光分解水產氫能之研究---含金鈦觸媒的製備、Fe2O3/Si元件設計與Fe2O3薄膜的製備

Abstract

本論文是針對光分解水的程序，分別在光催化和光電化學 系統中，進行光吸收體的開發研究。在光催化系統中，主要是 載負奈米金在K2La2Ti3O1 0 上以獲得高光反應效率的光觸媒； 在光電化學系統中， 主要是設計新型的Fe2O3/Si 雙重能隙薄 膜元件及製備p 型和n 型Fe2O3 薄膜。 在光催化系統方面，由於奈米金不但擁有高催化活性，且 在可見光區有一個表面電漿共振的光吸收峰，也許可提升在可 見光下的光反應活性。因此， 本研究選擇以K2La2Ti3O10 為光 觸媒擔體，探討奈米金的載負濃度、載負方法及前處理步驟， 對光催化水分解反應的影響。並且，以UV 或可見光為照射光 源，與文獻中已知的Ni/K2La2Ti3O1 0 光觸媒做比較。由研究結 果得知， 載負奈米金可增進K2La2Ti3O1 0 的光反應活性， 且以 初濕含浸法所製備的Au-i/K2La2Ti3O1 0， 其光反應活性較好。 在UV 光的照射下， Ni/K2La2Ti3O1 0 的光反應活性比 Au-i/K2La2Ti3O1 0 來得高， 可能原因是Ni/K2La2Ti3O1 0 擁有較 好的K2La2Ti3O1 0 結構，使得K2La2Ti3O1 0 擔體在吸光後可產生 較多的電子電洞對； 而在可見光的照射下， Au-i/K2La2Ti3O1 0 的光反應活性比較高，可能原因是奈米金在可見光下的表面電 漿共振可極化金顆粒表面的電子分佈，再藉由電子在奈米金與 K2La2Ti3O1 0 之間的傳遞， 減少再結合機率。 光催化水分解有兩項主要缺點： (1) 隨後之氫氧產物分 離； (2)光觸媒能隙值過大， 使得在太陽光下的的光能轉換效 率較低。因此，本論文研究兩極的光電化學系統，並以多重能 隙原理設計光電極，在考慮各種材料限制後，初步選擇氧化鐵 (Fe2O3 )和矽(Si)的雙重能隙薄膜元件， 再以銦錫氧化物(ITO) 薄膜為中間透光歐姆導電層。由文獻可知，該元件的材料組合 可使光電轉換效率由單一矽薄膜之24%提升至32%。由於矽 和ITO 的薄膜製備技術成熟， 再加上Fe2O3 為n 型半導體材 料， 故製備p 型Fe2O3 薄膜則成為製備此雙重能隙薄膜的關 鍵。本研究先進行製備p 型Fe2O3 塊材的研究， 利用Mg 原子 的摻雜，探討不同熱處理步驟對p 型Fe2O3 塊材性質的影響。 由研究結果可結論出，製備p 型Fe2O3 塊材的較佳處理步驟為 在3 大氣壓氧氣下以900℃ 持溫20 小時， 較佳的鎂摻雜濃度 為0.2 at%。 接下來， 本研究應用磁控射頻物理濺鍍法獲得p 型和n 型Fe2O3 薄膜， p 型和n 型摻雜物採用Mg 原子和Si 或Ti 原 子。而為了獲得光電轉換效率高的p 型Fe2O3 薄膜，本研究探 討不同濺鍍條件對薄膜性質的影響。由研究結果發現，電漿氣 體組成是影響p 型Fe2O3 薄膜性質的主因， 與Ar 或Ar/O2 混 合氣體做比較， 只有利用純O2 電漿氣體濺鍍Fe2O3 薄膜時， 才可獲得較高p 型光電流的Fe2O3 薄膜。高溫後處理會使Fe2O3 薄膜由p 型轉變為n 型，可能原因應為高溫後處理使薄膜壓縮 至較密堆積，減少氧原子間隙而形成n 型。在高壓氧氣下高溫 後處理比在常壓空氣下更能提高Fe2O3 薄膜的n 型光電流。改 變光源為UV 燈泡對n 型光電流的提升較p 型光電流大40 倍。 摻雜不同原子和不同濃度於Fe2O3 薄膜， 只影響Fe2O3 薄膜的 p 型和n 型光電流數值，對薄膜結構和能隙值影響不大。在本 研究的結果中，較高p 型光電流的Fe2O3 薄膜為，未經高溫後 處理的2at% Mg-Fe2O3 薄膜， 其p 型光電流密度在0.4V 的負 偏壓下為0.02 mA/cm2； 而較高n 型光電流的Fe2O3 薄膜為， 依序在常壓空氣和高壓氧氣下高溫後處理的1at% Ti-Fe2O3 薄 膜， 其光電流密度在0.6V 的正偏壓下為0.36 mA/cm2， 並由 效率公式的計算可得， 實際光量子效率φc o n v 和光能轉換效率 ηp r a c 分別為1.8%和0.44%。在ITO 基板上製備pn 接面和pin 接面的Fe2O3 薄膜元件，都無法獲得類似固態太陽能電池在照 光下的電流電壓曲線。與非晶矽薄膜的性質比較可發現，Fe2O3 薄膜的光/暗導電度比遠小於非晶矽， 推測應為Fe2O3 的間接 能隙性質所影響。 本研究主要的成果及貢獻包括：在光催化系統的研究中， 首先載負奈米金在K2La2Ti3O1 0 層狀鈦觸媒上， 發現奈米金可 提升K2La2Ti3O1 0 層狀鈦觸媒在可見光下的光反應活性。在光 電化學系統的研究中， 已設計出便宜的Fe2O3/Si 雙重能隙薄 膜光電極，可在太陽光下獲得較高的理論光電轉換效率。利用 Mg、Si 和Ti 原子的摻雜，可藉由磁控射頻物理濺鍍法製備出 具有p 型和n 型光電流的Fe2O3 薄膜。

