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.