Unique Preparation of Hexaboride Nanocubes: A First Example of Boride Formation by Combustion Synthesis

Nanocubes of LaB6 and Sm0.8B6 have been synthesized using low-temperature combustion synthesis, a technique that had only been used previously for the preparation of oxides. The hexaboride nanocubes were prepared using lanthanum nitrate or samarium nitrate, carbohydrazide, and boron powders. The furnace temperature for synthesis was kept at 320 degrees C, lower than the typical temperature values used in combustion processes for the preparation of oxides. After synthesis, the nanocubes were characterized using X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. The combustion process was analyzed using differential scanning calorimetry, which shows that the formation of the carbohydrazide and nitrate melts as well as the formation of a complex between metal ions and carbohydrazide are crucial steps for the reaction. The technique results in high-purity powders with a unique cubic morphology, in which the corners of the cubes can be used as point sources for efficient electron emission.