隨著石油價格的飆漲與使用化石能源所引起之環保議題，使得再生能源技術開發正逐漸受到大家的重視。其中利用光電化學電池將太陽能直接轉換成可儲存之化學能，因可解決太陽光能無法連續供應的問題，變成最近的研究焦點。光電化學電池中最主要的工作機制是照光後所產生的少數載子會從光陽極內部傳遞到電極表面，並和水溶液中的羥基(OH-)產生光水解產氧反應，同時在陰極端會有水解產氫反應。因此，要有良好的光水解表現，首先必須先選擇具有好的吸光特性之光陽極材料，且照光後要能產生足夠的電子電洞對以產生光水解反應。而常見的光陽極材料有二氧化鈦、氧化鎢、氧化鐵等過鍍金屬氧化物之半導體。因氧化鐵(α-Fe2O3, hematite)具有適當的能隙2.1 eV可吸收大部分的可見光，還有在水溶液中具有較佳的光電化學穩定性，所以氧化鐵是一個良好的光陽極材料。然而，從文獻中得知，氧化鐵在不同的製備方法下，其光水解表現差異甚大。本研究中分別以陽極電鍍法與鐵氧化法製備氧化鐵，以材料物理特性分析以及電化學分析，包括晶體結構、表面形貌、光學性質、光電流、電池電容(Mott-Schottky)、與介面阻抗分析來了解兩種氧化鐵之光水解性質差異並探討可能之影響機制。
根據實驗結果得知，兩製備法均可以獲得氧化鐵的結構，兩者的表面形貌都是由30 nm之奈米顆粒所構成，且以光吸收的角度來看，陽極電鍍法都是優於鐵氧化法的。可是實驗結果顯示，即使已使用了高能量強度之光源，以陽極電鍍法製備之氧化鐵還是無法產生光水解反應，而鐵氧化法卻可以產生光水解反應；其在1.23 VRHE AM1.5 92 mW/cm2下光電流為0.126 mA/cm2。從電池電容的分析結果可以得知，陽極電鍍法製備之氧化鐵的多數載子(電子)濃度比較高，約為1×1020 cm-3，同時其電容值在未照光與照光的情況下並不會有所改變。相對地，鐵氧化法製備之氧化鐵的多數載子(電子)濃度比較低，約為6×1019 cm-3，且其電容值會隨光源而變。此外，鐵氧化法製備之氧化鐵在正偏壓下會呈現費米能階釘住現象(Fermi level pinning)。從上述之分析結果可以得知，陽極電鍍法製備之氧化鐵在照光下沒有光電流產出(即無光水解現象)之原因，主要是由於材料內部之缺陷濃度所導致，和氧化鐵的結晶結構與表面形貌並無直接之關係。透過介面阻抗分析，我們也印證鐵氧化法的Fermi level pinning確實與高表面能態密度有關，但從此分析方法並無法得知有關光電流的大小、光水解反應之反應速度、表面能態位置及強度，這三者之間的關聯性。從能帶隨偏壓之變化來看，表面少數載子(電洞)濃度之高低，將決定了光水解反應的起始電壓。

Owing to the high oil price and environmental issues, the use of renewable energy have attracted much attention; particularly, using photoelectrochemical (PEC) cell for water splitting. PEC cell is a device that can convert the intermittent solar energy directly into that storable and clean chemical energy (H2) by means of dissociating water. PEC water splitting uses the semiconductor as photoelectrode, which are mainly transition metal oxides including TiO2, WO3, and Fe2O3. Upon the light incidence, minority carriers (holes in photoanode) may be generated and react with OH- of alkaline aqueous solutions to release oxygen molecules at anode and hydrogen molecules simultaneously at cathode. To maximize the cell performance, photoelectrode must have the appropriate band gap energy to absorb as many as possible of photons from solar irradiation, and generate electron-hole pairs that bear sufficient energy to split the water molecule. Among the potential materials, hematite (α-Fe2O3) has been recognized to have the most appropriate band gap energy (2.1 eV). In addition, it also appears to have high photoelectrochemical stability in electrolyte. However, the performance of hematite has not been satisfactory, and shows large performance discrepancy among hematite prepared with different methods. In this study, we prepared two hematite films using respectively the anodic electrodeposition and Fe oxidation method. The differences of their characteristics in water splitting were then analyzed with XRD, SEM, UV-vis absorption spectra, photocurrent, Mott-Schottky and electrochemical impedance spectroscopy (EIS) measurements.
