以溶凝膠法製作摻錫氧化鐵薄膜並研究其 在光電分解水之應用

On the fabrication of Sn-doped Fe2O3 thin films by sol-gel process and their application to photoelectrochemical water splitting

簡戩(Chien Chien)

指導教授 ： 林景崎

國立中央大學/工學院/機械工程學系/碩士(2015年)

摘要

本論文於披覆摻氟氧化錫(F-doped SnO2, FTO)玻璃基板上，以溶凝膠法搭配旋轉塗佈製程成長氧化鐵薄膜，並探討其應用於光電化學分解水產氫之功能。研究內容在於改變前驅物中螯合劑(polyvinylpyrrolidone, PVP)之濃度、旋轉塗佈層數製作多層氧化鐵薄膜，進而施以二次熱處理研究錫自FTO擴散至氧化鐵薄膜之情形，探討此多層膜氧化鐵之晶體結構、光學性質和光電化學特性。結果顯示: 上述製程所得之多層氧化鐵薄膜，經掃描式電子顯微鏡(SEM)觀察到具有多孔表面之結構；經X光繞射分析(XRD)及拉曼(Raman)光譜分析得知: 此多層氧化鐵薄膜經550 ℃退火2小時後，皆呈現為多晶赤鐵礦(α-Fe2O3, hematite)且未出現二次相；經偏壓0.6 V (vs. SCE)光電化學測試結果顯示: 當螯合劑濃度與鐵之比例為2 (PVP/Fe = 2) ，且經旋轉塗佈8層之薄膜試片較其他層數薄膜具有較高之光電流(0.015 mA/cm2)。若此經550 ℃退火之8層薄膜再經750℃二次退火之試片，偵測其光電流，發現增高至0.03 mA/cm2，究其原因:係由錫離子自FTO基板擴散至氧化鐵薄膜中，提高其載子濃度所致。 為了解錫離子之作用，故意製作錫摻雜自氧化鐵薄膜。製程如同上述，僅在前驅溶液中添加含二價及四價之氯化錫溶液來造成薄膜摻雜。在2.5、5、7.5及10 at.% 四種摻錫氧化鐵薄膜中，經550 ℃退火2小時後，含7.5 at.%之錫摻雜薄膜測得最佳之光電流密度(0.023 mA/cm2 bias 0.6 V vs. SCE)；以X光光電子能譜儀(XPS)分析可得知:無論摻雜二價或四價錫離子，薄膜在經過退火後均已轉變成四價錫離子之摻雜。

關鍵字

光電化學法 ； 溶膠凝膠法 ； 氧化鐵薄膜 ； 錫摻雜 ； 二次退火

並列摘要

In this study, the sol-gel and spin coating methods are used to fabricate α-Fe2O3-based thin films on fluorine-doped SnO2 (FTO) glass substrates which is used as a photoelectrode for photoelectrochemical (PEC) water splitting. In the first part, effects of polyvinylpyrrolidone (PVP) concentrations and spin-coating times on the microstructural, morphological, optical and electrochemical and PEC properties of pure α-Fe2O3 thin films are investigated. In the second part, we attempted to prepare Sn-doped α-Fe2O3 thin films for PEC water splitting via two types of impurity-doping (i.e., re-annealed at higher temperature or directly added tin chloride to our precursor solutions). The results from scanning electron microscope showed that the pure α-Fe2O3 thin films revealed a porous films on the surface of sample. After annealing at 550 oC in air ambient, all samples belonged polycrystalline hematite structure and no secondary phase via X-ray diffraction patterns and Raman spectra. From PEC performance, the sample deposited via PVP/Fe concentration ratio of 2 and spin-coating times of 8 times which has a higher photocurrent of 0.015 mA/cm2 than other samples. In the section of Sn-doping, we found that the specimen with re-annealing at 750 oC has a better PEC response due to the high temperature of re-annealing allowed the tin ion diffused from the substrate into iron oxide crystalline which increased the carrier concentration leading a higher PEC performance. On the other hand, we found that the 7.5% Sn-doped α-Fe2O3 thin films has a higher PEC response than other samples which prepared via directly added tin chloride to our precursor solutions. It is noticed that no matter whether added divalent or tetravalent tin ion, after annealing, they all became tetravalent tin ion and bonding with iron which were analyzed by X-ray photoelectron spectroscopy.

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