Silver grafted graphitic-carbon nitride ternary hetero-junction Ag/gC3N4(Urea)-gC3N4(Thiourea) with efficient charge transfer for enhanced visible-light photocatalytic green H2 production

Highlights: • Ag/gC₃N₄(U)/gC₃N₄(T) photocatalyst is successfully synthesized for the first time. • Ag/gC₃N₄(U)/gC₃N₄(T) forms S-scheme heterojunction and boosts H₂ production. • Existence of N-vacancies improve carrier charge separation and transportation. • The highest H₂ evolution rate over Ag/gC₃N₄(U)/gC₃N₄(T) was 10.1 mmol g⁻¹ h⁻¹. • Ag/gC₃N₄(U)/gC₃N₄(T) exhibited excellent reusability and chemical stability. The efficient and visible-light-active graphitic carbon nitride (gC₃N₄) has attracted attention for green H₂ production from solar energy through water splitting. However, the photocatalyst suffers faster recombination of photogenerated electron-hole pairs and a low visible light absorption efficiency. In this work, the ternary Ag-grafted gC₃N₄-gC₃N₄ heterostructures were synthesized using thermal polycondensation of urea and thiourea, followed by photo-deposition of the silver from silver nitrate. The formation of S-scheme heterojunction was confirmed through XRD, VB XPS, UV–Vis, Mott-Schottky, EPR and PL analyses. The X-ray photoelectron spectroscopy revealed the presence of nitrogen vacancies with Ag grafting. The H₂ production rate was the highest at 10.1 mmol g⁻¹h⁻¹ with 2.5Ag/gC₃N₄-gC₃N₄, which was 3 and 8 times higher than that over gC₃N₄-gC₃N₄ and gC₃N₄, respectively. The addition of Ag mainly contributed to enhance the photocatalytic activity of the heterojunction due to its dual-function. Firstly, silver is attributed to enhance the localized surface plasmon resonance, which broadened the visible light absorption and secondly, the abundant electron capture due to the high Schottky barrier. Moreover, the existence of nitrogen vacancies further improved the separation of charge carriers and helped in carrier transportation. This work provides a new S-scheme heterostructure strategy for the sustainable utilization of solar radiation in the production of H₂ from water splitting at higher rates.