**Davide M. S. Marcolongo 1, Francesco Nocito 1, Nicoletta Ditaranto 1, Michele Aresta <sup>2</sup> and Angela Dibenedetto 1,3,\***


Received: 5 August 2020; Accepted: 18 August 2020; Published: 31 August 2020

**Abstract:** In the present paper, we report the synthesis and characterization of both binary (Cu2O, Fe2O3, and In2O3) and ternary (Cu2O-Fe2O3 and Cu2O-In2O3) transition metal mixed-oxides that may find application as photocatalysts for solar driven CO2 conversion into energy rich species. Two different preparation techniques (High Energy Milling (HEM) and Co-Precipitation (CP)) are compared and materials properties are studied by means of a variety of characterization and analytical techniques UV-Visible Diffuse Reflectance Spectroscopy (UV-VIS DRS), X-ray Photoelectron Spectroscopy (XPS), X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), and Energy Dispersive X-Ray spectrometry (EDX). Appropriate data elaboration methods are used to extract materials bandgap for Cu2O@Fe2O3 and Cu2O@In2O3 prepared by HEM and CP, and foresee whether the newly prepared semiconductor mixed oxides pairs are useful for application in CO2-H2O coprocessing. 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 shows 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. This work is a rare case of full characterization of photo-materials, using UV-Visible DRS, XPS, XRD, TEM, EDX for the surface and bulk analytical characterization. Surface composition may not be the same of the bulk composition and plays a key role in photocatalysts behavior. We show that a full material knowledge is necessary for the correct forecast of their photocatalytic behavior, inferred from experimentally determined bandgaps.

**Keywords:** CO2–H2O photo-co-processing; VIS-light driven reactions; CO2 reduction; photocatalysts properties
