Abstract
Bare and silver-loaded TiO2 multiple-walled nanotube arrays (MWNTAs) were prepared by template-assisted atomic layer deposition (ALD), and ultrasound-aided photochemical reduction was employed to synthesize silver nanoparticles on the nanotubes. The MWNT contains more than one concentric tubes with different diameters. They offer larger reaction surface area than conventional single-walled tubes. Increasing the wall number of nanotubes (i.e., more material) still keeps their high specific surface area characteristic. The pore size of home-made anodized aluminum oxide (AAO) can be tuned by the applied voltage, electrolyte composition, reaction temperature, and pore-widening time. The home-made AAO was chosen as the template to fabricate TiO2 MWNTAs, due to its highly ordered nanoporous array structure. The pore size ranging from 20 nm to 150 nm was successfully fabricated. To fully duplicate the AAO structure, ALD with excellent conformability and thickness control ability was employed to deposit TiO2 into the pores of AAO. Gaseous precursors titanium (IV) tetrachloride (TiCl4), trimethylaluminum (TMA), and H2O were selected for the ALD process. They were alternatively introduced into the chamber and chemically reacted with each other to form nanolaminate of TiO2 and Al2O3 on AAO. The thickness of nanotube wall and the gap span of MWNTAs were accurately controlled by the ALD cycle numbers of TiO2 and Al2O3. Silver nanoparticles were further deposited on the TiO2 nanotubes by ultrasound-aided photochemical reduction method under UV light. The TiO2 MWNTAs were characterized by X-ray diffraction for their crystallinity and phase, by scanning electron microscopy and high resolution TEM for their morphology, and by UV-visible light absorption spectroscopy for the optical property. To examine the efficiency of both bare and Ag-loaded TiO2 MWNTAs in photodecomposition test, single-, double- and triple-walled TiO2 MWNTAs with or without Ag-loading and TiO2 nanorod arrays were selected as photocatalysts with the same reaction area (π/2 cm2). A solution of 10 ml containing 2 ppm methylene blue (MB) was tested for photodegradation. After the test, the concentration of MB was finally reduced to less than 10 % of the original concentration by triple-walled TiO2 MWNTAs and Ag-loaded single- and double-walled ones. It was demonstrated that the triple-walled TiO2 MWNTAs showed the best activity among the unloaded TiO2 samples. It could be attributed to the multi-heterojunction of three coexisting phases (anatase, rutile, and brookite) and larger reaction surface area. The result also reveals that Ag does enhance the efficiency of photocatalyst. It is attributed to Ag particles serving as electron traps to retard recombination rate of electrons and holes.