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

Abstract

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.