 2.2 eV are generally attested in literature for Cu2O [40], though quantum size effect is widely recognized able to markedly influence its energy gap. In this work, Eg,opt values equal to 2.047 and 2.495 eV for C-Cu2O and

S-Cu2O sample, respectively, are found in accordance with literature [20,36,39]. This is also true for the synthesized sample, where a UV-shifted value is justified by quantum size confinement and where a pronounced tail, recorded before gap and reflected in absorption spectrum, is attributed to crystalline disorder and broad size dispersion.

Absorption spectra for In2O3 samples are shown in Figure 4 as red traces which do not show large differences at a first examination: both commercial and synthesized samples absorb radiation mainly in the near UV range (below 450 nm). Tauc Plots (Figure 6b) enhance features previously not evident, such as a large tail in the S-In2O3 trace or its steeper rise with respect to the C-In2O3. Extracted Eg,opt values are equal to 3.091 eV in C-In2O3 and to 3.610 eV in the S-In2O3 sample. The determination was performed considering that a direct allowed electronic transition occurs, as for recent studies [25,54]. Though band gap nature and electronic structure for In2O3 are somewhat controversial, a fundamental band gap ranging from 2.6 to 2.9 eV is now commonly accepted [25], and it slightly differs from the optical gap, attested within 2.3 and 3.8 eV [30,34], therefore values measured in the present work result in accordance with literature. The 0.6 eV difference observed has been deemed coming from the preparation technique. In this case, its contribution affects band structure mainly through the introduction of lattice defects, especially oxygen vacancies, and creation of mid-gap states, a phenomenon which is commonly found in *n*-type wide gap semiconductor oxides [54], the effect appears more pronounced in the S-In2O3 sample, where the trace shows a large tail extending towards low energies.

Both the Fe2O3 samples exhibit good light harvesting properties in the whole UV-Visible region, with significant absorption up to 600 nm, higher in S-Fe2O3 sample, as shown by analysis of red traces in Figure 5. Four different regions are commonly identified in absorption spectra [31] and the fundamental band gap is recognized at 2.2 eV [26,55], with discussion whether its direct or indirect nature and thus its coincidence with the optical band gap. Furthermore, complication can arise in Eg,opt determination because it usually merges with an exciton absorption, which is produced by an indirect transition between 3d–3d orbitals and dominates spectra over 550 nm, giving hematite its typical red color [55]. The exciton is clearly observed in Tauc Plots as the low intensity shoulder with onset at 2.1 eV (Figure 6c). Because of this indirect nature, absorption below 2.1–2.2 eV is considered not able to produce useful separated electron-hole pairs, suffering from very fast recombination, a widely recognized drawback in α-Fe2O3 [26,55]. Therefore, to study photocatalysis-useful optical absorption, a direct allowed transition has been assumed for Tauc Plots. Extracted Eg,opt equal 2.772 and 2.875 eV for S-Fe2O3 and C-Fe2O3 sample, respectively. Although these values result unusually larger than the commonly accepted and measured ones [31,32,36,39], such a discrepancy can be explained by peculiar interacting electronic levels in iron atoms and presence of lattice defective particles. All these elements can lead to mixing between the standard direct gap and higher energy features (Ligand to Metal Charge Transfer (LMCT) processes occurring at 2.9–3.1 eV) [55]. Aggregation of very small particles towards polycrystalline clusters formation was cited able to enhance this mixing and can be also identified as the source for the large tailing trend in S-Fe2O3 trace.

Table 4 lists single metal oxide samples, with indication of optical energy gap extracted values and corresponding absorption wavelength. Except for iron oxide, S-samples show higher optical energy gaps than C-ones. Moreover, S-samples reveal a tailing trend more pronounced than in C-samples, indicative of broader size dispersion and lower average size (10–50 nm).


**Table 4.** Optical energy gap and corresponding absorption wavelength in binary oxide samples.

<sup>1</sup> Calculated by the relation λ = 1240/Eg,opt [18].

#### *2.4. Band Structure Evaluation and Discussion*

Possible photocatalytic activity performances in solar fuels production by utilization of ternary metal oxides prepared in this work has been outlined on the basis of electronic band structure of binary and ternary mixed metal oxides samples. In fact, the energy position of band edges corresponds to redox potential of electrons and holes in semiconductor materials, thus it determines the redox behavior of photogenerated charge carriers and their possible transfer to adsorbed chemical species, as needed for photocatalytic reaction to occur [6,7,10,11].

An estimation for VBM and CBM is thus fundamental in characterization of semiconductors, and this task is usually accomplished by combination of theoretical considerations and of experimental data obtained by use of different techniques [10–12]. For energy band structure evaluation in the present work, experimental data for VB edges, as determined by XPS, and for Eg,opt, as determined by UV-Visible DRS, have been used in combination with theoretical arguments, found in literature, about energy band structure of specific semiconductor metal oxide considered. Band structures plots are shown in Figure 7 for ternary metal oxides, and they are reported as composed by those of binary metal oxides and compared to redox potentials involved in solar fuels production [2,11].

