Biomass as a Green Source of Dopants: A Review on In-Situ Synthesis of P-N Co-Doped ZnO for Photocatalytic Dye Degradation

Synthetic dyes, particularly azo dyes, pose significant environmental and health risks due to their persistence, toxicity, and potential carcinogenicity. Zinc oxide (ZnO) is a promising photocatalyst for wastewater remediation, but its wide bandgap and rapid charge recombination limit its practical efficacy. Furthermore, conventional doping methods often rely on hazardous chemical precursors, undermining the sustainability of the overall approach. This review introduces a novel and sustainable paradigm: the utilization of biomass-derived precursors as green reagents for the in-situ synthesis and simultaneous phosphorus-nitrogen (P-N) co-doping of ZnO nanoparticles. We critically analyze how the intrinsic biochemical composition of biomass, rich in P, N, and other heteroatoms, facilitates this one-pot, eco-friendly functionalization. This integrated strategy merges the performance enhancement offered by advanced co-doping, such as extended visible-light absorption and suppressed charge recombination, with the core principles of green chemistry and circular economy. It offers a dual benefit: creating highly effective photocatalysts for the degradation of persistent pollutants and valorizing abundant agricultural or biological waste streams. Our comprehensive evaluation goes beyond description to critically assess the underlying mechanisms, comparative efficacy, scalability challenges, and future research directions of this emerging field. This review underscores the unique contribution of biomass-mediated synthesis to advancing sustainable nanotechnology for environmental applications.