Engineered synthesis of a novel bixbyite-structured high-entropy oxide (Ce0.2Zr0.2Yb0.2Er0.2Gd0.2)2O3.4 as a stable and high-performing visible-light-active photocatalyst for multifunctional pollutant degradation

This study presents a novel bixbyite-structured High Entropy Oxide (HEO), (Ce₀.₂Zr₀.₂Yb₀.₂Er₀.₂Gd₀.₂)₂O₃.₄, synthesized through a straightforward and efficient co-precipitation method in a carbonate environment, as a next-generation photocatalyst for the removal of water pollutants under visible light irradiation. Calcination at 750 °C induced the formation of the entropy-stabilized HEO bixbyite single-phase as displayed by the diffraction pattern, characterized by a homogeneous distribution of the cations. The photocatalytic activity of (Ce₀.₂Zr₀.₂Yb₀.₂Er₀.₂Gd₀.₂)₂O₃.₄ was assessed by examining the influence of pH, photocatalyst dosage and irradiation time on the degradation of methylene blue (MB). The optimal conditions for MB degradation were at pH = 11, with a catalyst dose of 1 g L⁻¹ and a reaction time of 180 min, achieving an MB degradation and mineralization efficiency of approximately 99 and 45 %, respectively. ANOVA analysis revealed that the MB removal efficiency was significantly influenced by the individual positive effects of irradiation time and pH, further confirming that the photocatalytic performance of (Ce₀.₂Zr₀.₂Yb₀.₂Er₀.₂Gd₀.₂)₂O₃.₄ is strongly dependent on the adsorption of the target contaminant onto its surface. Analysis of the (Ce₀.₂Zr₀.₂Yb₀.₂Er₀.₂Gd₀.₂)₂O₃.₄ band structure and reactive oxygen species involved in MB photodegradation (superoxide and hydroxyl radicals) suggested that this material acts as a direct band gap semiconductor. Moreover, the catalyst exhibited good stability and reusability over multiple cycles. The (Ce₀.₂Zr₀.₂Yb₀.₂Er₀.₂Gd₀.₂)₂O₃.₄ photocatalyst was also capable of degrading gallic acid (a colorless pollutant), achieving near-complete degradation of this pollutant after 180 min of irradiation time without altering the solution pH and excluding possible photosensitization phenomena of (Ce₀.₂Zr₀.₂Yb₀.₂Er₀.₂Gd₀.₂)₂O₃.₄ by MB molecules. These results highlight the potential of (Ce₀.₂Zr₀.₂Yb₀.₂Er₀.₂Gd₀.₂)₂O₃.₄ as a highly efficient and durable visible light-driven photocatalyst for water purification, particularly in the removal of dye pollutants, but also in the degradation of other water contaminants like gallic acid at the spontaneous pH of the polluted water, offering a promising solution for multipurpose wastewater treatment.