Abstract
This thesis study mainly focus on the photo-electrocatalytic de-colorization of dye. Which was conducted in the electrochemical cell, containing WO3．0.33H2O electrode and carboxylic-modified P25 electrode. The WO3．0.33H2O electrode acts as cathode and carboxylic-modified P25 electrode was employed as anode. In addition, the TiO2 and WO3．0.33H2O were modified to enhance the visible-light photo-response to extend the practical application on environmental protection. The WO3．0.33H2O, were prepared by the conventional hydrothermal method. The as prepared WO3．0.33H2O exhibits n-type semiconductor and hexagonal structure showing the tunnels. The textural analysis were carried out by XRD, Raman, TGA, UV-Visible-NIR spectrophotometer and the linear sweep voltammetry (LSV) by electrochemical analyzer. According to the photo-electrochemical analyses, we found that proton-intercalation via negative potential biases can enhance the rate constant of methylene blue (MB) photo-electrocatalytic de-colorization on n-type tungsten trioxide hydrate, rather than the positive potential bias in promoting the charge separation of photo-excited electrons/holes. Due to the generation of W(V) and W(IV) hydroxyl species with smaller band gaps (in comparison with W(VI)) and the electrochemical activation of WO3．0.33H2O, negatively biased proton intercalation into WO3．0.33H2O clearly enhances its photo-electrocatalytic activity for MB de-colorization, including its visible-light photo-electrocatalytic activity. A possible scheme for promoting the photocatalytic activity of WO3．0.33H2O through proton intercalation under constant potential biases is proposed and discussed. In addition, this study employs the experimental design strategy including the fractional factorial design (FFD), path of the steepest ascent (PSA) and central composite design (CCD) coupled with the response surface methodology (RSM) to promote the photocurrent density and photo-electrocatalytic activity on tungsten trioxide hydrates (WO3．0.33H2O) which prepared by microwave-assisted hydrothermal synthesis (MAHS). On the other hand, for the TiO2, the visible-light-driven photocatalytic activity of Degussa P25 TiO2 is effectively promoted by modifiers containing carboxylic groups through impregnating cyclohexanol and a low-temperature heating treatment. The textural analysis were characterized by using XRD, SEM, TEM, PL, ATR-FTIR, and UV-Visible DRS. From the results, the superficial TiO2 modified with the organic modifiers creates certain low band-gap states which enhance its visible-light activity. The carboxylic-modified TiO2 dispersed on the P25 surface shows the p-type semiconductor characteristics, confirmed by the photocurrent responses under the open-circuit state and blue-light irradiation. Accordingly, a p-n heterojunction between the superficial modified TiO2 and underneath P25 enhances the separation of photo-excited electron/hole pairs, resulting in the higher photocatalytic activity. During the photo-electrocatalytic de-colorization test, the photo-excited electrons are further withdrawn by the positive potential bias towards the graphite cathode through the electric circuit, which favors the oxygen reduction reaction (ORR) of two-electron transfer process effectively generates H2O2 for organic pollutants decomposition. Therefore, methylene blue (MB) can be efficiently de-colorized on the carboxylic-modified P25 under the visible-light irradiation meanwhile the de-colorization rate is enhanced under the photo-electrocatalytic mode. Finally, in this work we try to explain the contribution of the dye-sensitizing effect on the photocatalytic de-colorization of dyes dissolved in aqueous media using an anatase TiO2 (A-TiO2) as photocatalyst. The photocurrent-voltage (J–V) curves of a typical dye-sensitized solar cell (DSSC) with the A-TiO2 photo-anodes adsorbed with various dyes, including methylene blue (MB), orange G (OG), rhodamine B (RhB), metanil yellow(MY), acid black 24 (AB24), and N719, are employed to demonstrate the sensitizing characteristics of dyes on A-TiO2. Unlike RhB, the MB, OG, MY, and AB24 cannot work as the photo-sensitizer on A-TiO2 as observed from the J–V curves of DSSCs, reasonably due to the unmatched band position between dyes and A-TiO2. In the photocatalytic de-colorization test for these dyes on A-TiO2 in aqueous media, dye adsorption onto A-TiO2 becomes the key factors affecting the photocatalytic de-colorization rate because the generation of oxidants species (e.g., O2−, OH．or photo-excited holes) occurs on the A-TiO2 surface. The contribution of dye-sensitizing on the photocatalytic de-colorization of RhB is negligible since the dye-sensitizing/electron-injection/dye-regeneration cycle cannot be completed in the degradation media while adsorbed RhB molecules are unstable when few photo-generated electrons have been injected into the conduction and of A-TiO2, probably leading to the decomposition of RhB.