Improving the photocatalytic activity of CdS/SrTiO3 by surface modification with metal oxide for hydrogen evolution

氫氣具有高熱值 高能量轉換率及燃燒後不會排放污染性氣體等特性，、故其被視為新世代的主要替代燃料。而光催化產氫為利用太陽光、水以及光觸媒作為原料以合成氫氣的技術，於該反應過程中不會產生污染物，因此，該技術被視為對環境友善且具有發展潛力之產氫技術。 由於光催化產氫成效至今仍因光觸媒對於太陽光能之應用率偏低及其電子與電洞易再結合而抑制觸媒還原產氫之能力，因此，為提升光催化氫氣產率，則需拓展觸媒光能應用率及增加觸媒催化活性。 為了提升觸媒對太陽光能之應用率，本研究使用沉澱法合成可見光下具催化活性之硫化鎘(CdS)觸媒，並探討鍛燒氣氛對其晶相轉換之影響。研究結果指出當控制鍛燒氣氛為氮氣環境下時 由於可避免 CdO 不純物生成，，因此使硫化鎘於 600 oC 下鍛燒得以轉換成催化活性佳之六角晶格結構。 為了增加觸媒之催化活性，本研究藉由複合半導體以及披覆金屬以延緩觸媒受光激發產生之電子-電洞對的再結合速率。本研究選用於紫外光照下具良好之還原產氫能力的商業化鍶酸鈦(SrTiO3)觸媒做為內層材料，並使用沉澱法將 CdS 複合於 SrTiO3 觸媒上，探討不同之 SrTiO3 複合比修飾觸媒之成效 研究結果指出 CdS/SrTiO3 觸媒之催化活性有隨 SrTiO3 的複合。比增加而逐漸提升之成效，其中，當 SrTiO3 的複合比為 3 wt.%時，最具有光催化活性。此現象歸因於 CdS 與 SrTiO3 之還原電位差，CdS 受光激發產生之電子得以轉移至 SrTiO3 之導電帶上，進而達到分離電子-電洞對之成效。然而，當複合比增加至 5 wt.%時，CdS/SrTiO3 觸媒之活性則會因整體之吸光性質偏向紫外光波段而抑制其在於模擬太陽光照下之還原能力，故其展現了較差之催化表現。本研究進一步使用金屬修飾 CdS/SrTiO3 觸媒之表面，探討分別使用鉑(Pt)以及銅(Cu)做為電子接受角色時，其對於提升觸媒活性之成效。研究結果指出當以 Pt 做為三接面觸媒之修飾層時，由於其相較於 Cu 有較大之陰電性，故其對於電子之吸引力較佳，進而使電子-電洞對得以有效地分離。本研究亦探討不同之 Pt 披覆量對於三接面觸媒活性之影響，由研究結果指出 Pt/CdS/SrTiO3 觸媒之催化活性隨著 Pt 披覆量增加而逐漸提升，然而，當披覆量超過 0.2wt.%時，則會抑制觸媒之活性。藉由 TEM 之分析結果顯示當 Pt 披覆量為 0.5 wt.%時，由於 Pt 容易團聚於觸媒表面並形成電子-電洞之再結合中心，進而抑制觸媒於模擬太陽光照下之催化活性 本研究成功地利用複合半導體改質及金屬表面修飾改質 CdS。，當 CdS 複合 3 wt.% SrTiO3 及披覆 0.2 wt.% Pt 時，由於具備拓展光能應用率及增加觸媒催化活性，因此有效地提升 CdS 光催化活性，由 120 μmolh-1g-1 提升至 681 μmol h-1g-1，提升幅度約為 4.7 倍。Hydrogen is regarded as the major alternative fuel in future due to the it’s advantage such as high heating value and high energy conversion, and it will not produce greenhouse gas after hydrogen combustion. Photocatalysis is an environmentally friendly and prospective technology for hydrogen evolution. The major resources of photocatalytic system are only solar energy, water and photocatalyst. However, the photocatalytic activity of photocatalyst is always limited due to the less utilization of sunlight energy and rapid photo-excited electron-hole recombination rate of photocatalyst. In order to enhance the utilization of sunlight energy of photocatalyst, visible-light-driven cadmium sulfide (CdS) which has well activity for hydrogen production was synthesized by deposition precipitation method in this study, and the effect of calcination atmosphere on CdS crystalline structure conversion was investigated. The XRD analysis results indicated that when calcined the as-prepared CdS under nitrogen atmosphere, it could avoid the CdO impurity production and facilitate the active hexagonal CdS conversion. For photocatalytic activity enhancement, the CdS was modified by strontium titanate (SrTiO3) composite and platinum (Pt)/copper (Cu) coating sequentially for photo-excited electron-hole separation. In this study, the UV-light-driven SrTiO3 which has well activity for hydrogen production was chosen as the internal layer material of triple-junction photocatalyst, and the CdS was composing on the SrTiO3 by deposition precipitation method. The results showed that the photocatalytic activity of CdS/SrTiO3 was increased with increasing SrTiO3 composite ratio, and CdS/SrTiO3 revealed better photocatalytic activity with 3 wt.