近年來，隨著人為活動增加排放大量溫室氣體，溫室效應加劇，導致全球各地氣候異常、海水酸化等問題，其中二氧化碳是影響最為嚴重的溫室氣體。因此，在2015年聯合國氣候變化大會，各國都同意減少二氧化碳的排放。目前最主要的二氧化碳處理技術是二氧化碳捕獲與封存，但是二氧化碳捕獲與封存有一個最大的問題就是全球的封存量有限，所以將二氧化碳轉化成碳氫燃料是目前最有潛力的技術，然而二氧化碳轉的過程需要提供額外的能源，若能運用太陽光作為能量的來源是一最佳的方法，因此，利用光觸媒催化還原二氧化碳是現在最有吸引力的潛在技術。

　　氧化亞銅(Cu2O)是一種P型半導體，其能隙值小、製造成本低、無毒且環境友善，適合被選用來當光催化還原二氧化碳。為了降低電子電洞對再結合率和提升光催化效率，石墨烯被選用來修飾氧化亞銅，由於良好的物理特性，像是比表面積大、導電性佳與高光穿透率。本研究利用氧化亞銅在可見光下光催化還原二氧化碳，並摻入不同的比例的還原氧化石墨稀，來提升光催化能力。經過20小時的反應，二氧化碳成功轉化成甲醇，其產率9.76、47.91、335.26與12.41 μmole/g-cat個別對應到Cu2O、Cu2O/1%rGO、Cu2O/5%rGO與Cu2O/10%rGO。其中石墨稀摻入量為5%時，擁有最好的光催化效果。

　　光觸媒的晶相對光催化效果的影響在本篇研究亦有探討，在經過20小時候，(100)、(111)與(110)晶相的氧化亞銅的甲醇產率分別為 4.35、26.81、355.26 μmole/g-cat。其中結晶面相為(110)的氧化亞銅有最佳的光催化還原二氧化碳效果。此外本研究還探討不同亞化態的銅觸媒是否會影響光催化效果。

In the past decades, the global warming, which causes the serous global climate change problem and ocean acidification, tends to be more and more severe with increasing of greenhouse gases such as methane, nitrous oxide, and especially carbon dioxide. A global agreement to reduce greenhouse gases has been reached on the COP21 in Paris in December, 2015. Hence, reduction of CO2 emission is a big and global issue. In order to reduce CO2 emission, there are two main technologies developed; one is carbon capture and storage and the other is CO2 conversion. There is a problem for carbon capture and storage, i.e., very limited sites for CO2 storage in the world. CO2 conversion has a potential to reduce CO2 to hydrocarbon. However, the extra energy is required for reduction of CO2 because of a positive ΔG value. Therefore, CO2 conversion by using photocatalytic reactions via solar energy, is one of potential and eco-friendly attractive technology.
Cuprous oxide (Cu2O), a p-type semiconductor with a narrow band gap (2.0-2.2 eV), is low cost, nontoxicity, and environmental friendliness. It is suitable to be used as photocatalysts in the photoreduction of CO2, but the rate of electron-hole pairs recombination is high due to the narrow band gap. Therefore, the incorporation of graphene onto Cu2O may reduce the rate of electron-hole recombination and enhance the performance of photocatalysts. In this study, Cu2O was chosen to photocatalytic reduction of CO2 under visible light (λ>420). Different amounts of reduced graphene oxide (rGO) were decorated onto Cu2O to enhance the photocatalytic activity. After visible irradiation for 20 hours, CO2 can be converted into methanol by Cu2O and Cu2O/x%rGO (x = 1, 5, 10). The yields of methanol are 9.76, 47.91, 335.26, and 12.41 μmole/g-cat observed for Cu2O, Cu2O/1%rGO, Cu2O/5%rGO, and Cu2O/10%rGO composites, respectively. Among all the prepared samples, Cu2O/5%rGO has the superior activity of CO2 photoreduction because the proper amounts of incorporated rGO could decrease electron-hole recombination and avoid block of light to reach surface of Cu2O nanocrystals.
The effect of facets on photocatalytic activity of photocatalysts were also studied. The yields of methanol are 4.35, 26.81, and 355.26 μmole/g-cat after 20 hours under visible irradiation, which corresponding to cubic Cu2O/5%rGO nanocrystals with (100) facets, octahedral Cu2O/5%rGO nanocrystals with (111) facets, and rhombic dodecahedra Cu2O/5%rGO nanocrystals with (110) facets, respectively. The rhombic dodecahedra Cu2O/5%rGO shows surpassing photocatalytic performance due to the positive charged (110) surface of particles. Moreover, different oxidation states of copper decorated with rGO were discussed in this study.

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