Cascaded Photon Upcycling in an Upconversion-Plasmonic Fabry-Pérot Cavity for Broadband Solar Hydrogen Production...

Solar-driven hydrogen evolution is limited by the poor ability of conventional photocatalysts to utilize the visible-near-infrared (Vis-NIR) region, which accounts for ~95% of the solar spectrum. Here, we design an upconversion-plasmonic Fabry-Pérot cavity (Al/NaYF₄:Yb³⁺,Tm³⁺/Au/TiO₂) to achieve cascaded photon upcycling for efficient solar hydrogen production. In this architecture, the NaYF₄:Yb³⁺,Tm³⁺ layer serves as the dielectric medium of the cavity, enabling multiple light reflections and enhanced NIR-to-UV/Vis upconversion. The upconverted photons, together with the incident light, are further concentrated by the adjacent Au layer via surface plasmon resonance, promoting hot-electron generation and injection into TiO₂. As a result, the optimized structure achieves a broadband absorption efficiency of 60.57% and a hydrogen evolution rate of 19.86 mmol·g^-1·h^-1 from polylactic acid wastewater, 124 times higher than that of pristine TiO₂. This work provides a scalable strategy for broadband solar harvesting and plastic-waste-to-hydrogen conversion.