K/Na Co-Doped Carbon Nitride: A Robust Luminescent Material with High Thermal Endurance and UV Durability

Abstract

Carbon nitride materials are promising for photocatalysis and optoelectronics owing to their earth-abundant composition and tunable electronic structure, yet they often suffer from limited charge separation and poor photothermal stability. However, achieving high-temperature luminescence retention together with long-term UV durability, especially in polymer-processable forms, remains challenging. Here, we introduce a dual-alkali co-doping concept (K/Na) that enables a carbon nitride phosphor with simultaneously improved thermal endurance and UV durability, and we further demonstrate its processability into transparent polymer composites. We report a potassium/sodium co-doped carbon nitride (CN–K/Na) synthesized by a solid-state route using 3-AT, NaOH, and KOH. Structural analyses indicate that K/Na incorporation modulates the local structure of the CN framework, which is consistent with synergistic regulation of defects and charge transport. The co-doped material shows a narrowed optical bandgap (2.23 eV), enhanced photoluminescence, and an increased TRPL lifetime (τ_avg ≈ 1.42 ns, from multi-exponential fitting), and significant thermal stability, retaining 90.8% emission at 475 K and ~75% at 625 K. Under extended UV exposure, the emission exhibits a non-monotonic evolution, reaching 1.7× of the initial intensity and maintaining 43% after 192 h, indicating notable optical durability. When incorporated into a polymer matrix, CN–K/Na effectively suppresses UV-induced degradation of PMMA, maintaining stable emission with minimal spectral drift. The improved UV durability of the composites is consistent with UV attenuation by the filler and reduced photodegradation of the polymer matrix. These results establish K/Na co-doping as a practical and scalable route to robust carbon nitride luminescent materials and UV-durable transparent composites for optoelectronic applications.