Green Synthesis of Z-Scheme SnO ₂ /CdS Heterostructures: Density Functional Theory Calculation and Photocatalytic CO ₂ Reduction to Methanol and Hydrogen

Converting CO₂ into valuable fuels through photocatalysis offers a sustainable strategy to address the energy shortage and decrease greenhouse gas emissions. This work studied bio-based Z-scheme SnO₂/CdS heterojunction photocatalysts (5.7 nm, 120.6 m² g^-1; conduction band: -0.96 V) synthesized using analytes found in <i>Aegle marmelos</i> via microwave irradiation for efficient CO₂ reduction into value-added chemicals. The heterojunction demonstrates a notably higher CO₂ adsorption ability (0.4 mmol g^-1) and photocurrent response (0.74 μA cm^-2) than both CdS QDs(bio) and SnO₂(bio). It also improves charge carrier dynamics, as evidenced by 60 and 56.1% reductions in PL intensity and 48.9 and 59.2% increases in charge carrier lifetime, respectively. The Fermi level difference between CdS (-4.51 eV) and SnO₂ (-5.52 eV) creates an internal electric field that promotes charge separation and transport, inhibits recombination, and protects CdS QDs(bio) from photocorrosion (SO₄^2-). Density functional theory analysis reveals that SnO₂ incorporation into CdS reduces the bandgap (2.38 → 2.15 eV), introduces additional electronic states near the Fermi level (<i>E</i>_F), and shifts the conduction band density of states closer to the <i>E</i>_F compared to CdS QDs, indicating a higher density of active states that enhance interfacial charge transfer. The optimized 0.50SnO₂/CdS QDs(bio) heterostructure exhibits outstanding photocatalytic activity for CO₂ reduction, achieving methanol and hydrogen production rates of 675.9/139.5 μmol g^-1 h^-1 (apparent quantum yields of 3.51 and 0.24%) while maintaining its structural and morphological stability.