Fe- and Ni-Stannate-Integrated g-C ₃ N ₄ : A Dual S-Scheme Heterojunction for Boosted Hydrogen Evolution via Water Splitting

Dual S-scheme heterojunctions have emerged as a highly promising photocatalytic architecture, offering significant improvements in charge transfer efficiency while simultaneously mitigating electron–hole recombination. In this study, we construct a transition metal-based, titanium-free (Ti-free) dual S-scheme heterojunction at the interface of graphitic carbon nitride (g-C 3 N 4 ) and metal stannates of iron (Fe) and nickel (Ni). The FeSnO 3 /g-C 3 N 4 /NiSnO 3 photocatalyst is fabricated via a straightforward ultrasonication-calcination approach and comprehensively characterized using a range of physico-chemical, optical, and electrochemical techniques. The synthesized photocatalyst was loaded with ∼0.38% (w/w) Pt used as a cocatalyst. The photocatalyst shows a relatively enhanced hydrogen (H 2 ) production rate of 1350 μmol·(g·h) −1 during water splitting, outperforming NiSnO 3 /g-C 3 N 4, FeSnO 3 /g-C 3 N 4, and g-C 3 N 4 by factors of 2.0, 2.3, and 6.5, respectively. Remarkably, it also exhibits exceptional stability, with less than a 5% decline in activity after ten reaction-regeneration cycles. The underlying dual S-scheme charge transfer mechanism is validated through advanced techniques, including Kelvin probe force microscopy, electron paramagnetic resonance, and radical scavenging experiments. These results highlight the synergistic potential of anchoring Fe- and Ni-based stannates onto g-C 3 N 4, offering the robust and cost-effective route to high-performance photocatalytic materials for sustainable H 2 production.