Sustainable low temperature preparation of Mn_3−xCo_xO₄ (0 ≤ x < 3) spinel oxide colloidal dispersions used for solar absorber thin films

The preparation of pure crystalline oxide nanoparticles (with controlled composition, size and shape) and formation of stable suspensions free of complex organic precursors was developed and optimized at room temperature (or below 100 °C). This reproducible water and ethanol synthesis and solution stabilization of oxide nanoparticles is based on Mn_3−xCo_xO₄ (0 ≤ x < 3) composition materials. To our knowledge, this is the first study on the complete Mn–Co–O spinel system synthesized at low temperature. The main hydrodynamic parameters, as well as the physical and chemical properties that control the oxide precipitation and nanoparticle size and morphology were characterized in detail for the family end member Mn₃O₄ and used for the other compositions. X-ray diffraction and Scanning Electron Microscopy images showed the influence of the alkaline solution concentration, pH, temperature and solvent on the nanoparticles properties. Neutron diffraction was used for determining the cationic distribution in two compositions, i.e. CoMn₂O₄ and MnCo₂O₄. While the tetrahedral site is mainly occupied by Co²⁺, four types of cations were determined for the octahedral sites. Zeta potential and rheological measurements were performed to determine the stability region of nanoparticles in aqueous solution. This innovative and low cost process was used to produce homogenous and crystalline metal oxide thin films that can be used as solar absorbers in various applications. Their optical properties were characterized. A second absorption edge, due to cobalt and observed in the visible region, is attributed to an intermediate band gap, which is a very important feature, especially for future solar cells. This sustainable synthesis of oxide nanoparticles and thin film preparation procedure is applicable to other oxide families. - Graphical abstract: Display Omitted - Highlights: • Sustainable low temperature synthesis of oxide nanoparticles. • Stabilization of colloidal dispersions free of organic precursors or surfactants. • Influence of hydrodynamic parameters and physicochemical properties. • Thin films preparation by the dip-coating method and their optical properties. • Promising method for enhanced efficiency in the preparation and use of nanomaterials