銳鈦礦二氧化鈦中摻雜氮原子以及同時存在氧空缺與氫原子時對其光吸收行為影響之理論分析與模擬

Abstract

摻氮之銳鈦礦二氧化鈦是否會造成能隙值的縮減以及光吸收曲線的紅移是目前研究上的一大爭議。本研究運用第一原理計算，探討銳鈦礦二氧化鈦中摻入氮原子後之特性。我們利用生成能、電子態密度、電荷密度分佈以及光吸收係數等計算，來了解銳鈦礦二氧化鈦摻入氮原子後之相對穩定度、電子性質以及光學性質之變化。 研究結果顯示，隨著系統中的摻氮濃度愈高，氮原子所需的生成能愈大，摻雜愈不易。此外，製備摻氮二氧化鈦的過程中，氮原子傾向於在已摻入二氧化鈦中之氮原子的同一個八面體上生成，其可能會造成氮原子之群聚現象。 當系統中之摻氮濃度較低時，能隙值幾乎沒有變化。隨著摻氮濃度的增加，能隙值之變化量會增加，隨著氮原子排列方式之不同，能隙值可能會增大也可能會減小，並沒有明顯之趨勢。 從光吸收係數曲線之分析可以發現，隨著摻雜濃度的增加，光吸收係數曲線會逐漸出現類似紅移之現象。但是，即便在能隙值沒有縮減之情形下也可能出現紅移之現象。我們認為，紅移現象的發生與能隙值是否變小並無絕對之關係。缺陷能態以及價帶上緣之電子態密度的重新分布（Redistribution）才是造成摻氮二氧化鈦之光吸收曲線有類似紅移現象的主要因素。因此，我們也認為實驗文獻中以Tauc plot決定能隙值之作法並不適用於含有缺陷能態之結構中。 在高摻氮和高氧空缺濃度（N:Vo = 2:1）的系統中，我們也可觀察到光吸收曲線有紅移的現象發生，不過其主要是由高局域態電荷密度所造成。氧空缺的存在相對於純摻氮之二氧化鈦結構的光吸收的行為並沒有顯著增強的效果。 含有N:H之二氧化鈦結構的特性與含有氧空缺的系統類似。當TiO2-x(NH)x中的N:H濃度上升至x = 0.250時，高局域態電荷密度導致光吸收係數曲線有類似紅移之現象。不過，其光吸收量相較於純摻氮系統而言，依舊沒有顯著增強的效果。

Band gap narrowing and red-shift of optical absorption spectra were fiercely debated for N-doped anatase in the past ten years. In this study, the properties of N-doped anatase were discussed using first principle calculation. In order to realize the variations of stability, electronic and optical properties of N-doped anatase, I apply formation energy, charge density of state, charge density distribution, and optical absorption coefficient to this research. According to my calculation, the formation energy of N-doped anatase was increased as the system concentration of nitrogen was elevated. This result was lead to difficult doping of nitrogen atoms into the lattice of anatase. Besides, the doped nitrogen atoms preferred to grouped or clustered with the pre-doped nitrogen atoms in the same tetrahedral site under the nitrogen doping process. The band gap was unchanged when the doping concentration of nitrogen atoms was low. When the doping concentration was increased, the band gap variation was also getting obvious. Under the analysis of optical absorption spectra, the red-shift-like behavior of optical absorption spectra became apparent as the doping concentration was elevated, even though without the band gap variation. Our results show that the red-shift-like behaviors do not seem to have clear correlation with the change in the electronic band gaps. I proposed that defect state and redistribution of charge density of the valence band top were the main factors to the red-shift properties of N-doped anatase. On the other hand, I supposed that the band gap defined by tauc plot was inappropriate for defected structure of anatase. In the system under high concentration of doped-nitrogen atoms and oxygen vacancies（N:Vo = 2:1）, the red shift of optical absorption spectra was observed due to high charge density of the localized states. The optical absorption efficiency does not obviously enhanced by oxygen vacancies in N-doped anatase. Properties of the TiO2 involved N:H is similar to the N-doped anatase contain vacancies. When the concentration of x in the TiO2-x(NH)x raise to 0.250, the red shift of optical absorption spectra was observed due to high charge density of the localized states. Nevertheless, The optical absorption efficiency does not obviously enhanced by hydrogen bonding to the nitrogen in N-doped anatase.