The Preparation And Modification Of TiO₂ Nanotubes

TiO₂ nanotubular structures have long been of great interest due to their high surface area, good optical & electronic properties, stable physical & chemical properties and non-toxic. In the present work, the self-organized nanotubular layers were fabricated on Ti and Ti-4Zr-22Nb-2Sn alloys by anodization method. The possible effects, such as: voltage, anodization time, concentration of NH4F, pH value, electrode spacing, on the morphology of TiO₂ nanotubes were investigated. The phase composition and micro-structure of nanotubes were also studied by TEM, SEM, XRD, and XPS in detail. Furthermore, in order to improve the applications of TiO₂ nanotubes, they were further modified by different methods to obtain some particular functions. Five kinds of modifications were discussed as follows: (1) Improving biocompatibility of Ti substrate: The TiO₂ nanotubes were prepared on the pure Ti substrate, and then the bioactive Ca-P layer was further prepared on the surface of TiO₂ nanotubes by biomimetic growth method. The objective of this work is to discuss the effect of TiO₂ nanotube microstructural features and immersion time on apatite layer formation mechanism and morphology. (2) Drug loading and release system:The drug of minocycline hydrochloride (MH) was loaded in the nanotubes by physical adsorption method.The release effects were studied in phosphate buffer solution (PBS). (3) Magnetization of TiO₂ nanotubes: The iron oxide nanoparticles were loaded onto self-organized TiO₂ nanotube layers by electrodepositing method. We discussed the effect of depositing voltage and time on the morphology of nanoparticles. (4) The electrocatalytic property of Ag-TiO₂ nanotubes: Silver (Ag) nanoparticles were successfully assembled in self-organized TiO₂ nanotubes by the polyol process. The effects of nanotubular phase structure and deposition time on the morphology of Ag nanoparticles were investigated. Moreover, the electrocatalytic property of Ag-doped nanotubes was also studied. (5) TiO₂-SnSe semiconductor composition: SnSe nanoparticles assembled to vertically aligned TiO₂ nanotubes were obtained by pulsed electrodepositing. The relationship between the morphology of SnSe nanoparticles and the deposition parameters was investigated. The phase composition, micro-structure, and pulsed electrodepositing mechanism of SnSe nanoparticles were also studied in detail.The results can be listed as follows: (I) The self-organized TiO₂ nanotubes with single-size and two-size-scale diameters could be fabricated by anodizing Ti and Ti-4Zr-22Nb-2Sn sheets, respectively. After annealing at different temperature, the nanotube layer could be transformed from amorphous phase to anatase (annealed at 500°C) and to rutile (annealed at 700°C). A possible physical model was also reported according to the evolution of nanotubes during the whole anodization process. For single-size nanotubes, the porous structure could be formed at the initial stage, and then transformed to tubular structure with the anodization time. For two-size-scale nanotubes, the single-size nanotubes could be evolved to the ones with two-size-scale diameters after additional anodization. (II) The results showed that the nanopores and longer nanotubes exhibited the lower diffusion rate of Ca²⁺ and PO43- ions, Therefore, the Ca and P ions in the SBF were apt to be deposited on the tube surface to form thick Ca-P layer. Besides, the thickness of Ca-P layer increased with the immersion time. TiO₂ nanotube arrays provided more nucleation sites for Ca²⁺ and PO43- ions, leading to the formation of Ca-P layer with bond structure. (III) The release rate of MH from TiO₂ nanotubes in PBS increased with tube length. The accumulative release efficiencies of MH-nanotube (4.1μm) and MH-nanotube (5.7μm) could reach up to 90.7 and 87.9% within 194 h, respectively, while the nanotubes with length of 1.5μm and 2.8μm had relatively lower release efficiencies of 48.2 and 59.1%. The sustaining release time could last at least 150 h. The factor that affected the drug release rate significantly was the higher aspect ratio of nanotubes, which could effectively retard and accelerate the drug release. (VI) The obtained nanoparticles consisted of iron nanocrystalline (Fe) and magnetite (Fe₃O₄). The hematite (α-Fe2O3) structure was obtained by annealing in air at 450°C. Furthermore, the TiO₂ nanotubes loading magnetic nanoparticles exhibited good ferromagnetic properties at room temperature. (V) The results showed that Ag atoms reacted with the small amount of ruitle TiO₂ to form the silver titanates for Ag-TiO₂/500°C coatings. The basal plane of Ag (1 1 1) became the main plane, and the Ag doped anatase TiO₂ composites exhibited excellent catalytic activity in electrocatalytic ethanol oxidation both in adic and alkaline media. (VI) The SnSe particle diameter and the corresponding current density increased with"on"and"off"time ratio. The deposited SnSe was polycrystalline and of orthorhombic structure and its composition was slightly Sn-rich. In a word, after different surface modifications, the TiO₂ nanotubes are promising for direct usages in several technical applications, such as energy, environment, and biomedical applications.