利用電子、電洞和自由基捕捉劑探討氧化亞銅晶體於光催化的晶面效應

Facet-Dependent Photocatalytic Properties of Cu2O Crystals Probed by Electron, Hole and Radical Scavengers

朱婕瑜(Chieh-Yu Chu)

指導教授 ： 黃暄益

國立清華大學/理學院/化學系所/碩士(2017年)

摘要

氧化亞銅的晶面效應已經藉由光催化實驗被證實，具有單一晶面之氧化亞銅立方體在甲基橙光降解中沒有光催化活性，而氧化亞銅菱形十二面體的催化活性遠遠優於八面體和立方體。為了更進一步去探討不同晶面之氧化亞銅，其光降解速率為何具有如此大的差異性，本篇研究藉由添加電子、電洞、氫氧自由基以及超氧自由基捕捉劑於光催化實驗中，並比較各種活性物種的光降解速率。當添加電子捕捉劑於光催化實驗中，其結果顯示立方體、八面體以及菱形十二面體的光降解速率分別為1%、13%和65%；而添加電洞捕捉劑時，立方體、八面體以及菱形十二面體的光降解速率分別為3%、69%和97%，根據此實驗數據，認為氧化亞銅立方體不具光催化活性是由於電子及電洞沒有轉移至表面層，所以無法產生自由基進行光催化反應；氧化亞銅菱形十二面體因為同時具有電子及電洞轉移至表面層產生自由基，因此光催化活性較好；而氧化亞銅八面體主要藉由電子產生自由基進行光降解，此結果輔助我們建構出三種晶面之氧化亞銅具有不同程度的能帶彎曲。此外，為了量測不同晶面之氧化亞銅的能帶，利用紫外光光電子能譜(Ultraviolet photoelectron spectroscopy , UPS)以及反射式紫外線/可見光光譜儀計算氧化亞銅的價帶及導帶，並進一步模擬出氧化亞銅三種晶面的能帶圖。然而，三種晶面所得到的能帶圖結果非常相近，這也解釋了半導體的晶面效應被忽略是因為光譜方法無法完整解釋光催化晶面效應的實驗結果，那就是說從光譜實驗所呈現的氧化亞銅立方體能帶圖無法說明為何它無光催化活性。

關鍵字

晶面效應 ； 氧化亞銅 ； 光催化

並列摘要

The significant difference between photocatalytically inactive Cu2O cubes and the high photocatalytic performance of Cu2O rhombic dodecahedra (RD) in methyl orange (MO) photodegradation has been demonstrated previously, and we have further introduced various chemical scavengers to probe the facet-dependent photocatalytic mechanisms of Cu2O cubes, octahedra, and RD in order to reveal the contribution of different catalytic species in photodegradation of MO. With the addition of electron scavenger, the conversions of MO are 1%, 13%, and 65% after 90 min of irradiation using Cu2O cubes, octahedra and RD as photocatalysts, respectively. With the addition of hole scavenger under same conditions, the conversions of MO are 3%, 69%, and 97% using Cu2O cubes, octahedra and RD respectively as photocatalysts. Cubes remain inactive with and without the introduction of electron and hole scavengers, matching out previous assumption that the photogenerated electrons and holes do not reach the surface of Cu2O cubes. RD preserve relatively independent activity upon introduction of electron and hole scavengers, implying that both photogenerated electrons and holes could efficiently produce radicals, thus maintaining its activity when scavengers capture either electrons or holes. Interestingly, the photocatalytic activity of Cu2O octahedra is significantly quenched by electron scavenger but not hole scavenger, suggesting their activity mostly comes from photogenerated electrons. This could be interpreted as photogenerated electrons travel to surface to yield radical species, while holes are largely unavailable for photooxidation reaction.
Based on these experimental results, a modified band diagram with different degrees of band bending for Cu2O crystals is constructed to explain the photocatalytic performance of differently exposed crystal facets. Ultraviolet photoelectron spectroscopy (UPS) was also used to analyze the energy level at the surface. However, it was not able to give sufficient degrees of surface bend bending to explain why Cu2O cubes should be photocatalytically inactive. Semiconductor facet effects have largely been missed because the measured band energies do not explain or predict extremely different photocatalytic ability exhibited by the same material with different exposing crystal facets.

並列關鍵字

facet-dependent effect ； cuprous oxide ； photodegradation

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