具奈米結構赤鐵礦之製備及其在光電化學行為研究;On the Preparation of Nanostructured α-Fe2O3 and Evaluation of Their Photoelectrochemical Behaviors

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Title:	具奈米結構赤鐵礦之製備及其在光電化學行為研究;On the Preparation of Nanostructured α-Fe2O3 and Evaluation of Their Photoelectrochemical Behaviors
Authors:	黃茂嘉;Huang,Mao-Chia
Contributors:	材料科學與工程研究所
Keywords:	赤鐵礦;奈米結構;二次退火;陽極蝕刻;光分解水;光電化學;α-Fe2O3;nanostructure;re-annealed treatment;anodization;water splitting;photoelectrochemistry
Date:	2016-07-21
Issue Date:	2016-10-13 13:22:22 (UTC+8)
Publisher:	國立中央大學
Abstract:	1972年Honda與Fujishima率先提出光電化學法產氫，以太陽光照射二氧化鈦催化分解水釋放氫氣為一前瞻性之產氫方法。氫能源為一重要之新能源，將可解決人類面臨的能源危機及地球暖化之問題；本研究嘗試以氧化鐵奈米結構取代二氧化鈦應用於光催化分解水產氫，利用磁控濺鍍設備搭配一簡單、便宜及快速之電化學陽極蝕刻製程於摻氟二氧化錫(F-doped SnO2, FTO)玻璃基板成長氧化鐵奈米結構，除探討蝕刻溫度對其結構、光電化學特性影響外，並嘗試以二次退火熱處理提升氧化鐵奈米結構之光電化學表現及抗蝕性。研究結果顯示：於不同蝕刻溫度下，經20 V蝕刻2分鐘，退火550 ℃後持溫2小時後，以掃描式電子顯微鏡觀察試片表面呈現三種不同形貌，分別為薄膜(20 ℃)、奈米顆粒(40 ℃)及奈米柱(60 ℃)；若蝕刻溫度升至80 ℃則產生過蝕刻現象而無法應用於光電化學水分解。經由X光繞射圖譜及拉曼分析結果得知：經550 ℃退火之試片皆為純赤鐵礦，由光電化學反應結果顯示：奈米柱結構因具有適當的膜厚(330 nm)、較佳之光吸收率及較強的(110)峰值而具有較佳之光電流表現(0.59 mA/cm2 bias 0.6 V vs. SCE)。此外，將上述之奈米柱結構試片於大氣環境下分別進行二次退火600至800 ℃持溫20分鐘，由光電化學測試結果可知，經750 ℃二次退火之氧化鐵奈米柱具有最佳光電流密度(1.50 mA/cm2 bias 0.6 V vs. SCE)，雖然I(110)/I(104)由1.53降至0.80，但高溫促使FTO基板適量的錫離子之擴散至氧化鐵內部，因而增加載子濃度並降低質傳阻抗，相較於未二次退火之試片(0.59 mA/cm2 bias 0.6 V vs. SCE)，經二次退火熱處理750 ℃後，提升試片之光電流表現約2.54倍。;Renewable energy correlated to hydrogen utilization has been considered as one of potential solutions to solve nowadays urgent issues that humankind encountered in energy crisis and global warming. Photoelectrochemical (PEC) water splitting was a promising method to convert the solar energy into hydrogen by Honda and Fujishima in 1972. In this study, nanostructured α-Fe2O3 have been fabricated on fluorine-doped SnO2 (FTO) glass substrate by DC magnetron sputtering process using a simple, cheap and rapdily method of electrochemical anodization and considered to replace TiO2 for PEC water splitting application. Influences of anodized parameters (i.e., anodization temperature and anodization time) and re-annealed treatment on structural, optical, anti-corrosion and PEC characteristics have been investigated. The as-obtained samples after annealing at 550 ℃ for 2 h in air ambient were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectra, UV-visible spectra and electrochemical analysis. After annealing, all samples revealed only hematite signals by Raman spectra and XRD pattern. Result from SEM, different etching temperatures show various morphologies, i.e., thin film at 20 ℃, nanoparticles at 40 ℃ and nanorods at 60 ℃. However, high anodizing temperature (80 ℃) would lead a great fraction of FTO surface exposed and therefore compromises the photocatalytic activity of hematite electrodes. In PEC results, the α-Fe2O3 nanorods had a better performance of 0.59 mA/cm2 at 0.6 V vs. SCE compare to other nano-structures due to a suitable thickness (330 nm), higher absoprtion in visible wavelength and a higher intensity of (110) peak in XRD pattern. On the other hand, the α-Fe2O3 nanorods diameter increased ranging from 300 to 1500 nm with increasing re-annealed temperature ranging from 600 to 800 ℃ for 20 min, respectively. Based upon our observations, the α-Fe2O3 nanorods re-annealed at 750 ℃ for 20 min indicated a better PEC response with photocurrent density of about 1.50 mA/cm2 at 0.6 V vs. SCE. This value was about 2.54 times higher than the simply annealed at 550 ℃ for 1 h. Observed higher photocurrent density was attributed to Sn-doped on the surface of hematite through its diffusion from the FTO substrate in the re-annealing duration althougth the I(110)/I(104) of samples decreased from 1.53 to 0.80 with re-annealed temperature up to 750 ℃.
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