Mild Solvothermal Growth of Robust Carbon Phosphonitride Films

Solvothermal growth of thin (1–2 μm thick), uniform films of carbon phosphonitride on fused quartz substrates has been achieved by self-reaction of 1 m P(CN)3 in diphenyl ether at 185 °C for 7 days in a sealed Teflon-lined digestion bomb. The films are chemically similar to C3N3P powders thermoset from neat (solid) P(CN)3 based on XPS and IR spectra. A difference in bulk elemental composition relative to C3N3P—reduced nitrogen and increased oxygen content—is attributable to through-film (as opposed to surface-confined) hydrolysis of unreacted nitriles (P–CN) to hydroxyl species (P–OH). Nanoindentation analyses were performed to assess the mechanical properties of these CPN films. Minimal dependence of mechanical properties on thermal treatment was observed with annealing temperatures in the 100–400 °C range, resulting in an average reduced modulus of Er = 26.5 ± 1.5 GPa and hardness of H = 1.5 ± 0.1 GPa, which is on par with high performance polymers. Heating to 550 °C results in higher modulus and hardness (Er550 °C = 51 ± 2 GPa, H550 °C = 5.0 ± 0.4 GPa), likely due to an increased prevalence of oligomeric phosphine oxide cross-links formed by dehydration of P–OH functional groups.