Towards near-infrared photosensitization of tungsten trioxide nanostructured films by upconverting nanoparticles

Upconverting materials are currently explored in the field of solar energy conversion in order to extend the light harvesting properties of semiconductors to the near-infrared (NIR) region, their absorption being generally limited to the visible region of the solar spectrum. Here, we propose to photosensitize nanostructured films of tungsten oxide (WO3), a semiconductor widely investigated in photoelectrochemistry, photocatalysis and electrochromics, with NaGdF4:Er3+, Yb3+ upconverting nanoparticles (UCNPs). In order to do so, we fabricate nanocomposite films of WO3 and NaGdF4:Er3+, Yb3+ UCNPs (indicated as UCNP/WO3 films). Current–time measurements show that, under irradiation at λ = 980 nm, a relative increase in current of about 3% with respect to the dark current is observed in the UCNP/WO3 films. The UCNP/WO3 mol% ratio and the temperature of the thermal treatment of the nanocomposite films are both critical to simultaneously achieve photosensitization and charge carrier transport in the UCNP/WO3 films.