Eco-Friendly Coffee Waste-Based Carbon Dots Coupled to ZnBi-Layered Double Hydroxide Heterojunction: Enhanced Control of Interfacial Charge Transfer for Highly Efficient Visible-Light Catalytic Activity

Using a simple, low cost, and excellent efficient approach, carbon dots (CDs) were fabricated via a one-pot hydrothermal process of coffee waste. Amazingly, the combination of 2% CDs with ZnBi₂O₄ to form a new and excellent heterogeneous photocatalyst enabled the complete decomposition of 2, 4-dichlorophenoxyacetic acid (2, 4-D) into CO₂ and H₂O. The findings of this study provide a new perspective on the utilization of agricultural waste for creating products of scientific and practical significance. More than 91% of 2, 4-D (initial concentration of 30 mg/L) was completely decomposed and dechlorinated using 1.0 g/L of CDs (2%)-ZnBi₂O₄ at pH 4.0 after 120 min of exposure to visible light (with k = 0.0178 min⁻¹), and more than 86% of the decomposed 2, 4-D was mineralized into CO₂ and H₂O. There was no sign of catalyst deactivation after four cycles of reuse, demonstrating the durability and efficiency of CDs (2%)-ZnBi₂O₄. The significant improvement in the photocatalytic efficiency of CDs (2%)-ZnBi₂O₄ compared with that of bare CDs or ZnBi₂O₄ is due to the formation of defects at the interfaces of the heterojunction; therefore, the movement of photogenerated electrons at the interface between the two components is rapid. The mineralization of 2, 4-D by CDs (2%)-ZnBi₂O₄ upon exposure to visible light is induced mainly by the photogenerated holes, followed by O₂^•−, and finally OH^• radicals.