Electrochemical Preparation Of Highly Active TiO₂/SiO₂ Composite Film And Its Application In Photoelectrocatalytic Degradation

The treatment of toxic organic wastewater is one of the most important technicaldifficulties due to the lack of highly effective methods, which has caused severeproblems to our health and the environment. It has been demonstrated that thephotocatalytic degradation of organic pollutants over TiO₂ is the most efficient method todeal with wastewater, which is considered to be used in industrials. In this paper, methylorange (MO) was chosen as one of the representatives of the organic pollutions, and theTiO₂/SiO₂ composite film electrode was prepared through the cathodic electrodepositionmethod. The photocatalytic and photoelectrocatalytic degradation of MO over theTiO₂/SiO₂ composite film electrode was studied. The gavalic cell was introduced to thephotocatalytic system to form coupled photocatalytic system, and then we furtherdiscussed the mechanism of the coupled photocatalytic system and the effect of factorson the coupled photocatalytic system.The ITO electrode was put into the electrolyte solution containing SiO₂ nanoparticlesand Ti sol, and the Pt electrode was worked as counter electrode. The electrodepositionof the TiO₂/SiO₂ composite film electrode proceeded under -2 V for 90 min, and thenwashed, dried in atmosphere environment. After that the TiO₂/SiO₂ composite filmelectrode was calcined under 450℃for 2 hours. It was demonstrated that the SiO₂nanoparticles was embeded in the TiO₂ matrix by the characterization of XRD,SEM,XPS,AFM. The photocatalytic activity of the TiO₂/SiO₂ composite film was 40% higherthan that of the pure TiO₂ film.The composite film electrode showed very highly activity toward thephotoelectrocatalytic degradation of MO. It was further confirmed that when the Ti/Siatom ratio was 2:1, the electrodeposition time was 90 min, and the calcinationtemperature was 450℃, the prepared composite films owned the bestphotoelectrocatalytic activity. Both of electrode potential and the initial solution pH value affected the photoelectrocatalytic degradation of MO. It was found that the optimumelectrode potential was 0.8 V (vs. SCE), and the lower pH value favor to thephotoelectrocatalytic degradation of methyl orange.Due to the introduction of the galvanic cell into the photocatalytic process, the coupledphotocatalytic degradation efficiency of MO over the TiO₂/SiO₂ composite film coupledwith Pt electrode was enhanced about 3.25 times than that of the uncoupled system. Itwas found that larger counter electrode area resulted in higher coupled photocatalyticefficiency. The O₂ bulking could enhance the coupled photocatalytic efficiency greatly.The enhanced coupled photocatalytic efficiency of the counter electrodes obeyed theorder Pt＞Fe＞uncoupled＞ITO＞Ti.The addition of NaCl enhanced the coupled photocatalytic efficiency, whileunfortunately other inorganic salts (such as Na₂SO₄, Na₂HPO₄, NaClO₄, NaF, NaI)suppressed the coupled process. The function of the additional NaCl not only decreasedthe ohmic potential, but also functioned as catalyst. The experiments suggested thatduring the coupled photocatalytic process, the Cl^- could first adsorb on the surface of theTiO₂/SiO₂ composite film and then reacted with photogenerated holes to generated Cl·,ClO^-, ClO₃^-, which could directly oxidize the organic pollutions in the solution. So thecoupled photocatalytic efficiency was enhanced greatly. NaBr would play the same roleas NaCl in the coupled photocatalytic process, which could enhance the coupledphotocatalytic efficiency greatly.Coupled photocatalytic treatment could enhance the photocatalytic activity of theTiO₂/SiO₂ composite film about 2.8 times than that of the new prepared film. It wasmainly due to the increase of the physics water on the surface of the TiO₂/SiO₂ compositefilm during the coupled photocatalytic treatment, which could favor to more hydroxylfree radicals and the enhanced photocatalytic activity of TiO₂/SiO₂ composite film.