根據以往的氧化亞銅降解甲基橙的實驗，觀察到顯著的光催化晶面效應，氧化亞銅的立方體不具有光降解染料的能力，而菱形十二面體的氧化亞銅則具有最佳的活性表現，並且有修飾上金、氧化鋅、硫化鎘、硫化鋅與磷酸銀在立方體仍然保持無活性，我們認為氧化亞銅立方體{100}晶面表面具有高的能障，所以電子不容易傳遞到表面而沒有降解活性。當硫化銀沉積在氧化亞銅立方體上時，會使無活性的立方體變成具有高催化活性，在20分鐘內降解效率達到99%。加入電子與電洞捕捉劑後，甲基橙之降解活性都有受到抑制，代表電子與電洞能順利傳遞到表面進行染料降解。加入超氧化物與氫氧自由基捕捉劑後，亦活性受到抑制，表示在光降解過程裡兩者皆有參與其中。電化學阻抗量測也顯示經過硫化銀沉積後的氧化亞銅立方體，其電荷轉移電阻明顯變低。最後利用紫外光電子能譜以及反射式紫外-可見光光譜儀畫出三種晶面的氧化亞銅與硫化銀的能帶圖，當對準費米能階後，氧化亞銅的表面價帶往上彎曲靠近往下彎曲的硫化銀導帶，氧化亞銅的價帶上的電洞與硫化銀的導帶上的電子會傾向再結合，推測有效分開電子與電洞，因此降低了電子電洞對再結合效率可能是氧化亞銅立方體活性改變的原因。這例子展現氧化亞銅立方體接上某些半導體材料將有機會具有好的光催化活性。

On the basis of previous facet-dependent photocatalytic performances of Cu2O in the photodegradation of methyl orange (MO), Cu2O cubes have been known to be photocatalytically inactive, while Cu2O rhombic dodecahedra (RD) are highly active and octahedra are moderately active. Besides, we have also shown that Cu2O cubes remain photocatalytically inactive after surface deposition with Au, ZnO, CdS, ZnS and Ag3PO4 nanostructures. Thus, it is believed that the {100} faces of Cu2O present as a large barrier, preventing photogenerated electrons from reaching to this surface to produce radical species. Surprisingly, when Ag2S particles are decorated on Cu2O cubes, they become highly active, degrading MO almost completely in 20 min. To confirm this observation, CrO3 and Na2C2O4 were added as electron and hole scavengers. Degradation of MO was significantly suppressed, indicating electrons and holes can migrate to the surface to degrade the dye. Adding 1,4-benzoquinone and isopropanol to capture superoxide anion radicals (·O2–) and hydroxyl radicals (·OH) also yields catalytic activity suppression, showing they are the active species responsible for MO photodegradation. Electrochemical impedance spectra (EIS) measurements indicate significantly reduced charge transfer resistance after Ag2S deposition. In addition, ultraviolet photoelectron spectra (UPS) and UV‒vis diffuse reflectance spectra were collected to obtain energy levels of Ag2S and different Cu2O crystals. An upward band bending of the Cu2O valence band (VB) is close to the downward band bending of Ag2S conduction band (CB) after aligning the Fermi level. The photogenerated electrons from the CB of Ag2S then recombine with the photogenerated holes from the VB of Cu2O. Because electrons and holes are separated efficiently, the reduction of the electron-hole pair recombination efficiency may give rise to activity of Cu2O cubes. This remarkable example demonstrates Cu2O cubes can show good photocatalytic activity with proper selection of the deposited semiconductor materials.

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