Band-engineered heterostructure photocatalytic micromotors for autonomous water remediation

Photocatalytic micromotors (PCMs) are self-propelled micro- or nanoparticles that couple light-driven surface redox chemistry to motion for pollutant degradation. PCMs have emerged as active platforms for water remediation because they combine pollutant degradation with self-propulsion, enhancing mass transfer and pollutant–catalyst contact beyond passive photocatalysts. However, their practical performance is often limited by rapid charge recombination, weak visible-light response, insufficient redox potential, fuel-dependent propulsion, and instability in complex water matrices. Band-engineered heterostructures provide a promising strategy to address these limitations by tailoring interfacial charge transfer, strengthening built-in electric fields, improving reactive oxygen species generation, and sustaining the chemical gradients required for micromotor motion. This review critically examines Type-II, p–n, Z-scheme, and S-scheme heterostructure PCMs for the removal and degradation of emerging contaminants such as pharmaceuticals, dyes, antibiotics, and microplastics (MPs). Particular attention is given to how band alignment governs both photocatalytic activity and propulsion behaviour, linking charge separation, redox strength, reactive oxygen species pathways, and mass-transfer enhancement. Rather than simply cataloguing material systems, this review evaluates whether reported performance gains remain meaningful under realistic conditions, especially in studies relying on peroxide fuels, model pollutants, or idealized water matrices. Finally, key challenges and future directions are discussed, including fuel-free solar propulsion, scalable fabrication, catalyst recovery, environmental safety, and long-term operation in natural waters.