The Photocatalytic Behavior Of TiO₂ Nanofilm Electrode In Dye Solutions

The most recent researches in water purification concern the oxidation of organic substrates dissolved in water. As one of the new functional materials, titanium dioxide nano-semiconductor has been proven to be the most suitable for widespread environmental applications. Titanium dioxide is biologically and chemically inert; it is stable to photo and chemical corrosion, inexpensive and non-toxic. Furthermore, TiO₂ is photoactive, sensitive to UV or near visible light.The photocatalytic activity of TiO₂ has been widely studied and applied in wastewater treatment, air purification, deodorization and room sterilization. However the use of TiO₂ is limited due to the low quantum efficiency (< 5%) because of the recombination of the generated electrons and holes, which become the main barrier in the development of the photocatalytic technology.The electrochemical anodic oxidation method was used into preparation of TiO₂ photocatalytic nanoporous film in our laboratory by our team in recent years. In this paper, the experimental preparation parameters including the nature of electrolyte, the oxidation voltage, the oxidation temperature and the oxidation time were carefully controlled. Their influence on the structure, morphology and photocatalytic activity of the TiO₂ film was systematically evaluated. The internal relationship between the structure and property of the TiO₂ film was analyzed. The result showed that there is a relatively wide range in the selection of the preparation condition.The experimental results revealed that the optimum preparing conditions are that the concentration of electrolyte solution is 1.0 mol/l H₂SO₄, the oxidation voltage is 130V, the oxidation temperature is at room temperature, and the oxidation time is at about 5min. The film prepared under such condition has multi-porous morphology and it is constituted of a mixture anatase and trace rutile. It showed a high photocatalytic activity. The effects of doping metal ions into TiO₂ film by anodic oxidation method on photocatalysis are also studied. In this work, the bias was firstly applied to improve the performance of TiO₂ photocatalysis. The bias can accelerate the move of the photogenerated electron and hole. Their recombination is restrained and then the quantum yield of the semiconductor is increased. The result indicated that by varying the potential bias, the photocurrent density increased from micro-scale (μA) to milli-scale (mA) and when the applied anodic bias reached the value of 3.5V, the rate of increasing of the photocurrent density began to decrease. The change of photocurrent density with the applied bias follows the parabolic laws.A three steps electrochemical method called in the present work re-anodization technique was designed to incorporate transition metals with unfilled d-levels (Pt, Ir, and Ag) and (Fe, Mn, Cr, Co and Ni) into TiO₂ film. The experimental results showed that the photocatalytic properties of transition metal doped-TiO₂ greatly depend on the nature of the transition metal used as dopant.This work demonstrates that a common textile synthetic azo-dye, acid red G and acid orange 7 could be decolorized efficiently using transition metals doped- multi-porous TiO₂ electrodes produced by a re-anodization method. The rate of degradation was remarkably enhanced by the incorporation of transition metals into titanium dioxide but the doping with nickel was detrimental.The effect of transition metals concentration on the degradation of acid red G was also investigated and the results showed that the photocatalytic efficiency increases with the increase in the metal dopant concentration up to an optimum level (about 1.0 at %). Above the optimum metal doping concentration (about 1.0 at %), the photocatalytic activity of metal-doped TiO₂ was lower.The experimental absorption spectra demonstrated that the Pt-TiO₂ film electrodes have a significant increase of absorbance in visible light due to Pt deposition, which indicates a new possibility for improving TiO₂ photoresponse. The photodegradation of acid red G or acid orange7 (anionic dyes) was faster in acidic condition (pH 2.1) while in alkaline solution the degradation was lower (pH 12.2). At pH 7.1 an intermediate degradation was observed. At acidic pH, the TiO₂ catalyst surface is positively charged and acid red G molecules are bound on the catalyst surface enhancing then the photodegradation rate.Basing on the pervious work, a waste-water decontamination reactor is designed. It is composed of TiO₂ as working electrode, graphite as counter electrode, external power source to generate the applied bias, ammeter to monitor the direct current, transparent container and ultraviolet light. The experimental results showed that, after 3.0 h of illumination of acid red G 7.5 ppm (pH 2.1, using Pt-TiO₂ electrode), the decolorization was about 95% and the COD removal was about 86.3 %. The reduction of COD removal increased with increasing reaction time. The rate of COD removal was much lower than that of decolorization. This is due probably to the formation of smaller uncolored products during the degradation of acid red G, which continue to contribute to the COD of the solution. To achieve complete degradation of smaller organic compounds, longer irradiation time is required.The effect of the addition of the inorganic oxidants such as IO₄^- and H₂O₂ on the photodegradation of acid orange 7 by using Fe-doped TiO₂ multi-nanoporous prepared by the re-anodization method was also investigated. This work demonstrated that the addition of IO4" prior to UV irradiation significantly increases the degradation of acid orange 7. Moreover, the addition of propanol, CH₃-CHOH-CH₃ to remove OH°in acid orange 7 solution demonstrated that the photodegradation of AO7 was not significantly affected by the addition of propanol. This observation showed that the photodegradation of AO7 is due to the holes reaction and not due to OH°reaction. The photocatalytic activity of the TiO₂ was greatly modified in samples doped with Fe. The maximum increase of photocatalytic activity was found at an optimum concentration of approximately 1.0 at % for Fe.The photocurrent increased when 2-propanol was added to AO7 solution. The increase of the photocurrent is due to the photo-oxidation of 2-propanol on the surface of TiO₂ electrode by holes. The degradation rate of AO7 was not significantly affected by the addition of an excess of 2-propanol in the photocatalysis system while the rate was strongly affected in the system KIO₄/ UV. This indicates that the photocatalytic degradation of AO7 in acidic solution (pH= 2.4) proceeds via a direct oxidation of the organics by the photogenerated valence band holes while the OH°radical could be one of the major oxygenating species responsible for the photodegradation of AO7 in the photolysis of KIO₄.The catalyst's lifetime is an important parameter of the photocatalytic process, due to the fact that its use for a longer period of time leads to a significant cost reduction of the treatment. The results demonstrated that the transition metals-TiO₂ electrode represent a catalyst with good photocatalytic properties, probably suitable to use in a real wastewater applications for an economic point of view.The elimination of these azo dyes fulfils the conditions established for a first-order heterogeneous reaction kinetic model. Langmuir- Hinshelwood rate expression was employed to study the kinetic of the degradation of acid red G by UV / TiO₂ electrodes system. The adsorption equilibrium constant, the rate constant, and the initial degradation rates were determined for different electrodes.