自工業革命以來，煤炭、石油等不可再生能源使用量日益增長，隨之而來的還有大量的二氧化碳排放量，這些影響所帶來的威脅已令人類無法忽視。科學家開始著手研究減少二氧化碳排放以及開發其他的綠色能源，將這兩點串起的反應即為二氧化碳還原反應。此反應可將二氧化碳反應成甲烷、甲醛、一氧化碳等可再進行利用的產物，而利用光催化驅動此反應更令其具有環保價值。
本實驗合成出立方體及菱形十二面體兩種形狀的氧化亞銅，並於外層覆蓋上二氧化鈦形成半導體異質接面，並利用XPS、SEM、EDS等方式進行鑑定，確定二氧化鈦覆蓋薄層在氧化亞銅上以及晶型未受到破壞。接著利用此氧化亞銅-二氧化鈦奈米粒子作為光催化觸媒，並使用國家同步輻射中心BL24A於2017年架設的室壓X光光電子能譜 (Ambient Pressure X-ray Photoelectron Spectroscopy, APXPS) 實驗站進行臨場二氧化碳的紫外光光催化還原反應，觀察照光前後圖譜變化，並進行兩種催化劑的比較。
在二氧化碳光催化還原反應中，我們觀察到菱型十二面體形狀的氧化亞銅-二氧化鈦催化劑上出現了碳氫鍵譜峰的大幅度上升，而在立方體的樣品上則無類似的變化。我們進一步使用程序升溫反應進行質譜儀的測量，結果亦觀察到菱型十二面體的樣品甲醇相關訊號產生；同樣的實驗於立方體樣品則無相同結果。
為比較兩者活性差異，我們測量並繪製兩種催化劑的異質接面能階圖，結果發現菱形十二面體樣品之價帶差距(Valence band offset, VBO)要大於立方體樣品相對應數値，我們認為此差距會帶給受光激發的電子較大的驅動力使其容易傳遞至能階較低的二氧化鈦價帶。因此菱型十二面體較容易在照光後將電子傳遞至外層的二氧化鈦，幫助催化反應進行。此外亦有可能因異質接面產生時的能帶彎曲 (band bending)使氧化亞銅價帶下彎過多，造成電子傳遞能力下降。

Since the industrial revolution, the extensive usage of fossil fuel has resulted in a continuous increase of atmospheric concentration of carbon dioxide, generating a global temperature rise with a catastrophic effect on global weather. Therefore, the sequestration of carbon dioxide and the conversion of carbon dioxide into reusable carbon feedstock have received a world wide attention of scientific community. Photocatalytic reduction of carbon dioxide, capable of turning carbon dioxide into useful commodities such as methane, methanol, carbon monoxide, etc., offers a particularly appealing advantage by tapping the solar radiation as the energy source of reactions.
In this thesis, we report a photoreduction study of carbon dioxide on semiconducting, core-shell heterojunctions fabricated with nanostructured materials of cuprous oxide (core) and titanium oxide (shell). Two cuprous oxide structures are adopted for the anticipated difference in photoreductivity: the cubic structure terminated with (100) face, and the rhombic dodecahedron (r.d.) terminated with (110) face. The techniques such as secondary electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) are employed to characterize the heterojunction. Additionally, a rather unique, operando research technique, i.e. ambient pressure XPS (APXPS), is also used to track the change of reaction species on the photocatalyst surfaces. APXPS C 1s spectra show the Cu2O(r.d.)-TiO2 is more reactive than Cu2O(cube)-TiO2, as evidenced by a larger production of carbon species such as formate, carbonyl, carbon dioxide, aliphatic carbons on the surface. After prolong photocatalytic reactions, the chemical identity of accumulated surface species evolved into gas phase can be revealed with a quadrupole mass spectrometer by running the so-called temperature programmed reaction spectroscopy (TPRS) experiments. The production of methane and methanol at ~400 K is clearly noted.
The energy diagram relevant to the performance of photocatalysis is also obtained by XPS. The heterojunction of Cu2O(cube)-TiO2 has smaller valence and conduction band offsets than its counterpart of Cu2O(r.d.)-TiO2. The larger conduction band offset of Cu2O(r.d.)-TiO2 facilitates a better charge carrier separation, with electrons and holes ended up being more accumulated on TiO2 and Cu2O, respectively. It is argued that the larger band offset found for Cu2O(r.d.)-TiO2 plays an important role in enhancing its photoreduction capability.

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