Expanded solar absorption spectrum to improve photoelectrochemical oxygen evolution reaction: Synergistic effect of upconversion nanoparticles and ZnFe₂O₄/TiO₂

The key to increasing photocatalytic activity is to maximize solar spectrum utilization and minimize the recombination of photosynthetic carriers. Along with ZnFe₂O₄ for visible light absorption, we designed a novel heterojunction photoelectrode using upconversion nanoparticles (UCNPs) with near-infrared (NIR) light absorption to improve solar absorption efficiency. We fabricated a UCNP-decorated ZnFe₂O₄/TiO₂ photocatalyst (UCNPs-ZFO/TiO₂) by sequentially drop-casting ZFO nanoparticles and UCNPs onto TiO₂ nanopillars grown on a transparent fluorine-doped tin oxide (FTO) substrate. The UCNP-ZFO/TiO₂ exhibited a significantly higher photocurrent density of 0.795 mA cm⁻² compared to that of the pristine TiO₂ (0.260 mA cm⁻²). The applied bias photon-to-current efficiency of UCNP-ZFO/TiO₂ was 4.1 times higher than that of TiO₂, and its photoanode had the lowest charge transfer resistance and highest charge density among the prepared photoelectrodes. This implies that the combination of UCNPs and ZFO on the TiO₂ photoanode can be an effective strategy to significantly improve photoelectrochemical performance owing to the synergetic utilization of NIR light by UCNPs and that of visible light by the ZFO nanoparticles.