| In recent years, there are extensive reports regarding the photoelectrocatalytic (PEC) degradation of organic pollutants, more and more researchers devoted themselves to improving PEC efficiency including fabricating high efficiency TiO2 film electrodes. Ionic liquids are receiving an upsurge of interest as green solvents, they possess non-volatile, non-toxicity, wide electrochemical window, high conductivity and stability, and leading to preparing porous materials with high nucleation rate, strong stability and highly ordered structure. These inherent characteristics make them highly suitable as electrolytes for electrochemical reactions, providing a novel approach for TiO2 nanomaterials preparation. To our knowledge, there are few reports about fabrication of TiO2 film electrode in ionic liquids. This dissertation mainly included preparation of composite structural TiO2 oxide film electrode using ultrasonic assisted anodic oxidation method in ionic liquid based organic electrolyte, growth mechanism of composite structural TiO2 oxide film and PEC properties of TiO2 film electrodes mentioned above.(1) Preparation and characterization of composite structural TiO2 oxide film electrodeComposite structural TiO2 oxide film electrodes were prepared by anodic oxidation within ultrasonic bath in the organic electrolyte mainly included 1-n-butyl-3-methyl-imidazolium tetrafluoroborate ([BMIM]BF4) and ethylene glycol. The surface structure of TiO2 oxide film was controlled by changing experimental parameters, such as composition of electrolyte (ionic liquid and its concentration, water content, organic electrolyte), oxidation voltage, oxidation time and ultrasonic power. SEM images indicated that different structural TiO2 oxide films were obtained in different experimental conditions. Composite structural TiO2 oxide film electrodes were prepared in 0.6 wt% [BMIM]BF4-10 vol% H2O-EG electrolyte at 60 V applied voltage under 480 W ultrasonic power for 20 min. This composite film includes coral structural film layer and double-wall nanotube arrays layer. The coral structural film possesses excellent light trapping ability and short double-wall nanotube improves photoinduced electron transmission rate, so that this composite structural film highly improves the PEC ability of TiO2 film. In addition, the adhesion between the composite film and titanium substrate was strong, without dropping even in ultrasonic bath for long time. EDS patterns indicated that composite structural TiO2 film mainly contained titanium and oxygen elements. A small number of carbon elements maybe came from electrolyte. XRD patterns showed that TiO2 films fabricated by anodic oxidation were amorphous without being annealed, TiO2 films crystallized into the anatase phase after annealed at 400℃ and 500 ℃, into the anatase and rutile mixed phase after annealed at 600 ℃, into rutile phase after annealed 700 ℃.(2) The formation mechanism of composite structural TiO2 oxide film electrodeThe formation mechanism of composite structural TiO2 oxide film electrode was tentatively revealed, on the basis of surface morphological changes of TiO2 oxide film prepared in different time. The chemical reaction in growth process of composite structural TiO2 oxide film is similar to that in growth process of TiO2 nanotube arrays in F ions based electrolyte. Unique structural TiO2 oxide film was obtained due to the difference between BF4-ions and F-ions. In initial stage, compact oxide film was formed, and then nanotube array structural film was formed on account of chemical etching. When the balance between oxide formation and dissolution was reached, the surface of nanotube arrays were dissolved into coral structural film, and double-wall nanotube arrays were existed under this film. This composite structure would be damaged with reaction time extending. Double-wall nanotube arrays formation was related to contraction of titanium hydroxide during dehydration process. Coral structure was fabricated on account of complicated dissolution of oxide film by BF4-ions.(3) The PEC properties of composite structural TiO2 oxide film electrodesThe PEC properties of various composite structural TiO2 oxide film electrodes were investigated by PEC degradation of azo dye MO solution. The main factors of PEC degradation, for instance, surface morphology of TiO2 oxide film electrode, applied bias potential, concentration of Na2SO4, concentration of initial MO solution were analyzed. The results indicated that composite structural TiO2 oxide film electrode show excellent PEC properties. The removal efficiency 20 mg/L MO solution was higher than 95%, TOC removal efficiency was higher than 75% in 5 h-PEC degradation at 2.0 V bias potential.(4) The photoelectrocatalytic mechanism of MO solutionThe TOC removal efficiency of MO solution indicated that it’s very difficult to mineralize MO molecular completely, although the TOC removal efficiency was higher than 80%. In addition, the Liquid chromatography results of MO solution showed that MO molecular was indeed destroyed in PEC degradation. But PEC degradation was an complicated process, and the intermediates should be investigated further. |
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