金屬奈米結構增益表面電漿子於光水解反應之研究

Abstract

在能源議題日益重要的今日，乾淨、可重複利用的綠色能源一直是科學家積極追尋的目標，而其中最吸引注意的也許就是「氫能源」了，氫氣是能量密度極高的氣體，其進行氧化反應後可以釋放出大量的能量，有效的將化學能轉換成電能或其他型式的能量，是燃料電池中最常使用的燃料，同時其反應後的產物只有水，沒有額外的廢氣產生，十分符合綠色能源的需求，因此科學家一直透過各種方法希望有效地獲得氫氣，目前商業化的方法主要有利用藻類或微生物的生物產氫法、石化燃料產氫法、高溫高壓下的熱化學產氫法、電解水產氫法等。 近年來，由於光觸媒（photocatalyst）合成技術的突破，利用太陽能源產氫的光電化學電池（photoelectrochemical cell） 逐漸受到關注，這些光觸媒主要成分都是半導體，其基本原理是透過半導體吸收光能後使得價帶的電子躍遷至導帶，若導帶的電子能擴散到半導體表面，則可以和水溶液中的氫離子反應產生氫氣。此方法的優點是不需額外施加外加的能量，僅需照射太陽光就可以產生氫氣；然缺點是大部分用作觸媒的半導體具有較大的能隙，通常需要吸收紫外光才能有效使其電子電洞對分離，這表示僅有７％左右的太陽能量可以被有效使用，而太陽中佔大多數的可見及紅外波段卻被浪費了。本研究的初衷即是希望透過表面電漿子（surface plasmon）的共振效應，將光觸媒能使用的能量波段延伸至可見和紅外光。 本研究選用最為人熟知的二氧化鈦作為光觸媒，並利用簡單的濺鍍或蒸鍍方法在其表面製造奈米等級的金屬顆粒，這些奈米金屬粒子受到外加電磁波的驅動，在某些特定波段會出現表面電子集體震盪的情形，即形成所謂的表面電漿子，在此共振情形下，會有相當大量的熱電子（hot electrons）注入半導體中，因而大幅增加產氫效率。使用濺鍍或蒸鍍方法製造的奈米金屬粒子，粒子大小不一，並且是隨機的分布在二氧化鈦表面，其共振的波長是一個寬頻的分布，能利用太陽光中大多數的光譜能量，這也是大多數文獻中所能達到的效果；然而，我們想進一步比較一個只有某些特定波長的規則結構，是不是有可能透過增強其在這些特定波長的吸收強度，進而超越隨機金屬粒子的產氫效果，同時我們亦可以透過模擬軟體預測在何種結構下，會在特定的波長產生電漿子共振的效應，這也是在一般的隨機結構中難以達到的。因此我們在本研究中也引入了奈米壓印的技術，在二氧化鈦表面上製作大面積的規則奈米柱結構，透過照射不同光源下的光電流量測，我們可以比較規則結構和隨機粒子的產氫效率優劣，相信將來有機會作為未來增益結構的參考。

Nowadays, energy issues have become more and more important. Scientists keep searching for clear and reusable energy sources. Undoubtedly, “hydrogen energy” is the most attractive one among them. Hydrogen is a gas with high energy density. It will generate a great energy after Reduction-Oxidation and efficiently transforms the chemical energy into electric energy or energies with different forms. Besides, the only product of hydrogen after redox is water. No waste gas and no pollutant. Until now, the most common technologies acquiring hydrogen are biological hydrogen production, thermal chemical production, water electrolysis and so on. Electron-hole pairs are generated after these semiconductors absorb the sun-light. Theses free carriers will diffuse onto the surface, and electrons may react with hydrogen ions in the water and reduce them into hydrogen gas. However, most semiconductors in these cells have large band gaps. Only UV light can excite electrons from valence band to conduction band. However, UV light only take up about 7% in sun light, which is relatively small compared with visible light and infrared. In recent years, photocatalyst synthesis makes great strides. photocelectrochemical Cells therefore attracts more attraction. These cells are made of semiconductors. The principle for hydrogen production behind them is very simple. Our object is to extend the working wavelength of these semiconductors from UV light to visible light and infrared by the “surface plasmon resonance” effect, which comes from the collective oscillation of electrons induced by the electromagnetic waves. TiO2 is the most common semiconductors used for photocatalyst. In our research, we try to fabricate metallic nanostructures by simple physical deposition methods---evaporation and sputtering on the surface of TiO2. Then we measure the photocurrents of sample under illumination. If the surface plasmon resonance happens on certain wavelength, there will be a great amount of “hot electrons” injected into the semiconductor. A great enhancement of photocurrents will be expected at this situation. Another point in our research is to fabricate regular nanostructures, which can only have single resonance wavelength. We want to know if these regular structures can generate more photocurrents than random structures make. Although random structures may have a broad-band absorption, regular structures usually have a relatively high absorption peaks at resonance wavelength and may have better effects on current generation. We use “nanoimprint lithography” to fabricate regular structures with a large area. Furthermore, we can predict the resonance wavelength of these structures by simulation, which is almost impossible for random structures. We believe that we demonstrate a possible way to increase the efficiency of photocatalysis.