Understanding the remarkable luminescence enhancement via SiO₂ coating on TiO₂:Eu³⁺ nanofibers

TiO₂:Eu³⁺ nanofibers diameters can be tuned by changing the mixed solvent ratio are synthesized via the electrospinning technique. Concentration quenching phenomena and the relationship of the PL intensity ratio I(⁵D₀-⁷F₂)/I(⁵D₀-⁷F₁) with the Eu doping concentration are discussed. Notably, the luminescence intensity is enhanced by about 7.8 fold by coating SiO₂ gel layer on the TiO₂:Eu³⁺ nanofibers, which is successfully performed via a sol-gel process followed by calcination. Furthermore, these fibers are characterized systematically via X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (UV-vis DRS), Fourier transform infrared spectroscopy (FT-IR), and photoluminescence (PL) spectroscopy. Additionally, their Judd-Ofelt and spectral parameters are calculated to investigate their local structures. The relatively low Ω₂ value of the coated samples reveals a decrease in covalency in the Eu-O bonds and an increase in the symmetric nature of Eu³⁺, which indicate that the perturbation effect of the crystal field in the solid system is larger than that of the uncoated samples. FT-IR analysis indicates the formation of Ti-O-Si bonds, which provide the ligand field in the interface between the TiO₂:Eu³⁺ nanofibers and SiO₂ layer and can repair the surface unsaturated bonds. This reduces the selection rules for radiative transitions, thereby the state of the Eu³⁺ ions is converted from dormant to activated. Moreover, the silica coating stabilizes the surface of the TiO₂:Eu³⁺ nanofibers and eliminates the surface defects. Finally, a detailed mechanism is proposed to explain the luminescence enhancement behavior.