Intense Single Red Emission Induced by Near-Infrared Irradiation Using a Narrow Bandgap Oxide BiVO4 as the Host for Yb3+ and Tm3+ Ions

As a new way of producing a single red emission under near-infrared (NIR) irradiation, the possibility of using a narrow bandgap oxide as the host for the sensitizer Yb3+ ion as well as a suitable activator ion such as Ho3+, Er3+, or Tm3+ is considered, because the host can absorb the emissions of the activator ion higher in energy than the red emission. To verify this new concept, BiVO4:Yb3+,Tm3+ using a sol-gel method is prepared and its photoluminescence properties are characterized. BiVO4 has the bandgap of approximate to 2.4 eV so that, under NIR irradiation, it absorbs the blue and other higher-energy emissions of Tm3+. As a consequence, BiVO4:Yb3+,Tm3+ exhibits an intense single red emission under the NIR irradiation. The high-energy emissions absorbed by the host generate electron-hole pairs in the host, so that BiVO4:Yb3+,Tm3+ acts as a photocatalyst under NIR irradiation although the host alone cannot absorb the NIR light. NIR-induced upconversion luminescence (UCL) and photocatalysis in the region of red lights can be achieved by employing a semiconducting oxide as the host for sensitizer and activator lanthanide ions.