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.