Tailoring selective absorption performance based on tetrahedral/octahedral sites distribution via La doping in high-entropy (Fe0.25Co0.25Mn0.25Cu0.25)3O4

Solar-absorbing coatings that combine high solar absorptance and reduced infrared emittance are essential for high-temperature solar-thermal applications. However, achieving both properties simultaneously remains challenging. Here, we report spray-coated La-doped (Fe 0.25 Co 0.25 Mn 0.25 Cu 0.25 ) 3 O 4 coatings, fabricated from as-synthesized nanoparticles, with concurrently tailored solar absorptance and infrared emittance. La 3+ incorporation produces a slight expansion of the interplanar spacing and induces cation redistribution from tetrahedral to octahedral sites. These changes in site occupancy alter the cation valence distribution, resulting in a narrowed band gap and reduced local structural disorder. When the La 3+ concentration exceeds 0.5 mol%, a secondary LaFeO 3 phase is formed due to the limited solubility of La in the spinel lattice. At optimal La 3+ concentration of 0.5 mol%, the coating exhibits a solar absorptance of 94.7% (0.2–2.5 μm) and an infrared emittance of 84.0% (2.5–25 μm) derived from room-temperature reflectance, representing a modest reduction from 87.1% for the undoped coating. After thermal aging at 750 °C for 200 h, the coating exhibited a solar absorptance of 94.9% and a photothermal conversion efficiency of 89.0%, calculated using the emittance derived from reflectance measured at 750 °C. The findings provide a promising strategy for balancing solar absorptance and infrared emittance in solar-absorbing coatings for solar-thermal applications.