Oxygen Vacancy-Engineered SnO _2–x Coupled with Ultrathin CoAl-Layered Double Hydroxide: An Efficient S-Scheme Heterojunction for Photocatalytic and Electrocatalytic H ₂ Generation

In the present work, we developed a bifunctional photocatalytic and electrocatalytic system for hydrogen evolution by synthesizing a CoAl-LDH/SnO2–x nanocomposite via a hydrothermal route followed by formamide-assisted exfoliation. The integration of oxygen-deficient SnO2–x nanoparticles with ultrathin CoAl-LDH nanosheets resulted in the formation of an n–n type S-scheme heterojunction. XPS and EPR analyses confirmed the presence of oxygen vacancies, which play a key role in tailoring the electronic structure, enriching the electron density, and improving charge separation. Benefiting from these features, the composite exhibited a markedly enhanced hydrogen production rate of 727.21 μmol g–1 h–1, surpassing that of the pristine counterparts. In electrocatalytic HER, UCAL/SnO2–x-2 achieved a low overpotential of 137 mV at 10 mA cm–2, a small Tafel slope of 127 mV dec–1, and the lowest charge transfer resistance among all tested samples. Furthermore, the catalyst demonstrated long-term stability over 100 h and a high Faradaic efficiency of 85.3%. These results highlight the synergistic role of oxygen vacancies and S-scheme heterojunctions in enhancing catalytic activity, providing a robust and efficient strategy for the rational design of advanced photo- and electrocatalysts for sustainable hydrogen production.