Enhanced photocatalytic performance of Ag₃PO₄ modified g-C₃N₄ composites for hydrogen production

As a green and eco-friendly clean energy source, hydrogen energy has the advantages of high energy density and no pollution to the environment. Due to the direct utilization of solar energy, photocatalytic water splitting for hydrogen production is a promising technology. g-C₃N₄ could be utilized to achieve hydrogen through water splitting. However, g-C₃N₄ material has some disadvantages such as a small specific surface area, fast recombination of electron and hole pairs generated by photocatalysis, and insufficient photo-responsiveness, which greatly limits its photocatalytic performance. Ag₃PO₄, as one of the new silver-based photocatalysts, has the advantages of high quantum yield and narrow bandgap. Coupling polyhedral Ag₃PO₄ with two-dimensional g-C₃N₄ material to form a heterojunction to enhance photocatalytic stability should be feasible. In this paper, silver phosphate modified graphitic carbon nitride (Ag₃PO₄/C₃N₄) binary composite photocatalysts were prepared by a high-temperature calcination and in-situ deposition-precipitation method, which were employed for photocatalytic hydrogen production. The effects of Ag₃PO₄ loading, the type of sacrificial agents and the amount of sacrificial agent on hydrogen production were studied. Meanwhile, various techniques such as XRD, FI-TR, XPS, SEM, UV-vis DRS, PL, etc. were carried out to analyze the physical and chemical characteristics of the photocatalysts. The results show that when the Ag₃PO₄ mass loading is 4%, the hydrogen production rate of Ag₃PO₄-4/C₃N₄ is the highest, reaching 218.97 μmol, which is 3.1 times and 58.4 times higher than that of C₃N₄ and Ag₃PO₄, respectively. Compared with methanol, glycerol and lactic acid, triethanolamine is the best sacrificial agent for Ag₃PO₄/C₃N₄ photocatalyst, but an excessive triethanolamine cannot further improve the hydrogen production rate. No obvious absorption peak of Ag₃PO₄ is observed in the FT-IR spectra, which could be due to the low content of Ag₃PO₄. While with the increase of Ag₃PO₄ loading, the diffraction peak of C₃N₄ (002) crystal plane decreases gradually, reflecting an excellent dispersion of Ag₃PO₄ on C₃N₄ surface and an effective coupling of Ag₃PO₄ and C₃N₄. The shift in binding energies of the researched elements indicates a strong interaction between C₃N₄ and Ag₃PO₄. After loading a small amount of Ag₃PO₄, the morphology of C₃N₄ changes from the original rocky block shape to a multi angular flower shape, and its specific surface area increases. Moreover, Ag₃PO₄ is highly dispersed on the surface of C₃N₄ in the form of nanoparticles. Ag₃PO_4-4/C₃N₄ exhibits a high transient photocurrent intensity, confirming that C₃N₄ and Ag₃PO₄ form a heterojunction, effectively enhancing the separation of photo-generated electron-hole pairs in Ag₃PO₄-4/C₃N₄. The large specific surface area, enhanced light absorption characteristics and effective separation and transfer of photogenerated electron-hole pairs are important factors for efficient photocatalytic hydrogen production of Ag₃PO₄/C₃N₄ composites. Based on the characterization and experimental results, an S-type heterojunction photocatalytic hydrogen production mechanism is proposed. Under illumination, the cocatalyst H₂PtCl₆ in the reaction solution is reduced to Pt⁰. Due to the SPR effect of Pt metal, some e⁻ are transferred to Pt⁰, which combines with H⁺ to accelerate the photocatalytic hydrogen production reaction rate. At the same time, a small amount of Ag₃PO₄ is irradiated to precipitate Ag⁰ as a silver bridge, promoting the recombination of e⁻ generated by Ag₃PO₄ with h⁺ at the VB site of C₃N₄ photocatalyst. The h⁺ accumulated by Ag₃PO₄ photocatalyst at VB oxidizes triethanolamine to TEOA⁺. This S-shaped heterojunction charge transfer method helps to improve the separation speed of electron-hole pairs and enhance the photocatalytic hydrogen production performance.