Role of the Yb3+ concentration in the high efficient UV-blue up-conversion emission from hydrothermally grown Yb3+/Er3+-doped K2YF5 crystals

• Yb3+/Er3+ doped K2YF5 crystals were synthesized under hydrothermal conditions.
• Intense up-conversion to enhance TiO2 & Fe2O3 spectral response for water-splitting.
• Total infrared to UV-VIS up-conversion efficiency was calculated to be around 66%.
• Use of less common heavy RE versus light RE for equilibrating the balance problem.

High intense UV–VIS up-conversion luminescence from orthorhombic K2YF5 fluoride crystals doubly doped with Yb3+ and Er3+ has been obtained under near-infrared excitation at 980 nm. Fluoride crystals containing high Yb3+ concentrations, namely 20; 40 and 99 at.%, and low Er3+ concentrations have been synthesized under hydrothermal conditions. The role of the Yb3+ ions in the mechanism of the up-conversion process is clearly disclosed with absorption and up-conversion luminescence spectra from which one can see that high intense green (520–545 nm) and outstanding UV-blue (380–410 nm) emissions are greatly enhanced by increasing the concentration of Yb3+ ions whose aggregate acts as an infrared collector harvesting long wavelength infrared photons. Outstanding total infrared to UV–VIS up-conversion efficiency of 66% has been calculated for the sample with the highest Yb3+ doping level, i.e. 99 at.%. These strong UV-blue up-conversion emissions would have the potential use in enhancing spectral response of main semiconductor electrodes (such as TiO2 and Fe2O3), and these K2YbF5 crystals doped with Er3+ could be used in photo-electrochemical devices for sustainable production of hydrogen via photolysis of water, as a promising photonic approach which is currently discussed in the literature.