Plant-Derived Phytochemicals for the Synthesis of p–n Junction CuO/CdS Heterostructures for Photocatalytic Carbon Dioxide Reduction to Ethanol and Carbon Monoxide

Photocatalytic reduction of CO2 to value-added fuels is a promising strategy to mitigate the energy crisis and reduce greenhouse gas emissions. Herein, a bio-based zero-dimensional (0D) p–n heterojunction CuO/CdS photocatalyst (7.2 nm, 136.65 m2/g) with a conduction band of −1.12 V was synthesized using bio-analytes from Aegle marmelos via microwave irradiation. The p–n heterojunction enhanced the CO2 adsorption capacity (0.643 mmol/g) and photocurrent response (0.94 μA/cm2) compared to CdS QDs(bio) and CuO QDs(bio). Additionally, it improved charge carrier dynamics by reducing PL intensities by 73 and 67% and increasing decay times by 74 and 54.6%, respectively. The internal electric field generated by the Fermi level difference between n-type CdS (−4.21 V) and p-type CuO (−4.7 V) enhanced charge separation and transport, suppressed recombination, and prevented photocorrosion (SO42–) of CdS QDs(bio). Density functional theory (DFT) analysis revealed alterations in the density of states (DOS) of CdS within the band gap region due to the incorporation of CuO, further facilitating efficient charge separation and transport at the local junctions. The optimal 0.50CuO/CdS QDs(bio) heterostructure exhibited remarkable photocatalytic performance for CO2 reduction, achieving an ethanol (EtOH) and carbon monoxide (CO) production rate of 158.48/182.68 μmol/g/h (AQY 8.24/1.58%) while maintaining its structural and morphological stability. This work highlights the potential of bio-based p–n junction photocatalysts for efficient CO2 reduction into value-added chemicals.