| In21st century, energy shortage is a major problem for human being. For this reason, people turn to clean, efficient and renewable new energy from oil, coal and other non-renewable energy. Solar energy is inexhaustible, non-polluting and renewable, thus it should play a critical role in the future development of new energy. Hydrogen has the merits of high fuel value, non-pollution of combustion products and etc. Therefore, solar hydrogen by photocatalytic water splitting will be the future new energy alternative to oil, which has become one of current hot research topics.Hematite (a-Fe2O3), as an n-type semiconductor, has the desired property of a narrow band gap of2.0-2.2eV which in principle can capture～40%of the incident sunlight, together with low cost, and non-toxic, thus it is an ideal candidate material for photocatalytic decomposition of water to produce hydrogen. However, it also has the following disadvantages:(1) serious recombination of photo-generated carriers;(2) slow interface hole transfer;(3) poor conductivity and (4) slightly photocorrosion. The currently reported efficiency for the photocatalytic decomposition of water to hydrogen over a-Fe2O3is still far less than its theoretical maximum (12.9%). Therefore, a variety of methods have been taken for improving the efficiency. Yet it is still far from the requirements of practical application and also the adopted methods is rather complex and expensive. In this thesis, simple electrochemical reduction and an electro-deposition methods were adopted to modify a-Fe2O3in order to overcome its shortcomings and consequently to improve the performance of photoelectrochemical oxidation water or the efficiency of photoelectrochemical hydrogen production. The paper is divided into four parts: In the first part, the undoped and Ti-doped α-Fe2O3thin film electrodes were prepared on the surface of conductive glass (ITO) by sol-gel method, and the effect of preparation conditions on the photoelectrochemical performance of the material was optimized. It was verified that doping with Ti can greatly improve the photoelectrochemical oxidation of water for the α-Fe2O3film electrodes. On the basis of these results, the effect of dye sensitization, surface fluorination, Co3+immersion treatment and Al3+-doping on the photoelectrochemical oxidation of water of Ti-Fe2O3was investigated. The results show that both the dye sensitization and the surface fluorination could not enhance the performance, while Co3+immersion and Al3+-doping were able to significantly improve the performance, with a good photoelectrochemical stability.In the second part, the undoped and Ti-doped α-Fe2O3thin film electrodes were pretreated by electrochemical reduction at-1.2≤E<-0.6V versus saturated calomel electrode in the dark in aqueous1M NaOH electrolyte. This treatment could highly enhance the photoelectrochemical oxidation of water of the electrodes, with a good photoelectrochemical stability. Compared with the undoped α-Fe2O3, the enhancement is more significant for the Ti-doped. Meanwhile, the enhanced mechanism was studied in detail by scanning electron microscopy, energy dispersion spectroscopy, X-ray photoelectron spectroscopy, UV-Vis diffuse reflectance spectroscopy, electrochemical impedance spectroscopy, Mott-Schottky measurements and etc. The enhancement is mainly due to a significant decrease of charge recombination and a favorable photohole transfer and physically associated with the partial transformation of Fe2O3to Fe3O4and/or FeOOH which functions as a catalyst to facilitate the photoelectrochemical oxidation of water. The method is simple and economical, controllable, thus providing a novel approach for improving the photoactivity of water splitting over α-Fe2O3.In the third part, electro-deposition In3+on doped (Ti) ferric oxide to enhance its photocatalytic property. The tests demonstrate treatment materials’ photocurrent are1-2times of the doped (Ti) ferric oxide. when immerse the doped (Ti) ferric oxide in In(NO3)3solution for3hours,then heat the materials in furnace at450℃for30min, it also have the same photocurrent of materials treatment by electro-deposition In3+.Comparing with the immersed method, electro-deposition need less time, less In(NO3)3,and lower cost. By a series experiments we find that enhancement of the photocurrent was mainly of the increasing of the specific surface area and decreasing of the charge recombination.In the fourth part, the impact of applied potential on the interfacial charge-transfer rate constant during photoelectrochemical oxidation water over the two electrodes is investigated by electrochemical impedance spectroscopy. The results shows that increasing the applied anodic potential the interfacial charge-transfer rate constants for both electrodes were increased. The less increase in the magnitude of rate constant than that expected by theory indicates that not all of the applied potential drops across the Helmholtz layer but it does in both the space charge and Helmholtz layers simultaneously (Fermi level pinning). The photo-generated charge could be accumulated in the surface states, caused the re-distribution of potential in the interfacial, and increased the rate constant. Compared with the a-Fe2O3, the improvement of charge-transfer rate constant by anodic potential is more evident. |
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