Carbon nitrides: sp2- and sp3-bonded materials (11.00 - 11.30); Water Dynamics in Bacteria at High Pressure (11.40-12.10)


Paul McMillan

Time and Place

Thursday, 8 November 2018 - 11:00am
CSEC Seminar Room


1. Water Dynamics in Bacteria at High Pressure - Quasi-Elastic Neutron Scattering Studies Understanding how biological organisms function under extreme environmental conditions is critical for comprehending life in Earth’s deep biosphere. Understanding the dynamic behavior of intracellular water is a key aspect of this discussion. Water management inside prokaryotic organisms is still not fully understood. It has long been assumed that the bacterial cytoplasm could be approximated as a homogenous gel-like medium containing ionic and macromolecular components. However, recent models suggest a more dynamically organized structure with water-rich channels threading between macromolecular regions. Our quasi-elastic neutron scattering data for bacteria under high pressure conditions provide support for this new interpretation, and provide new insights into cytoplasmic dynamics in deep subsurface environments. 2. Carbon Nitrides : From High Pressure Synthesis to Novel Nanomaterials Carbon nitride compounds with high N:C ratios and graphitic to polymeric sp2-bonded structures have been known since work by Berzelius, Liebig, Wöhler and others in the early 1800's, and now receiving attention as potential materials for energy, catalysis and optoelectronics applications. In 1985 M. Cohen predicted that an sp3-bonded polymorph with the -Si3N4 structure should exist, that might have "superhard" properties competitive with diamond. Despite many attempts using high-P,T and CVD synthesis approaches, this phase has never yet been fully demonstrated. However, our work has shown the existence of sp3-bonded C2N3H with a defective wurtzite structure. We then studied effects of compression on a crystalline layered polytriazine imide (PTI) structure C6N9H3.HCl, itself synthesised under high-P,T conditions. The experimental results combined with DFT investigations suggest that C-N interlayer bonding occurs at high pressure, resulting in a pillared-layered material with mixed sp2-sp3 bonding. Laser-heated diamond cell experiments following compression of the triazine-based graphitic carbon nitride g-C3N4 showed crystallisation of a new phase above around 45 GPa. This was identified from AIRSS DFT predictions carried out by our colleagues Chris Pickard and Richard Needs (Cambridge), and revealed to have an unusual open-framework structure: the first of a new family of CxNy "zeolitic" phases. In recent work we have concentrated on the exfoliation and intercalation chemistry of PTI phases at ambient pressure, first showing the formation of few-layer CxNy nanoflakes in solution, and then the creation of a "deintercalated" PTI phase (C6N9H3) following Soxhlet extraction. Re-intercalation of HCl leads to formation of the PTI.HCl compound, that was previously only available from high-P,T synthesis.