Oxygen vacancy and p–n heterojunction in a g-C3N4 nanosheet/CuFeO2 nanocomposite for enhanced photocatalytic N2 fixation to NH3 under ambient conditions

Abstract

The process of photocatalytic nitrogen fixation alongside solar energy is a promising alternative strategy to existing industrial methods of ammonia production. Despite the mild and low-cost conditions, dissatisfactory yield rates have prevented its widespread development. In this context, we have fabricated a p–n heterojunction of g-C₃N₄ nanosheets/CuFeO₂ photocatalysts with different weight percentages of CuFeO₂ using a simple hydrothermal method and evaluated its photocatalytic performance for nitrogen fixation. The average crystallite size, lattice strain, and dislocation density were determined using XRD data. Morphological assessment using FESEM, TEM, and HRTEM images showed the presence of CuFeO₂ nanoparticles on the exfoliated g-C₃N₄ nanosheets. The g-C₃N₄ nanosheet/CuFeO₂15 (15 refers to the weight percentage of CuFeO₂) nanocomposite achieved optimal ammonium production (4560 μmol g⁻¹ L⁻¹), which was 4.3 and 2.5 times higher than that of pure g-C₃N₄ nanosheets and CuFeO₂, respectively. One of the important factors of an acceptable performance is related to oxygen vacancies in the nanocomposite structure, which was confirmed by XPS and EPR analyses. Oxygen vacancies allow the chemisorption and activation of nitrogen molecules and result in the unusually high efficiency of ammonia production by breaking the NN triple bond. Also, the matched energy band structure, p–n heterojunction formation, and separation of charge carriers, which are other important factors for improving photocatalytic activity, were investigated by electrochemical analyses. The optical properties of the nanocomposite confirmed that the g-C₃N₄ nanosheets after coupling with CuFeO₂ were able to absorb more light, which was in favor of photocatalytic activity. However, the aggregation of nanoparticles at higher weight percentages of CuFeO₂ reduced the active sites and subsequently the photocatalytic activity. Based on the obtained results and the stability of the prepared photocatalyst after successive use in nitrogen fixation, it is expected to undergo future development.