Carbon nitride materials have been of great interests to the scientific community due to their unique properties such as extreme hardness, low density, chemical inertness, biocompatibility, etc. According to theoretical calculations published elsewhere, some covalent carbon-nitrogen compounds (e.g., b-C 3 N 4 and cubic-C 3 N 4 ) are expected to possess a very high bulk modulus even exceeding that of diamond. However, due to the great thermodynamic stability of carbon and N 2 molecules, the experimental synthesis of crystalline carbon nitride with C 3 N 4 stoichiometry remains a challenge to date. In recent years much attention has been paid to the graphitic form of carbon nitride (g-C 3 N 4 ) which was successfully synthesized via the polycondensation of triazine-based compounds. During the polycondensation reaction the nucleus of tri-s-triazine (tri-ring of C 6 N 7 ) is energetically more stable than that of s-triazine (ring of C 3 N 3 ). Therefore, it is widely accepted that the tri-s-triazine nucleus forms the basic unit for the formation of the graphene-like sheet of g-C 3 N 4 . Graphitic carbon nitride is regarded as the most promising candidate to complement carbon materials in various potential applications. Apart from its application as the precursor for the synthesis of superhard carbon nitride phases, it has also been investigated as a mesoporous material, a high-performance tribological coating, a metal-free catalyst, the nitrogen source for the synthesis of metal nitrides, and as the precursor for the preparation of carbon nitride nano/microstructures. Vapor-grown carbon microfibers (VGCFs) are already widely used as efficient fillers in composites, and supporting substrates for the growth of nanoscale materials. However, the large-scale synthesis of nitrogen-rich carbon nitride 1D nano/microstructures, in particular the synthesis of microfibers with a large length-diameter ratio has not been reported yet. Although several experimental approaches for the synthesis of carbon nitride nano/microstructures have been published, these experiments are actually performed at the C and N atomic level (e.g., at high temperatures), which commonly results in low nitrogen incorporation (<15 at %) due to the high thermodynamic stability of N 2 . In this communication, we present graphitic carbon nitride, which exhibits high thermal stability, as a precursor for the synthesis of nitrogen-rich carbon nitride microfibers. The tri-s-triazine and/or s-triazine rings present in the precursor remain stable during the microfiber growth process, thus ensuring a high nitrogen content in the final product. Additionally, we demonstrate that the large-scale synthesis of carbon nitride microfibers can be realized via a thermal evaporation process.
Graphitic carbon nitride (C 3 N 4.4 ) was prepared via a ''stepby-step'' pyrolysis route from melamine (2,4,6-triamino-s-triazine, C 3 N 3 (NH 2 ) 3 ). Elemental analysis indicated the presence of residual hydrogen ($1.58 wt%) in the as-prepared product. Residual hydrogen atoms bind to the edges of the graphene-like C-N sheet in the form of C-NH 2 and 2C-NH bonds. The hydrogen terminated C-N sheet is energetically more stable, and thus has a high thermal stability. Figure shows the thermogravimetric analysis (TGA) result of the precursor carbon nitride. The product is relatively stable up to $715 8C, after which it starts to decompose and lose mass.
In order to synthesize carbon nitride microfibers, the precursor carbon nitride was placed in a quartz boat and heated to 680 8C under N 2 atmosphere (see Experimental). We found that after the precursor was held at 680 8C for one hour, there was still ca. 55% of the precursor left. However, when it was held at 720 8C for the same amount of time, there was no residual carbon left in the quartz boat. This means the nuclei of s-triazine and tri-s-triazine which are present in the precursor remain stable during the thermal evaporation. The precursor is evaporated as CN x (x > 1) nucleus vapor phase, rather than in N 2 form (residual carbon should be left in this case).