Synthesis of High-Quality Crystalline Carbon Nitride Oxide by Selectively Driving the High-Temperature Instability of Urea with Pressure

One-step synthesis using only physical tools is an appealing “green” method for the realization of technological materials. High-pressure conditions are particularly suitable in the attainment of extended supramolecular networks and in combination with high-temperature are effective in selecting specific reaction pathways to increase product quality. Here we show how pressures below 3 GPa and temperatures on the order of 420 K are effective for the synthesis, from urea crystal phase IV, of 2D graphitic carbon nitride oxide with enhanced crystalline quality. Angle-dispersive X-ray diffraction and Fourier transform IR spectroscopy show that the reaction becomes less selective at higher pressure with the formation of melamine and ammonium carbamate as byproducts. An anomalous positive slope is shown by the P−T instability boundary, likely arising from unfavorable hydrogen bonding with respect to the topochemical path, making urea an interesting case study for gaining insight into the role of hydrogen bonding in solid-state reactivity.