P–N Junction-Based Photocatalysts: Synthesis, Mechanism, and Their Applications

Abstract P–N junction photocatalysts have emerged as versatile and efficient semiconductor materials for solar-driven environmental and energy applications due to their intrinsic ability to promote directional charge separation via a built-in electric field. This review provides a comprehensive overview of the fundamental principles governing P–N junction formation, including band alignment, Fermi-level equilibration, and interfacial charge-transfer mechanisms, and correlates these features with photocatalytic performance. Particular emphasis is placed on the role of the internal electric field in suppressing electron–hole pair recombination and enabling spatial separation of redox reactions. The applications of P–N junction photocatalysts are systematically discussed across environmental pollutant remediation, energy conversion, air purification, and antibacterial activity. In pollutant degradation systems, P–N systems exhibit dual degradation mechanisms, employing an electron-driven reduction process for heavy-metal removal and reactive-oxygen-species-mediated oxidation pathways for the degradation of nonheavy-metal pollutants, such as dyes, pharmaceuticals, and pesticides. For energy applications, the review examines photocatalytic hydrogen (H2) evolution and CO2 reduction, emphasizing the importance of band structure engineering and multielectron transfer pathways in determining activity and product selectivity. Air purification and antimicrobial applications further demonstrate the effectiveness of P–N junctions in generating sustained oxidative species for degradation of gaseous pollutants and pathogen inactivation. A comparative mechanistic framework is presented to unify these diverse applications, illustrating how charge-carrier dynamics dictate reaction pathways. Finally, key challenges, including interfacial control, stability, and scalability, are critically evaluated, and future research directions focusing on advanced material design, in situ characterization, field implementation of P–N junction photocatalysts, and their potential application in sustainable environmental and energy technologies are proposed.