Bandgap-engineered carbon quantum dots: eco-friendly synthesis from citrus aurantium juice and enhanced photocatalytic degradation of malachite green

Conventional semiconductor quantum dots pose toxicity and environmental risks due to their heavy-metal content, limiting their sustainable use in photocatalysis and optoelectronics. To overcome this, we developed a green, one-pot hydrothermal method to synthesize carbon quantum dots (CQDs) from Citrus aurantium juice as a renewable carbon source, enabling controlled bandgap tuning and efficient visible-light degradation of malachite green. The CQDs, with diameters of 2–6 nm, exhibit blue–green photoluminescence under UV light. Incorporating urea and polyethylene glycol into the precursor modulates the bandgap from 2.5 to 3.85 eV. Under visible light, the degradation efficiencies reach 83.12% for pure CQDs, 96% with urea, and 77% with polyethylene glycol. These results confirm the method’s effectiveness for tailoring optical properties and enhancing photocatalytic activity. Overall, this study introduces a simple, cost-effective, and eco-friendly route for CQD synthesis with promising potential in photocatalytic and optoelectronic applications.