改質氧化鎢與二氧化鈦於有機染料光電催化降解之研究

Modification of Tungsten Oxide and Titanium Dioxide for Photo-electrocatalytic Degradation of Organic Dyes

蕭宇呈(Yu-Cheng Hsiao)

指導教授 ： 胡啟章

國立清華大學/工學院/化學工程學系/博士(2015年)

摘要

此篇研究中利用WO3．0.33H2O為陰極材料與羧基物種修飾二氧化鈦為陽極材料，將其組裝為一電化學反應器。將此電化學反應器運用於光電催化脫色反應中進行探討。此外，為了增加WO3．0.33H2O與二氧化鈦於環境保護的運用性，本研究將此兩材料加以修飾進而增加其可見光應答性。
WO3．0.33H2O部分係以傳統水熱法製備一具有穿隧結構之六方晶型的 n型半導體WO3．0.33H2O。利用XRD、Raman、TGA、UV-Visible-NIR 光譜儀與電化學分析儀進行試樣分析，進而探討此n型半導體WO3．0.33H2O之結構性質。常見的n型半導體係利用正偏壓的施予有效增加光激發電子與電洞的分離能力，達到提升光觸媒的光催化活性。然而，本研究所製備的WO3．0.33H2O可藉由負偏壓的施予使得氫離子嵌入WO3．0.33H2O結構中，促進該結晶產生具較小能帶的W(V)和W(IV)物種(相對於W(VI)而言)，此些物種可以有效增加WO3．0.33H2O在紫外光至可見光區的吸收能力而提高其光催化能力。本研究是以亞甲基藍的光電催化降解作為示範反應，並藉由光電化學分析了解WO3．0.33H2O可利用負偏壓的施予促進氫離子嵌入其結構進而增加對亞甲基藍的光電催化降解能力。此機制現象於文章內將加以詳述。此外，本研究亦利用實驗設計 (其中包含: 部分因素設計(FFD)、最陡上升路徑法 (PSA)與中央合成設計 (CCD))與微波輔助水熱法製備WO3．0.33H2O進行光電化學特性探討，以快速與有效方式提升WO3．0.33H2O之光電流與光電催化活性。
羧基物種修飾二氧化鈦部分，係藉由含浸法將羧基物種有效修飾於二氧化鈦上並結合正偏壓的施予，進而有效的提高二氧化鈦於可見光下的光電催化活性。進一步利用XRD、SEM、TEM、PL、UV-Visible DRS 光譜儀與ATR-FTIR加以探討經羧基物種修飾之二氧化鈦的結構性質。結果顯示，經有機羧基物種之表面修飾的二氧化鈦其可能為一可見光敏化劑或其可以與二氧化鈦產生一較小能係的物種進而提高二氧化鈦的可見光活性。然而，初步實驗結果也顯示分佈於P25表面之羧基物種修飾二氧化鈦，其在藍光激發與開環電路量測條件下表現p型半導體的光電應答訊號。因此，此p型之羧基物種修飾二氧化鈦將與n型P25形成p-n異質結構層，進而有效增加光激發電子與電洞的分離效果。光電催化脫色反應過程中，可利用正偏壓的施予進而有效將光激發電子由二氧化鈦的電極上傳遞至石墨對電極上，以提高光激發電子與電洞的分離效應。同時，於石磨對電極上，則傾向發生兩個電子轉移的氧氣還原反應，進而產生可將有機汙染物分解的過氧化氫。因此，亞甲基藍可以於可見光與光電化學的反應下有效的提高其光電催化脫色之活性。
總結上述之WO3．0.33H2O與羧基物種修飾二氧化鈦的反應機制，本研究將其組裝為一電化學反應器並將其有效運用於光電催化脫色反應中進行探討。最後，將進一步探討此電化學反應器對於不同酸鹼值溶液的適用性。
本研究的最後一部份主要係以銳鈦礦之二氧化鈦為主軸。藉由典型的染料敏化太陽能電池系統所測之光電流密度與電壓曲線圖中，釐清染料敏化機制與染料水溶液的光催化脫色反應間之關聯性。本研究所使用之染料為methylene blue (MB), orange G (OG), rhodamine B (RhB), metanil yellow (MY), acid black 24 (AB24), 與N719。實驗結果顯示，MB、OG、MY與AB24等染料對於銳鈦礦之二氧化鈦來說不具染料敏化特性，主要係因為染料與銳鈦礦之二氧化鈦間的能帶不匹配所造成之。相對於其他染料，RhB與二氧化鈦間的能帶雖然匹配而具有光敏化之特性，但染料於光催化脫色反應中因染料敏化/電子躍遷/染料再生等反應路徑並未完成，因此染料光敏化效應對於光催化脫色反應的貢獻極為微弱。最後，本研究並證明在光催化脫色反應中，染料與二氧化鈦間的吸附能力對於脫色反應性具有決定性的影響，主要係因為分解汙染物之活性物種(O2–、OH∙或光激發電洞)主要係產生於二氧化鈦表面。

關鍵字

光電催化反應 ； 三氧化鎢 ； 實驗設計法 ； 羧基修飾二氧化鈦 ； 染料敏化效應

並列摘要

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.

並列關鍵字

Photo-electrocatalytic reaction ； WO3．0.33H2O ； Experimental design strategy ； Carboxylic-modified P25 ； Dye-sensitizing effect

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