| 摘要: | 氫氣被預期將代替化石燃料作為未來社會的主要能量來源。由於其具有高重量能量密度，科學家希望可大量生產、運輸。在幾個可能的途徑中，太陽光分解水，─ 特別是光電化學水分解。在太陽光照射下施加小額偏壓，可完全分解水，並分別在陽極和陰極處釋放氧和氫 ─ 特別被寄予厚望。
本論文分為三個部分。第一部分解釋光電化學電池（PEC）的原理，包括半導體光電化學，半導體 - 液體介面和電荷載子物理的知識。雖然完整的水分解包含氧化和還原兩部分，本研究聚焦在陽極氧化水的四個電子轉移反應。，著眼於可見光吸收的適當的帶隙寬度、較高的理論光轉氫效率、極好的反應穩定性、不含重金屬、低成本的特色，我們選擇赤鐵礦 (a-Fe2O3) 作為目標材料。
第二部分研究著重在異價離子摻雜。我們在水熱法 (Hydrothermal synthesis) 合成過程中，摻入In3 +、Mn2+、Ca2 +或Ti4 +作為p / n型摻雜劑，並發現鈦離子摻雜的樣品有最佳的效果。同時，藉由高溫燒結 (> 550 ℃) 讓FTO（F:SnO2）的 Sn 從背面摻雜入Fe2O3，是解決其低導電性的常見方式。研究發現經由Sn4+摻入，在AM1.5G 光照、1.23VRHE的條件下，樣品光電流從近乎0.0 mA / cm2增加到0.42 mA / cm2；藉由Ti4+ 的摻雜，可進一步提升光電流至0.87 mA /cm2。透過EIS，XPS和國家同步輻射中心XAS分析，結果歸因於材料中的Fe2-xTixO3奈米微結構，有效提升Fe-O、Fe-M (M = Fe、Ti) 結構對稱性，(降低再結合速率)延長電子存活壽命以及擴散長度。
第三部分研究是透過浸潤法 (Dip coating)將磷酸（Pi）奈米顆粒或薄膜裝飾於表面。我們發現，相對低溫燒結 ( 300℃ ) 產生的表面磷酸覆蓋層有利於從空間電荷層尾部到固液介面的電洞傳輸。磷酸可橋接和取代表面-OH並佔據內在表面狀態。這種非金屬處理不具有塊材摻雜的效果。在EIS中，在低頻率範圍消除的半橢圓曲線，展現出Pi裝飾的表面效果；在J-V圖中，原已摻雜 Ti 的樣品並未因 Pi 的覆蓋而增益光電化學性質，甚至有弱化效果。這也解釋磷酸層作用於表面的現象。從XRD圖譜特徵峰判讀磷酸鹽層含有 Kx+2(PxO3x+1) 化合物。
本綜合研究探討Ti4+ 摻雜在氧化鐵的作用以及磷酸鹽在表面的功能，也重新提出幾個性質分析討論。Ti摻雜的樣品中，Ti4+ 不作為電子供給者（由Mott-Schotky），但具有調整近表面化學組成，並在氧化鐵內形成Fe-Ti-O奈米微結構。由磷酸鹽製成的磷，對於從塊材中到固液界面的電洞傳輸有所幫助，但我們不認為磷酸鹽會深入摻雜於氧化鐵晶格中。
;Hydrogen has been expected to replace fossil fuel as major energy fuels in future human’s community. With its high gravimetric energy density, grid production and transportation are desired. Within several possible pathways, solar water splitting, especially photoelectrochemical water splitting, can entirely utilize water to evolve oxygen and hydrogen at anode and cathode respectively beyond sunlight illumination with small external bias. This technique has been intensively studied and expected as promising method to link up fossil-fuel community in future.
In this thesis, the study is separated into three parts. First part interprets conceptual knowledge of photoelectrochemical cell (PEC), including general knowledge of semiconductor photoelectrochemistry, semiconductor-liquid junction, and physics of charge carriers. Though a complete water splitting combines oxidation and reduction, we here focus on four-electron process of water oxidation at photoanode. Comparing with several transition metal oxides such as TiO2, BiVO4, and WO3, hematite (-Fe2O3) is selected as target material owning to its proper band-gap width for visible-light absorption, dominating theoretical quantum efficiency and stability within reaction, environmental friendly character, and low-cost for production.
Second part of study lies on aliovalent-ion doping. Foreign-ion doping to hematite has been approved to enhance conductivity and lifetime of charge carriers. We wet-chemically impregnated In3+, Mn2+, Ca2+ and Ti4+ as p-/n-type dopants during hydrothermal synthesis and found promising improvement with titanium ion doping. Meanwhile, the self Sn-doping from back contact (F:SnO2) is an intrinsic way to eliminate drawbacks of synthesized hematite. With Sn4+ assisted, the photocurrent increases from nearly 0.0 mA/cm2 to 0.42 mA/cm2 at 1.23 VRHE under AM 1.5G illumination. Additional improvement was by help of Ti4+ which enhances photocurrent to 0.87 mA/cm2 at 1.23 VRHE under light harvesting. By EIS, XPS, and XAS, the results are ascribed to near-surface nano TiO2 and/or composition heterogeneous (Fe2-xTixO3) in material which may prolong electron survival lifetime as well as diffusion length.
The third part of study was to decorate surface with phosphate (Pi) nanoparticles or thin film by dip coating. The surface overlayer produced by mild-annealing was found to facilitate hole transportation from tail of space-charge layer to liquid site. Phosphorus can bridge and replace surface –OH and take up intrinsic surface states. This nonmetal treatment presents no effect of bulk doping and set its own debate on semiconductor-liquid junction. In EIS, the eliminated semi-ellipsoid exhibits surface effect of Pi decoration; In J-V diagram the overlayer also emphasize its role in reaction by evidence of ignorable increase of photocurrent in Ti-doped (surface-activated) electrodes. XRD patterns indexes Kx+2(PxO3x+1) complex as composition of overlayer itself.
This study addresses a different conclusion toward debate on role of Ti4+ in doped hematite and capability of phosphate ion in bulk iron-oxide. Ti4+ in Ti-doped Fe2O3 didn’t act as electron donor (by Mott-Schotky) but tune near-surface composition and formed TiO2 nanostructure within Fe-O crystal. Phosphorus made by phosphate salt serves sound ability for hole transportation from bulk to solid-liquid interface. However, phosphate surface doping is not suggested. The conclusion contributes to renewed suggestions on several aspects of Fe2O3 photoanode. |
