光觸媒、高介電與鐵電材料奈米複合結構之形成與分析

Abstract

本研究藉由鈦金屬薄片在水基以及乙二醇電解液中陽極氧化合成三種不同形貌及結構的二氧化鈦奈米管陣列，並利用其作為反應模板，在水熱法環境下與含有鋇離子及鹼離子的前驅物在160至180 °C反應後可以得到鈦酸鋇與二氧化鈦的奈米複合結構。根據X光繞射分析、掃描式電子顯微鏡和X光電子能譜儀的分析，這些奈米複合結構可依照它們的形式分為四項：鈦酸鋇薄膜、花狀複合結構、雙層複合結構以及多孔複合結構。 研究中我們同時利用鈦酸鋇與二氧化鈦的多孔複合結構以及二氧化鈦奈米管陣列作為原子層沉積技術生長氧化鉿的模板，氧化鉿在填滿多孔結構後形成奈米管狀電容。在使用阻抗分析儀以及等效電路分析不同結構的氧化鉿奈米管狀電容電性後，我們發現使用特定二氧化鈦奈米管陣列作為模板生長氧化鉿所形成的電容具有最佳的表現：若是使用長度570 nm的二氧化鈦奈米管作為模板生長氧化鉿，這些排列整齊的電容陣列具有800-900 nF.cm-2的電容值，若是使用長度4 μm的二氧化鈦奈米管作為模板，可得到1.7 μF.cm-2的電容值，這些數值都明顯超越了擁有相同介電層厚度的氧化鉿薄膜電容。推測氧化鉿奈米管狀電容與二氧化鈦奈米管陣列所形成的複合結構中，不同的氧化鉿奈米柱電容之間有並聯的效果，造成整體電容值顯著的提升。 本文的另一項主題為探討鐵電基材之極化方向對於二氧化鈦光催化能力的影響。垂直排列的鐵電性鈦酸鋇奈米柱陣列是藉由兩階段的水熱法合成而得，第一步先於FTO基材之上生長排列整齊的二氧化鈦金紅石奈米柱，第二步在維持形貌的狀況下將其轉換為鈦酸鋇奈米柱。接下來使用原子層沉積技術在此具有鐵電性之鈦酸鋇奈米柱陣列間隙以及孔洞間填入二氧化鈦，形成薄膜狀之鈦酸鋇與二氧化鈦複合結構，二氧化鈦在結構之中可提供光致激發的電子電洞對並做為載子運輸的途徑。實驗結果顯示當鈦酸鋇奈米柱具有指向FTO基材的極化方向時，將有助於二氧化鈦光致激發的電子流入FTO基材，造成光電流密度值顯著的提升。

In this study, titania (TiO2) nanotube arrays of different structures and morphologies were fabricated by anodic oxidation of titanium foils in aqueous ammonium sulfate or ethylene glycol electrolytes. After the anodizing process, the nanotube arrays were adopted as templates to fabricate barium titanate (BaTiO3) and TiO2 nanostructure composites using the hydrothermal method with barium hydroxide precursors at 160-180 °C. Based on the SEM, XRD and XPS analyses, the nanostructure composites can be classified into four types according to their forms: BaTiO3 thin film, flower-like BaTiO3/TiO2 composite, BaTiO3/TiO2 double-layer composite and porous BaTiO3/TiO2 composite. This study also reports the use of atomic layer deposition (ALD) to fabricate hafnium dioxide (HfO2) nanotubular capacitors in the prepared TiO2 nanotube arrays or porous BaTiO3/TiO2 composites. In other words, the TiO2 nanotube arrays or BaTiO3/TiO2 composites were used as templates with their pores filled with HfO2 by ALD. After using the impedance analyzer and equivalent circuits to investigate the electrical properties of the HfO2 nanotubular capacitors, it is found that the capacitors with specific TiO2 nanotube (template) dimensions performed the best – 800-900 nFcm-2 was achieved with 570-nm-long TiO2 nanotubes and 1.7 μFcm-2 with 4-μm-long TiO2 nanotubes. These capacitance values are significantly higher than those of HfO2 film capacitors of the same dielectric layer thickness. It is believed that the HfO2 nanotubular capacitors formed within the TiO2 nanotube arrays behave like capacitors in parallel connection; hence, the significant increase in capacitance. Another topic of this study was to examine the influence of the polarization direction of a ferroelectric substrate on the photocatalytic performance of TiO2. A two-step hydrothermal treatment was adopted to grow vertically aligned, ferroelectric BaTiO3 nanorod arrays on conductive FTO substrates. This involved growing rutile phase TiO2 nanorods first and then converting them into BaTiO3 nanorods. The gaps and pores between the ferroelectric BaTiO3 nanorods were then filled with TiO2 by ALD, forming film-like BaTiO3/TiO2 composites. The TiO2 component within the composites could provide both photoinduced charge carriers and a conduction path for the charge carriers. It is found that when the polarization of the BaTiO3 nanorods was poled downward toward the FTO substrate, the photoinduced electrons from the TiO2 were encouraged to flow to the FTO substrate, resulting in a significant increase in the photocurrent density.