A graphitic-C3N4 derivative containing heptazines merged with phenyls: synthesis, purification and application as a high-efficiency metal-free quasi-green phosphor for white LEDs

Abstract

Because rare-earth elements are scarce, expensive, and unsustainable, it is of great significance to develop rare-earth-free (even metal-free) luminescent materials as phosphors for LEDs. Here, a graphitic-C₃N₄ (g-C₃N₄) derivative containing some heptazines merged with phenyls has been synthesized via thermal polymerization of melamine and quinazoline-2,4(1H,3H)-dione at an optimal mole ratio of 18 : 1. In comparison with g-C₃N₄ synthesized from melamine only, the photoluminescent (PL) emission colour changed from blue to green, the maximum emission wavelength (λ_em,max) changed from 467 nm to 508 nm, and the PL quantum yield (PLQY) increased from 8.0% to 24.0%. It was further purified via vacuum sublimation, and a product with yellowish green emission (λ_em,max = 517 nm) and PLQY up to 45.5% was obtained. This sublimated product had high thermal stability and low thermal quenching; its thermal decomposition temperature was as high as 527 °C, and its relative PL emission intensity at 100 °C was 90.8% of that at 20 °C. Excited by blue light chips (λ_em,max ≈ 460 nm), cold, neutral and warm white LEDs can be fabricated using the sublimated product and orange-emitting (Sr,Ba)₃SiO₅:Eu²⁺ as phosphors. The good performances of these white LEDs (for example, the CIE coordinates, color rendering index and correlated color temperature were (0.31, 0.33), 84.4 and 6577 K, respectively) suggest that the low-efficiency blue-emitting g-C₃N₄ had been successfully converted into a high-efficiency metal-free quasi-green phosphor.