Evanescent waves modulate energy efficiency of photocatalysis within TiO₂ coated optical fibers illuminated using...

Coupling photocatalyst-coated optical fibers (P-OFs) with LEDs shows potential in environmental applications. Here we report a strategy to maximize P-OF light usage and quantify interactions between two forms of light energy (refracted light and evanescent waves) and surface-coated photocatalysts. Different TiO₂-coated quartz optical fibers (TiO₂-QOFs) are synthesized and characterized. An energy balance model is then developed by correlating different nano-size TiO₂ coating structures with light propagation modes in TiO₂-QOFs. By reducing TiO₂ patchiness on optical fibers to 0.034 cm²/cm² and increasing the average interspace distance between fiber surfaces and TiO₂ coating layers to 114.3 nm, refraction is largely reduced when light is launched into TiO₂-QOFs, and 91% of light propagated on the fiber surface is evanescent waves. 24% of the generated evanescent waves are not absorbed by nano-TiO₂ and returned to optical fibers, thus increasing the quantum yield during degradation of a refractory pollutant (carbamazepine) in water by 32%. Our model also predicts that extending the TiO₂-QOF length could fully use the returned light to double the carbamazepine degradation and quantum yield. Therefore, maximizing evanescent waves to activate photocatalysts by controlling photocatalyst coating structures emerges as an effective strategy to improve light usage in photocatalysis.