銳鈦礦二氧化鈦的光催化活性受氧空缺存在以及機械應變影響之理論分析與模擬

Abstract

本研究嘗試運用第一原理計算(First Principles Calculations)，藉由氧空缺生成能、態密度、能帶結構及光吸收係數等計算結果來探討結構中氧空缺及機械應力對二氧化鈦的電子性質及光學性質的影響。在含有氧空缺之二氧化鈦的性質上，研究結果顯示氧空缺之間相對位置的不同將使其產生相異的交互作用，並會導致迥然不同的電子及光學性質，而在相異的氧空缺濃度下氧空缺的排列傾向及其所展現出之各種性質也將有所差異。在TiO2-x的系統中，當氧空缺濃度x提升時，系統的光吸收係數將隨之上升。然而當氧空缺濃度x > 0.125時，氧空缺的排列傾向將從B類排列轉變為A類排列，使得能隙中產生缺陷能態的機率大幅上升，成為電子電洞對再結合的中心而使載子生命週期縮短，進而對光電轉換效率產生負面影響。另一方面，本研究亦嘗試運用第一原理計算來分析機械應力對銳鈦礦結構二氧化鈦薄膜(thin film)性質的影響，其中我們所探討之應力方向平行於(001)、(010)及(101)三個面，並將機械應力範圍設定在8%以內，討論二氧化鈦薄膜與基材間的晶格係數差異(lattice mismatch)所造成的晶格變形(lattice deformation)對二氧化鈦系統之電子結構及光學性質的影響。結果顯示在應力平行於(001)面時，[100]及[010]方向之雙軸張應力和[100]方向之單軸張應力會使二氧化鈦之能隙值下降，並使吸收光譜往長波長方向移動。在平行(010)面施加[001]方向之單軸壓應力，以及平行(101)面時施加[010]方向之單軸張應力與[1 ̅01]方向之單軸壓應力，皆會得到相似的結果。其餘方向之施加應力則不會使光催化活性得到改善。本研究結果可對二氧化鈦光觸媒的相關研究及材料製程提供更多可行的方向及參考。

This study tried to use First Principles to study the effects of induced oxygen vacancies and mechanical strain on formation energy of oxygen vacancies, electronic density of states, band structure and optic absorption coefficient. Our results showed that in oxygen defective TiO2, the relative sites of vacancies have strong influence on the interaction between vacancies, which leads to totally different electronic and optical properties. For different oxygen vacancy concentration, the tendency of oxygen vacancy configuration differs, and this also makes difference on the properties. For TiO2-x system, when x, the concentration of oxygen vacancy, increases, the optical absorption coefficient also increases. However, when x is larger than 0.125, the configuration tendency of oxygen vacancies switches from B-type to A-type, and this leads to higher probability of defect level. Defect level may become the recombination center of electron-hole pairs and the life time of carriers is largely shortened. This has negative effects on photovoltaic conversion. On the other hand, we also used First-Principles Calculations to study the effects of mechanical strain on thin film anatase TiO2. We discussed the stress parallel to (001), (010) and (101) planes. The mechanical strain we induced on the system is no larger than 8%. We use this system to discuss how the deformation that induced by lattice mismatch affects the electronic and optical properties. Our result shows that when the stress is parallel to (001) plane, the biaxial tensile stress on [010] and [100] direction and uniaxial tensile stress on [100] direction decreases the band gap, and the absorption spectrum shift toward longer wavelength side. We also found out that if we induce uniaxial compressive stress on [001] direction parallel to (010) plane or uniaxial tensile stress on [010] direction and uniaxial compressive stress on [1 ̅01] direction parallel to (101) plane, there will be similar results. Otherwise, there is no significant improvement on photocatalysis ability. Our results may give an aid to the process of TiO2 photocatalyst.