Two-dimensional (2D) materials have emerged as promising cocatalysts for photocatalytic applications, particularly in dye degradation under visible light. Among these, the combination of graphitic carbon nitride (g-C₃N₄) and molybdenum disulfide (MoS₂) has demonstrated significant potential due to their synergistic effects in enhancing charge separation, promoting reactive oxygen species (ROS) generation, and maintaining reusability. This system contrasts with traditional titanium dioxide (TiO₂)-based photocatalysts, which suffer from limited visible light absorption and rapid charge recombination. The g-C₃N₄/MoS₂ heterostructure presents a viable alternative for industrial wastewater treatment, addressing key challenges in efficiency, stability, and scalability.

The photocatalytic performance of g-C₃N₄/MoS₂ is primarily attributed to its ability to enhance charge separation. g-C₃N₄, a metal-free semiconductor, possesses a moderate bandgap of approximately 2.7 eV, enabling visible light absorption. However, its photocatalytic efficiency is often hindered by fast electron-hole recombination. The introduction of MoS₂ as a cocatalyst mitigates this issue. MoS₂ exhibits metallic or semiconducting properties depending on its phase, with the 1T phase being highly conductive and the 2H phase acting as a semiconductor. When coupled with g-C₃N₄, MoS₂ forms a heterojunction that facilitates electron transfer from g-C₃N₄ to MoS₂, thereby reducing recombination rates. Studies have shown that the optimized g-C₃N₄/MoS₂ system can achieve a charge separation efficiency up to three times higher than pure g-C₃N₄.

Reactive oxygen species generation is another critical factor in dye degradation. The g-C₃N₄/MoS₂ system promotes the production of hydroxyl radicals (·OH), superoxide radicals (·O₂⁻), and holes (h⁺), which are responsible for breaking down organic dyes. The conduction band of g-C₃N₄ is positioned at approximately -1.1 eV versus the normal hydrogen electrode (NHE), while the valence band lies at +1.6 eV. MoS₂, depending on its phase, can further modulate these energy levels. The 1T phase of MoS₂, with its high conductivity, accelerates electron transfer to adsorbed oxygen molecules, forming ·O₂⁻. Meanwhile, the 2H phase contributes to hole retention on g-C₃N₄, facilitating direct oxidation of dyes or water to produce ·OH. The combined effect results in a 50% increase in ROS generation compared to g-C₃N₄ alone.

Reusability is a practical consideration for industrial applications. The g-C₃N₄/MoS₂ system exhibits excellent stability over multiple photocatalytic cycles. Unlike TiO₂, which often suffers from photocorrosion or deactivation, g-C₃N₄/MoS₂ maintains its structural integrity and catalytic activity. Experiments have demonstrated that after five consecutive cycles of dye degradation, the system retains over 90% of its initial efficiency. This durability is attributed to the robust interfacial interaction between g-C₃N₄ and MoS₂, as well as the chemical stability of both materials under visible light irradiation.

In contrast, TiO₂-based systems face inherent limitations. TiO₂ has a wide bandgap of 3.0-3.2 eV, restricting its activity to ultraviolet light, which constitutes only 5% of solar radiation. Although doping or sensitization can extend its absorption into the visible range, these modifications often introduce instability or reduce catalytic efficiency. Additionally, TiO₂ suffers from high electron-hole recombination rates, necessitating the use of expensive co-catalysts like platinum. The g-C₃N₄/MoS₂ system outperforms TiO₂ in visible light utilization, charge separation, and cost-effectiveness, making it a superior candidate for large-scale applications.

The industrial wastewater treatment potential of g-C₃N₄/MoS₂ is significant. Textile and dye industries generate vast quantities of polluted water containing toxic organic dyes, which are resistant to conventional biological treatments. The g-C₃N₄/MoS₂ photocatalyst can degrade complex dyes such as methylene blue, rhodamine B, and Congo red with efficiencies exceeding 85% under visible light within 2 hours. Its scalability is further supported by the ease of synthesis; g-C₃N₄ can be produced via simple thermal polymerization of urea or melamine, while MoS₂ is obtained through hydrothermal or exfoliation methods. The low-cost raw materials and mild synthesis conditions make this system economically viable for industrial adoption.

However, challenges remain in optimizing the mass production and immobilization of g-C₃N₄/MoS₂ for continuous flow systems. Powdered photocatalysts require post-treatment separation, which can be energy-intensive. Recent advances propose immobilizing the catalyst on substrates like glass fibers or polymer membranes to facilitate recovery and reuse. Additionally, the long-term stability of g-C₃N₄/MoS₂ under varying pH and temperature conditions in industrial effluents requires further investigation.

In summary, the g-C₃N₄/MoS₂ heterostructure represents a breakthrough in visible-light-driven photocatalysis for dye degradation. Its enhanced charge separation, efficient ROS generation, and exceptional reusability outperform traditional TiO₂-based systems. With continued development in scalable synthesis and immobilization techniques, this 2D material cocatalyst holds immense promise for addressing industrial wastewater treatment challenges sustainably and cost-effectively.