Boron and phosphorus-based materials : from synthesis and reactivity to properties

Overview & Synopsis

This thesis deals with boron- and phosphorus-based materials, their synthesis, the study of their reactivity with transition metals and their properties. The work focuses on the combination of boron and phosphorus, which enables the emergence of strong, directional covalent bonds in solids. These bonds, uncommon in inorganic solids, are responsible for specific properties in boron or phosphorus compounds, whether mechanical (high hardness), transport (superconductivity) or catalytic (long life and modified selectivity). While binary compounds based on boron or phosphorus have been well studied, particularly in the last ten years for these properties, the combination of boron and phosphorus has given rise to only relatively marginal work, not least because the synthesis of the corresponding compounds is very difficult. Indeed, the organization of a network of covalent bonds in a crystalline structure requires significant energy input, which traditionally takes place through solid-state synthesis under harsh temperature and/or pressure conditions. It is therefore necessary to develop new synthesis strategies to push back the limits of existing syntheses. Molten-salt synthesis is one such strategy, giving access to liquid-phase syntheses where reactions are accelerated, in a temperature range inaccessible by conventional solvents, opening the way to metastable solids, including nano-objects for novel compositions. In this work, we first present the development of a molten-salt synthesis of cubic boron phosphide (c-BP) nanocrystals. For the first time, this material is obtained in the form of nano-objects as small as 5 nm. Using a wide range of X-ray scattering and spectroscopic techniques, we demonstrate that new atomic environments arise at this scale, due to defects and surface states. The optical and (photo) electrochemical properties of these nanocrystals are then discussed. Molten-salt synthesis is then extended to the boron subphosphide B12P2 and crystallization methods under very high pressures, explored using in situ X-ray diffraction. We then continue our exploration of boron- and phosphorus-based compounds in molten salts by tackling ternary iron-boron-phosphorus compounds, with properties of interest for electrocatalysis of water oxidation and superconductivity. Fe5B2P, the only compound in this system with a crystalline structure specific to a ternary phase, is targeted as a priority. We highlight the crucial role played by the nature of the precursors in these syntheses, the most effective approach being to use a redox reaction between a metal salt and a borohydride for rapid incorporation of all the chemical elements into the same structure. Another strategy using the c-BP nanocrystals described above as a single source of boron and phosphorus was discussed, with in situ X-ray diffraction under synchrotron radiation showing in particular that a pure Fe5B2P phase was obtained. Finally, we studied the Mo-B-P system, in particular the Fe5B2P isostructural compound Mo5B2P, which has great potential for electrocatalysis of hydrogen production from water. We have demonstrated by in situ X-ray diffraction how the ternary phase crystallizes directly from the metal formed in situ, and not from the corresponding binary boride and phosphide phases, unlike the silicoboride Mo5B2Si. This particular behavior illustrates how the high covalency of borophosphides directly influences reaction pathways.