**2. Results and Discussion**

#### *2.1. Synthesis, Composition, and Size of Ternary Oxides*

Binary oxides powders used in this work were either commercial samples (C-Oxide) or synthesized in the laboratory (S-Oxide). Ternary oxides powders were fabricated through HEM and CP techniques, as reported in the Materials and Methods section. Such mixed oxides are labelled in Table 1 according to the preparation technique employed and numbered according to increasing Cu/In or Cu/Fe molar ratio, as measured by EDX spectrometry and calculated considering all copper, indium, and iron present in the form of Cu2O, In2O3, and Fe2O3, respectively.


**Table 1.** Composition of Mixed-Oxide samples prepared by High Energy Milling (HEM) or Co-Precipitation (CP), listed by molar ratio.

Preliminary Transmission Electron Microscopy (TEM) measurements were carried out on ternary mixed oxides containing Cu2O and In2O3. TEM micrographs show that both techniques (HEM and CP) were able to produce single particles whit linear size below 100 nm, which aggregate into sub-micrometric clusters, which show some differences. While in HEM prepared samples, clusters are produced by association of particles upon collision, with relevant presence of amorphous phase and no precise morphology, in CP prepared samples clusters are produced by stacking of well-formed particles of cubic morphology [24], which is common to crystal lattice of both component binary oxides [25,40], and particles clearly show a core-shell coverage.

Powder X-Ray Diffraction (XRD) measurements, used to study crystallinity and crystal phase of selected ternary oxides samples, allowed determine that, while samples prepared by HEM miss any crystallinity, samples prepared by CP show small size crystalline domains, with Cu2O is in its usual cubic phase and Fe2O3 in its rhombohedral α phase (hematite).
