CuOx/TiO2複合型光觸媒材料性質、光降解及光電特性之研究

Materials Characteristics, Photodegradation and Electrical Properties of CuOx/TiO2 Composite Photocatalysts

趙柏凱(Bo-Kai Chao)

指導教授 ： 郭俞麟

大同大學/工程學院/材料工程學系所/碩士(2009年)

摘要

本研究以Cu(hfac)2‧2H2O作為前驅物，利用初濕含浸法與市售之TiO2 (Degussa P25)粉末均勻混合並進行煅燒程序，製備CuOx/TiO2複合型光觸媒。利用p-type半導體CuOx的摻雜與n-type之TiO2形成p-n 接面，以促進電子電洞對分離，藉以增加TiO2之光催化活性。研究中並分析在不同添加濃度及煅燒溫度參數下所製備之CuOx/TiO2複合型光觸媒，其晶體結構、表面形態、化學鍵結，以及光學性質的差異，並探討製程參數對CuOx/TiO2複合型光觸媒的影響。
研究結果顯示，CuOx的添加並未改變TiO2之晶格結構，僅以顆粒與TiO2之表面相鄰；在化學特性(XPS, Raman)的分析中，發現存在於TiO2表面之Cu氧化物的型態包括CuO、Cu2O，以及Cu(OH)x，證實Cu(hfac)2‧2H2O前驅物中之Cu原子可與結晶水中之O原子以及TiO2表面所附著之OH基進行鍵結；光學特性方面，CuOx的添加有效地增加TiO2於可見光波段的吸收，並降低激發螢光強度，顯示CuOx的添加能有效的抑制電子電洞對再結合。
光降解測試中，以亞甲基藍及重鉻酸鉀作為模擬污染物，分別進行光氧化及光還原的光降解實驗，結果發現添加濃度為本研究中之最重要參數。光氧化實驗中，使用日光燈為燈源的條件下，不同煅燒溫度下之1%-CuOx/TiO2光觸媒的反應速率常數約介於為1.9 ×10-3至2.3 ×10-3 min-1之間，高於TiO2(P25)之1.4 ×10-3 min-1；而1%-CuOx/TiO2光觸媒之光降解效率介於39.4 %至46.1 %之間，亦高於P25之30.7 %。而在BLED光源下的光降解表現，1%-CuOx/TiO2光觸媒的反應速率常數為7.0×10-4至1.3 ×10-3 min-1之間，高於P25之2.0×10-4 min-1；而1%-CuOx/TiO2光觸媒之光降解效率為22.0 %至40.2 %之間，亦高於P25之5.4 %。顯示在1%之CuOx添加下，能有效地增加TiO2在可見光區段下之光降解效率。但過量Cu元素的摻雜，卻使光降解的效果大幅下降，推測可能因為Cu(OH)x的形成，佔據了TiO2表面的活性基，而降低TiO2之光降解效率。在光還原的應用中，則以添加濃度為1%及煅燒溫度為300℃的條件下所製備之光觸媒效果最佳，反應速率常數可達2.3×10-3 min-1，此結果亦優於P25之1.0×10-3 min-1。
在染料敏化太陽能電池(DSSC)的應用上，本研究製備之CuOx/TiO2複合電極，大幅的降低DSSC的效率。推測其原因，CuOx為一能隙較小之半導體，當CuOx受到擬太陽光源的照射時產生電子電洞對，而染料受激發之電子可能與CuOx所產生的多數電洞載子進行再結合，而無法有效的使電子移動至TiO2的傳導帶當中。實驗結果指出，以CuOx的添加改良TiO2電極為一不可行的方法。

關鍵字

光觸媒 ； 光降解 ； p-n 接面 ； 染料敏化太陽能電池 ； TiO2

並列摘要

CuOx/TiO2 composite photocatalysts with different CuOx dopant concentrations were prepared by Cu(hfac)2‧2H2O as a precursor via incipient wetness impregnation method. Due to the combination of p-type CuOx and n-type TiO2, p-n junction rigion were formed at the interface of CuOx and TiO2 particles. The formation of the p-n junction could reduce the recombination of electrons and holes, and enhance the photocatalysis activity. In this study, crystal structure, surface morphology, chemical properities, and optical properities of CuOx/TiO2 photocatalysts were investigated.
According to the XRD analysis, the doping of the CuOx did not change the microstructure and surface morphology of TiO2, indicating that CuOx particles were adhered onto TiO2 surface. In the chemical analysis(XPS, Raman), the type of CuOx were CuO, Cu2O, and Cu(OH)x, revealing that Cu atom in Cu(hfac)2‧2H2O precursor could combine with the hydroxyl radical on the surface of TiO2 to form the Cu(OH)x. In the optical properties, the absorption edge of CuOx/TiO2 composite photocatalysts were extended to longer wavelengths, and the doping of CuOx were successfully decreased the recombination of electrons and holes from PL analysis.
In photodegradation test, Methylene Blue and K2Cr2O7 were used as the pollutants for the photo-oxidation and photo-reduction processes, respectivity. For photo-oxidation of Methylene Blue, the reaction rate constants of 1%-CuOx/TiO2 photocatalysts prepared at various temperatures were between 1.9 ×10-3 and 2.3 ×10-3 min-1 under the fluorescent lamp, which are greater than that of P25 (1.4 ×10-3 min-1), and the photodrgradation efficiency of 1%-CuOx/TiO2 composite photocatalysts were between 39.4 % and 46.1 %, were also greater than P25 (30.7 %). Under the BLED lamp, the reaction rate constants and the photodrgradation efficiency of 1%-CuOx/TiO2 photocatalysts were also higher than P25. However, at heavy CuOx doping concentration, excessive CuOx increased the formation of the Cu(OH)x, which could cover the surface of TiO2, and leaded to decrease on the photocatalytic activity of photocatalysts. In the photo-reduction process, the optimum parameter in this study was 1%-CuOx/TiO2 calcined at 300℃.Under fluorescent lamp, the reaction rate constants was 2.3×10-3 min-1，greater than that of P25 (1.0×10-3 min-1).
For Dye-Sensitized Solar Cells (DSSC) application, the doping of CuOx decreased the efficiency of DSSC. The reason for the lower efficiency could be due to the recombination of electrons from dye and holes from CuOx. As CuOx is p-type semiconductor with a narrow energy band gap, pairs of electrons and holes could be easily produced under irrdation of solar simulator. Electrons injected from the excited dye molecules could recombined with the holes from CuOx. Therefore, generated electrons from dye could not quickly transfer through TiO2 layer to reach the ITO substrates for the further redox reaction of electrolyte. This result indicated that CuOx/TiO2 composite electrodes were not suitable for DSSC use.

並列關鍵字

TiO2 ； photocatalysts ； photodrgradation ； p-n junction ； Dye-Sensitized Solar Cells

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