Bandgap and adsorption engineering of carbon dots/TiO₂ S-scheme heterojunctions for enhanced photocatalytic CO₂ methanation

S-scheme heterojunctions have garnered significant interest in photocatalytic CO₂ conversion to valuable products (e.g., CH₄) due to their enhanced charge separation and robust redox capabilities. Carbon dots (CDs), with their tunable band structures and light absorption ranges, show particular promise in constructing efficient S-scheme photocatalytic systems. Nevertheless, the critical roles of CDs' band alignment and surface adsorption properties in determining heterojunction configuration, charge carrier kinetics, and ultimately CO₂ activation/product selectivity distribution remain insufficiently explored. Herein, we construct four CDs/TiO₂ heterojunctions using CDs synthesized from varied carbon sources, in which S-scheme heterojunctions were successfully constructed based on cost-effective coal pitch (C-GQDs, 1.75 nm), glucose (G-CQDs, 1.84 nm), and acetone (CQDs-X, 1.82 nm) carbon sources, whereas Type-I heterojunctions were formed by carbon black based CDs (GQDs-A, 1.92 nm). Systematic investigations reveal that both the band structure and adsorption characteristics of CDs play important roles in the charge transfer path and separation efficiency, CO₂ adsorption and activation capacities, and product selectivity in photocatalytic CO₂ reduction. Remarkably, the introduction of CDs significantly broadens the photo-response range compared to fresh TiO₂, and in particular, the C-GQDs/TiO₂ exhibits exceptional performance with a CH₄ production rate of 32.7 μmol·g⁻¹·h⁻¹, surpassing TiO₂ by 6.3-fold and outperforming GQDs-A/TiO₂, CQDs-X/TiO₂, and G-CQDs/TiO₂ by factors of 3.8, 2.7, and 2.3, respectively. This heterojunction simultaneously achieves 72.6 % CH₄ selectivity and 98.1 % hydrocarbons selectivity (encompassing CH₄, C₂H₆, C₂H₄, and C₃H₈). In contrast, composites incorporating GQDs-A, CQDs-X, or G-CQDs exhibit substantially diminished CH₄ selectivity (<40.0 %). The high CH₄ production rate and selectivity of C-GQDs/TiO₂ can be attributed to its unique S-scheme heterojunction structure, higher reduction potential, and well-matched CO₂ and H₂O adsorption and activation capabilities. This study provides unique insights into the efficient photoreduction of CO₂ to CH₄ driven by the S-scheme heterojunction electron transfer pathway in CDs/TiO₂ photocatalysts.