| Summary: | Various thicknesses of TiO<sub>2</sub> films were prepared by the sol–gel method and spin-coating process. These prepared TiO<sub>2</sub> films exhibit thickness-dependent photoelectrochemical performance. The 1.09-μm-thickTiO<sub>2</sub> film with 20 spin-coating layers (TiO<sub>2</sub>-20) exhibits the highest short circuit current of 0.21 mAcm<sup>−2</sup> and open circuit voltage of 0.58 V among all samples and exhibits a low PEC reaction energy barrier and fast kinetic process. Photoelectrocatalytic (PEC) degradation of methyl orange (MO) by TiO<sub>2</sub> films was carried out under UV light. The roles of bias, film thickness, pH value, and ion properties were systematically studied because they are the four most important factors dominating the PEC performance of TiO<sub>2</sub> films. The optimized values of bias, film thickness, and pH are 1.0 V, 1.09 μm, and 12, respectively, which were obtained according to the data of the PEC degradation of MO. The effect of ion properties on the PEC efficiency of TiO<sub>2</sub>-20 was also analyzed by using halide as targeted ions. The “activated” halide ions significantly promoted the PEC efficiency and the order was determined as Br > Cl > F. The PEC efficiency increased with increasing Cl content, up until the optimized value of 30 × 10<sup>−3</sup> M. Finally, a complete degradation of MO by TiO<sub>2</sub>-20 was achieved in 1.5 h, with total optimization of the four factors: 1.0 V bias, 1.09-μm-thick, pH 12, and 30 × 10<sup>−3</sup> M Cl ion content. The roles of reactive oxygen species and electric charge of photoelectrodes were also explored based on photoelectrochemical characterizations and membrane-separated reactors. Hydrogen peroxide, superoxide radical, and hydroxyl radical were found responsible for the decolorization of MO.
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