**4. Conclusions**

The experiments carried out in this work have confirmed that the properties of photocatalysts based on mixed oxides strongly depend on the starting materials and the way the binary- and mixedoxides are prepared. Even their real composition depends on the technique used for their preparation. Careful analyses are necessary to confirm their composition that can or cannot match the stoichiometric ratio used in their preparation. The surface composition can be different from bulk and this can influence the reactions, should they occur on the surface or into channels in the bulk.

The electronic properties of the photo-materials change with their composition and mode of synthesis. Using two different techniques (HEM and CP), the fundamental properties of the photocatalysts have been measured, including the band-gap and electrochemical potential. Some of the catalysts prepared, based on their band gap and value of E, have been tested in the gas-phase photoreduction of CO2 + H2O. The experimental results show that the synthetic technique influences the photoactivity of the materials that can correctly be foreseen on the basis of bandgap experimentally derived. Of the mixed oxides prepared and described in this work, only Cu2O@In2O3 prepared by co-precipitation from synthesized binary oxides have shown positive results in CO2–H2O photo-co-processing. Preliminary results show that the composition and synthetic methodologies of mixed-oxides, the reactor geometry, the way of dispersing the photocatalyst sample, play a key role in the light driven reaction of CO2–H2O. Hydrogen plus reduced species of CO2 (in lower amount) have been observed, depending on the geometry of the reactor used and the photocatalyst used. In order to observe the formation of reduction products it is necessary that the catalyst is finely dispersed

(thin film) and well illuminated. Massive amounts of photocatalyst are not active, at least under the illumination technique used in this work, most likely because the number of photons that reach the photoactive centers is quite low.

This work is a rare case of full characterization of photo-materials, using UV-Visible DRS, XPS, XRD, TEM, and EDX for the surface and bulk analytical characterization. We show that surface composition may not be the same of the bulk composition and plays a key role in photocatalysts behavior and a full material knowledge is necessary for the correct forecast of their photocatalytic behavior, inferred from experimentally determined bandgaps. Coupling UV-Vis DRS and XPS with EDX is necessary for getting the correct information about the composition of the materials and their surface-bulk characterization. Further studies are planned in order to discover the most active species and the best performing reactor geometry under best illumination conditions, using the systems which gave positive results so far. All of the systems described above are even under evaluation for discovering how their properties are changed with addition of partners such as noble metals or hole scavengers and attribute the correct role to each component of the photomaterial.

**Supplementary Materials:** The followings are available online at http://www.mdpi.com/2073-4344/10/9/980/s1, Figure S1: Comparison of CP-Fe2O3 XRD patterns with HEM-Cu/Fe-1 (a) and CP-Cu/Fe-1 (b) along with peak positions for reference diffraction patterns. Figure S2: High resolution XP spectra of all the samples. (a) C-Cu2O sample: (a1) Cu 2p3/<sup>2</sup> spectral region, (a2) Cu LMM Auger transition and (a3) O1s spectral region. (b) S-Cu2O sample: (b1) Cu2p3/<sup>2</sup> spectral region, (b2) Cu-LMM Auger transition and (b3) O1s spectral region. (c) C-In2O3 sample: (c1) In3d spectral region, (c2) In MNN Auger transition and (c3) O1s spectral region. (d) S-In2O3 sample: (d1) In3d spectral region, (d2) In MNN Auger transition and (d3) O1s spectral region. Fe2p spectral region for (e1) C-Fe2O3 and (e2) S-Fe2O3. Figure S3: H2 evolution with time by using CP-Cu/In mixed oxides under VIS light irradiation.

**Author Contributions:** Conceptualization, M.A. and A.D.; synthesis methodology and GC analytical techniques, F.N.; synthesis and spectroscopic characterization of photomaterials, D.M.S.M.; XPS analysis, N.D.; writing—original draft preparation, D.M.S.M.; writing—review and editing, M.A. and A.D.; supervision, A.D.; project administration, A.D.; funding acquisition, A.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received MIUR-IT funding. It was executed within the frame of a preliminary work of an international collaboration that has brought to a common EU-Project application.

**Acknowledgments:** The authors thank Roberto Comparelli (Istituto per i Processi Chimico-Fisici, Consiglio Nazionale delle Ricerche, c/o Dipartimento di Chimica, Università di Bari) and Teresa Sibillano (Istituto di Cristallografia, Consiglio Nazionale delle Ricerche, Bari) for performing and interpreting preliminary TEM and powder XRD measurements, respectively.

**Conflicts of Interest:** The authors declare no conflict of interest.
