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

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 degrees 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 degrees 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.