New directions in carbonitride research: synthesis of resin-like dense-packed C3N4 using a hydrogen-free precursor

Abstract

In this work, a new synthetic approach towards binary carbonitride (C₃N₄) is presented. Using C₂N₂O, a hydrogen-free inorganic polyisocyanate obtained by polymerisation of molecular cyanogen isocyanate (NC–NCO), a phase-pure 3D-structured C–N-network was prepared by a carefully controlled polycondensation reaction. Only gaseous CO₂ was released, providing amorphous and porous C₃N₄ at T = 400 °C. Further densification of this inorganic cross-linked polymer was achieved by annealing in the presence of elemental mercury as a catalyst. Network densities of up to 2.3 g cm⁻³ were obtained. C₃N₄ thus obtained was analysed using several experimental techniques (elemental analysis, IR- and Raman spectroscopy, thermal analysis, MS studies, electronic conductivity measurement, EPR spectroscopy). From the combined results, it follows that this material with the composition C₃N₄ is best described as an inorganic resin-like thermosetting network, structurally related to the well-known melamine-based resins (polytriazines). Within this picture, a reasonable reaction path of formation has been derived. The thermal stability of this binary C–N material turned out to be disappointingly low, with the onset of decomposition at T = 500 °C. The results obtained are compared with recent findings on hydrogen-containing C–N–H samples (i.e. “melon”) and further discussed in the context of C–N– as well as of general inorganic polymer chemistry. Some preliminary results towards the high-pressure transformation to crystalline C₃N₄ are presented.