Green preparation of nitrogen vacancies enriched g-C3N4 for efficient photocatalytic reduction of CO2 and Cr(VI)

Abstract

Introducing vacancies has emerged as one of the valid strategies to modulate the photocatalytic performance of graphitic carbon nitride (g-C₃N₄). Introduction of nitrogen vacancies into g-C₃N₄ can create defect energy levels and trap electrons, consequently accelerating the separation of e⁻/h⁺ pairs and effectively boosting photocatalytic activity. Nitrogen vacancies can also serve as adsorption active sites, enhancing the adsorption capacity of the catalyst towards the target molecule (CO₂). In this study, a series of g-C₃N₄ with abundant nitrogen vacancies were prepared using a green and facile strategy using sodium bisulfite treatment. Successful introduction of nitrogen vacancies endows with the photocatalyst more active sites, optimizes the band structure, significantly boosts the separation of photoexcited carriers, thereby remarkably enhancing photocatalytic CO₂ and Cr(VI) reduction. On the 11CN photocatalyst (0.5 g g-C₃N₄ was treated by 11 g sodium bisulfite), the generation rate of CO and CH₄ is 5.74 μmol·g⁻¹·h⁻¹ and 1.30 μmol·g⁻¹·h⁻¹, respectively, which is 3.19 times and 8.29 times higher than those on the reference g-C₃N₄ (CO: 1.37 μmol·g⁻¹·h⁻¹, CH₄: 0.14 μmol·g⁻¹·h⁻¹). Under irradiation by three distinct monochromatic lights, the apparent quantum yield (AQY) of 11CN is also superior to that of the reference g-C₃N₄. Moreover, photocatalytic Cr(VI) reduction experiments were performed on the catalysts to demonstrate the universality of the catalysts The results show that the photocatalytic reduction rate constant of Cr(VI) by 11CN is 1.79 times higher than that over the reference g-C₃N₄. Stability of the catalyst was verified through cycling experiments, and the samples exhibit promising practical application prospect. The mechanism of photocatalytic CO₂ reduction and the transformation pathway of intermediate products were elucidated using in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). This study introduces a new strategy of introducing nitrogen vacancies into g-C₃N₄-based photocatalytic materials, providing an effective approach to enhance the photocatalytic activity of g-C₃N₄ in photocatalytic CO₂ and Cr(VI) reduction.