Thermal annealing synthesis of titanium-dioxide nanowire-nanoparticle hetero-structures

Crystalline TiO₂ nanowire-nanoparticle hetero-structures were successfully synthesized from titanium foils by using a simple thermal annealing method with the aid of CuCl₂ at the atmospheric pressure. Nanowires were grown from Ti foils by simply annealing Ti foils at 850 °C. Then, TiCl₄ was delivered to TiO₂ nanowires so as to precipitate TiO₂ nanoparticles on nanowire surfaces. At 750 °C reaction temperature, nanoparticles of tens of nanometers in diameter were well distributed on pre-grown nanowire forests. Nanoparticles were likely to be precipitated by TiCl₄ decomposition or oxidation and that require high temperatures above ∼650 °C. Electron microscopy, X-ray diffraction, and UV-vis spectroscopy analyses show they have the rutile polycrystalline structure with a slightly enlarged bandgap compared to that of bulk TiO₂. The influence of key synthesis parameters including reaction temperature, reaction time, and quantity of supplied materials on the incorporating nanoparticles was also systematically studied. The optimum reaction condition in the present paper was identified to be 750 °C annealing with repetitive 20 min reactions. A higher reaction temperature yielded larger diameter particles, and higher loading of Ti produced dense particles without changing the particle size. Finally, this method could be utilized for synthesizing other metal oxide nanowires-nanoparticle hetero-structures.