Persistent luminescence warm-light LEDs based on Ti-doped RE2O2S materials prepared by rapid and energy-saving microwave-assisted synthesis

Persistent luminescence (PeL) materials find many applications nowadays, such as in emergency lighting, medical imaging, and enhancement of solar cell devices. However, the majority of PeL materials are synthesized by time- and energy-consuming processes such as conventional solid-state methods. Here, a series of Ti-doped RE2O2S materials (RE: La, Gd, and Y) were successfully prepared by an energy-saving microwave-assisted solid-state synthesis. This versatile method allows obtaining oxysulfide materials starting from primary precursors (e.g. oxides and elemental sulfur) in a one-step 25 minute process. The PeL materials were characterized using synchrotron radiation X-ray powder diffraction (SR-XPD), scanning electron microscopy (SEM), X-ray absorption spectroscopy (SR-XAS), and infrared absorption spectroscopy (FTIR), and the spectroscopic properties were studied using photoluminescence spectroscopy (PL) and thermoluminescence (TL). SR-XAS at the sulfur K-edge of RE2O2S suggests that elemental sulfur (S0) is oxidized in a first step forming sulfate-like (SVI) species, then is gradually reduced to form oxysulfide (S2-). The RE2O2S:Ti,Mg2+ materials show a broad emission band in the orange-red spectral region, assigned to the transitions of Ti3+/TiIV ions. The persistent luminescence of the Gd and Y2O2S:Ti,Mg2+ materials is long-lasting, being visible for ca. 5 h with the naked eye. Lastly, these photonic materials were shown to act as emitting layers for PeL warm white-LEDs, presenting great potential for applications such as self-sustained security lighting.