From XRD analysis, both samples appear with hematite crystal structure, and their morphology appears to be the aggregate of nanoparticles with the size of about 30 nm. In UV-vis absorption check, the sample with anodic electrodeposition
III
always shows better absorption than that with Fe oxidation. Nevertheless, the performance of solar water splitting can only occur in cell with photoanode prepared with Fe oxidation method, the photocurrent can reached 0.126 mA/cm2 (at 1.23 VRHE, AM1.5G, 92 mW/cm2). In contrast, no water splitting was observed from the cell using anodic electrodeposition films, no matter how high intensity of incident light. From Mott-Schottky plots, high level concentration of electrons (~1×1020 cm-3) was derived from anodic electrodeposition sample, and no capacitance change can be detected between dark and illumination condition. On the other hand, Fe oxidation method shows lower concentrations which is about 6×1019 cm-3, and capacitance tends to vary with light incidence. Besides, the Fermi level pinning was observed when the sample is subjected to a positive bias. From the aforementioned results, it is evident that the concentration of bulk defects may be the cause for no water splitting observed in the sample prepared with anodic electrodeposition, and has nothing to do with morphology and crystal structure of iron oxides. From the results of EIS analysis, it is clear that Fermi level pinning is directly caused by the high surface state density. Despite that, there seems no explicit correlation between the magnitude of photocurrent, the rate of water splitting, and surface defect state level and its density. From the change of band diagram with bias, it is found that the excess minority carrier concentration concerns more about the onset voltage of water splitting.

1. Tilley, S.D., et al., Light-Induced Water Splitting with Hematite: Improved Nanostructure and Iridium Oxide Catalysis. Angewandte Chemie International Edition, 2010. 49(36): p. 6405-6408.
2. Cesar, I., et al., Translucent Thin Film Fe2O3 Photoanodes for Efficient Water Splitting by Sunlight:  Nanostructure-Directing Effect of Si-Doping. Journal of the American Chemical Society, 2006. 128(14): p. 4582-4583.
3. Cornell, R.M. and U. Schwertmann, The Iron Oxides 2004, Wiley-VCH Verlag GmbH & Co. KGaA.
4. Sivula, K., F. Le Formal, and M. Grätzel, Solar Water Splitting: Progress Using Hematite (α-Fe2O3) Photoelectrodes. ChemSusChem, 2011. 4(4): p. 432-449.
5. Kennedy, J.H. and J.K.W. Frese, Photooxidation of Water at α-Fe2O3 Electrodes. Journal of The Electrochemical Society, 1978. 125(5): p. 709-714.
6. Maruska, H.P. and A.K. Ghosh, Transition-metal dopants for extending the response of titanate photoelectrolysis anodes. Solar Energy Materials, 1979. 1(3–4): p. 237-247.
7. Butler, M.A., Photoelectrolysis and physical properties of the semiconducting electrode WO3 Journal of Applied Physics, 1977. 48(5): p. 1914.
8. Dare-Edwards, M.P., et al., Electrochemistry and photoelectrochemistry of iron(III) oxide. Journal of the Chemical Society, Faraday Transactions 1, 1983. 79(9): p. 2027.
9. Reichman, J., The current-voltage characteristics of semiconductor-electrolyte junction photovoltaic cells. Applied Physics Letters, 1980. 36(7): p. 574.
10. Quinn, R.K., R.D. Nasby, and R.J. Baughman, Photoassisted electrolysis of water using single crystal α-Fe2O3 anodes. Materials Research Bulletin, 1976. 11(8): p. 1011-1017.
11. Ponomarev, E.A. and L.M. Peter, A comparison of intensity modulated photocurrent spectroscopy and photoelectrochemical impedance spectroscopy in a study of photoelectrochemical hydrogen evolution at p-InP. Journal of Electroanalytical Chemistry, 1995. 397(1–2): p. 45-52.
12. Upul Wijayantha, K.G., S. Saremi-Yarahmadi, and L.M. Peter, Kinetics of oxygen evolution at α-Fe2O3 photoanodes: a study by photoelectrochemical impedance spectroscopy. Physical Chemistry Chemical Physics, 2011. 13(12): p. 5264.
13. Klahr, B., et al., Water Oxidation at Hematite Photoelectrodes: The Role of Surface States. Journal of the American Chemical Society, 2012. 134(9): p. 4294-4302.
14. Spray, R.L. and K.-S. Choi, Photoactivity of Transparent Nanocrystalline Fe2O3Electrodes Prepared via Anodic Electrodeposition. Chemistry of Materials, 2009. 21(15): p. 3701-3709.
15. JI, B., A study of α-Fe2O3 thin films for the oxygen evolution reaction in water photolysis 2010, NATIONAL UNIVERSITY OF SINGAPORE.
16. McDonald, K.J. and K.-S. Choi, Photodeposition of Co-Based Oxygen Evolution Catalysts on α-Fe2O3Photoanodes. Chemistry of Materials, 2011. 23(7): p. 1686-1693.
17. Enache, C.S., Y.Q. Liang, and R. van de Krol, Characterization of structured α-Fe2O3 photoanodes prepared via electrodeposition and thermal oxidation of iron. Thin Solid Films, 2011. 520(3): p. 1034-1040.
18. Morin, F., Electrical Properties of α-Fe2O3. Physical Review, 1954. 93(6): p. 1195-1199.