| It is difficult for fossil energy to support the future development of human society. Moreover, the consumption of fossil energy has brought a lot of environmental problems, so the finding of renewable clean energy is imminent. Solar energy is one of the world's most abundant renewable clean energy, and hydrogen is a clean energy carrier with high energy densities. Photoelectrochemical (PEC) water splitting provides a promising technology to generate hydrogen using solar energy,like the photosynthesis of plants, which can convert the solar energy into chemical energy in one step. To realize the application of PEC cells, three fundamental requirements should be meted: first one is that PEC cells should exhibit a high solar-to-hydrogen efficiency, second one is that PEC cells should show good stability,the last one is that the cost of PEC cells should be cheap. According to the U.S.department of energy, if a PEC cell can realize large-scale practical application, the solar-to-hydrogen efficiency must reach the least value of 10%. The semiconductor material is the key component and must absorb enough of solar light.When 10% is taken as a least value for a practical device, it is hard for a single photoelectrode to achieve this value, while the combined photosynthesis systems show the better chance of realizing such a goal. So far, there are many p-type semiconductors with narrow ban gaps which have achieve high photocurrent, however,these p-type electrodes often suffer poor water oxidation ability and can't split water without applied bias. For n-type semiconductor,?-Fe2O3 and Ta3N5 are the most investigated photoanodes with red light response, but their solar energy conversion efficiency is still low.The band gap of ?-Fe2O3 is about 2.0 eV and its solar-to-hydrogen efficiency can reach as high as 16%, combined with its high PEC stability as well as economy and non-toxic, ?-Fe2O3 is a promising photoanode material for application in the combined photosynthesis systems. However, because the conduction band of ?-Fe2O3 is lower than the potential of water reduction, an applied bias is needed to realize the water splitting, moreover, the saturated photocurrent of ?-Fe2O3 is still low now.These two disadvantage factors limit the application of ?-Fe2O3 in the combined photosynthesis systems.In this paper, we aim to improve the solar-to-hydrogen efficiency of ?-Fe2O3 photoanode. Against the two issues of high over potential and low saturated photocurrent, we employed ion doping, surface corrosion and heterojunction to improve the performance of ?-Fe2O3 photoanode. The main research contents are as follows:The ?-Fe2O3 photoanode was fabricated by a facile hydrothermal method,and its PEC performance was improved remarkably after Ti ion doping.Generally,it is difficult to grow doped ?-Fe2O3 samples directly by hydrothermal methods. In this paper, we successfully fabricated transparent Ti-doped ?-Fe2O3 photoanode by adjusting the FeCl3 concentration and pH value of the precursor solution. It was found that Ti4+ had two kinds of effect, shielding and doping. Ti4+prevented the hematite films from dissolving into acid solution under hydrothermal conditions and thicker films were obtained in the solution with Ti4+. Moreover, the results in this paper indicated that the increase of carrier concentrations, rather than morphology change or surface passivation, played a main role in the photocurrent improvement after doping.The photocurrent onset potential was cathodic shifted by a facile surface corrosion method,and a new mechanism was proposed to explain this phenomenon. Surface corrosion of Ti4+ doped hematite photoanodes using diluted acid solution resulted in a favorable decrease of the onset potential for water oxidation.The photocurrent onset potential was shifted cathodically by as much as 100 mV. In previous reports, the cathodic shift of the onset potential on a ?-Fe2O3 photoanode generally came from accelerating water oxidation kinetics, passivating surface states and/or ions adsorption. We propose a new mechanism that the cathodic shift of onset potential in this experiment is due to the suppressing of back reaction.A Cr-doped SrTiO3/Ti-doped ?-Fe2O3 heterojunction was constructed to improve the charge separation efficiency and decrease over potential of a?-Fe2O3 photoanode. The segregation TiOx on the Ti-doped ?-Fe2O3 photoanode was used as Ti source to prepare Cr-doped SrTiO3. Cr-doped SrTiO3 can form staggered relative band positions with ?-Fe2O3, and an effective built-in potential can form. This approach provides a facile way to achieve Cr-doped SrTiO3/Ti-doped ?-Fe2O3 heterojuction electrodes. The onset potential was shifted negatively for about 100 mV and the shift is nonreversible and stability. We demonstrated that the improved photoelectrochemical properties were attributed to the enhanced charge separated efficiencies,which was induced from the build-in potential forming by the Cr-doped SrTiO3/Ti-doped ?-Fe2O3 heterojunction. |
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