Sol-gel synthesized rare-earth high-entropy oxide for efficient pH dependent photocatalytic dye degradation

Abstract

High-entropy oxides (HEOs) are increasingly recognized as multifunctional materials that contain various cations in nearly equal proportions, forming stable, single-phase crystalline materials. Their high configurational entropy, complex defect chemistry, and structural flexibility make them a promising option for photocatalytic applications. This study reports the first synthesis of a rare-earth high-entropy oxide with the formula (Er_0.2Yb_0.2Sm_0.2Gd_0.2Dy_0.2)₂O₃, using a simple sol-gel citrate method. The resulting powder was characterized by a single-phase bixbyite structure and nanosized particles, Raman spectroscopy confirmed the rare-earth oxide framework through characteristic M - O vibrational modes. Optical analysis showed an optical band gap of 4.5 eV. The photocatalytic degradation of methylene blue (MB) under UV light was tested over a broad pH range of 3-12 and the catalyst demonstrated effectiveness, with a degradation rate of 94% within 90 min of exposure. The degradation was fastest under mild acidic conditions, likely due to increased dye adsorption and radical formation. Nanoscale morphology coupled with numerous surface-active sites, and entropy-stabilized defect environment contributed to effective charge separation and enhanced photocatalytic performance. Catalyst stability and recyclability tests at pH 7 confirmed retained photocatalytic activity over three cycles, and retention of the main crystalline phase in the system. This shows the potential of rare-earth bixbyite-type HEOs as stable photocatalysts for wastewater treatment.