S-scheme p-n junction Na0.6CoO2/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 Na0.6CoO2/g-C3N4 S-scheme photocatalyst via electrostatic self-assembly technology. A significant hydrogen production rate of ∼ 0.294 mmol g-1h−1 was achieved on the optimal NCO-20/CN, which was ten times higher than that of pure g-C3N4. In-situ XPS showed that the electrons in Na0.6CoO2/g-C3N4 had different flow directions without and with illumination, demonstrating a built-in electric field being formed through Na0.6CoO2 and g-C3N4 interaction. DFT calculations and ultraviolet photoelectron spectroscopy verified that g-C3N4 and Na0.6CoO2 possess the energy band structures conforming to the heterostructure of S-scheme. In-situ Kelvin probe microscope studies show that Na0.6CoO2 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 Na0.6CoO2 (−30 mV), the average surface contact potential difference of Na0.6CoO2/g-C3N4 reached 320 mV, yielding a higher efficiency of photogenerated electron separation. This work provides direct evidence on the existence of a built-in electric field and an electron flow direction for heterostructure materials.