Thermal transformation of metal oxide nanoparticles into nanocrystalline metal nitrides using cyanamide and urea as nitrogen source

Preformed metal oxide nanoparticles with varying crystallinity and different particle sizes and shapes can thermally be transformed into nanocrystalline metal nitrides in the presence of cyanamide or urea as nitrogen sources. In the case of anatase nanoparticles with crystallite sizes of 5, 10, and 20 nm, respectively, only the 5 nm sized TiO2 transformed completely into TiN. According to powder X-ray diffraction (XRD) and transmission electron microscopy measurements (TEM), the TiN particles are rather uniform in size and shape with a crystallite size of 3-5 nm, depending on the nitrogen source. In contrast to titania, vanadium oxide nanoparticles of much larger sizes and with different shapes reacted to nanocrystalline VN. However, the morphological features of the precursor particles are not transcribed into the final metal nitrides with an average crystallite size of 15 nm. Crystallinity plays a role too, as only amorphous tantalum oxide could be converted into nanocrystalline TaN. Furthermore, aluminum oxide gamma-Al2O3, gallium oxide gamma-Ga2O3, and niobium oxide nanoparticles formed AlN, GaN, and NbN, respectively, whereas HfO2 transformed into a hafnium oxide nitride compound. With the exception of GaN and TaN, all other metal nitrides contain amorphous carbon as byproduct. However, by adjusting the metal oxide-to-nitrogen source ratio, the carbon content can be minimized.