Fabrication Of Functionalized TiO₂ Nanotube Arrays And Their Photocatalytic Application

Titanium dioxide （TiO₂） possesses the advantages of low cost, non-toxicity, high efficiency, chemical inertness and photostability, so it is still the most promising photocatalyst. However, the widespread technological use of TiO₂ is retarded by its wide band gap, which requires ultraviolet irradiation that accounts for only 4–5% of the spectrum of solar energy for photocatalytic activation. Additionally, TiO₂ has the disadvantages of high recombination rate of the photogenerated electron–hole pairs and poor adsorbability to most of organic contaminants. Quite a few different methods have been developed to solve the problems, such as ion doping or noble metal deposition for improving the separation and transport of photocarriers during photocatalysis, semiconductor composition for enhancing the visible light absorbance and restraining the recombination of the photogenerated carriers, or surface hydrophobic modification such as surface molecular imprinting for enhancing the adsorbility to organic pollutants.TiO₂ nanotube （TiO₂ NT） arrays prepared by anodization has attracted increasing attention due to its large surface area, tunable size, high orientation, stable physical/chemical properties, and repetitious recycle. Here, we have prepared some TiO₂ NT array composites: （1） CuInS₂ modified TiO₂ NT array which extended the absorption of TiO₂ in the visible light; （2） Anthracene-9-carboxylic acid （9-AnCOOH）-imprinted TiO₂ NT array which enhanced the adsorbility to the target organic pollutant and improved its photocatalytic effiency; （3） An in situ electrodeposition technique coupled with photocatalytical reduction was developed to produce reduced graphene oxide film with large-area, uniform, continuous, crystalline properties, high-quality graphene films on the TiO₂ NT arrays. The detail is listed as below:（1） Fabrication, characterization and photoelectrochemical properties of CuInS₂-TiO₂ NT: a pulsed electrodeposition technique, based on a multipulse sequence of potentials of equal amplitude and duration, has been proposed for the preparation of CuInS₂ nanoparticles onto the TiO₂ NT surface. Compared with the unmodified TiO₂ NT, the absorption of CuInS₂-TiO₂ NT in the visible light region remarkablely increased, and the zero-current potential negatively shifted from–0.62 to–0.79 V, reducing the recombination rate of the photogenerated electron–hole pairs. The maximum photocurrent was obtained on CuInS₂-TiO₂ NT under 300 sequences with weak photoluminescence emission, indicating good separation efficiency of electron-hole. After 160 min, CuInS₂-TiO₂ NT can entirely remove 2,4-D.（2） Fabrication, characterization and photoelectrochemical properties of TiO₂ NT using surface molecular imprinting modification: in this study, a new molecularly imprinted film （MIF） layer modified TiO₂ NT array （MIF-TiO₂ NT） photocatalyst was prepared through constructing an imprinted TiO₂ film layer onto the surface of a TiO₂ NT array using a sol-gel method with 9-AnCOOH as the imprinting molecule. In comparison with the unmodified TiO₂ NT, the MIF-TiO₂ NT showed a higher adsorption capacity for the target contaminant and an enhanced photocatalytic activity in the contaminant photodegradation. After 40 min, thin-layer modified TiO₂ NT can totally remove 9-AnCOOH.（3） Fabrication, characterization and photoelectrochemical properties of reduction oxidation graphene/TiO₂ NT composite: in this work, owing to the ease-of-operation, cost-effective, and environment-benign properties, photoreduction-assited electrochemical reduction method was applied to prepare graphene/TiO₂ NT composite. The composites exhibited increased charge separation and transport properties, extended light absorption range and intensity as well as improved adsorptivity to 9-AnCOOH. Therefore, the composite displayed a significantly enhanced photocatalytic activity compared to the bare TiO₂ NT. The adsorptivity of 26-cycles TiO₂ NT （0.24 mg/L） is as six times as the bare TiO₂ NT （0.04 mg/L）, and the removal efficiency of 26-cycles TiO₂ NT （100%） is as two times as the bare TiO₂ NT （50%）.