% SrTiO3. The enhanced activity was attributed to the well e--h+ pair separation. Due to the reduction potential difference between CdS and SrTiO3, the photo-excited electron migrated from the conduction band (CB) of CdS to the CB of SrTiO3 efficiently, then facilitated the H2O reduction (H+ to H2). When increasing the composite ratio of SrTiO3 to 5 wt.%, the photocatalytic activity of CdS/SrTiO3 was decreased because of the narrowed utilization of visible light energy. In this study, Pt and Cu was chosen as the outer layer material of triple-junction photocatalyst, respectively. In a comparison between Pt and Cu, the electronegativity of Pt is higher than that of Cu, resulting in better photo-excited electron transfer from CdS and SrTiO3 conduction band to the Pt species and higher photocatalytic activity. Besides, it was found that the photocatalytic activity of Pt/CdS/SrTiO3 triple-junction photocatalyst enhanced as Pt loading weight percentage increased, and the Pt/CdS/SrTiO3 with 0.2 wt.% Pt exhibited better photocatalytic activity under simulated sunlight irradiation. However, when increasing the loading amount of Pt to 0.5 wt.%, the photocatalytic efficiency was decreased. The TEM analysis results verified that the excess Pt species would aggregate on the surface of CdS/SrTiO3 and become a charge recombination center, thus inhibit the photocatalytic activity of Pt/CdS/SrTiO3 for hydrogen evolution.摘要.........i Abstract.........iii 總目錄.........v 圖目錄.........ix 表目錄.........x 第一章 序論 .........1 1-1 研究緣起與目的 .........1 1-2 研究架構與內容 .........2 第二章 文獻回顧 .........4 2-1 氫能源簡介 .........4 2-1-1 氫氣特性 .........4 2-1-2 產氫能源技術 ......... 5 2-2 光觸媒原理 ......... 6 2-2-1 光觸媒催化反應之基本原理.........6 2-2-2 光觸媒光催化反應之應用 .........9 2-2-3 光催化產氫光觸媒種類 .........12 2-3 提升光觸媒光催化活性之方法......... 14 2-3-1 拓展觸媒對於可見光波段之吸光性 .........15 2-3-2 延緩觸媒之電子-電洞對之再結合速率.........20 2-4 文獻總結與研究方向 .........29 第三章 研究方法 .........30 3-1 藥品與實驗設備儀器 .........30 3-1-1 實驗藥品 .........30 3-1-2 實驗氣體 .........31 3-1-3 實驗設備 .........31 3-2 觸媒製備 .........33 3-2-1 CdS 製備 .........33 3-2-2 CdS/SrTiO3 製備 .........33 3-2-3 Pt/CdS/SrTiO3 製備 .........34 3-2-4 Cu/CdS/SrTiO3 製備 .........34 3-4 觸媒特性分析 .........35 3-4-1 結構鑑定 ......... 35 3-4-2 型態與物理性鑑定.........36 3-4-3 光學特性分鑑定.........36 3-3 光催化產氫測試 .........37 第四章 結果與討論 .........39 4-1 鍛燒氣氛對硫化鎘觸媒特性及氫氣產率影響 .........39 4-1-1 硫化鎘之 X 光射線繞射分析.........39 4-1-2 硫化鎘氫氣產率之評估 .........40 4-1-3 小結 .........42 4-2 以鈦酸鍶修飾硫化鎘對觸媒特性及氫氣產率影響.........43 4-2-1 硫化鎘/鈦酸鍶複合半導體之 X 光射線繞射分析 .........43 4-2-2 硫化鎘/鈦酸鍶複合半導體之掃描電子顯微鏡分析 .........44 4-2-3 硫化鎘/鈦酸鍶複合半導體之光學性質分析 .........45 4-2-4 硫化鎘/鈦酸鍶複合半導體之 X 光光電子能譜儀分析 .........46 4-2-5 硫化鎘/鈦酸鍶複合半導體之螢光分光光譜分析.........49 4-2-6 光催化水解氫氣產率評估之探討 .........50 4-2-7 小結.........52 4-3 銅金屬氧化物改質於硫化鎘/鈦酸鍶複合半導體.........52 4-3-1 銅金屬氧化物改質複合半導體之 X 光射線繞射分析 .........53 4-3-2 銅金屬氧化物改質複合半導體之 X 光光電子能譜儀分析 ......... 53 4-3-3 光催化水解氫氣產率之評估 .........57 4-3-4 小結.........59 4-4 鉑金屬氧化物改質於硫化鎘/鈦酸鍶複合半導體 .........59 4-4-1 光催化水解氫氣產率之評估 .........59 4-4-2 鉑金屬氧化物改質複合半導體之 X 光射線繞射分析 .........60 4-4-3 鉑金屬氧化物改質複合半導體之 X 光光電子能譜儀分析 .........61 4-4-4 鉑金屬氧化物改質複合半導體之穿透式電子顯微鏡分析.........65 4-4-5 鉑金屬氧化物改質複合半導體之螢光分光光譜分析 .........66 4-4-6 小結 .........68 第五章 結論與建議 .........69 5-1 結論.........69 5-2 未來研究與建議 .........71 參考文獻 .........7