Solid-State Synthesis and Solution-Processable MB₆ Nanocrystals

Metal hexaborides (MB₆ ) are of high technological interest as they show a wide range of bulk physical properties depending on the metal ion in the structure. Boron-rich compounds like CeB₆ and LaB₆ possesses low work functions (2.5-2.7 eV), while CaB₆ and SrB₆ possess high-temperature thermoelectric properties. ¹The MB₆ is reported to have very high hardness (9.5 on the Mohs hardness scale) with nanoindentation hardness of 16-18 GPa. ² The existing literature reports the solid-state synthesis of MB₆ nanocrystals below 500 °C, but so far, no studies have been reported on solution-processable MB₆hexaborides. Herein we report a solid-state synthesis of LaB₆ and SrB₆ nanocrystals performed at 440 °C and a novel post-processing technique of treating the nanopowders obtained from solid-state reaction to form a colloidally stable ink. Through the surface modification of the nanocrystals, we can exchange the surface ligands from inorganic short-chain ligands to organic long-chain ligands, making them solution processable in both polar and non-polar solvents. The colloidally stable MB6 nanocrystals have great potential to be used as thermoelectric devices or ultra-hard protective coatings, which can be fabricated at low cost by solution-based mass production processes.<br/>References<br/>(1) Takeda, M.; Fukuda, T.; Miura, T. Thermoelectric Properties of Metal-Hexaborides. International Conference on Thermoelectrics, ICT, Proceedings 2002, 2002-Janua, 173–176. https://doi.org/10.1109/ICT.2002.1190293.<br/>(2) Qin, M.; Yan, Q.; Liu, Y.; Wang, H.; Wang, C.; Lei, T.; Vecchio, K. S.; Xin, H. L.; Rupert, T. J.; Luo, J. Bulk<br/>High-Entropy Hexaborides. J Eur Ceram Soc 2021, 41 (12), 5775–5781. https://doi.org/10.1016/J.JEURCERAMSOC.2021.05.027.