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

The synthesis of early transition nanocrystals using NaBH4and the respectivemetal oxides at atmospheric pressure was studied at temperatures between 400and 1000°C. Reaction products were analyzed by x-ray diffraction, the crystallitesize was determined after Rietveld refinement of diffraction patterns, while themorphology was analyzed by scanning and transmission electron microscopy. Forall the investigated systems the lowest temperature to complete the synthesis was700°C and the reaction occurred in three subsequent steps: (i) decomposition ofNaBH4, (ii) formation of crystalline ternary species Na-M-O and Na-B-O, (iii)conversion of intermediary species to MB2and NaBO2. Syntheses carried out atT>700°C only caused coarsening of the powders. The synthetized boride pow-ders had the morphology of highly agglomerated nanocrystals. TiB2had a specificsurface area of 33.5 m2/g and crystallite diameter of 12 nm. Both ZrB2and HfB2had a platelet-like morphology with crystallite diameter around 45 nm and speci-fic surface area of 25.0 and 36.4 m2/g, respectively. Finally, NbB2and TaB2powders had a crystallite diameter around 5 nm with specific surface area of 21.1and 11.4 m2/g, respectively. The goal of this synthesis is the use of cheap rawmaterials and moderate temperature conditions