Synthesis of group IV and V metal diboride nanocrystals via borothermal reduction with sodium borohydride

The synthesis of early transition nanocrystals using NaBH4 and the respective metal oxides at atmospheric pressure was studied at temperatures between 400 and 1000°C. Reaction products were analyzed by x‐ray diffraction, the crystallite size was determined after Rietveld refinement of diffraction patterns, while the morphology was analyzed by scanning and transmission electron microscopy. For all the investigated systems the lowest temperature to complete the synthesis was 700°C and the reaction occurred in three subsequent steps: (i) decomposition of NaBH4, (ii) formation of crystalline ternary species Na–M–O and Na–B–O, (iii) conversion of intermediary species to MB2 and NaBO2. Syntheses carried out at T > 700°C only caused coarsening of the powders. The synthetized boride powders had the morphology of highly agglomerated nanocrystals. TiB2 had a specific surface area of 33.5 m2/g and crystallite diameter of 12 nm. Both ZrB2and HfB2 had a platelet‐like morphology with crystallite diameter around 45 nm and specific surface area of 25.0 and 36.4 m2/g, respectively. Finally, NbB2 and TaB2 powders had a crystallite diameter around 5 nm with specific surface area of 21.1 and 11.4 m2/g, respectively. The goal of this synthesis is the use of cheap raw materials and moderate temperature conditions.