Modifying Carbon Nitride through Extreme Phosphorus Substitution

A glassy carbon phosphonitride material with bulk chemical composition roughly approximating C3N3P was synthesized through a high-pressure, high-temperature process using a pure P(CN)3 molecular precursor. The resulting material (hereafter referred to as "HPHT-C3N3P") was characterized using a variety of techniques, including X-ray scattering, pair distribution function analysis, 31P, 13C, 15N magic-angle spinning nuclear magnetic resonance spectroscopies; X-ray photoelectron spectroscopy, and Raman and IR spectroscopies. The measurements indicate that HPHT-C3N3P lacks long-range structural order with a local structure predominantly composed of a sp2, s-triazine-like network in which phosphorus atoms substitute for bridging nitrogen sites found in related C3N4 materials. The HPHT-C3N3P sample exhibits semiconducting properties, with electrical transport dominated by variable-range hopping. The high phosphorus content of HPHT-C3N3P (approaching 13 at. %) is associated with a major decrease in the optical absorption edge (∼0.4 eV) and a ∼1010-fold increase in electrical conductivity, as compared to previously-reported P-doped graphitic g-C3N4 (0.6-3.8 at. % P). The HPHT-C3N3P sample is considerably harder than layered g-C3N4 and exhibits superior thermal stability up to ∼700 °C in air. These results demonstrate a remarkable range of tunable properties possible for C3N4-related materials through elemental substitution and provide valuable information to guide the design of new materials.