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Abstract:
We report the size-dependent structural, surface, colloidal, and thermal properties of the well-known refractory material titanium diboride prepared at the nanoscale by a size-controlled inorganic molten-salt technique. A combined analysis of the powder X-ray diffraction data through a modified Williamson-Hall plot, a size-strain plot, and Rietveld fitting methods indicates that TiB2 nanocrystals with average crystallite sizes smaller than D approximate to 22 nm prefer to form defects at the titanium site rather than experience strain. The resulting composition is Ti1-xB2 [x = 0.03(1)-0.08(1)]. The size-induced defect formation is accompanied by anisotropic lattice contraction that decreases primarily the cell parameter c. Transmission electron microscopy revealed that the nanocrystals are embedded in an amorphous matrix. A comparison between the electron spin resonance spectra of bulk and nanosized samples indicated that only the nanosized samples yield observable signals. This signal is associated with unpaired electrons trapped in the boron-oxygen species that surround the Ti1-xB2 nanocrystals. These species provide stability to the Ti1-xB2 nanocrystals in aqueous dispersions, as evidenced by electrokinetic measurements. Upon heating under an Ar atmosphere, the boron-oxygen species evaporate from ca. 1000 degrees C, and the Ti vacancies in the interior nanocrystalline core vanish. TiB2 nanocrystals show excellent chemical stability against decomposition up to 1500 degrees C.
