Polyaniline/ZnO nanocomposites for environmental remediation: Synthesis strategies, interfacial interactions, & structure-property relationships

The rise in emerging contaminants and water pollution calls for energy-efficient, sustainable remediation strategies. In this context, polyaniline/zinc oxide (PANI/ZnO) nanohybrids have emerged as promising multifunctional materials owing to their synergistic interfacial interactions and modified electronic structure. This critical review analyses PANI/ZNO heterojunctions, with a strategic focus on protonation effects, charge transfer, band alignment, and Fermi-level equilibrium. Type II heterojunction between p-type PANI and n-type ZnO enhances visible light absorption due to its band gap between 1.5 and 3.0 eV, and improved charge separation, which ultimately improves the photocatalytic pollutant removal efficiency up to 95–99 % as well as adsorption capacities up to ∼400 mg g−1. This review also correlates crystallinity, morphology, and interfacial bonding, and summarizes synthetic strategies, including in situ polymerization, sol-gel processing, electrochemical deposition, and hydrothermal synthesis. A systematic discussion of critical challenges, including long-term stability, photocorrosion, and scalability, is presented. Environmental sustainability, including catalyst reusability, utilization of solar light, and integration into a continuous-flow system, is discussed. Future perspectives should focus on advanced materials design based on band gap and mechanistic considerations, controlled protonation, and core-shell structures to enable next-generation PANI/ZnO for scalability.