Hydrothermally engineered NiO–ZnO–In2O3 ternary heterojunction for enhanced visible-light photocatalytic degradation of methylene blue and rhodamine B

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The development of efficient and stable photocatalysts for wastewater remediation remains a significant environmental challenge. In this study, NiO, NiO–ZnO, and a ternary NiO–ZnO–In₂O₃ heterojunction were synthesized via a hydrothermal method and evaluated for the photocatalytic degradation of methylene blue (MB) and Rhodamine B (RhB) under UV–visible-light irradiation. Structural and morphological analyses confirmed the formation of well-crystallized heterostructures with reduced particle size and improved surface uniformity. Optical studies revealed a pronounced red shift with band gap narrowing from 3.2 eV (NiO) to 2.4 eV for the ternary heterojunction, enabling enhanced visible-light absorption. Among the studied photocatalysts, NiO–ZnO–In₂O₃ exhibited superior performance, achieving 96% degradation of MB and 89% of RhB within 100 min, following pseudo-first-order kinetics. The enhanced activity is attributed to efficient heterojunction formation, which facilitates charge separation and suppresses electron–hole recombination, resulting in increased generation of reactive oxygen species. Scavenger experiments identified superoxide radicals (•O₂⁻) as the dominant reactive species, with additional contributions from hydroxyl radicals (•OH) and holes (h + ). The photocatalytic efficiency was further influenced by catalyst dosage and alkaline pH, while reusability tests demonstrated excellent stability over four cycles. Overall, these findings highlight the effectiveness of ternary heterojunction engineering in enhancing photocatalytic performance and underscore its potential for practical wastewater treatment applications.