This thesis was focused on the development of novel photo-converters by the photo-catalytic and photo- electrochemical systems for water splitting to hydrogen. In the photo-catalytic system, the effects of gold loading on the perovskite titanate substrate (K2La2Ti3O10) for water splitting were studied. In the photo-electrochemical system, the device of Fe2O3/Si multiple bandgap films was designed and the preparations of the p-type and n-type Fe2O3 films were studied. In the photo-catalytic system, the factors of the loading processes and pretreatment procedures on Au/K2La2Ti3O10 were investigated. Also, a preliminary comparison of the activities of nano-gold and the reduced (then partially re-oxidized) nickel for water splitting under UV or visible light were made. It was found that Au/K2La2Ti3O10 prepared by an incipient wetness impregnation process possessed a better activity for water splitting than that prepared by a deposition process. This was because a better crystallinity of K2La2Ti3O10 was preserved from the impregnation process than that from the deposition. Moreover, the activity of Au/K2La2Ti3O10 can be increased significantly, after the reduction of gold ions to nano-gold metal through some pretreatment processes. When compared with the best metal–titanate catalyst reported in the literature (i.e., Ni/K2La2Ti3O10), Au/K2La2Ti3O10 possessed a lower hydrogen production rate in UV region and a higher one in visible region. This may be because Ni/K2La2Ti3O10 preserved a better crystallinity of K2La2Ti3O10 to produce more electron-hole pairs in UV region and Au/K2La2Ti3O10 had an absorption in the visible region from plasma resonance on the nano-gold surface, rather than on the Ni surface. There were two bottlenecks in the photo-catalytic system, which were the separation of the mixing product (H2/O2) and the low theoretical quantum efficiency of the photo-catalyst under solar light. To improve the quantum efficiency for water splitting, the device of multiple bandgap films in the photo-electrochemical system was designed. Fe2O3 and Si were chosen as the top and bottom materials, and ITO was chosen as the transparent middle layer for ohmic contact. The theoretical quantum efficiency of this device for solar-to-electricity was raised to 32% from 24% of the single crystal Si solar cell. Due to the n-type intrinsic behavior of Fe2O3 and the maturity of the Si and ITO technology, the p-Fe2O3 film preparation was the key research. The p-Fe2O3 film was fabricated by RF magnetron sputtering process and the p-Fe2O3 target. The preparation of p-Fe2O3 pellets was studied for the p-Fe2O3 target preparation. From our results of the p-Fe2O3 pellets preparation, it was demonstrated that the p-Fe2O3 pellet could be made by Mg doping, under the conditions of three atmospheres oxygen pressure at 900℃ for 20 h. In order to fabricate p-type and n-type Fe2O3 films with good photo-efficiencies, some sputtering factors were investigated. The p-type dopant of Fe2O3 films was the Mg atom and the n-type one was the Si or Ti atom. The results were shown that the plasma composition was an important factor for sputtering the p-Fe2O3 film. The Fe2O3 films with p-type photocurrent were obtained only by sputtering in the O2 plasma gas. The pure Fe2O3 films transformed from p-type to n-type after the post-annealing treatment, due to the release of the interstitial O2 atoms by structure compression. The better n-type photocurrent can be obtained by post-annealing in three atmospheres oxygen pressure. Moreover, the more Mg doping levels the Fe2O3 film contained, the higher p-type photocurrent was obtained. On the contrary, the rising effect of the n-type photocurrent was obtained by doping with the Si and Ti atoms. It has been concluded that the higher p-type photocurrent density of 0.02 mA/cm2 at -0.4V was obtained by the 2at% Mg-Fe2O3 film which was sputtered under O2 plasma gas. The higher n-type photocurrent density of 0.36 mA/cm2 at +0.6V was obtained by the 1at% Ti-Fe2O3 film which was sputtered under O2 plasma, followed by the post-annealing treatments in air and in three atmospheres oxygen pressure. The higher real quantum yield of solar-to-electricity and quantum efficiency of solar-to-hydrogen for the above-mentioned 1at% Ti-Fe2O3 film were 1.8% and 0.44%, respectively. The devices of pn and pin junction Fe2O3 films were fabricated on ITO glass substrate by RF magnetron sputtering process. None of them had the phenomenon of solid state solar cell. Compared with the amorphous Si film (a-Si), the quantum yield of the Fe2O3 film was much lower than that of the a-Si film, probably due to the indirect bandgap property of the Fe2O3 film. The main contributions of this thesis were listed as below: (1) the quantum yield of K2La2Ti3O10 was increased in the visible region by gold loading, (2) the cheaper device of the Fe2O3/Si multiple bandgap films was designed to obtain the higher theoretical quantum efficiency, (3) the Fe2O3 films with the higher p-type and n-type photocurrents were sputtered by doping with Mg and Ti atoms, respectively.