| Summary: | 碩士 === 國立成功大學 === 化學工程學系 === 88 === Titanium dioxide is widely applied in photocatalysis to carry out the redox reaction. Recently, the usage of photoenergy is a significant subject of study in photocatalytic reaction. The usage of photoenergy of tungsten oxide is larger than titanium dioxide due to its smaller energy band-gap. However, the recombination of electron hole pair owing to inadequate reduction potential of conduction band of tungsten oxide results in bad phototcatalysis performance. In this study, using bias potential to improve tungsten oxide photocatalytic capability is investigated. Meanwhile, the influence of the electrode preparation condition and photoelectrocatalytic reaction factors is discussed.
The photocatalyst electrode was prepared by deposited the semiconductor on conductive substrate with reactive sputtering. XRD, SEM, Potentiostat/Galvanostat, UV/VIS sepectrophotometer, and profilometer were used to analyze the characteristics of photoelectrode. An Arc lamp for simulating ultraviolet of sunlight used to induce photoelectrocatalytic decolorization of methyl orange. Mehtyl orange is an azo compound in dye wastewater.
The prepared TiOx/Ti and WOy/Ti electrode show the excellent crystal structure without calcination. The TiOx film prepared by reactive sputtering is very uniform and it presents a slight blue shift. The blue shift effect means the photocatalyst is needed UV light with larger energy to excite electron hole pair. The best photocurrent of the TiOx/Ti electrode was obtained at the deposited condition with 300 W sputtering power and 3.2´10-2 mbar total sputtering pressure containing 25 % oxygen partial pressure. The results indicate that WOy/Ti film electrode also has excellent photoactivity by applying a bias potential and irradiation simultaneously. Decolorization of methyl orange proceeded through oxidation reaction promoted by the hydroxyl radical (OH·) that was generated from the reaction of holes and hydroxyl groups or water adsorbed on the surface of WO3. Decolorization of methyl orange proceeded through reduction reaction promoted by the electron is not observed. The photoactivity of WOy/Ti film electrode decreases quickly in the alkaline solution due to the WO3 was dissolved.
The pH value of the aqueous solution, concentration of methyl orange, and bias potential are the major factors affecting the photoelectrocatalytic decolorization of methyl orange but the effect of power of UV light is minor. The photocurrent efficiency is evaluated by the ratio of concentration change of methyl orange and photocurrent. The photocurrent efficiency is about 1.2% and 12% in alkaline and acidic solution, respectively. The low photocurrent efficiency results from deactivation of hydroxyl radical. Increasing bias potential results in higher reaction rate and lower photocurrent efficiency. Based on the experiment results, the mechanism of photoelectrocatalytic reaction is proposed and a semitheoretical kinetic rate equation is obtained. Theoretical analysis ones correlate well with experimental results. The rate determined step is methyl orange reacted with hydroxyl radical. Langmuir- Hinshlewood adsorption of methyl orange is suitable to this system. The semitheoretical kinetic rate equation is represented as following
（I-1）
where Ri is decolorization rate of methyl orange (mM/min). P, E, and [MO] represent power of UV light, applied bias potential, and concentration of methyl orange, respectively. a and b are the reaction order of power of UV light and applied bias potential, respectively. The value of rate constants including Kp, K’, and K6 calculated from experimental data are
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