Modulating the H-bond strength by temperature for the high pressure synthesis of nitrogen rich carbon nanothreads
Carbon nanothredas are among the most attracting new materials produced under high pressure conditions. Their synthesis can be achieved by compressing crystals of aromatic molecules exploiting both the anisotropic stress produced by the unidirectional applied force and that intrinsic to the crystal arrangement. We explored here the transformation of pyridine into a nitrogen rich carbon nanothread crystal varying pressure and temperature conditions with the twofold purpose of disclosing the microscopic mechanism of the transformation and optimizing yield and quality of the crystalline nanothreads produced. The best conditions for the synthesis were identified in the 14-18 GPa range for temperatures between 400 and 500 K with a product yield greater than 30%. The comparison of experiments performed in different P-T conditions allowed to understand the role of high temperature, which is necessary to weaken or even destroy the complex H-bond network characterizing the pyridine crystal and preventing the correct approach of the aromatic rings for the nanothreads formation. X-ray diffraction data attest for an excellent 2D hexagonal packing of the nanothreads over several tens of microns, whereas the sharp absorption lines observed in the IR spectrum strongly support a substantial order along the threads. Diffraction results suggest a polytwistane structure of the threads deriving from a Diels-Alder [4+2] polymerization involving molecules arranged in a slipped parallel configuration along the pyridine crystal a and b axes. Electron microscopy evidences an arrangement of the nanothreads in bundles of tens of nanometers.