Ta和Nb摻雜赤鐵礦納米棒束作為有效的光催化水分解樣品之電子結構

Abstract

水為清潔，可再生之能源，將其轉化為氫氣被認為是理想之方式以減少化石燃料之使用與環境汙染問題。利用太陽能透過光電化學(Photoelectrochemical(PEC))之方式分解水進行能量轉換而產生氫氣為綠色能源之重要步驟。赤鐵礦為地球豐富之資源且擁有顯著的優點，如無毒性，成本低，化學性質穩定。特別的是赤鐵礦的能隙，可以吸收太陽光譜裡大部分的可見光。然而，它的導電率低，電子-電洞複合速率快，導致赤鐵礦成為非常低效率之PEC水分解的光電陽極，這大大限制了其用於太陽能轉換的實際應用，故常藉由添加第三元素的方式，來改善原先的效率，使其較具有應用價值。 在本次論文裡，我們利用X光吸收光譜（XAS）及原位X光吸收光譜（in-situ XAS）來幫助我們了解各個樣品在不同退火溫度、摻雜濃度、摻雜元素的情況下，在電子結構中所表現出來的差異。研究結果發現，樣品在接受500 ˚C退火的樣品都較差於接受750 ˚C退火的樣品，是因為受到納米棒束結構中γ- Fe2O3的影響。而在排除γ- Fe2O3混雜的因素後，我們討論摻雜濃度與提高α- Fe2O3薄膜光催化性能之間的關係，透過較為方便的水溶液成長法便可以簡單的將Nb5+、Ta5+及其多於的電子和d0空軌域帶入樣品中，並且在10mg的摻雜濃度之下達到最佳的光電效率，爾後便隨濃度的增加呈反向變化。然而在同樣的濃度下，兩種摻雜劑都能夠達到最佳的效果，但其光電流以及照光in-situ XAS的變化卻有所差異，經過整理後發現，由於Ta外層較Nb容易捕獲電子，因此較可能形成電子-電洞複合中心，使得光生電子-電洞對的傳遞受到不利的影響。

Photoelectrochemical (PEC) water splitting is an attractive approach converting sun light to hydrogen, which is one of the most important renewable energies. Owing to the narrow band gap (about 2.1 eV), abundance, low cost, and high chemical stability, hematite (α-Fe2O3) has been considered as a promising candidates for the pivotal component of a PEC photoanode. However, the conduction band is lower than hydrogen evolution potential. Nanostructuring/doping is a way to improve the photocatalytic performance of α- Fe2O3 film. In this investigation, we use X-ray absorption spectroscopy (XAS) and in situ X-ray absorption spectroscopy (in situ XAS) to measure the electronic structures for understand the annealed temperature and doping effects for PEC improvement of the hematite. The results show that the samples annealed at 500˚C are not as efficient as the samples annealed at 750˚C due to the intermixture of γ- Fe2O3. After removing the γ- Fe2O3 intermixing factor, the relationship between the doping concentration and the photocatalytic activity of the α- Fe2O3 thin film is discussed. It is found that Nb and Ta are the transition-metal cations with empty d orbitals. There are doped with a concentration of 10mg for increasing donor density and meanwhile with the unique nanostructure well preserve for efficient charge transfer. When the concentration increase, the morphology of O K-edge obviously changes. Compared with the O K-edge of oxide, we can find the oxides form with the high dopant concentration, which affect the photocatalytic activity of the sample. However, the in situ XAS in the same dopant concentration with the different dopant element (Nb, Ta) show the different result. It is found that the Ta outer layer unoccupied state is more easier to trap electrons than Nb, so it will form recombination center that could affect the transformation of the photo-generated electron-hold pairs.