**1. Introduction**

Combustion of fossil fuels (fossil-C) is actually the main source (80.2% as for 2018) to fulfil human hunger for energy, but natural resources are not infinite and are expected to get exhausted in 160 y or so. Moreover, the use of fossil-C is responsible for the emission of 37 Gt/y of CO2 and other green-house gases considered to be the origin of climate change. According to Earth System Research Laboratory's—ESRL's Global Monitoring Laboratory, in January 2020, atmospheric concentration of

CO2 reached 412 ppm [1]. However, avoiding massive and continuous CO2 emission and utilization of alternative sustainable primary energy sources is necessary [2]. On the other hand, CO2 represents a readily available building block for chemicals and source of carbon for fuels, which can be produced through a conversion driven by C-free energy sources [3]. Solar radiations carry a quantity of energy to Earth surface, sufficient to be considered to perform CO2 conversion [4]. This can be realized by means of photocatalysis [5], in a semiconductor-assisted light-driven process during which light is absorbed and converted into chemical energy, such as CO, CH3OH, CH4, Cn-species, or even H2, produced in water-splitting [6,7].

Unfortunately, for the moment, the photocatalytic processes still suffer low efficiency and are not ready for an industrial commercially viable application [5], despite the research started with work by T. Inoue [8] and J.M. Lehn [9] that dates back to 40 years ago. Searching for the best photocatalysts [10], a variety of semiconductors have been studied, ranging from those based onto Group 4 elements to more classical Group 3–5 semiconductors [11,12], Group 6 chalcogenides [13] and more "exotic" semiconductors [14]. Among the latter materials, Earth-abundant transition and post-transition metal oxides such as TiO2, Fe2O3, Cu2O, CuO, ZnO, NiO, Ta2O5, Ga2O3, In2O3, and WO3 revealed very promising to act both as photo-catalysts [10–12] or co-catalysts [15]. They are usually cheap and yet widely used as chemo-catalysts in industrial applications, easily fabricated at micro- and nano-size and can absorb light in the UV-Visible region [16,17]. Further, they can be efficiently coupled to enhance properties such as visible light absorption [18], band edge levels' position, and photogenerated charge transfer, and separation processes [15]. Copper oxides, both CuO and Cu2O, were recently demonstrated able to convert CO2 or water into solar fuels under VIS-light irradiation [19]. These *p-type* semiconductors, that can efficiently be used either bare [20] or as co-catalysts [21–24], are affected by high recombination rate and photodegradation. The formation of a heterojunction by coupling with a suitable *n-type* semiconductor, is a widely adopted strategy to inhibit charge recombination, enhance stability and provide alternative energy levels to carry out photocatalytic reactions [11,12]. The formation of such junctions even at very small particle size is one of the keys for leading to fabrication of active photocatalysts. The semiconductor coupling strategy can also be adopted for copper oxides, in which properties as photostability and charge separation are found to be significantly affected by coupling with *n-type* metal oxides [19]. At the same time, the addition of copper oxides is useful to shift the absorption spectrum of semiconductor partner towards visible range [12,18].

In the present work, two different *n*-type metal oxides were selected as potential partners for Cu2O: indium oxide, In2O3, and iron oxide in the form of hematite, α-Fe2O3. Thanks to band energy levels position, affinity towards CO2 and electronic properties, both these oxides are recognized in literature as potential photocatalysts or cocatalysts for solar fuels production [12,25,26]. In particular, indium oxide has been recently experimented for CH4 and H2 photocatalytic production, coupled with other semiconductors or cocatalysts [27,28], or even with a thermal input during the reaction [29,30]. Instead, iron oxide is widely experimented for photocatalytic dye degradation [31] and water cleaning [26], but recently water splitting application is under study [32].

To the best of our knowledge, semiconductor pairs composed of Cu2O with In2O3 or α-Fe2O3 are very scarcely characterized and tested for solar fuels production. Thin films are generally preferred over powders and particles. For the latter, 2D semiconductors as graphene or carbon nitride have been used as platforms for achieving enhanced charge separation and better lattice matching [10–12]. This is especially true for the Cu2O-In2O3 pair, where lattice parameters mismatch can hinder the formation of heterojunctions, but samples prepared by hydrothermal/co-precipitation methods are possibly active in photocatalytic degradation and hydrogen evolution [33,34]. In the literature, the Cu2O-Fe2O3 pair is reported to a bigger extent than that with Cu2O-In2O3 and samples prepared by solvothermal, co-precipitation or electrodeposition methods were tested for photocatalytic degradation [35], hydrogen production [36,37] and CO2 reduction to carbon monoxide, methanol and methane [38,39]. A point to mention is that the Cu/In,Fe-mixed-oxides were not always fully characterized and often they were

added with other compounds (especially noble metals) and hole scavengers, adding complexity to the already not clearly defined system and making difficult to understand the role of each partner.

In this work, mixed-oxide nano-particles were prepared through two different techniques: High Energy Milling—HEM and Co-Precipitation—CP. Starting oxides were either commercial samples or synthesized in our laboratory. Neat nano-sized powder samples were analyzed by Energy Dispersive X-Ray spectrometry (EDX), characterized by UV-Visible Diffuse Reflectance Spectroscopy (UV-VIS DRS), and X-ray Photoelectron Spectroscopy (XPS) and then tested in gas phase CO2–H2O co-processing under VIS-light irradiation, at room temperature, without addition of noble metal co-catalysts or hole scavenger species to evaluate the properties of the single mixed oxides. Evaluation and tailoring of properties of cited materials, with particular attention to electronic band structure and optical absorption, was the final goal of this work, targeting a correlation among properties of the materials and their photocatalytic activity in co-processing H2O and CO2 under VIS-light in different reactor geometries.
