One-step ultrasonic-assisted synthesis of Ni-doped g-C3N4 photocatalyst for enhanced photocatalytic hydrogen evolution

Polymeric graphite carbon nitride (g-C3N4), as a nonmetallic organic semiconductor, inexpensive, stable, and active photocatalyst, has received significant consideration in the last two decades. However, the prospective uses of g-C3N4 are constrained by poor charge separation and low visible light absorption efficiency. This paper demonstrates Ni-doped g-C3N4 photocatalysts prepared via a facile ultrasonic-assisted route using nickel acetate and melamine as precursors. Several characterization techniques such as XRD, XPS, HRTEM, UV–vis diffuse reflectance spectra, and electrochemical experiments were used to study the relationship between structure and photocatalytic properties. XRD indicates that several peaks associated with NiO, and metallic Ni (Ni0) have developed, and the defect structure in the high-doped samples is responsible for reducing the photocatalytic activity. HRTEM demonstrated the existence of metallic nickel (Ni0) in the Ni-doped g-C3N4 photocatalysts that could be established through the reduction of Ni2+ in the samples doped with a small amount of Ni and containing a higher amount of g-C3N4. XPS revealed that Ni2+ is the major nickel species in the low-doped samples, whereas metallic nickel and NiO are the predominant nickel species in the high-doped samples. Importantly, the Mott-Schottky analysis verified the negative shift of the flat band potential of Ni-doped g-C3N4, proving that doping with Ni causes the surface band of g-C3N4 to bend more deeply, which leads to enhance charge separation efficiency and boosts the photocatalytic efficiency. Photocatalytic hydrogen production rates of Ni-doped g-C3N4 photocatalysts were assessed through water splitting experiments and demonstrates that the optimized Ni-doped g-C3N4 (2.4 wt-% Ni) exhibited the highest activity that reached 995 μmol h−1, which by a factor of about 3 times in comparison of pure g-C3N4 (369 μmol h−1). This remarkable improvement can be ascribed to the role of the Ni(II) ion in accelerating the electron transport rate and enhancing the visible light absorption of g-C3N4.