以局部密度泛函理論探討二氧化鈦奈米柱的生長

Abstract

由於單一方向之單結晶半導體奈米柱在光電設備中的潛在應用性，它的合成方式實為一種具有前瞻性的技術，例如：染料敏化太陽能電池(DSSCs)。前人的研究中提出一種簡單快速的熱合成方式，利用導電玻璃(FTO)作為基材，合成出只延著(001)面生長，單一方向之單結晶金紅石相二氧化鈦奈米柱。在此熱合成方法的框架之下，陸續有許多研究提出其奈米柱可能的成長機制，然而至今仍有非常多尚未被釐清的地方。 在此研究中，我們以局部密度泛函理論，利用電腦模擬軟體Vienna ab-initio simulation package (VASP)去探討前述熱合成實驗中的二氧化鈦奈米柱成長機制。模擬過程中根據原子層沈積理論(ALD)建構金屬前驅物與水的吸附模型，進而研究吸附物在二氧化鈦金紅石相結晶上吸附的差異。由於表面型態對於吸附的差異影響重大，模擬中我們建構了五個表面，分別是(001)，(100)，(101)，(110)及(111)面，放上鈦的來源：四氯化鈦以及氧的來源：水，去模擬二氧化鈦奈米柱之成長過程，從過程中判斷其中分子是否有吸附的選擇性。本研究根據理論化學的方法尋找主導二氧化鈦奈米柱成長的機制，從結果中我們發現(001)面有著高度的來源獲得可能性，進而解釋二氧化鈦奈米柱只延著(001)方向生長的原因。我們認為了解單一方向之單結晶二氧化鈦奈米柱的成長機制有助於提升往後鈦材料在未來的應用，且提供了一個以理論化學去印證實驗的例子。

Synthesis of aligned single-crystalline semiconductor nanorods is promising nowadays because of their potential applications in photovoltaic devices, for example in dye-sensitized solar cells (DSSCs). For DSSCs, a facile hydrothermal method was developed to grow oriented, single-crystalline rutile TiO_2 nanorod films on transparent conductive fluorine-doped tin oxide (FTO) substrates. After that, a growth model of rutile TiO_2 nanorod was proposed, but still many points in the mechanism are unclear. In this study, we try to face the growth mechanism with computer modeling with simulating software, Vienna ab-initio simulation package (VASP), which is based on density functional theory (DFT). And the simulation mode was depended on atomic layer deposition (ALD) theory. The metal precursor and water adsorbing preference are different depending on different surface of metal oxide. Because the surface morphology plays an important role in adsorptions, five different termination of rutile: (001), (100), (101), (110) and (111) were built and be simulated the result of adding precursor of titanium and water on different surfaces to be checked if there are selectivity or not. This research is based on theoretical methods and focus on elucidating key factors of the TiO2 nanorods growth mechanism. According to the simulation results, (001) surface gains high preference of growth. Understanding of growth of oriented TiO2 nanorods and mechanism are helpful to increase performance of titanium applications in the future.