In all band structure schemes (Figure 7), the left side refers to Cu2O energy bands, both for Cand S-samples. These were determined by considering only VBM and Eg,opt, where this last equals the fundamental Eg [19]. A big discrepancy observed in energy gap values and band position has been ascribed to CuO impurities, which were detected in C-samples used as precursor for HEM, and to quantum size confinement effect. Both of these arguments concur to the whole S-Cu2O sample structure being moved upward in energy with respect to the C-Cu2O sample, for which a different plot for Cu2O and CuO is not possible. Moreover, it is worth noting that both CuO and Cu2O are *p-type* semiconductors materials [19,50], but the unpredictable formation of junctions between Cu2O and CuO can affect the Fermi level position in C-Cu2O [50], thus hiding the *p-type* character expected in the schemes of Figure 7a,c, which is on the contrary easily detectable in schemes of Figure 7b,d, referred to S-Cu2O.

Band structures schemes for C-In2O3 and S-In2O3 samples are reported in Figure 7a,b, respectively, where theoretical arguments found in literature about somewhat controversial and long debated electronic structure were considered [25]. In this frame, the direct optical gap measured involves a level which lays 0.81 eV below the VBM determined by XPS. Thus, the fundamental Eg for In2O3 equals 2.28 and 2.80 eV for C-In2O3 and S-In2O3 samples, respectively, in accordance with a maximum possible 2.9 eV value [25]. Also, schemes report correct *n-type* semiconductor behavior in both the samples [25,54].

In the case of Fe2O3 samples, band structure schemes are plotted in Figure 7c,d for C-Fe2O3 and S-Fe2O3 samples, respectively. While optical energy gap is considered to occur between the VBM determined by XPS and a level above the CBM, the fundamental energy gap involving the real CBM equals the energy required for exciton formation [55]. This energy has been measured by studying the corresponding optical absorption with Tauc Plot procedure, with assumption of an indirect transition (n equals 2) which incorporates exciton absorption and thus reflects the interested energy difference [31]. Fundamental Eg equals 2.01 and 1.97 eV for C-Fe2O3 and S-Fe2O3 samples, respectively, values slightly

lower than common 2.1 eV expected for exciton in hematite [55]. As for the In2O3 samples, expected *n-type* semiconductor character results evident in all α-Fe2O3 samples [26,55].

**Figure 7.** Band structures schemes for Cu2O/In2O3 pair, prepared by (**a**) HEM and (**b**) CP, and for Cu2O/Fe2O3 pair, prepared by (**c**) HEM and (**d**) CP.

Results obtained by this method are shown in Table 5, where band edges and energy gaps, both optical and fundamental are pointed out.


**Table 5.** Resume of energy gaps and band edges levels in semiconductor metal oxides.

From data in Table 5, some photocatalytic activity result in solar fuels production can be expected only for the CP prepared Cu2O/In2O3 samples, whose band scheme structure is shown in Figure 7b. This pair could reveal useful in H2 generation coming from photocatalytic water splitting, a reaction in which it is able to participate thank to proper band levels alignment of two metal oxide components. In particular, electrons excited in Cu2O can move into In2O3 CB and leave behind holes in VB, a level adequate to oxidize water molecules. At the same time, the electrons photogenerated in In2O3 are in a level adequate to reduce H<sup>+</sup> and produce H2, and this level also collects electrons coming from Cu2O

CB, while the photogenerated holes move into Cu2O VB, where they can oxidize water molecules. On the other hand, large charge carrier recombination occurring in Cu2O could be responsible for widely recognized poor activity in general.

At the same time, CO2 reduction can show different trend whether a 1e- or multielectron-transfer is considered. In fact, the CB potential of the two components, results not negative enough for one electron transfer to CO2 molecule (−1.90 V vs. NHE is needed) [2]. As a matter of fact, this is not the reaction we are considering: the Proton Coupled Electron Transfer (PCET) is the process that should operate in the present case.

On the basis of similar considerations, poor activity for solar fuels production can be foreseen for other oxides pairs studied in the present work which do not provide the potential for one-electron transfer to CO2 molecule and do not show a proper level alignment for water splitting. Additionally, the presence of CuO impurities can be at the origin of enhanced charge carrier recombination, a phenomenon recognized in this material, up to withdrawn all photogenerated charge carriers from reaction or charge carrier separation processes. More, in both Cu2O/Fe2O3 pairs (Figure 7c,d), the not proper level alignment adds to their relative positions, which results not adequate for photogenerated charge carrier separation, as in HEM prepared pair (Figure 7c), or even in enhanced interband recombination, as in CP prepared pair (Figure 7d).
