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 Na 0.6 CoO 2 /g-C 3 N 4 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 0.6 CoO 2 /g-C 3 N 4 , which was ten times higher than that of pure g -C 3 N 4 . In-situ XPS shows that the electrons in Na 0.6 CoO 2 / g -C 3 N 4 had different flow directions without and with illumination, demonstrating a built-in electric field being formed through Na 0.6 CoO 2 and g -C 3 N 4 interaction. DFT calculations and ultraviolet photoelectron spectroscopy verified that g -C 3 N 4 and Na 0.6 CoO 2 possess the energy band structures conforming to the heterostructure of S-scheme. In-situ Kelvin probe microscope studies show that Na 0.6 CoO 2 and g -C 3 N 4 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 -C 3 N 4 (60 mV) and Na 0.6 CoO 2 (−30 mV), the average surface contact potential difference of Na 0.6 CoO 2 / g -C 3 N 4 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.