以近室壓光電子能譜術探討氧化鋅/氧化亞銅奈米顆粒異質接面的二氧化碳光催化還原反應

Ambient Pressure X-ray Photoemission Spectroscopy Study of Photocatalytic Reduction of CO2 on ZnO/Cu2O Nanoparticle Heterojunction

黃子誠(Tzu-Cheng Huang)

指導教授 ： 楊耀文 ； 黃暄益

國立清華大學/理學院/化學系所/碩士(2020年)

摘要

本篇論文主要探討氧化鋅/氧化亞銅所組成的第二型半導體異質結構進行光催化還原二氧化碳的表現之研究。本實驗選用兩種預期有不同光催化活性的氧化亞銅結構，分別為（100）面的立方體(cube)氧化亞銅與（110）面的菱形十二面體(rhombic dodecahedron, r.d.)氧化亞銅，並利用近室壓光電子光譜術（APXPS）對光催化劑表面上反應物種的變化進行表徵。我們準備了一系列樣品，例如ZnO (7％)/ Cu2O (cube)、ZnO (7％)/ Cu2O (r.d.)、ZnO (20％)/ Cu2O (r.d.)以及ZnO (40％)/ Cu2O (r.d.)。由APXPS C 1s光譜顯示出在菱形十二面體氧化亞銅系列的樣品表面上生成了大量碳中間體（例如甲酸酯，羰基和甲氧基），這證明了ZnO / Cu2O（r.d）比ZnO / Cu2O (cube)更具反應活性。此外，在Cu2O (r.d.)的系統中，相較於低氧化鋅負載量(7%)的產物以甲醇為主，數據顯示當提高ZnO的負載量(40%)會導致生成甲烷的比例提高。我們使用APXPS的數據建構出與光催化劑表現相關的能帶圖，經由計算後，與ZnO / Cu2O (cube)系統的數值相比，發現ZnO/Cu2O (r.d.)系統中，氧化亞銅與氧化鋅的價帶位能差(conduction band offset, ∆ECBO)與導帶位能差(valence band offset, ∆EVBO)皆來的大，但氧化亞銅的價帶與氧化鋅的導帶之間的能量差卻比較小，此因素導致光電子電洞對有更高機率的在此兩能階中再結合，讓氧化亞銅的導帶累積更多的光激發電子幫助催化反應；亦有可能是在ZnO / Cu2O (cube)系統中異質接面產生時氧化亞銅價帶下彎過多，造成電子傳遞能力下降。

關鍵字

近室壓光電子能譜術 ； 半導體異質結構 ； 二氧化碳光還原反應 ； 臨場量測

並列摘要

In this thesis, we report a photocatalytic reduction study of carbon dioxide on semiconductor heterojunctions constructed from nanoparticles of cuprous oxide and zinc oxide with cuprous oxide of two different facets. The samples investigated include ZnO (7%)/ Cu2O (cube), ZnO (7%)/ Cu2O (r.d.) and ZnO (40%)/ Cu2O (r.d.). Two types of cuprous oxide nano-crystals were selected based on the anticipated difference in photocatalytic performance: the cubic structure terminated with (100) faces, and the rhombic dodecahedron (r.d.) terminated with (110) faces. Ambient pressure X-ray photoelectron spectroscopy (APXPS) was used to track the change of reaction species on the photocatalyst surfaces. The ZnO/Cu2O (r.d.) is found to be more reactive than ZnO/Cu2O (cube) as evidenced by a larger production of carbon species such as carbonate, formate, carbonyl, and methoxy on the surface when the nano-catalysts were exposed to gaseous carbon dioxide and water of mbar pressure and illuminated with UV photons. Further, increasing ZnO loading from 7% to 40%, on Cu2O (r.d.) alters reaction pathway, yielding more methane instead of methanol. The energy diagrams useful in discerning the catalytic performance of photocatalysts are also constructed by means of APXPS data. The semiconductor interfaces of ZnO/Cu2O (r.d.) and ZnO/ Cu2O (cube) are all belonged to type II heterojunction. The valence band maximum (VBM) of Cu2O (r.d.) is found to be located at 1.28 eV lower than the conduction band minimum (CBM) of ZnO, whereas the corresponding energy level difference for ZnO/ Cu2O (cube) is increased to 1.83 eV. We speculated that the smaller energy difference brings about a faster interfacial recombination rate between the holes residing in VBM of Cu2O (r.d.) and electrons residing in CBM of ZnO when the materials are illuminated with UV photos, resulting in an improved charge separation with electrons accumulated at Cu2O (r.d.) and holes at ZnO, as depicted in the so-called Z-scheme.

並列關鍵字

APXPS ； semiconductor heterojuction ； CO2 photoreduction ； in-situ detection

參考文獻

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