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
Antonietta Mancuso, Katia Monzillo, Olga Sacco, Luca Spiridigliozzi, Viviana Monfreda, Andrea Muscatello, Gianfranco Dell’Agli, Serena Esposito, Vincenzo Vaiano
Abstract
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 .