人工光合作用被視為是能夠解決地球暖化與能源需求增加的方法之一，如同自然界光合作用，藉由光催化的方式在光觸媒上進行二氧化碳還原成碳氫化合物，如甲醇等有用的燃料，可同時減少二氧化碳及解決能源需求一直增加的問題。本研究主要探討以氧化石墨烯成為光觸媒進行二氧化碳轉換的優異性，並利用X-ray繞射分析、掃描式電子顯微鏡、高解析穿透式電子顯微鏡、紫外-可見光光譜儀、光電子光譜儀、原子力顯微鏡、紫外光電子能譜儀和電化學循環伏安及線性伏安法了解光觸媒特性。
光觸媒效能部分，首先以修改式Hummer’s氧化石墨烯(Hummer’s GO, GO-1)及改良式氧化石墨烯(Improved GO, iGO)，了解不同的氧化參數造成二氧化碳光轉換效率的影響，在實驗上的參數包括在GO-1製程中加入的過錳酸鉀含量，以及在iGO製程中加入的磷酸含量。不同操作參數之結果顯示，在GO-1製程中當過錳酸鉀量增加，能隙及氧化程度將增加，但隨著氧化石墨烯的缺陷增加，光催化還原產率也下降，而在iGO製程中隨著量增加，能隙及氧化程度有所增加，但磷酸在合成過程中，有效的保護氧化石墨烯表面，使缺陷減少，光催化還原產率也隨之增加，但當氧化過程中加入過多磷酸反而降低能隙及氧化程度，產率也將下降，磷酸的添加可以避免氧化石墨烯基面形成C=O之缺陷，所以iGO製程相對於HGO製程可能有較少的缺陷形成在氧化石墨烯基面上，並在三倍磷酸添加的條件下(GO-3)，將有最較少的缺陷，進而幫助還原反應，維持穩定性。以二氧化碳及水氣連續通入不鏽鋼之反應器中，利用300瓦之鹵素燈照射在光觸媒上，再經由自動注射系統注入氣相層析儀-火焰離子偵測器(Gas Chromatography/Flame Ionization Detector, GC/FID)偵測產物，其中，甲醇產率最高將可達到0.172 μmol g-1-cat hr-1，產率相對於商用二氧化鈦 (Degussa P25)有近五倍的提升。為進一步增加其效能，吾人以一步合成的水熱法，藉由聯銨輔助還原二硫化鉬合成在氧化石墨烯奈米表面上，使增加可見光驅動活性，以作為光觸媒複合材料，因而實驗中測得之各產物為甲醇、乙醇及乙醛，二氧化碳轉換成碳氫化合物的總產率達1.8 μmol g-cat-1 hr-1，為iGO產率的10倍、為P25的45倍，顯示出二硫化鉬/氧化石墨烯奈米複合材料光觸媒在催化二氧化碳還原的傑出表現。

Artificial photosynthesis is one of the solutions to solve global warming and mitigate the rising demands of energy consumption. Photocatalytic conversion of carbon dioxide (CO2) to hydrocarbons such as methanol makes possible simultaneous solar energy harvesting and CO2 reduction, resulting in solution for both the energy demands and environmental problems. This work describes a promising photocatalyst based on improved graphene oxides (iGOs), which have high photocatalytic conversion efficiency of CO2 to hydrocarbon fuels.
Improved Hummer’s method has been applied to synthesize the GO based photocatalyst for the enhanced catalytic activity. The photocatalytic CO2 to methanol conversion rate on the pristine improved graphene oxide is 0.172 μmole g-1-cat. h-1 under visible light, which is four-fold higher than the pure TiO2 (P25). On the other hand, we have also synthesized a composite catalyst based on molybdenum disulfide-iGO system.The MoS2 nanosheet decorated improved graphene oxide (iGO) hybrid nanostructures are fabricated by a facial one-step hydrazine-assisted hydrothermal method. The photophysical properties of the synthesized photocatalysts have been investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), UV-Vis spectrometer, Ultraviolet photoelectron spectroscopy (UPS), cyclic voltammetry (CV), linear sweep voltammetry (LSV) and X-ray photoelectron spectroscopy (XPS). Enhanced visible light-driven activity for the CO2 photoreduction to solar fuel has been achieved. The average apparent CO2 reduction to solar fuel formation rate of MoS2 nanosheet decorated iGO composite is more than 10 times higher than the pristine iGO; or 40 times that of TiO2 (P25). The MoS2 nanosheet decorated iGO composite nanostructures makes an outstanding contribution to the excellent photocatalytic CO2 reduction.

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