Nitrogen-Doped Reduced Graphene Oxide Covalently Coupled with Graphitic Carbon Nitride/Sulfur-Doped Graphitic Carbon Nitride Heterojunction Nanocatalysts for Photoreduction and Degradation of 4-Nitrophenol

4-Nitrophenol (4-NP) is a high-priority industrial pollutant and is known to cause adverse effects to the human body. Owing to this, detoxification of water contaminated with 4-NP is highly essential. The photocatalytic degradation of 4-NP is considered an efficient method. However, for this purpose, mostly expensive reagents that can cause adverse effects on the environment are used. Therefore, for the treatment of 4-NP contaminated water, an eco-friendly method is considerably required. To meet this requirement, here, we have synthesized nitrogen-doped reduced graphene oxide (NrGO) covalently coupled with carbon modified porous graphitic carbon nitride/sulfur-doped graphitic carbon nitride (g-C3N4/Sg-C3N4) isotype heterojunction (g–g PSCN) nanocatalysts. The photocatalytic performance of the prepared NrGO/g–g PSCN nanocatalysts has been examined by the reductant-free reduction and H2O2-assisted degradation of 4-NP. The results of the photocatalytic performance suggest that the catalytic activity is strongly influenced by the presence of the NrGO content. For the 4NrGO/g–g PSCN nanocatalyst, the highest catalytic activity is observed, which is 12.25 and 2.92 times higher than that of g–g PSCN toward the reduction and degradation of 4-NP, respectively. The influence of various environmental factors such as solution pH, temperature, and the presence of competitive ions on the degradation rate of 4-NP have also been investigated. The reactive species involved during the reactions have also been explored, and a plausible mechanism has been proposed for both reactions. The durability and stability of the nanocatalysts have been examined, and the obtained results reveal that the nanocatalysts can endure the experimental conditions even after eight successive cycles. This approach opens up an avenue for the fabrication of graphitic carbon nitrite-based metal-free heterogeneous nanocatalysts with high catalytic performance.