Titanium Nanoparticles Stabilized by Ti-C Covalent Bonds

Nanosized titanium particles have attracted extensive research interest because of their diverse applications in, for instance, catalysis, energy conversion, and biomedical engineering. Of these, titanium colloids have been used as effective catalysts for the hydrogenation of titanium and zirconium sponses and as a powerful activator for heterogeneous hydrogenation catalysts. Titanium nanoparticles have also been used as doping agents to improve the hydrogen storage and exchange properties of sodium alanate, which may find important applications in mobile devices. Additionally, titanium nanoparticles formed at the metal-tissue interface may play an important role in the long term stability of titanium-based biomedical implants. So far various effective protocols have been developed for the preparation of titanium particles. For instance, pulse laser deposition and laser ablation have been used to prepare fine powders of a wide range of transition metals including titanium. Yet the core size of the resulting particles is typically of the order of (sub)micrometer. Nanometer-sized titanium particles have been prepared from plasmas in mixtures of titanium tetrachloride, argon, and hydrogen. However, dispersion and chemical processing of the particles remain a challenge. Wet chemistry methods have also been employed to synthesize titanium nanoparticles. For instance, Bonnemann et al. used triethylhydroborate as the reducing agent to reduce TiCl4 into Ti colloids. However, these colloids tended to agglomerate which rendered it difficult to evaluate accurately the size and crystalline structure of the particles. In fact, so far, no clear transmission electron microscopy (TEM) image has been reported of these Ti colloids, to the best of our knowledge. Thus, in this study, we report the synthesis of stable nanometer-sized titanium particles where the structures of the nanocrystals were unambiguously revealed by highresolution transmission electron microscopy (HRTEM) measurements. We believe that this is the first of its kind. The particles were passivated by virtue of the strong Ti-C covalent linkages (bond strength 423 kJ/mol). This is largely motivated by recent success of using diazonium compounds as precursors to passivate transition-metal nanoparticles by the strong metal–carbon covalent bonds. We used biphenyl-stabilized titanium nanoparticles as the illustrating example. The biphenyldiazonium compound was synthesized by following a literature synthetic protocol. Briefly, a calculated amount of 4-aminobiphenyl was dissolved in ice cold 50% fluoroboric acid. Then a 1:1 stoichiometric amount of sodium nitrite was added into the reaction vessel to generate biphenyldiazonium fluoroborate. The solution was allowed to mix for several minutes. The product was then washed thoroughly with cold fluoroboric acid and ether to remove any impurities. The titanium nanoparticles were then synthesized. In a typical reaction, 0.2 mmol of TiCl4 was dissolved in 20 mL of tetrahydrofuran (THF). The solution was purged with nitrogen, and the inert atmosphere was maintained for the duration of the synthesis. After 10 min of vigorous stirring, the biphenyldiazonium compound synthesized above was added directly into the reaction vessel. The mixture was allowed to stir for 2 h. Then 5 mmol of superhydride (5 mL of a 1 M solution in THF) was added into the reaction vessel. The solution color changed immediately from brown to dark red, signifying the formation of titanium nanoparticles. The solution was then allowed to stir for 24 h. Excessive free ligands were removed by centrifugation with the addition of ethanol, methanol, and acetonitrile to the synthetic solution. The purified product was denoted as biphenylprotected titanium (Ti-BP) nanoparticles. Like other monolayer-protected nanoparticles, the Ti-BP particles were soluble in apolar organic solvents such as dichloromethane, THF, toluene, and chloroform but insoluble in polar solvents such as methanol and ethanol. Importantly, while titanium nanopowders have been known for spontaneous combustion upon exposure to air, the obtained Ti-BP nanoparticles were stable under ambient conditions. Thus, no particular protection was needed in the purification process. The purified particles then underwent thorough characterizations (experimental details in Supporting Information). Figure 1A depicts a representative TEM micrograph of the resulting Ti-BP nanoparticles. The inset shows the corresponding particle size histogram. From the micrograph, it * To whom all correspondence should be addressed. E-mail: schen@ chemistry.ucsc.edu. † These authors contributed equally to this work. 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