A Size‐Dependent Analysis of the Structural, Surface, Colloidal, and Thermal Properties of Ti_1–<i>x</i>B₂ (<i>x</i> = 0.03–0.08) Nanoparticles

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 TiB 2 nanocrystals with average crystallite sizes smaller than D ≈ 22 nm prefer to form defects at the titanium site rather than experience strain. The resulting composition is Ti 1– x B 2 [ 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 Ti 1– x B 2 nanocrystals. These species provide stability to the Ti 1– x B 2 nanocrystals in aqueous dispersions, as evidenced by electrokinetic measurements. Upon heating under an Ar atmosphere, the boron–oxygen species evaporate from ca. 1000 °C, and the Ti vacancies in the interior nanocrystalline core vanish. TiB 2 nanocrystals show excellent chemical stability against decomposition up to 1500 °C.