Plasmon and Upconversion Mediated Broadband Spectral Response in TiO₂ Inverse Opal Photocatalysts for Enhanced Photoelectrochemical Water Splitting

Harvesting low-energy photons by strategically exploiting the photocatalytic properties of plasmonic and upconversion nanocomponents is a promising route to improve solar energy utilization. Herein, a rationally designed 3D composite photoanode integrating NIR-responsive upconversion nanocrystals (UCNs) and visible-responsive plasmonic Au nanoparticles (NPs) into 3D TiO2 inverse opal nanostructures (Au/UCN/TiO2) has been shown to extend the solar energy utilization in the UV–vis–NIR range. The NIR-responsive properties of NaYF4:Yb3+-based UCNs doped with Er3+ or Tm3+ ions, and the effect of an alternating sequential introduction of UCN and Au, have been assessed. With an extended overlap between the emission of Er-UCN and the characteristic SPR band of Au, our ternary Au/Er-UCN/TiO2 hybrid nanostructure unveiled a notable 10-fold improvement in photocurrent density under UV–vis–NIR illumination compared with a pristine TiO2 reference. The Au incorporation was confirmed to play a key role in enhancing the efficiency of light harvesting and to synergistically facilitate the energy transfer from UCNs to TiO2. This work further dissected plausible mechanistic pathways combining collected photoelectrocatalytic results, with electrochemical impedance measurements and transient absorption spectroscopic measurements. The synthesis and catalytic performance of our Au/UCN/TiO2 and the underlying mechanism here proposed are expected to reflect extended applicability in analogous applications for efficient solar-to-energy sustainable platforms.