Eco‐Friendly ZnO/Chitosan/Cinnamomum Composite: A Sustainable Catalyst for the Photocatalytic Degradation of Methylene Blue

A novel ternary nanocomposite comprising zinc oxide, chitosan, and Cinnamomum sp. phytochemicals (Ci─ZnO–Chitosan) was successfully synthesized via a sustainable green route using cinnamon bark extract as a dual reducing and capping agent. Comprehensive physicochemical characterization using FTIR, XRD, SEM, EDX, and UV–vis, and uniformly embedded hexagonal wurtzite ZnO nanoparticles (30–80 nm). The photocatalytic efficacy of the nanocomposite was rigorously evaluated through the degradation of methylene blue (MB) under UV irradiation. Systematic condition profiling revealed that the system (UV+H 2 ​O 2 ​+catalyst) achieved 59.1% degradation in 120 min, which escalated to 88.3% at 225 min and peaked at 91.4% under optimized mild acidic conditions (pH 5). Kinetic evaluation demonstrated rapid degradation following a pseudo‐first‐order model ( k = 0.01576 min −1 , t 1/2 ​ = 44 min, R 2 = 0.9976) at an optimal initial dye concentration of 20 mg/L. Conversely, higher concentrations (40–50 mg/L) induced unfavorable MB H‐aggregation, evidenced by a hypsochromic shift to 600–615 nm. These H‐aggregates are photocatalytically inactive, reducing the effective concentration of MB monomers available for ROS attack; additionally, the inner filter effect limits light penetration to the catalyst surface, together sharply reducing efficiency. Control experiments confirmed that dark adsorption, photolysis, and catalyst‐free oxidation yielded negligible degradation, highlighting the vital role of the semiconductor catalyst.