Sulfur-Doped g-C₃N₄ Heterojunctions for Efficient Visible Light Degradation of Methylene Blue

The discharge of synthetic dyes from different industrial sources has become a global issue of concern. Enormous amounts are released into wastewater each year, causing concerns due to the high toxic consequences. Photocatalytic semiconductors appear as a green and sustainable form of remediation. Among them, graphitic carbon nitride (g-C₃N₄) has been widely studied due to its low cost and ease of fabrication. In this work, the synthesis, characterization, and photocatalytic study over methylene blue of undoped, B/S-doped, and exfoliated heterojunctions of g-C₃N₄ are presented. The evaluation of the photocatalytic performance showed that exfoliated undoped/S-doped heterojunctions with 25, 50, and 75 mass % of S-doped (g-C₃N₄) present enhanced activity with an apparent reaction rate constant (k_app) of 1.92 × 10^-2 min^-1 for the 75% sample. These results are supported by photoluminescence (PL) experiments showing that this heterojunction presents the less probable electron-hole recombination. UV-vis diffuse reflectance and valence band-X-ray photoelectron spectroscopy (VB-XPS) allowed the calculation of the band-gap and the valence band positions, suggesting a band structure diagram describing a type I heterojunction. The photocatalytic activities calculated demonstrate that this property is related to the surface area and porosity of the samples, the semiconductor nature of the g-C₃N₄ structure, and, in this case, the heterojunction that modifies the band structure. These results are of great importance considering that scarce reports are found concerning exfoliated B/S-doped heterojunctions.