S-scheme p-n junction Na_0.6CoO₂/g-C₃N₄ heterostructure as an efficient photocatalyst for green hydrogen production: fabrication, characterization and mechanisms

Probing the spatial separation and transport process of photogenerated charges at nanoscale interfaces is essential for understanding catalytic reaction mechanisms on heterostructure photocatalysts. Here, we developed a p-n junction Na_0.6CoO₂/g-C₃N₄ S-scheme photocatalyst via electrostatic self-assembly technology. A significant hydrogen production rate of ∼ 0.294 mmol g⁻¹ h⁻¹ was achieved on the optimal Na_0.6CoO₂/g-C₃N₄, which was ten times higher than that of pure g-C₃N₄. In-situ XPS shows that the electrons in Na_0.6CoO₂/g-C₃N₄ had different flow directions without and with illumination, demonstrating a built-in electric field being formed through Na_0.6CoO₂ and g-C₃N₄ interaction. DFT calculations and ultraviolet photoelectron spectroscopy verified that g-C₃N₄ and Na_0.6CoO₂ possess the energy band structures conforming to the heterostructure of S-scheme. In-situ Kelvin probe microscope studies show that Na_0.6CoO₂ and g-C₃N₄ both have a self-induced electric field effect, and their combination significantly strengthens the built-in electric field and improves the space separation of photogenerated electrons. Compared with the change of the surface photovoltage of g-C₃N₄ (60 mV) and Na_0.6CoO₂ (−30 mV), the average surface contact potential difference of Na_0.6CoO₂/g-C₃N₄ reached 320 mV, yielding a higher efficiency of photogenerated electron separation. This work also provides direct evidence on the existence of a built-in electric field and an electron flow direction for heterostructure photocatalyst materials.