Single Cobalt Atom-Integrated Z-Scheme CdS@PCN-222 Heterojunction for Enhanced Syngas Production from Solar-Driven CO2 Reduction

In this study, a series of CdS@PCN-222-Co with different weight percentages of CdS (denoted as PCS-x, where x = 10, 20, and 30 wt %) were synthesized via a solvothermal method for enhanced photocatalytic CO₂ reduction under visible light. PCN-222-Co with a nanorod structure, featuring single cobalt (Co) atoms in porphyrin rings, served as an efficient photocatalyst with Co as active sites, while CdS nanoparticles facilitated charge separation through the formation of a Z-scheme heterojunction. Among the composites, PCS-20 (i.e., 20 wt % CdS@PCN-222-Co) demonstrated the highest photocatalytic activity, achieving CO and H₂ production rates of ∼636.42 and 1361.81 μmol·g^–1·h^–1, respectively, in 4 h. Transmission electron microscopy (TEM) revealed a reduction in the size of the composite nanorods compared with bare PCN-222-Co, indicating morphological changes during the composite formation. Furthermore, an investigation using elemental mapping confirms the presence and high distribution of Co single atoms in the composite. A combination of UV–visible spectroscopy, X-ray diffraction, BET surface area analysis, and electrochemical studies revealed that the enhanced performance is due to the synergistic effect of the Z-scheme heterojunction and the well-aligned band structure between CdS and PCN-222-Co, which promotes efficient charge separation, migration, and strong redox reactions. Furthermore, Mott–Schottky, steady-state, and time-resolved photoluminescence analyses confirmed the improved charge carrier dynamics in the composite, with PCS-20 displaying the lowest charge transfer resistance and highest photocurrent density along with an improved carrier lifetime of ∼2.12 ns compared to bare CdS which is ∼1.07 ns. The study also identified the optimal reaction conditions, confirming the necessity of triethanolamine (TEOA) as a sacrificial agent and highlighting the stability of the composite over multiple cycles. These findings provide a promising strategy for designing efficient photocatalytic systems for the sustainable conversion of CO₂ into valuable products through photocatalysis.