Study On The Photoelectrocatalytic Capability Of TiO₂ And Its Application

The decolorization of textile wastwater has been attracting many research fellows' interests. During the last two decades, alternative methods for dye treatment have been investigated, including chemical, physical and biological methods. Due to the stability of modem dyes, conventional biological and physical treatment methods are ineffective, because the dye can't be completely mineralized with these methods.Since Fujishima and Honda found that the water could be decomposed to hydrogen (H₂) and oxygen (O₂) on the TiO₂ electrode, photocatalysis had become a promising advanced oxidation technology and already been successfully used for decontamination of organic materials in aqueous solutions. TiO₂ is thought to be the most promising semiconductor, due to its high photocatalytic activity, nontoxicity,low cost and chemical/photocorrosion stability to light illumination. The band gap of the TiO₂ is 3.2 eV, when illuminated with lightλ＜400 nm. Photoexcited electrons (e^-) and holes (h⁺) are generated on the surface of the TiO₂. The valance band holes (h⁺) are strongly oxidizing reagent and can react with organic materials in aqueous solutions. The efficiency of the photocatalysis depends on the recombination rate of the photogenerated electrons (e^-) and photogenerated holes (h⁺).In this study, nanostructured titanium dioxide has been prepared by improved sol-gel chemical methods. In the process of preparation TiO₂, acetic acid was used instead of nitric acid and hydrochloric acid, because acetic acid is a good ligand. It can restrain the hydrolysis of Ti(OBu)₄ efficiently; and finally obtained more stable and small TiO₂ nanoparticles, the diametres of the nanoparticles was about 10 nm.In order to improve the degradation rate of the organic, some anions such as NO₃^-, Cl^-, HCO₃^- and SO₄^2- were put into aqueous solutions and testified that the selection of a supporting electrolyte for the complete degradation of pollutants was important to the photoelectrocatalytic (PEC) process. The presence of the chloride (Cl^-) and sulfate (SO₄^2-) could decrease the recombination of the electron-hole pairs in the photoetectrocatalytic process. As a result of improved change separation, a concomitant increase in the overall photocatalytic activity of TiO₂ is observed. The study found that the Cl^- in the solution has different capability with others, the transmittance of the solution with NaCl supporting electrolyte is bigger than those with other electrolytes' solution. Cl^- could significantly increase the degradation rate of eosin B. Through the experiments, we demonstrated that Cl^- acted not only as a supporting electrolyte in the aqueous solutions but also as an indirect oxidizing agent in the photoelectrocatalytic oxidation process. Cl^- absorbed on the surface of the TiO₂ can be oxidized by the holes (h⁺) and generates other highly oxidizing species, which can react with some groups in the dye molecule and finally destroy the conjugated structure of the dye into micro-molecules that is easy to degrade, and finally lead to the degradation of the dye.At the same time, we studied the effect of potential. Under a positive applied potential, the electron is driven more efficiently to the counter electrode, decreases the recombination rate of the electrons and holes, and increase the lifetime of electron-hole pairs. It can also attract organic negative ions to the surface of the TiO₂ film electrode where the holes can react with H₂O/OH^- to give OH- radical with great oxidant power, which improves the efficiency of oxidizing organic contaminants.Titanium dioxide (TiO₂) film was immobilized on the indium-tin oxide (ITO) glass and used as a test photo-anode. Using Pt wire as a photo-cathode, saturated Ag/AgCl electrode as the reference electrode, eosin B as target pollution and NaCl,Na₂SO₄, NaNO₃, NaHCO₃ and Na₂CO₃ were selected as electrolyte, the photoelectrocatalytic behavior of the system has been thoroughly investigated. The experiments tested that the chroma removed rate of the wastwater wsa more than 98% in 30 minutes, under the irradiation of UV, and with the potential of+4.38V in the presence of 0.5mol/L Cl^-. The photo-anode (TiO₂/ITO) applied in the experiment could be used continuously more than 300 minutes, but the decompose rate of the dye (eosin B) didn't decrease obviously. This proved that the stability of the TiO₂ that was used as photo-anode in the experiment was rather well. Thus, the system has a good applied foreground in the practical application.